Potential Fertilizer Value of
Gamma-Irradiated
Sewage
Sludge on Calcareous Soils
AGRICULTURAL EXPERIMENT STATION • BULLETIN 692
ACKNOWLEDGEMENTS
This publication is based on research conducted by the New Mexico
Stale University Agricultural Experiment Station (Las Cruces, NM
88003) in cooperation with Sandia National Laboratories, Applied
Biology and Isotope Utilization Division (Albuquerque, NM 87IXS)
under Contract No. AC04-76ET-33626 with the U.S. Department ol
Energy, Albuquerque Operations Office, (Albuquerque, NM 871 15).
New Mexi
available i
CONTENTS
Introduction
Purposes
Study 1
Study 2
Study 3
Study 4
Study 5
Methods
Study 1
Study 2
Study 3
Study 4
Study 5
Results and discussion
Study 1
Study 2
Study 3
Study 4
Study 5
Summary and conclusions
References
Appendix
1
5
6
6
6
6
6
6
6
10
12
14
15
17
17
22
25
27
30
37
38
47
Potential Fertilizer Value of
Gamma-Irradiated
Sewage
Sludge on Calcareous Soils
Bobby D. McCaslin and George A. O'Connor*
Increasing volumes of sewage sludge, environmental constraints and economics associated with alternative disposal
methods have worked simultaneously to increase the interest
in utilizing or disposing of sewage sludge on land. One of the
most readily available and generally cheapest means of utilizing sewage sludge is by application to cropland. The beneficial
effects of application of sewage products to cropland have
long been known; their use by Greek and Roman farmers is
indicated by writings of Theophrastus (327-287 B.C.).
Now sewage products are appreciated for their nitrogen
(N), phosphorus (P), potassium (K). zinc (Zn), iron (lei.
copper (Cu), manganese (Mn), and other constituents (4,17,
20,27,31,36-42,46,47,51-53,55,58,62, 66,67). The United
States Environmental Protection Agencies (EPA) Rules and
Regulations 40CFR-257 (25) state the following:
The application of sewage, as well as other solid wasles, to the land
surface or incorporation wilhin the root zone of crops may provide
significant benefit through the addition of organic matter, nitrogen,
phosphorus and certain other essential trace elements to the soil.
Specifically, land application of solid waste coupled with good
management techniques for enhancement of parks and forests and
reclamation of poor or damaged terrain is a desirable land manage
ment technique.
Application of solid waste to agricultural lands may also be an
environmentally acceptable method of disposal. However, when improperly managed, the application of solid waste to agricultural lands
can create a potential threat to the human food chain through the
entry of toxic elemental compounds, and pathogens into the diet . . .
'Associate professor and professor, respectively. Department of Crop ami Soil
The greatest risks from cropland utilization of sewage
sludge are the potential introduction of toxic levels of certain
organic toxicants, viable pathogens, and heavy metals into
the human food chain. In regard to the organic toxicants risk
I PA regulations 40CFR257 (25) Section 257.3-5. 2 stales:
The proposed criteria required that solid waste containing pesticides
and persistent organics, when applied to land used for the production of food chain crops, not result in levels of these substances
in excess of the tolerances set pursuant to the authorities of the
Federal Food, Drug and Cosmetic Act...
Organic toxicants can be broken down microbially or
inactivated by chemical bonding in the soil. The soil mecha:an be used under present regulations.
ni sms for ina ictivatit
do not contain significant levels of
Many se<vag e sludj
o i ganic l< ixic ants. Under present regulations, even if sewage
lificant levels of organic toxicants they
sli idges d( ) c<mtain
•jd to croplands. However, proper manageca n still he ; .pplied
ontrols must be used to assure subsem cut and m<m i t o r i i
x)d never exceed acceptable contents
qi lent to:tic levels
•
Federal
Food, Drug and Cosmetic Act.
in
the
1
as publish led
T h e r e is risk ol viable pathogens in processed sewage
wastes, including chlorinated effluents and digested sewage
sludges. Ihe review by Burge and Marsh (12) indicates that
the sludge still contains viral, bacterial, protozoan, and helminthic agents of disease. However, they also point out that
the great majority of illnesses associated with sewage appear
to have been caused by application of raw or inadequately
treated sewage materials to crops which, in turn, were consumed raw; contamination of private water supplies from
septic systems; or consumption of raw shellfish grown in
sewage-polluted waters.
The greatest hazard from pathogens that are in land
applications of sewage results from their movement in surface
runoff (Burge and Marsh, 12). There appears to be little
danger, either to workers or the people of surrounding
communities, when chlorinated sewage materials are used for
irrigation or when anaerobically digested or pathogenreduced sludges are applied to soils if the crops produced are
not consumed raw by humans. There are a number of
alternatives for further treatment of sludge-carried pathogens
that survive the primary sewage treatments of anaerobic
digestion, aerobic digestion, etc. These treatment alternatives
include ail or mechanical dewatering, gamma or beta ray
irradiation, pressure-heat treatment, incineration, and heat
drying. Heat treatments result in complete inactivation of
pathogens, but they may pose air pollution and high-energy
consumption problems (64).
The use of nuclear wastes as a radiation source to kill
pathogens in sewage sludge has been studied at Sandia
National Laboratories, Albuquerque, New Mexico. The
results arc adding a new dimension to sludge utilization in the
United States. The use of ionizing radiation for the destruction of pathogens in sewage has been reviewed by
Gejrrard (28), Ballentine (2), and Reynolds (64). The radiation process being developed at Sandia, using cesium-137
separated from nuclear wastes, has a favorable cost/benefit
potential, as outlined by Morris (56). (lamina raws from
cesium-137are sufficiently energetic (660 Kev) to kill pathogens, but the energy is much lower than would be required to
induce radiation in the sludge itself. The cesium-137 has been
shown to be very effective in reducing the number of sludgecarried pathogens to very low levels, including Ascaris ova
(9,69,72,73). Gamma irradiated sludge is free of pathogen
hazard and weed seeds; thus it has considerable potential for
use as a fertilizer in agriculture or as a soil conditioner for
land reclamation (65.68,64. 69,57,70,71 ).
Transfer of heavy metals to the soil-plant system through
sludge has received much interest, particularly with respect to
acid soils. Concentrations of plant-available metals found in
sewage sludges are frequently higher than concentrations
found in soils, and some arc potentially available to crops
growing on treated soils. Thousands of papers have been
published on all aspects of cropland utilization of sludges,
and most states in the United States have ongoing research
projects that include heavy metal information, consequently,
a thorough review for this publication of all related literature
is not possible. In the United States, however, the greatest
concerns have been focused on cadmium-its subsequent
movement into the food chain and associated public health
and
environmental
problems
(5-8,14,16,18,19,21-24,
29,30, 32-35,44,49,50,54).
The EPA regulations 40CFR-257 (25) for heavy metals
singles out cadmium. The regulation states that for food
chain crops, the annual application of cadmium (Cd) from
solid waste should not exceed 0.5 kilograms per hectare
(0.45 lb/A) on land used for production of tobacco, leafy
vegetables, or root crops grown for human consumption.
The regulations also state that on soils (as in New Mexico)
that have a pH greater than 6.5 the cumulative application of
cadmium from solid waste should not exceed the levels in
table 1.
Table 1. M a x i m u m levels of cadmium that can be added to the soil, by
level of soil cation exchange capacity, according to 4 0 C F R 2 5 7
Soil Cation Exchange
Capacity
(meq/100g)
5
5-15
15
Maximum Cumulative Applicatio
of Cadmium with
Background Soil pH > 6.5
kg/ha
lbs/A
5
10
20
4.5
8.9
17.8
There are no cadmium restrictions when: the only food
chain crop produced is animal feed; the pH of the soil is
maintained at 6.5 or greater when food-chain crops are grown;
there is a facility operating plan demonstrating how the
animal feed will be distributed, to preclude ingestion by
humans; and future property owners are notified, by a stipulation in the land record or property deed, that the property
has received solid waste at high cadmium application rates.
No other heavy metal restrictions have been published at
this time, however, a proposed supplemental EPA regulation
(40CFR258, Distribution and Marketing of Sewage Sludge
Products) is being reviewed for possible adoption. These
regulations should further del ine restrictions on cadmium and
toxic organics and include restrictions on lead (Pb). These
also will probably require a process, such as irradiation
01 heal treatment, to essentially eliminate pathogens.
Since most metallic elements are less available to plants on
neutral to high-pH soils than on acid soils, it is generally
recommended that the p l l of soils amended with sewage
sludge l^e maintained al near neutral or higher pll ( l o t . However, little information is available on the effect of the application of sewage sludge carrying trace elements or heavy
metals to calcareous soils thai have a high pi I of 7.5 or more.
Utilization of sludge cm cropland with calcareous soils
(generally 7.5 or higher pll) and gamma irradiation of the
sludge greatly reduce the two major risks (pathogens and
heavy metals) to the human food chain.
Application of sludge would be beneficial to approximately 1.4 million hectares (3.5 million acres) of land in the
southwestern United States that arc deficient in various
essential plant micronutrients. The \w:c<.\ is especially acute
with iron in these soils because of the high p l l , lime, ami
bicarbonate contents (59), but zinc, manganese, and copper
have also been shown to be deficient.
McCaslin and
Rodriguez (49) reported that where sewage effluent in New
Mexico was used over a 40-year period as the only source for
irrigating crops, it supplied sufficient nutrients so that no
fertilizer was required. Furthermore, the beneficial increases
in Fe, Zn, and Cu in plant tissues were not accompanied by
detectable increases in toxic metals (Cr, Cd, Ni and Pb).
PURPOSES
This publication summarizes live studies covering gammaradiation effects on the availability o f sludge elements to
plants and the response of crops to sludge application on
selected calcareous soils in New Mexico. Reports on the live
studies have been published previously, as noted by
references.
Study 1 (50) was to examine effects o f gamma-irradiation
on plant-available nutrients and toxicants and to examine
plant response to sewage sludge.
Study 2 (49) was to study the potential phytotoxicity
problem of RDSS and to examine the micronutrient fertilizer
value of RDSS on a typical Fe-deficient soil from New
Mexico.
Study 3 (47) based on the results of study 2. was designed
to further evaluate the efficiency of RDSS as a soil-applied
iron fertilizer by direct comparison with the best iron fertilizer known at the time.
Study 4 (60) was to evaluate effect of irradiated sewage
ami water extracts of the sludge on Fe, Zn, and P availability
to plants in an attempt to identify the significance of natural
chelates in the sludge in making soil-micronutrients more
available to plants.
Study 5 (61) was designed to determine whether essential
metals were coming from the sludge directly, in readily
available forms, or indirectly, as metals released from soils by
reaction with the sludge.
METHODS
Study 1 (McCaslin and Titman, 50)
Characterization of Sludge Products - Approximately 400
liters (106 gallons) of each raw and anacrobically digested
liquid sewage sludge, from the Albuquerque sewage treatment plant, were collected and thoroughly mixed then
divided into halves. One-half of each of the digested (DSS)
and undigested (USS) sludge samples were gamma-irradiated
(RDSS and RUSS, respectively); the other half of each sample
remained untreated. The sludges were then air dried and
shipped to New Mexico State University.
For chemical analysis, sewage sludges were dried in a
forced-air oven at 25 °C, (77 °F) and ground to pass a
lO-nicsh screen in a stainless steel Wiley mill. The ground
samples were analyzed for total nitrogen according to
Bremner ( 10) and for organic carbon content by the WalkleyBlack method (l). Total metal analysis of the sludge was
performed using a nitric + perchloric acid wet digestion (63).
Water-soluble elements were determined by mixing deionized
water with 50 g (1.75 oz.) of sludge until an approximate
saturation paste was obtained (150 percent water by weight
lor digested sludge and 250 percent water by weight lor
undigested sludge). Saturation pastes were equilibrated lor
24 hours and then suction filtered. Sludges were prepared
lor DTPA (diethylenetriaminepentacetic acid)-extractable
metals determination by shaking 25 g (0.875 oz)ol sludge
with 150 ml DTPA extractant lor 2 hours. For 0.1 N IK I
(hydrochloric acid) extractable metal determination, sludges
were prepared by shaking 2g (0.07 oz) sludge with IO()-ml
0.1 N HCI. All extracts were analyzed for Fe, Cu, Mn. Zn.
Pb, Cr, and Cd by atomic absorption spectophotometry.
Plant-Available Sludge Elements
A greenhouse experiment was established with II treatments (table 2) in a live
replication randomized-block design. Each of the sludges was
added to soil at rates sufficient to supply 255 and 509 kg/ha
(500 and 1,000 lb/A) of elemental N, based on total N
contents of the sludge.
An untreated soil and (wo commercial fertilizer treatments, 1 1 2 kg N/ha (100 lb N/A) plus
45 kg P/ha (40 lb P/A) and 224 kg N/ha (200 lb N/A) plus 90
kg P/ha (80 lb P/A), were included as checks in the experiment. Fertilizers or sludges were thoroughly mixed with 3.5
kg (7.7 lb) of soil and placed in 17.8 cm (7 in) plastic pots
with drainage holes in the bottom. The soil used in the
experiment was from the surface 30 cm (0 to 12 in) of
Glendale clay loam soil (Typic Torrifluvent) at the New
Mexico State University Plant Science Research (enter.
Selected chemical properties of the soil are given in table 3.
light grain sorghum (Sorghum bicolor var. Capitan) seeds
were planted in each pot and thinned to three plants 2 weeks
after germination. Plants were grown with natural illumination in a greenhouse for approximately 2 months (September
25 to November 19, 1976). Twice weekly pots were watered
by hand to approximately 90 percent of field capacity.
Plants were harvested from the soil surface, washed, dried
at 70 °C (158 °F), and weighed. Three replicate plant tissue
samples were analyzed for total nitrogen (Bremner, 10) and
digested with nitric + perchloric acid (63) for analysis of total
V. K. Fe, (ii. Mn. Zn, Cr, Cd, and Pb.
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Following harvest, soil samples were taken from the surface
to the bottom of each pot. These samples were air-dried,
sieved (2 mm, 0.08 in), and analyzed for extractable P
(IN NaHC0 3 ), K (NH 4 OAC). NH 4 , and N 0 3 : pH; electrical
conductivity; and DTPA-extractable Fe, Mn, Zn, Cu, Cd, Pb,
and Cr (66).
Study KMcCaslin and Rodriguez, 49)
Sludge
Anaerobically digested sewage sludge was collected from the Albuquerque sewage treatment plant by Sandia
National Laboratories personnel and gamma-irradiated (150
Krad). In the laboratory, the RDSS was dried at 25 °C (77
° F) in a forced air oven, ground, and stored in a plastic barrel.
Soils - The soil used was from Lea County, New Mexico.
Bulk samples of this soil were obtained randomly from the 0
to 25 cm (0 to 9.75 in) depth. The soil was air-dried, crushed
to pass a 5-mm (0.2- in) sieve, and stored in plastic barrels.
The Lea soil was classified as a fine-loamy, mixed, Petrocalcic
Paleustoll. Chemical properties are given in table 4. The Lea
soil is known to be severely iron-deficient. Sorghum plants
growing in the Lea soil typically develop iron chlorosis
symptoms very early (7 to 10 days after germination) and
usually die before significant growth occurs.
Table 4. Selected properties of Lea, Harvey, and Glendale soils used in
studies 2 to 5
Item
Property
Texture
pH
CaCO,.%
Organic C, %
Lea
Fine sandy
loam
7.8
11.0
0.57
Elements extract ed by DTPA-Soil Test
Fe, ppm
2.0*
Zn, ppm
3.0
P, ppm
6.0"
Soil
Harvey
Very fine
sandy loam
7.7
0.5
0.43
4.8
1.8
13.0*
Glendale
Clay
7.9
8.8
0.90
9.4
2.0
27.5
Experimental Design
Nine treatments (table 5) were
selected for the first planting. The RDSS was applied al
constant increments of 10 metric ton/ha (4.5 t o n / A ) up to
50 metric ton/ha (22.3 t o n / A ) on a surface area basis. The
treatments included as checks were: untreated soil, soil
sludge ratio o f 2:1 (soil/RDSS) by dry weight, pure RDSS.
and commercial fertilizer 200 kg N/ha (178 lb N/A). The
normal rate o\' N recommended for sorghum in the
Lovington, New Mexico, area is 1 70 kg/ha ( I 50 l b / A ) .
The RDSS and fertilizer (urea) were thoroughly mixed
with 2 kg (4.41 lb) of soil and placed in a plastic pot with
drainage holes in the b o t t o m . The nine treatments were
applied in a randomized complete block design, with four
replications. Ten grain sorghum seeds were planted in each
pot ami thinned to two plants live to seven days alter
germination.
Table 5. Treatment descriptions of greenhouse experiment in study 2
Amount of RDSS or Fertilizi
Added to 2 kg (4.4 II)) Soil
Soil nly
RDSS 10 metric ton/ha (4.46 ton/A)
RDSS 20 metric ton/ha (8.93 ton/A)
RDSS 30 metric ton/ha (13.39 ton/A)
RDSS 40 metric ton/ha (17.86 ton/A)
RDSS 50 metric ton/ha (22.32 ton/A)
/A)
RDSS 381.5 metric ton/ha (170
Ratio 2:1 (soil/RDSS
Pure RDSS (no soil)
200 kg N/ha (178 lb N/A)
16.51 g (0.57 07) RDSS
33.02 g (1.16 07) RDSS
49.54 g (1.73 oz) RDSS
66.05 g (2.31 oz) RDSS
82.56 g (2.89 07) RDSS
630.00 g (22.05 oz) RDSS
1,260.00 g (44.10 oz) RDSS
Plants were grown with natural illumination in ,i green
house for l() weeks, from October 29, 1976, to January 8.
1977.
Pots were initially watered to approximately 90
percent of field capacity (as deteimined by a pressure plate at
1/3 bar), then later twice per week.
Plants were harvested at the soil surface and weighed to
13
determine total dry-weight yields. Plant tissue samples were
ground and stored in sealed plastic bags for later chemical
analysis. Alter plant harvest, soils samples were taken from
each pot, air dried, sieved (2 mm, 0.08 in), and thoroughly
mixed. They were stored in scaled plastic bags for later
chemical analysis. Plant tissue and soil analysis were completed using the same procedures outlined in study I.
After collection of soil samples from each pot from the
first planting, the soil surface was mixed to loosen the surface
as if by tillage for the second planting period (January 14,
1977, to March 25, 1977). Seed of the same grain sorghum
variety was then planted in each pot without new application
of the treatments. The same watering, sampling, and analysis
procedures for plant and soil, were used as for the first
planting. For the third planting (from April 3, 1977, to
June 13, 1977), the same procedures were followed as for the
second planting.
Study 3 (McCaslin et al., 47)
Experimental Design
The soil used (Lea soil) was the
same as that used in study 2 (table 4). The 14 treatments
selected are presented in table 6.
The RDSS treatments were applied at constant increments
of 10 metric ton/ha (4.46 ton/A), up to 40 metric Ion/ha
(17.86 ton/A). Fertilizer Fe was added in the form of iron
chelate, active ingredient technical sodium ferric ethylenediamine di-hydroxyphenylacetate. The compound is sold
commercially as "Sequestrcnc 138 Fe" (S-138 Fe) and
contains 6 percent Fe by weight. The distributors recommend applying up to 5.6 kg/ha (5 lb/A) of the commercial
(S-138 Fe) material at planting, or when deficiencies first
appear.
However, because the Lea soil is severely Fedeficient, the iron treatments were applied at constant
increments of 10 kg/ha (8.92 lbs/A), up to 30 kg/ha (26.76
lb/A) of S-138 Fe, plus an additional high-iron treatment of
5 kg Fe/ha equal to 83.34 kg/ha of S-138 Fe (75 lb Fe/A).
The 83.34 kg/ha of S-138 Fe would not be economically
practical (cost = $3,000 per hectare); however, this treatment
was included for comparison purposes. All the Fe treatments were applied with and without the application of the
14
N-P fertilizer treatment. An untreated soil and one N-P
fertilizer treatment alone (240 kg N + 50 kg P 2 O s /ha;
21 4 lb N/A + 44 lb P2 Os /A) were included as checks.
Since the amounts of S-138 Fe used were very small,
each iron treatment was mixed with small amounts of the,
same soil (8 to 10 g; 0.28 to 0.35 oz) passed through a 60mesh screen, and mixed uniformly into each pot at planting
time. The RDSS or fertilizers (urea and superphosphate
materials) were thorougly mixed with 2 kg (4.4 lb) of soil
and placed in a plastic pot with drainage holes in the bottom.
Fen grain sorghum seeds were planted in each pot and
thinned to two plants at 5 to 7 days after germination.
Plants were grown with natural illumination in a greenhouse
(different from experiment 1 ) for 10 weeks, from September
16, 1977, to November 14. I()77. The watering and sampling
procedures were the same as described for the first planting
in study 2.
Plant tissues from treatments soil alone, fertilizer, and 10,
20, and 30 kg/ha (8.9, 17.8, and 26.7 lb/A) of S-138 Fe with
and without fertilizer, were composited over replications
before analysis due to the small amount of plant material.
The remainder of the treatments and replications were
collected individually after sorghum harvest. Soil samples
were collected and plant tissue and soil analysis followed
procedures in study 1. An analysis of homogeneity of variances was done, using Bartlett's test to compare the 14 means.
Study 4 (O'Connor, McCaslin and Sivinski, 60)
Sludge and sludge extract treatments were applied with
and without inorganic Fe in an attempt to detect effects of
natural chelates in the sludge in making micro-nutrients more
available to plants. Plants were grown on the amended soils
in the greenhouse for two months and later analyzed for
Fe, Zn, and P contents. Changes in extractable nutrient
contents of each soil were related to treatments and to plant
tissue nutrient contents.
Three soils were used including surface samples of the
Glendale series (Typic Torrifluvent), the Lea series (Petrocalcic Paleustoll, same as in study 2), and the Harvey series
(Ustollic Calciorthid). Selected properties of each soil are
given in table 4.
16
The greenhouse experiment] was designed to include three
replications of all treatments in a randomized block design.
Treatments included irradiated sewage sludge added at 0,
22.4, and 44.8 metric ton/ha (0, 10, and 20 t o n / A ) , water
extracts o\ the sewage sludge rales equivalent to 0, 22.4 and
44.8 metric ton/ha (0, 10, and 20 t o n / A ) , and inorganic Fe as
FeSO< at 0 and 20 kg/ha (0 and 17.8 l b / A ) . The result was
12 treatments per replication including each of the sludge
treatments with and without the FeSO„ (total of 36 pots).
Sewage sludge was thoroughly mixed with the entire 3.6 kg
( 7 / ) lbs) o\ soil used in each 17.8 cm (7 in.) plastic pot. Sewage sludge extract was prepared by equilibrating a 2:1 (water:
sludge) suspension lor 24 hours with each rate of sludge, and
filtering through a 0.22 micron Millipore filter. The solution
phase, expected to contain soluble chelating agents (but lew
nutrients) was then applied uniformly to the surface of each
soil. Inorganic Fe as FeS() 4 was mixed with the entire 3.6 kg
(7.9 lb) of soil. N and P were applied to all pots in blanket
application of ( N l l „ )2 HPO, equivalent to 200 kg N/ha
(197 lb N / A ) and 230 kg P/ha (and 205 lb P/A).
All pots were thoroughly wetted w i t h about 5 cm ( 1.95 in)
of tap water, to leach salt associated with sludge treatments,
and then covered and left to drain. The next day, each pot
was weighed to determine the " p o t holding capacity."
Subsequent irrigations (twice per week) were sufficient to
bring each pot back to its pot-holding capacity.
Sorghum (Sorghum bicolor vat. Savanna) was used as the
test crop and thinned to three plants per pot. Plants were
grown for two months in the greenhouse and harvested
at the soil level. Plant tissue and soil analysis were completed
using procedures outlined in study 1.
Study 5 (O'Connor,
Watson <v Jahn. 61)
The same three soils used in study 4 were used in the
greenhouse study (table 4). The Lea sandy loam (Petrocalcic
Palcustoll) was known to be Fe-deficient; the Harvey fine
sandy loam (Typic Haplargid) was reported by a local farmer
to respond to Zn applications and the Glendale clay (Typic
Torrifluvent) was sufficient in both Fe and Zn. Bulk samples
o f each soil were divided into 500 g (1.1 lb) portions and
17
held in plastic zip-lock bags, 6 bags per soil. Three bags per
soil were treated with 500p C of carrier-free 59p e a n t j (| le
other three bags per soil were treated with 500 juC of carrierfree 65zn. In all cases, radioisotopes were added in sufficient
tap water to wet the soils to approximately 1/3 bar moisture.
Each bag of soil was then kneaded thoroughly and set aside
for 2 days. Flic bags were then opened and the soil allowed
to air-dry in the greenhouse. Dried soils were rewetted,
kneaded ami again set aside to equilibrate. This wetting and
drying procedure was continued for a total of 10 cycles
covering 40 days to equilibrate the radioisotopes with the
soils. Preliminary studies suggested that the specific activity
of each clement in each soil would change little after about
30 days equilibration.
Following equilibration, 10 g (0.35 oz)of each soil-isotope
mix was extracted with DTPA to determine the element's
specific activity (the amount of tracer activity per unit
amount of Zn and Fe). The DTPA soil test was chosen as the
extractant because water or dilute salt solutions failed to
extract detectable amounts of nonradioactive Fe or Zn, and
because the soil test was regarded as being representative of
the labile pool of Fe and Zn utilized by plants in calcareous
soils (Lindsay and Norvell, 45). DTPA extracts of each
soil-isotope mix were analyzed for total Fe or Zn by atomic
absorption spectrophotometry, and for 59fe or 65zn by
gamma scintillation using a Nal crystal.
Sludge treatments (0, 22.4, and 44.8 metric tons/ha, [0,
10 and 20 ton/A I) were applied to each soil-isotope mix
following the specific activity determinations. The sludge
used was secondary, anacrobically digested municipal sewage
sludge from Albuquerque, New Mexico (table 7). The sludge
had been air-dried and gamma-irradiated (I megarad) to
further reduce pathogens. Dried and ground sludge was
mixed thoroughly with the dried and ground isotope-labelled
soils and (NH4 )2IIP()4 fertilizer in plastic bags. The fertilizer
additions were equivalent to 200 kg N/ha (178 lb N/A) and
230 kg P/ha (205 lb P/A). Each soil-sludge mixture was
placed in small plastic pots and covered with about 2 cm
(0.78 in) of nonradioactive soil. Each pot was watered to
saturation and allowed to drain for two days with the surface
18
covered to minimize evaporation.
Each pot was then
weighed to determine the pot-holding capacity. Subsequent
water additions (every lew days) were sufficient to bring each
pot back to the appropriate pot-holding capacity weight.
Pots were watered frequently to maintain the soils near their
pot-holding capacity, and to minimize soil moisture effects
associated with the sludge treatments.
Twenty-five sorghum {Sorghum bicolor var. Savanna) seeds
were planted in each pot and allowed to germinate. Ten days
after germination, each pot was thinned to 15 seedlings of
approximately uniform size. Plants were harvested at ground
level after 21 days and immediately rinsed in dilute (0.1 N)
hydrochloric acid and then in distilled water. Plants were
dried in a forced air oven at 21 °C (70 ° F ) lor 3 days then
ground in a Wiley mill to pass a 40 mesh stainless steel
screen. Dried plant material was weighed and then analyzed
in plastic vials lor 59Fe or 6 5 / n content. Samples of plant
material were digested (nitric + perchloric acid), as in all the
other experiments, and analyzed for Fe or Zn. These data
were then used to calculate the specific activity o f 5(^Fe or
6 5 / n in the plant material.
Following harvest, soil from each pot was air-dried and
extracted w i t h DTPA.
These extracts were analyzed for
59pe, 6 5 z n , and total Fe and Zn. Final specific activities of
59Fe and 65Zn in the soil extracts were calculated for
comparison with the initial values, determined before sludge
and fertilizer addition.
RESULTS A N D
DISCUSSION
Study 1
Chemical Composition of Sludge
The total analyses of
the dried sludges used in the study show a difference between
undigested (USS) and digested (DSS) in nutrient concentration of N, organic carbon, pH, electrical conductivity, C/N
ratio, Pb, Cd, Cr, Fe, Zn, Cu, and Mn (table 7). The digested
sewage sample did not come from the same batch oI sewage
as the undigested sample. Since sludges differ in nutrient
concentration with sampling time, the variation in total
19
•o -5
If
< S
D
DC D
OC I -
5
chemical analysis between digested and undigested sludges is
partially confounded by different times of production. The
analyses do show higher concentrations o f metals in the
digested sludge, which is the result of the loss o f organic
matter by microbial decomposition.
The radiation process, however, had no statistical effect on
total element concentration of sewage sludge, comparing
undigested (USS) to irradiated undigested (RUSS) and
digested (DSS) to irradiated digested (RDSS) (table X).
Three extractants were selected to give three degrees ol
strength of extractabilily of sludge elements; they did not
necessarily represent plant available heavy metals. However,
if there were no effects of radiation treatment on any o f
these extracts it would suggest little effect on plant uptake.
The data in tables 8 and 9 indicate no significant differences
in extractability as a result of irradiation.
Table 8. H C 1 . D T P A , and H 2 0 extractable Fe, Z n , Cu, M n , Cr, Cd, and
Pb f r o m undigested, dried sewage solids, of which one half was
radiation treated (RUSS) and half untreated (USS)
Undigested (USS)
795
35.0
0.50
radiated Undigested (RUSS)
HCI
4248
971
DTPA
795
352
38.0
0.50
DTPA
1 sludge elemental conc<
Table 9. H C 1 , D T P A , and H 2 0 extractable Fe, Z n , Cu, M n , Cr, Cd, and
Pb from digested, dried sewage solids, of which one-half was
radiation treated (RDSS) and half untreated (DSS)
Extracts
Fe
Digested (DSS)
HCI
3389
DTPA
156
H20
2
Total
14671
Zn
Cu
Mn
Cr
Cd
Pb
1580
437
11
577
147
6
150
14
2
74.0
0.6
0"'
25
8
0
525
72
0.02
26
678
24
8
512
74
1684
Irradiated Digested (RDSS)
HCI
3204
1528
DTPA
166
421
1132
560
161
236
144
14
379
68.0
0.6
Effect of Radiation on Plant Nutrient Availability
Significantly more sorghum dry matter was produced in all
undigested sewage sludge treatments and in the 67.2 metric
ton/A rates of the digested sewage than in the check treatments (tabic 10). The two high-rate (40.3 metric ton/ha
[17.9 tons/A]) treatments of USS gave the highest yields,
followed by the two high rates of DSS and the two low rates
of USS. There was no significant difference between the two
low rates of DSS and the checks. Also, no significant growth
differences occurred between any corresponding radiation
and untreated sludge treatments, indicating little or no effect
from the radiation treatment with respect to growth response
for sorghum.
The sorghum tissues were then analyzed to determine
whether nutrient concentrations varied, thus testing whether
radiation had significant effects on elemental concentrations
in tissues that perhaps were not displayed in plant growth
22
Table 10. The average sorghum dry matter produced per sludge and
fertilizer treatment after growing approximately t w o months
in greenhouse conditions
Treatment
Yield (g/pot)*
.159c"
142c
242c
1.770b
2.973a
1.782b
2.443a
.554c
1.480b
.426c
1.438b
Check
11 2-45-0 kg/h;) (100 40-0 lb/A)
224 90 0 kg/hij (200-80-0 lb/A)
USS 20.2 metric ton/ha (9 ton/A)
USS 40 3 met. i<: ton/ha (18ton/A)
RUSS 70.2 metric ton/ha (9 ton/A)
RUSS 40.3 me trie ton/ha (18 ton/A)
DSS 33.6 metr ic ton/ha (15 ton/A)
DSS 67.2 metr ic ton/ha (30 ton/A)
RDSS 33.6 me trie ton/ha (15 ton/A)
RDSS 67.2 me trie ton/ha (30 ton/A)
ificantly different
differences.
No significant increases (P > .05) o f elemental
concentrations in plant tissues resulted from corresponding
radiation-treated and untreated sludges for the elements analyzed by orthogonal contrasts (appendix table l ) . The total
nutrient uptake (i.e. elemental concentration in tissue m u l t i plied by the total dry-matter yield) indicated an effect of
sludge applications on total heavy metals removed from the
soil (Appendix Table 2). The highest yielding treatments
removed the most metals.
Since the application rate of each sludge was based on the
nitrogen content of the sludge, the total amount o f heavy
metals added per pot varied with sludge type as well as w i t h
amount added. More metals were added per pot in the DSS
treatments. Extractable soil Cu, Cd, Pb, Fe, Zn, Mn, and P
were significantly higher for the DSS treatments than for the
USS treatments, which is consistent w i t h the fact that more
o f these elements were added initially to the soil in the DSS
treatments.
Kirkham (43) found little effect of 400 Krads o f beta23
irradiation on availability of heavy metals to plants, which is
consistent with results of present studies. Rosopulo et al.
(65) found little effect of using gamma-irradiation on subsequent heavy metal availability to plants. Also Beck et al.
(3) showed that a 300 Krad gamma-irradiation treatment had
no detectable effect on mineralization of sewage nutrients,
indirectly supporting the results of the other studies. These
three studies support the results of study 1 and strongly
suggest that the results of using sludges on soils should not
differ greatly whether sludges are irradiated or left untreated.
The results of study 1 also show a general lack of any phytotoxicity in the sludge used and show very good growth
response by sorghum to the sludge additions on the New
Mexico soil that was used.
Study 2
Sludge The RDSS used is characterized by a slightly acid
reaction, almost 12 percent organic-carbon content, with C/N
ratio of 10/1 plus a very high CEC compared to the soil used,
and a very high salt content (table 11). Most of the total
concentrations of micronutrients and heavy metals concentrations are in general agreement with RDSS values reported
in table 7 and would be typical of dried, secondary-treated
sewage solids from the Albuquerque sewage treatment plant.
However, notable variations in Pb and Cd contents were
observed among batches of sludge, emphasizing the need to
characterize each sludge batch used.
Plant Material Sorghum plants in the RDSS-treated Lea
soil were actively growing, deep green, and healthy-looking.
Plants in the pure RDSS and 2:1 (soil/RDSS) ratio treatments died in the first growth period, and some in the pure
RDS treatment died during the second growth period.
Death was probably due to the high salt levels indicated by
high electrical conductivity values, greater than 5 mmhos/cm.
However, during the third growth period, the plants in the
pure RDSS treatment were the largest plants. Plants growing
in the control (soil and fertilizer) treatments were stunted,
light-green to yellow in color, and very unhealthy looking,
typical symptoms of iron deficiency. These symptoms were
more obvious in subsequent growth periods.
24
Table 1 1 . Chemical characteristics of gamma irradiated, digested sewage sludge (RDSS)
Parameter
Organic carbon (%)
pH (saturated paste)
EC (mmhos/cm)
CEC (me/100 g RDSS)
Total N (%)
NO,-N (ppm)
Total P (%)
Total K (%)
Total Fe (ppm)
Total Zn (ppm)
Total Mn (ppm)
Total Cu (ppm)
Total Cr (ppm)
Total Ni (ppm)
Total Pb (ppm)
Total Cd (ppm)
DTPA Fe (ppm)
DTPA-Zn (ppm)
DTPA-Mn (ppm)
1 1.92
6.53
7.84
59.48
1.13
1.00
2.46
0
13,405
1,513
296
1.796
233
207
179
13
77
415
2.45
Total dry-matter yields show significant RDSS effects for
the successive plantings (figure 1). Yields from the first
planting were significantly increased by RDSS up to 40
metric ton/ha (17.9 tons/A).
No first-harvest yields were
obtained in the 2:1 (soil/RDSS) or the pure RDSS treatments, probably due to the high salt contents as measured
by electrical conductivity.
The second and third harvest
followed trends similar to those for the first harvest, except
that higher yields were obtained w i t h 2:1 (soil/RDSS) ratio
and pure RDSS treatments. The soil alone and fertilizer
treatments produced the lowest yields.
Ihe increase in yield after each successive planting
indicated that crops continued to respond favorably to RDSS
applications, for at least three plantings alter treatment,
principally at the higher RDSS application rates. However,
these rates may have supplied greater amounts o f nutrients
than were needed initially. Results show that RDSS was an
excellent source of nutrients for sorghum grown in this soil.
25
20
30
RDSS M.T/ho
40
TREATMENTS
Fig. 1 . E f f e c t o f
the three
letter are
Duncan's
50
200-0-0
kgZha
21
SOIL
PURE"
RDSS
RDSS
RDSS rate o n sorghum d r y - m a t t e r yields f o r each of
harvests. W i t h i n harvests the bars having a c o m m o n
n o t s i g n i f i c a n t l y d i f f e r e n t at the 5 percent level by
M u l t i p l e Range Test.
Chromium, Cd, Ni, and Pb were not detectable in sorghum
tissue digests, even from the pure RDSS treatment (detection
limit was 0.25 ppm). Iron concentration in plant tissue was
not significantly affected by treatments at the first harvest
(probably due to the large variation among replications,
appendix table 3). Iron levels at the second and third harvest
were affected by some RDSS treatments, but there was no
consistent trend. Iron concentrations in plant tissue tended
to decrease from the first to the third harvest (particularly
from the first to second harvest) this was similar to the trend
followed by soil DTPA-cxtractable Fe (appendix figure I).
However, statistical comparisons over time and across
experiments were not done.
26
Zinc concentration levels in the first harvest were affected
by RDSS treatments, with tissue concentrations increasing as
RDSS rates increased (appendix table 3). I evels at the second
and third harvests were affected in a manner similar to that
described lot Fe, except] foi the pure RDSS treatment where
concentrations increased rather than decreased. The amount
o f DTPA-extractable soil Zn also tended to decrease after
successive harvests for each treatment (appendix figure 2).
Manganese levels were either not affected or were reduced
by the RDSS treatments, except for an increase by the pure
RDSS treatment in the second and third harvest (appendix
table 3). There was no significant effect of treatments on Cu
level at any of the three successive harvests.
Iron, Zn, Mn, and Cu concentrations were below the
tolerance levels o f agronomic crops suggested by Melsled (27)
with the exception of Zn at the third harvest in the pure
RDSS treatment, where the suggested level of 300 u g/g was
exceeded by 52.55 ju g/g.
In almost all harvests, plant tissues from pure RDSS
treatments had the highest concentrations of most of the
elements. Plants from the soil-only treatment had concentrations of elements similar to plants grown on pure RDSS
but produced lower total growth compared with all othei
treatments, except soil plus fertilizer nitrogen. Due to the
lower total g r o w t h , the plants grown on soil only had
relatively lower total uptake.
Study 3
Yields for all RDSS treatments, 20 metric ton/ha (8.<>
t o n / A ) and higher, were significantly different from each
other and significantly higher than the rest o f the treatments
(table 12). S-138 Fe treatments were not affected by the
addition of N-P fertilizer. The highest S-138 Fe treatment
(5 kg/ha elemental Fe, (4.5 l b / A ) ) yield was not statistically different from the lowest RDSS level (10 metric
ton/ha, (4.5 t o n / A ) ) and was significantly higher than the
rest of the S-138 Fe treatments. Generally, yields tended to
increase as the S-138 Fe and RDSS rates increased.
Iron concentrations in plant tissue were not affected by
treatments, probably due to dilution in plant tissue from
27
Table 12. Dry-sorghum yields, iron concentration and uptake in plant
tissue, and DTPA-extractable iron after short-term application of iron chelate and RDSS treatments
Treatments
Soil only
RDSS 10 metric to n/ha'
RDSS 20 metric ton/ha
RDSS 30 metric ton/ha
RDSS 40 metric to n/ha
F3
S-138 Fe 10kg/ha :' +F
S-138 Fe 20 kg/ha + F
S-138 Fe 30 kg/ha +F
S-138 Fe 5 kg/ha
Fe-Active + F
S-138 Fe 10 kq/ha
S-138 Fe 20 kg/ha
S 138 Fe 30 kg/ha
S-138 Fe5 kg/ha
Fe-Active
Plant Tissue
Fe-Conc*
Fe-Uptake*
q/pot
p/fl
Dry Yields*
g/pot
Soil
Fe-DTPA*
ppm
0.74g
2.90d
4.38c
6.28b
7.54a
1.16efg
1.36efg
1.85e
1.74ef
66.82a
29.46a
30.58a
31.02a
36.19a
30.33a
33.87a
26.19a
35.84a
49.45d
84.9 I d
131.66c
194.26b
271.08a
35.18d
46.06d
48.45d
62.36d
4.57e
7.99d
12.32c
16.16b
19.99a
4.40e
4.43e
4.72e
4.91e
3.4UI
0.93fq
1.33efg
1.76ef
33.77a
33.86a
38.60a
27.57a
116.82cd
31.49d
51.34d
48.52d
5.36e
4.55e
4.59e
4.67e
3.46d
34.48a
118.77cd
5.03e
ollow
, the
significantly different at the 5 pery Duncan's Multiple Range Test,
ic ton/ha 4 2.24 - ton/A
luals N-PjOj (250 B0) kg/ha; S-138 F i equals Sequestrene - 138 Fe (G
it).
a X 0.892 - lb/A
increased growth. Iron uptake showed significant differences
related to yields and followed the same trend as yields.
Concentrations of DTPA-extractable soil Fe resulting from
the application of all RDSS treatments were significantly
different from each other and significantly higher than the
rest of the treatments. The highest S-138 Fe application level
(5-kg/ha elemental Fe, (4.5 lb/A)) with or without fertilizer
and all RDSS rates were the only treatments which resulted
in soil Fe concentrations equal to or above the recommended
sufficiency level of 5 ppm (45).
These data indicated deficiency of available Fe in the Lea
soil and the superiority of soil-applied RDSS as a source of
28
Fe compared with S-138 Fe. The Fe-DTPA extractable Fe
values followed yield rather closely, with a correlation
coefficient of 0.88 (P = 0.1 ). Fe-DTPA was almost as closely
related to total Fe uptake, with a correlation coefficient of
0.87 (P = 0.1). DTPA-extractable soil Fe is typically closely
associated with yields on soils low in plant available Fe.
Study 4
Yield data are presented in table 13 and support other
greenhouse data collected in similar studies (studies 1 to 3)
where sludge alone had been added to soils. Irradiated sludge
Table 13. Average (of three replicates) sorghum dry-matter yields for
each treatment on the three soils
Treatment
Sludge
Sludge
Sludge
Sludge
Sludge
Sludge
Sludge
Sludge
Sludge
Sludge
Sludge
Sludge
O ' , Fe O 1
O, Fe 20
22.4. Fe O
22.4, Fe 20
44.8, Fe O
44.8, Fe 20
extract O, Fe O
extract O, Fe 20
extract 22.4, Fe 0
extract 22.4, Fe 20
extract 44.8, Fe 0
extract 44.8. Fe 20
Lea
5.6cde"
7.2 cd
20.2b
18.4b
27.2a
28.1a
2.8e
4.3de
4.2de
6.1cde
6.8cd
8.0c
Soil
Harvey
g/pot'
6.9b
6.6b
21.0a
20.1a
20.6a
20.5a
6.7b
6.8b
5.8b
9.3b
10.6b
10.3b
Glendale
22.7c
24.6c
23.7c
26.1abc
29.1 abc
26.7abc
24.7c
25.3bc
29.5abc
25.7abc
31.8ab
32.1a
•g/pot x 0.35 = oz/pot.
line leu
Sludge or sludge extract rates in r
Fe rates in kg/ha (kg/ha x 0.892 li./A).
ificantly different at P - 0.05.
[Metric ton/ha : 2.24 - ton/A).
significantly increased sorghum yields on the initially Fedeficient Lea soil. Sludge also significantly increased yields
on the initially Zn-deficient Harvey soil. Sludge extracts
applied with and without inorganic Fe, and Fe alone, were
29
ineffective on either soil. The Glendale soil was regarded as
sufficient initially in both Fe and Zn and was not significantly improved by sludge treatments.
Inorganic Fe is rapidly hydrolyzed and precipitated as
insoluble Fe 2 0 3 in calcareous soils. Thus, inorganic Fe
additions are often a waste of money on such soils. Sludge
extracts were hypothesized to contain natural chelating
agents which might bind with soil Fe or Zn, or with added Fe,
to improve metal availability to plants. However, increased
crop growth appears to be a result of increased Fe and Zn
that comes directly from the sludge in available forms.
Yields for each soil were generally well correlated with soil
tests of available Fe and Zn following harvest. Soil test Fe,
Zn, and P are given in table 14. Available Fe was marginal to
deficient (< 5 ppm) in the Lea soil for all treatments except
sludge treatments. In the sludge treatment, available Fe was
significantly increased in proportion to the amount of sludge
added. Inorganic Fe and sludge extract additions had no
significant effect on extractable Fe.
Similar trends in extractable Zn and P were also observed.
Extractable Zn was below the critical level (1 ppm) in a few
cases, but these values were not significantly different from
other values considered adequate. Thus, although available
Zn was greatest in the sludge treatments, it appears that
plants were responding primarily to increased Fe availability
rather than to increased Zn availability. The same logic can
be used to interpret the P data. Greatest extractable P levels
were observed in the sludge treatments, but more than
adequate P was present in all treatments as a result of blanket
P fertilizer applications.
Extractable Fe in the Harvey soil was again greatest in the
sludge treatments, but was well above the critical level
(> 5ppm) in all treatments. Extractable soil Zn, on the other
hand, was near the critical level of 1 ppm in all but the
sludge treatments. This soil would be classified as sufficient
in Zn by the soil test used, but reports by the owner-farmer,
and our data showing a substantial response to increased Zn
levels, would suggest that the soil is at least marginally
deficient in the element. Plant-tissue contents of Zn were
almost doubled in the sludge treatments (table 15), further
30
§ 2 2 5ri3 3 ™ .
supporting the contention that Zn may limit sorghum yield
in unamended Harvey soil. Plant P uptake (table I 5) was also
increased in the sludge treatments, reflecting the increased
soil test P levels, but adequate P was present in the tissue of
plants from all treatments.
Soil test data for the Glendale soil reflect the overall high
fertility of the soil. All three nutrients are present in adequate
amounts in the control treatments, by the soil tests, used.
Sludge again increased soil test Fe, Zn. and P levels, there was
no significant effect on yield. Interestingly, plant tissue analysis (table 16) suggested that P availability was limited in the
Glendale soil despite the very high soil test levels. P in the
tissue (table 16) was below the deficient level of 0.2 percent
in all treatments except the sludge treatments. In the latter,
plant P was almost doubled, suggesting that sludge may have
improved P availability to plants in the Glendale soil. Plant P
levels were also high in sludge treated Lea and Harvey soils
but were not nearly as dramatically improved in these soils as
in the Glendale. P was deficient in the tissue of only the
Glendale non-sludge-amended soils.
Study 5
Contrary to previous greenhouse results obtained with
samples of the same soils and with similar treatments (studies
1 to 4), there was little effect of sludge additions on sorghum
yield within soils (table 17). Even with 15 plants per pot,
yields were low and similar in all treatments. Metal uptake
values were variable and also failed to consistently reflect the
effects of sludge treatments reported previously (studies 1 to
4). Apparently, the very short growth period (21 days)
of this study was insufficient for expression of treatment
effects. Flic minimal treatment effect on yield or metal
uptake was not regarded as significant in the present study.
In fact, the overall uniformity and health of plants probably
encouraged relatively uniform scavenging of labelled metals
in all treatments. Thus, plants in all treatments were regarded
as "drawing" from similar labile pools of metals, and it is
likely that no special "stress-induced" (Brown et al.. ( I I )
uptake mechanisms were operative.
Soil- and plant-specific activity data for Fe and Zn are
M
33
Table 17. Average yields and metal uptake of sorghum grown in each
treatment
Zn
Fe
Soil
Sludge
Rate
trie ton/ha'
Glendale
0
22.4
44.8
Harvey
0
22.4
44.8
Lea
0
22.4
44.8
Metal
Uptake
Yield
Yield
Metal
Uptake
g/pot'
1.90a*'
1.68a
1.37a
g/pot
125a
90.0a
95.1a
g/pot
1.53a
1.69a
1.52a
g/pot
58.3a
77.6a
94.4a
1.15a
0.75b
1.00a
116a
66.9a
70.9a
1.45a
1.04b
0.95b
53.3a
61.7a
50.9a
1.86a
1.34a
1.43a
107a
88.8a
109a
1.48a
1.4 7a
1.45a
45.8a
56.6a
58.7a
1
Metric ton/ha : 2.24 - ton/
'g X 0.035 - o;
given in tables 18 and 19, respectively. Despite the preliminary 40-day equilibration period, final soil-specific
activities (in the c o n t r o l t r e a t m e n t s ) were lower than initial
soil-specific activities. T h e changes in specific activities over
t h e course of the e x p e r i m e n t were p r o n o u n c e d in the Fe
t r e a t m e n t s ( t a b l e 18), b u t were relatively m i n o r in the Zn
t r e a t m e n t s (table 19). Final specific activities of the sludgea m e n d e d soils were always less than c o n t r o l s , reflecting the
addition of metals in the sludge. T h e latter changes were
e x p e c t e d , as specific activities were calculated by dividing
D T P A - e x t r a c t a b l e labelled metal (essentially s o i l - ' , s Z n oi
Soil-59Fe) by total D T P A - e x t r a c t a b l e metal (labelled metal
plus metal supplied by sludge). S o m e of the changes in
soil-specific activities in the c o n t r o l s can similarly be attrib u t e d to increases in D T P A - e x t r a c t a b l e Fe and Zn c o n t e n t s
from initial to final analysis ( c o l u m n s 5 and (> of tables 18
and 19). However, in no case was this increase sufficient to
totally explain the substantial r e d u c t i o n s in I e-specilic
activities of the c o n t r o l s .
35
i?gg
Specific activities of elements in the controls were expected to remain relatively constant throughout the study,
to allow comparisons of plant and soil element-specific
activities. Ibis would have facilitated identification of the
source of metals taken up by the plants. Flic substantial
changes in soil-specific activities, especially in the Fe treatments, negated such direct comparisons. Additionally, plant
Zn-specific activities (control treatments) were found to be
approximately 10 times as great as soil Zn-specific activities
(table l ( >). Apparently sorghum plants withdrew Zn from a
much more limited fraction of soil-Zn (enriched with 65Zn)
than was extracted by DTPA. Thus, the assumption that
DTPA extracts Zn from the same labile pool as plants was
not met. Direct comparisons of (DTPA extractable) soil
metal-specific activities with plant metal-specific activities
would, therefore, be inappropriate, regardless of the constancy of soil-specific activities.
Difficulties with DTPA-specific activity measurements
negated direct identification o f metal source, but indirect
assessments were deemed possible according to the following
logic.
If the increase in DTPA-extractable Fe and Zn in
sludge-amended soils is the result of sludge-born metals, the
change in plant metal-specific activities should be proportional to the change in DTPA-extractable soil levels. Thus,
plant metal-specific activities in the sludge-amended soils
should be approximately equal to the plant-specific activities
o f the controls multiplied by the ratio of DTPA-extractable
metal contents o\ the control soil to that in the sludgeamended soil. For example, the specific activity o f Fe in
plants grown in Glendale soil treated w i t h 22.4 mt/ha of
sludge can be calculated as follows:
0.1 3 X 1 0 V / g (control plants) X 7 - 0 S B/ml(DTPA-control soil)
8.90 g/ml(DTPA-sludge soil)
= 0 . 1 0 X K ) V 7 g (sludge plants)
Specific activities of elements in plant tissue were calculated
in this manner for all sludge treatments (last column of tables
18 and 19).
The agreement between actual and calculated values is
37
reasonably good. This lends credence to the hypothesis (hat
the main benefit o f sludge is the available Fe and Zn added to
each soil in the amendment.
Sludges and other organic amendments have also been
suggested as sources of natural chelates thai sequester soil
metals into tonus more available to plants (Chaney and
Giordano, (13)).
Sludges may also lower soil p l l , which
could increase the solubility and availability o f native soil
metals,
both of these suggested benefits, however, would
likely affect the same pool o f metals extractable in DTPA.
Thus, the specific activity of the metals made more available
would be similar to that in the DTPA extract. Plants responding to an increase in this available pool would be
expected to have specific activities similar to the specific
activity o\ the DTPA extract.
Flic data in tables 17 and
18 do not support such a hypothesis.
Long-term benefits of sludge additions may include effects
of natural chelates and (at least) localized increased acidity
on metal availability. The results o f this short-term study
and study 4 suggest that the dominant benefit o f sludge is as
a source o f readily available metals.
SUMMARY AND
CONCLUSIONS
Information from the studies indicates that the radiation
process of reducing sewage sludge pathogens that is being
developed by Sandia National Laboratories, Albuquerque,
New Mexico, does not significantly increase the chemical
extractability or plant uptake of a broad range oi nutrients
and heavy metals. Therefore, results o f experiments using
sludges on calcareous soils should not differ greatly with or
without radiation treatment.
However, radiation treatment greatly facilitates the hand!
ing of sewage for experimentation, because precaution
against pathogen contamination are eliminated ami weed
seeds are killed. Rosopulo et al. (65) and, recently, Kirkham
(43) studied the effects of sludge irradiation on plant
nutrient uptake and found results agreeing with those
presented herein.
39
Yield increases, over and above that expected from the N
and P inputs of the sewage, are typified in the greenhouse
results presented herein, especially for the known micronutrient deficient soils of New Mexico. Results indicate that
sewage sludge is an excellent Zn, Fe. and perhaps P fertilizer.
Also the data strongly indicate that the nutrients come
directly from sources within the sewage and not from indirect effects of the sewage on the soil (i.e., sewage does not
make the soil-borne nutrients more plant-available).
Sewage products may have special potential for use on
calcareous soils in New Mexico and other parts of the South
west. For instance, the lack of K in sewage products is not a
problem because most New Mexico soils contain sufficient K
for good crop growth and K is not routinely added as a
fertilizer. The naturally high pll (7.5 to 8.0 or higher)
greatly reduces availability to plants of many problem heavy
metals.
Other researchers have published evidence that
sewage sludge could greatly improve the physical properties
of soils (26). The data suggest a definite potential for utilizing gamma-irradiated sewage sludge as a fertilizer on soils in
New Mexico within the EPA guidelines for land application
of sewage. However, deVries and Tiller (20) indicate that
increased metal uptake in plants from sewage-sludge additions is more pronounced in greenhouse-grown plants than in
field-grown. More field testing is needed in New Mexico
before complete recommendations are made for the use of
sewage sludge on cropland, especially if it is used for
vegetable crops that could go directly into the human food
chain.
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J E E C N O
| £ E <
~ E
E»
i ?
O O .- ^J
iBdd--
# cr or cr oc or.
CN
cr o.
o
' h i 15
ill
RDSS
Kg/ha
Fig. 1.
DTPA-ext actable iron in the Leo soil after each harvest, short-ti
s within harvests having a common letter are not sig
: 5% level by Duncan's Multiple Range Test.
Fig. 2.
DTPA extractable zinc in the Lea soil after each harvest, short-term application; bars within harvests having a common letter are not significantly
different at 5% level by Duncan's Multiple Range Test.
Soil/RDSS RDSS Kg/ho