Soil Fertility
Nick Andrews
nick.andrews@oregonstate.edu
(503)-678-1264 x149
http://smallfarms.oregonstate.edu
Soil Fertility
• Fundamentals of organic fertility
management
• Organic matter
• Cover cropping
• pH and lime
• Nitrogen management
• New fertilizer calculator
Theories of Soil Fertility
• Baptista van Helmont (1579-1644)
– Water as principle growth factor
• Justus von Liebig (1803-1873)
– Mineral salts and Law of the Minimum
• 20th Century - William Albrecht, JI Rodale,
Sir Albert Howard, Lady Eve Balfour
– Biological health of the soil
– The Living Soil (1943)
ORGANIC FERTILITY
--A Systems Approach-• Build long term soil fertility – add organic material & use
cover crops
– Improve soil tilth
– Increase nitrogen mineralization rates
• Review soil test results
• Determine crop nutrient requirements
• Credit N-availability from previous manure and compost
applications
• Credit N-availability from previous crops and cover crops
• Fertilize as a supplement to soil building practices
Rodale Farming Systems Trial
Nitrogen from red clover and ammonium sulfate in corn
130
40
Environmental
Loss
N15 to plant
Organic - red clover
18
18 5
Respiration Rate
17
17
55
38
N15 to microbes
47
N15 to soil
140
120
100
80
60
40
20
0
Conventional - ammomium sulfate
Adapted from: Harris, G., Hesterman, O., Paul, E., Peters, S., and R. Janke. 1994. Fate of legume and
fertilizer nitrogen-15 in a long-term cropping systems experiment. Agronomy Journal. 86:910-915.
ORGANIC PRODUCTION
§ 205.203 Soil fertility and crop nutrient management
practice standard.
(a) The producer must select and implement tillage and cultivation
practices that maintain or improve the physical, chemical,
and biological condition of soil and minimize soil erosion.
(b) The producer must manage crop nutrients and soil fertility
through rotations, cover crops, and the application of plant
and animal materials.
(c) The producer must manage plant and animal materials to
maintain or improve soil organic matter content in a manner
that does not contribute to contamination of crops, soil, or water
by plant nutrients, pathogenic organisms, heavy metals, or
residues of prohibited substances.
Soil Type
Web Soil Surveys or printed
County Soil Surveys can be used.
• Physical properties: ie.
texture, depth, water
holding capacity,
susceptibility to
erosion, etc.
• Chemical properties:
ie. pH, CEC, etc.
• Vegetative
productivity: capability
classes, crop yields,
etc.
http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx
Soil Texture
SAND : SILT : CLAY
Clay
(<0.002mm)
Silt
(0.002-0.05mm)
Sand
(0.05-2mm)
Influences water movement,
fertility, erosion, compaction,
capping, etc..
Soil Structure
• Aggregation of soil particles
from biological activity and
influences:
• Soil porosity & permeability
• Water movement & holding
capacity
• Facilitates root growth
• Improves environment for
beneficial organisms
• Role of organic matter
Soil aggregation is important for
getting water into soil
Building Soils for Better Crops. Chapter 4.
Organic Matter
• Improves quality of most soils
• Breaks down to form humus
(natural glue)
• Strengthens aggregates
• Improves water holding capacity
• Releases plant nutrients
• Provides food for beneficial
organisms
• “Feed plants by feeding the soil”
The Meat & Bones of Organic Matter
It’s not all the same
Pool
Biologically
Active
Size/Age (yrs)
Small/1-5
Protected
intermediate/
5-30
Stable
Large/
50-10,000
Functions
The Meat:
nutrient mineralization,
macro-aggregation,
disease suppression
The Bones:
soil structure, porosity,
water relations
Micro-aggregation, CEC,
fate of ionic and non-ionic
compounds, color
A good sense
of humus
• Decomposing organic
matter: organic
compounds similar to
plant hormones are
sometimes formed.
• Plant response =
similar to auxins
• (photo: left without
humic acid; right with
humic acid)
Building Soils for Better Crops. Chapter 4.
Adding organic matter
• Low C/N ratio, high nutrient
value: raw manure, blood
meal, feather meal, less
contribution to organic matter,
use sparingly, do not exceed N
and P requirements
• Medium C/N ratio, lower
nutrient value: compost, leaf
mulch and cover crops, add in
large quantities
• High C/N ratio, can immobilize
N: straw, bark & sawdust
Cover Crop Opportunities
• Reduce erosion
• Protect soil structure
• Fix N and improve nutrient availability
• Supply N without increasing soil P
• Reduce nitrate leaching
• Increase soil organic matter
• Improve mycorrhizal winter survival
• Reduce weed pressure
http://www.myco-labs.com/
• Provide nectar & pollen for beneficial
insects
Cover Crop
Challenges
• Cost of seed ~$40/A
• Cost of planting &
incorporation ~$20/A
• Complicates & delays
spring bed preparation
• Possible need for fall
irrigation
• Equipment availability
• Requires management
effort
Four ways to time cover crops
a
Cover crop
b
c
d
January
Main crop
Cover crop
Main crop
Main crop
Main crop
Main crop
Cover crop
Main crop
Cover crop
Main crop
July
January
July
January
Adapted from Building Soils for Better Crops, F. Magdoff & H van Ess,
Available online: http://www.sare.org/publications/handbooks.htm
Selecting cover crops
•
•
•
•
To increase N use legumes
To reduce leaching use grasses
To increase organic matter use grasses
To reduce weeds plant higher seed rates and
plant early
• To feed beneficial insects incorporate after
flowering
• To combine benefits use mixtures
Nitrogen fixation
• Atmosphere contains
78% N but it is
unavailable to plants
• Rhizobia spp. colonize
legume roots and
convert N to NH3
• They exchange NH3
for carbohydrates
www.agnet.org
Nitrogen fixation
• N-fixation is highest when pH is near neutral, N
is low and other nutrients are plentiful.
• Rhizobia must be fresh – inoculate just before
seeding with sugar water
• Be sure to use Rhizobia that are compatible with
your crop – clover grp, pea & vetch grp, etc.
Soil sampling
•
•
•
•
•
•
•
Whole field sampling
Management Unit Approach (PNW 570-E)
10-20 cores / unit
6-8” deep, 12” for nitrate
Mix in a clean bucket and send ~1qt to lab
Take samples at same time ea. year
pH, SMP buffer, OM, NO3, P, K, Ca, Mg, S,
B
Soil pH is important to plant
growth
no lime
lime
As shown by alfalfa growth from David Swanson’s
field in Lane County during 1926
Courtesy John Hart, OSU Crop & Soil Science
Why is soil pH important?
• Mn and Al reach toxic
levels as soil pH
decreases
• Nutrient availability
changes with soil pH
• Growth of both higher
plants and bacteria
change as soil pH
changes
• Herbicide activity is
influenced
Adapted from John Hart, OSU Crop & Soil Science
pH Scale
Courtesy John Hart, OSU Crop & Soil Science
Soil pH and Alfalfa Yield
Yield, t/a
6
3
0
5.5
6
6.5
Soil pH
Courtesy John Hart, OSU Crop & Soil Science
120
Relative Yield, %
100
80
Beets
Spinach
60
Rutabaga
40
Turnip
Radish
20
Lettuce
Green bean
0
5.6
5.8
6
6.2
6.4
6.6
6.8
Soil pH
Courtesy John Hart, OSU Crop & Soil Science
Courtesy John Hart, OSU Crop & Soil Science
N-Cycle
Fertilizers and
bacteria supply
organic or
mineral N
Microbes form
Soluble Organic
Nitrogen
Dynamic
Plant uptake,
volatilized, fixed to
soil or leached
Nitrifying bacteria
convert NH4+ to NO3-
Driven by
microbes
Bacteria eat
organic residues &
form NH4+
Nitrogen Availability
Immobilization
Inorganic
NH4+, NO3→ Organic N
Fertilizer
C/N
15:1
Mineralization
Organic N →
Inorganic
NH4+ & NO3
Microbes have ~10:1 C:N ratio
Cumulative available N from
an organic source
Year 1
= available N
2
+
3
+
+
4
+
+
+
5
+
+
+
+
Compost yield benefit for seven yr.
15 to 20 % of compost N recovered
One time compost N applied in Year 1 = 2200 kg N ha-1
ASP composts
ATW composts
No compost
Grass yield
-1
(Mg dry matter ha )
14
12
10
8
6
1
2
3
4
5
6
7
Years after compost application
Compost Sci. Util. 11(3): 265-275. 2003
Compost N found in soil after 7 yr
was 33 % of N applied
5
-1
Soil nitrogen (g kg )
0 to 7.5 cm
ASP composts
ATW composts
no compost
4
3
a
a
aa
2
b
aa
b
b
1
0
Application
3
6
7
Years after compost application
Compost Sci. Util. 11(3): 265-275.
2003
Dan’s recipe for matching nutrient
supply from organic amendments
to crop needs
• Know your organic amendment
• Know field history of organic amendment
application
• Make your best guess, then soil test to see
if the plan is working
• Learn from what the data tells you and
amend the plan
“Book values” for manure nutrients. Test
manure for more accuracy!!!
Fertilizing with Manure
http://cru.cahe.wsu.edu/CEPublications/pnw0533/pnw0533.pdf
Fertilizing with Manure
http://cru.cahe.wsu.edu/CEPublications/pnw0533/pnw0533.pdf
N from compost & manure
Cumulative N mineralized
(% of annual N applied)
100
Fresh manure
80
50-20-10-5-5 %
of remaining N
60
40
Compost
10-10-10-10-10 %
20
of remaining N
0
2
4
6
8
10
12
Year of continuous application
Courtesy of Dan Sullivan (OSU)
www.tilth.org/resources
http://smallfarms.oregonstate.edu
Supplemental fertilizers
Organic fertilizer management is complex
• Organic fertilizers are relatively expensive so a systems
approach is important for economic as well as regulatory
or philosophical reasons.
• Organic fertilizers all have different nutrient ratios and Navailability, that can change over time (ie. manure loads).
Organic fertility management
is complex
• Fertilizer prices and nutrient requirements
vary dramatically.
• Whenever prices or nutrient requirements
change the ideal fertilizer program will
change.
Organic fertility
management is complex
• The calculations involved to figure
all this out are confusing, time
consuming and error prone.
• The calculator does the
calculations correctly and quickly.
Organic Fertilizer Calculator
What does the calculator do?
• Price/lb of mineral nutrients, making it easier
to find the most cost effective product for one
nutrient.
• Nutrients provided by a fertilizer or a
program, ie. 2000lbs of alfalfa meal provides
50 lbs total N, 5 lbs N after 28 days, 12 lbs N
after 125 days, 10 lbs K & 40 lbs P.
www.tilth.org/resources
http://smallfarms.oregonstate.edu
What does the
calculator do?
• Calculates the cost of each
material and the total
cost/acre of the fertilizer
program.
• Matches your fertilizer plan
with your soil nutrient
requirements
• Determines how much of a
plentiful material you should
apply?
www.tilth.org/resources
http://smallfarms.oregonstate.edu
www.tilth.org/resources
http://smallfarms.oregonstate.edu
What doesn’t it do?
• Calculate nutrient
requirements.
• Estimate environmental losses
to evaporation or leaching.
• Estimate the value of microorganisms in some materials.
• Determine nutrient release
very slowly over a number of
years.
• Indicate how easy a product is
to use.
Accounting for N conserved by
immediate tillage
NH4-N conserved with tillage
Fertilizing with Manure
http://cru.cahe.wsu.edu/CEPublications/pnw0533/pnw0533.pdf
Plant-Available Nitrogen (PAN)
%PAN & Total %N
120
100
%PAN
80
60
28 day PAN
40
Full Season PAN
20
0
-20 0
5
10
15
20
Total %N
www.tilth.org/resources
http://smallfarms.oregonstate.edu
Plant-Available Nitrogen (PAN)
Amendment total N
Amendment C:N
% dry wt.
Plant-available N estimate
28 days
full season
% of total N
% of total N
Uncomposted materials
1
35
<0
0
2
18
0
15
3
12
15
30
4
9
30
45
5
7
45
60
6
<6
60
75
7
<6
60
75
8+
<6
60
75
Composts
1
30
0
5
2-3
15-10
5
10
Table 1. Plant-available nitrogen (PAN) estimates
www.tilth.org/resources
http://smallfarms.oregonstate.edu
How to start
• Download the calculator and instructions
from http://smallfarms.oregonstate.edu –
it’s on the home page
• Get a good fertilizer recommendation
from a soil lab and/or using OSU
Extension publications
• Estimate N from cover crops and
previous manure or compost and
subtract from the fertilizer recommend’n
• Check the fertilizer analyses of your
products
• Get prices and % dry matter from your
supplier
• NOW YOUR READY!
Sample calculations
• How much of this fertilizer
should I apply?
• How do these two
fertilizers compare?
• What’s the cheapest
source of available N, P, K,
etc.?
• Does this program match
my fertilizer
recommendation?
www.tilth.org/resources
http://smallfarms.oregonstate.edu