A Field Exercise to AcquaintStudentswith Soil Testing
as a Measure
of Soil Fertility Status andField Variability
S. L. Taylor, G. V. Johnson,* and W. R. Raun
ABSTRACT
A laboratoryexercise wasdevelopedfor a senior-level soil
fertility class to acquaintstudentswiththe effect of field variability on uses of soil testing. Studentpairs took 25 random
surface (0-15 cm)soil samplesfroma 25002 sampling rea,
a
fromfour 25 by 25 msections within the samplingarea, and
single point samplesfromthe center of each of 25, 5 by 5 m
grids of one of the sections. Eachset of 25 randomsamples
werecombined,mixed,and the compositeanalyzedas a single
sample. Sampleresults werecomparedbetweenpairs of students samplingthe samearea. Also, split samplesfromthe
samecompositesampleweresent to the laboratoryto determinethe variability within the laboratory.Soil sampleswere
analyzedfor pH,buffer index(BI), nitrate-nitrogen(NO3-N),
phosphorus
(P), andpotassium(K). Results weregiven to
dentsto assess whateffect different soil samplingprocedures
hadon soil test results andhowthis wouldinfluencefertilizer
and lime recommendations.
The numberofsubsamplesneeded
to obtain a good compositewas between15 and20, based on
results of this excercise. Surfacecontourmaps,developedfrom
the results of grid sampling,wereusedto illustrate the variability of the nutrients throughoutthe field. Samplingand
travel to andfromthe off-campus
site wereeasily completed
in
a 2-h lab period. This exercise promoted
student interest in
field workandimprovedtheir understanding
of soil testing
andfertilizer recommendations.
A
PPLICATIONS
of fertilizer nutrients are often based on
soil test results; therefore, it is importantfor studentsto
understand the proper procedure for soil sampling, interpreting test results, and the influenceof field variability. The
soil test results and interpretation are onlyas goodas the initial soil samplecollected; therefore, the first step is to obtain
a good(representative) soil sample. The numberof samples
to collect, which area to exclude, and the samplingdepth to
get a good soil sample, have been studied. Johnson and
Allen 0994) stated that 20 cores taken randomly from the
field of interest are needed to obtain a good soil sample,
while avoiding unusual spots within the field. Adettmji
(1994) found that 25 to 30 samples were needed to estimate
field variability in a newlycultivated field, while 30 to 40
samples were needed for a continuously cultivated field,
indicating that continuouscultivation increases field variability. Adetunji(1994) further noted that samplestaken at
depth of 0 to 20 cm had the highest correlation with crop
nutrient uptake. Workby Diaz et al. (1992) estimated that
to 20 samples were needed from a sampling area to obtain a
representative sample with 95%confidence and 20%rela-
tive error, excluding highly variable border areas such as
bordering canals and areas close to roads.
A field exercise was designed in which students were
asked to collect soil samples randomly from within five
flagged areas and at each of 25 points in a 5 by 5 mgrid of
one of the areas. The collected soil samplesweresent to the
OklahomaCooperative Extension Service soil testing laboratory for analysis. Soil test results werethen returned to the
students and they were asked to determine field variability
from the point grid area. Theyalso estimated the numberof
samplesrequired to obtain a goodsoil sample. Limeand fertilizer recommendationswere madefor a specific crop at a
given yield goal according to OSUSoil Test Interpretations
(Allen and Johnson, 1993).
The objectives of this teaching-learning exercise were to
acquaint students with strategies neededto acquire a representative soil sample, to familiarize themwith field variability, and to develop an understanding of the use of soil
test results as a guide for soil pHand nutrient management.
MATERIALS AND METHODS
Soil Sampling
A cultivated field was divided into four areas (designated A, B, C, and D) each approximately 25 mby 25 m, with
area B further divided into 25 squares approximately5 mby
5 m(Fig. 1), before students arrived at the field. In addition
to the field site, materials neededincluded soil probes, plastic buckets, plot flags, tape measure,and samplecontainers.
Students did soil sampling in pairs, and samples were collected as follows:
25
A
D
C
Department
of Agronomy,
369AgriculturalHall, Oklahoma
State Univ.,
Stillwater, OK74078.Contribution
fromthe Oklahoma
Agric.Exp.Stn.
Received24 Sept. 1996.*Corresponding
author(gvj@soilwater.agr.
okstate.edu).
Published
in J. Nat.Resour.LifeSci. Educ.26:!32-135
(1997).
132¯ J. Nat. Resour.Life ScLEduc.,Vol. 26, no. 2, 1997
Fig. I. Fieldlayoutforsampling
areas.
m~
1. Twocomposite samples of the entire area (Sections
+ B + C + D), from two pairs of students (Samples
and 2), to obtain representative samplesof the entire
area and comparesampling variability between student pairs.
2. A composite sample of Section A (Sample 3).
3. A compositesample of Section B, split into two separate samples(Samples4 and 5), and repeated by a different pair of students (no. 6 and 7), to comparetesting variability within the lab and samplingvariability
amongstudent pairs.
4. Section C sampled using above approach for Section
B (no. 8, 9, 10, and ll).
5. Section D was sampled using the methodfor sampling
Section A (no. 12).
6. Single-point samplesobtained near the center of each
grid area within Area B (no. 13-37), to be used for
contouring and determining the number of samples
required to obtain a representative compositesample
for an area.
7. Profile soil samples--soil was obtained from the center of Grid 13 from depths of 0 to 15, 15 to 30, 30 to
45, and 45 to 75 cm(no. 38, 39, 40, and 41, respectively), to comparevertical distribution of mobile(N)
and immobile(P) nutrients.
All composite samples were taken from 25 randomcores
as described by Johnsonand Allen (1994), placed in a plastic bucket, mixed, and a portion placed in a soil samplebag.
Point samples from the grid areas and the deep samples
were collected and placed into the sampling bags directly.
All sampleswere sent to the state soil testing lab and analyzed for pH (1:1 soil/water), SMPBuffer Index (BI),
NO3-N(saturated CaSO4 solution extract), P, and
(MehlichIII extract) (Southern Coop.Series, 1984).
Interpretationof Soil Test Results
Results fromthe testing laboratory were given to the students in a tabular format and they were asked to complete a
report. Students were asked to note and discuss differences
in soil test results fromthe compositesamplesof the entire
area (Samples 1 and 2), and an average of the composite
samples taken from each separate area (A + B + C + D)
(Table 1). Differences were also noted in the results
obtained for the samples taken from the same areas, but
from different pairs of students. Oncestudents were familiar with the soil test results, they wereaskedto makefertil-
Table 1. Soil test results and fertilizer recommendations
(2.7 Mg-1
ha
wheat yield goal) for composite samples of an entire area and onefourth sections of the area (A, B, C, and D).
Fertilizer
recommendations
Soiltest results
Sampleno.
NO3-N P
-- kg ha -t
To~larea
46
1
2
60
Avg.
53
SectionA
40
3
SectionB
4
36
5
36
Avg.
36
6
40
7
41
Avg.
41
B Avg.
38
SectionC
8, 9, 10, 11 Avg.54
Section D
12
46
Sections A,B,C, andD
Avg.
45
K
pH
BI
P205 K20
-- kg ha -I --
--
N
54
70
62
409
467
438
5.2
5.3
5.3
6.6
6.7
6.7
34
20
37
9
0
2
0
0
0
49
451
5.5
6.7
40
13
0
48
50
49
57
54
56
52
397
402
400
421
429
425
412
5.5
5.5
5.5
5.4
5.4
5.4
5.5
6.8
6.8
6.8
6.8
6.7
6.8
6.8
44
44
44
40
39
39
42
14
12
13
6
9
7
10
0
0
0
0
0
0
0
69
383
5.2
6.7
26
0
0
67
532
4.9
6.5
34
0
0
59
445
5.3
6.7
35
5
0
izer recommendationsfor winter wheat (Triticum aestivum
L.) with a 2.7 Mgha-~ yield goal (40 bushel acre-I) using
current soil test calibration tables (Allen and Johnson,
1993).
For results of samples taken from various depths in the
profile, students were askedto provide interpretations related to; (i) nutrient cycling, (ii) historical fertilizer application
of immobileand mobile nutrients, (iii) agronomicreasons
for sampling by depth, and (iv) howsampling by depth may
change fertilizer recommendationscompared with surface
(0-15 cm) sampling alone.
Students were provided with blank grids to enter results
of grid point sampling, as illustrated for the NO3-N
results
in Fig. 2. Fromthis students were asked to develop contour
isolines basedon actual field variability for all nutrients (pH
and BI contoured grids were provided as examples by the
instructor), as illustrated in Fig. 3. Oncethe grids and contouting were completed, students were asked to makefertilizer recommendations
for the entire area based on averaging
25 m
25 m
72
39
28
39
43
59
59
32
37
36
50
34
30
25
102 45
40
28
29
5O
37
47
40
73
31
Fig. 2. Point grid soil sampleresults for NO3-Nin Section B (expressed
as kg ha-t).
37 4("38----
,.30
30/
25
Fig. 3. Field variability of NOrNresults of point grid sampling and
appropriateisolines (in kg ha-I).
J. Nat. Resour.Life ScLEduc.,VoL26, no. 2, 1997¯ 133
Table 2. Soil test results for samplesfromdifferent depths at the same
point location.
Sample
Soiltest results
No.
Depth
38
39
40
41
cm
00-I
5
15-30
30-60
60-90
NO~-N
-~
~ .kg
32
11
4
3
P
K
pH
BI
321
237
355
343
5.5
6.0
6.2
6.2
6.9
7.0
6.9
6.9
ha
60
32
13
8
all samples(Table 1), and for distinctly different areas identified by contours using variable rates for the area (Fig. 4).
The point grid soil sampling was also used to determine
the numberof samples required for a representative composite sample of the area. The methodused was to randomly select point sampleresults and average these to represent
the area. Results of two samples were selected first, then
three, four, and so on, until all 25 sampleswere included in
the mean.Meansfor all nutrients were then plotted on separate x, y, scatter plots (performedby an instructor) to illustrate the difference in meansas a function of the numberof
randomsamples taken to makeup a composite sample (Fig.
5 for NO3-N).Fromvisual observation of the graphs, the
students were asked to estimate the number of samples
needed to obtain a composite sample representative of the
area. The exercise took one 2-h lab period for sample collection, with the results and report being handedout the following lab period. Students were given 1 wkto complete the
lab report.
RESULTS AND DISCUSSION
Results from the lab exercise and report showedthat students becameacquainted with the proper procedure for taking soil samples and the numberof samples required to get
a good compositesoil sample. They also developed a better
understanding of field variability and were able to identify
fertilizer requirements for a given crop from a soil test
report. Different randomsamples taken from the samearea,
but by different pairs of students, showed that NO3-N
changedby 14 kg ha-l from one sample to the other, with a
change of 16 and 58 in the P and K values, respectively
(Table 1, Sample1 vs. 2). Fertilizer recommendationsfor
each separate sample and for the average of the two samples
showedthat soil test values varied considerably from one
student pair to another. However,the differences in fertilizer recommendations were smaller (Table 1) and may have
resulted in the same amountbeing applied in a field scale
situation, whereapplication rates are commonly
roundedto
the nearest 10 or 20 units.
Whencomposite samples were split and sent to the laboratory as two separate samples, as in the case with the two
pairs of students sampling Section B, the results indicate
there is more variation associated with repeated samplingof
the same area (average of 4 and 5 vs. average of 6 and 7)
than with repeated analysis of the same sample (Samples
vs. 5 and 6 vs. 7) by a laboratory. Using K values as an
example,differences in soil test values were greater between
the average of split samples for each pair of students (425
vs. 400) than betweenthe split samples makingup the average for each pair (402 vs. 397for pair l; 429vs. 421for pair
2).
Results of the deep sampling (Table 2) showed higher
surface soil values for N and P, indicative of past fertilizer
additions. For the immobile nutrients P and K, low P and
high K levels at depths below 30 cm indicate this soil was
inherently deficient in P and fertile in K. Lowconcentrations of NO3-Nbelow 30 cm are a result of good past N
management
that has involved fertilizer inputs that were not
excessive. The contour mapsprepared by the students for all
nutrients allowed them to becomeacquainted with actual
field variability and the differences in variability fromnutrient to nutrient, with NO3-Nand K being the most variable
and pHand P the least. Figure 3 illustrates the contour map
developed by the students for NO3-N.Concerns about the
increasing use of commercialfertilizers have stimulated the
interest in variable rate fertilization of fields, that is, applying nutrients to only those areas in need. Thepoint grid sampling allowed students to imagine the use of variable rate
technology and recommend
the use of different rates of fertilizer to separateareas of the field basedon soil test results.
For Section B, the average surface soil NO3-Nwas 36 and
41 kg ha-1 for the two randomly sampled composites
(Samples 4 and 5, and Samples6 and 7; Table l), and the
average of the 25 grid samples was 44 kg ha-1. Both pro6O
55
Z
SO
0
Z
45
28
391 43
32
37 I 36
50
34
30 I 25
45
40
28 i 29
37
47 I 40
39
31
-1
N Rate, kg ha
[]
[]
0-25
45-55
Fig. 4. Fertilizer Nrecommendation(in kg ha-~), based on field variability using the point grid method.
134 ¯ J. Nat. Resour.Life Sci. Educ.,Vol. 26, no. 2, 1997
4O;
350
5
10
15
20
25
Numberof Samples
Fig. 5. MeanNO3-Nvalues for randomly selected samples from point
grid area to estimate the numberof samples needed to obtain a
good sample.
vided good estimates of the average but masked the large
variability found within this section (29-102 kg ha"1; Fig. 2
and 3). Students were asked to make variable fertilizer recommendations based on contouring of the point grid soil
sampling for a 2.7 Mg ha~' wheat yield goal as illustrated in
Fig. 4. From a comparison of soil test means, calculated
from an increasing number of samples, students were able to
visually identify that 15 to 20 samples may be required to
obtain a reliable composite sample for a field (Fig. 5).
This exercise was used at Oklahoma State University in
a senior level soil fertility course. Student interest in this
exercise was high and they especially enjoyed getting to the
field.