UNIVERSITY OF CALIFORNIA
Division of Agriculture and Natural Resources
http://anrcatalog.ucdavis.edu
Publication 7249
SOIL FERTILITY MANAGEMENT
FOR ORGANIC CROPS
MARK GASKELL, UC Cooperative Extension Farm Advisor, Santa
Barbara and San Luis Obispo Counties; RICHARD SMITH, UCCE Farm
Advisor, Monterey and Santa Cruz Counties; JEFF MITCHELL, UCCE
Vegetable Crops Specialist, Kearney Agricultural Center, Parlier; STEVEN
T. KOIKE, UCCE Farm Advisor, Monterey and Santa Cruz Counties;
CALVIN FOUCHE, UCCE Farm Advisor, San Joaquin County; TIM
HARTZ, UCCE Vegetable Crops Specialist, UC Davis; WILLIAM HOR-
WATH, Professor of Soils and Biogeochemistry, UC Davis; and LOUISE
JACKSON, UCCE Vegetable Crops Specialist, UC Davis.
Growers need sound information to guide their
management decisions on organic vegetable pro-
duction practices in California, yet pecific informa-
tion is scarce. The Organic Vegetable Production in
California Series consists of publications written by
farm advisors and specialists from the University
of California’s Division of Agriculture and Natural
Resources. Each publication addresses a key aspect
of organic production practices applicable to all veg-
etable crops.
Organic soil fertility management is guided by the
philosophy of “feed the soil to feed the plant.” This
basic precept is implemented through a series of prac-
tices designed to increase soil organic matter, biologi-
cal activity, and nutrient availability. For the current
list of approved practices for organic certification,
see the USDA National Organic Program (NOP) Web
site at http://www.ams.usda.gov/nop/indexIE.htm.
Adding organic materials such as cover crops, crop
residues, and composts to cultivated soils over time
builds soil organic matter and improves the ability
of the soil to supply nutrients. The ultimate goal is a
healthy, fertile, biologically active soil with improved
structure and enhanced nutrient availability. Organic
management practices strive to optimize diverse bio-
logical processes in the soil to create a complex envi-
ronment that ensures adequate nutrition to the crop.
For a discussion of related soil management practices,
see Soil Management and Soil Quality for Organic Crops
(UC ANR Publication 7248, http://anrcatalog.ucdavis.
edu/pdf/7248.pdf).
ROLE OF ORGANIC MATTER
AND HUMUS
Increasing soil organic matter is a key aspect of
an organic production system. The formation and
decomposition of soil organic matter are fundamental
life-promoting processes that store and release energy
derived from photosynthesis. Soil organic matter is
mainly the product of microbial and faunal decompo-
sition of plant residues. The decomposition of plant
residue leads to the formation of humic substances,
which constitute 70 to 80 percent of the organic mat-
ter in most soils. The remaining soil organic matter is
termed “light fraction” or “particulate organic matter”
and is composed of a continuum of material ranging
from recently deposited litter to highly decomposed
unrecognizable plant residues. Soils with higher clay
contents in temperate climates generally have the
most soil organic matter. In California, organic matter
typically makes up 1 to 3 percent of the dry weight
of cultivated agricultural soils and 4 to 6 percent of
untilled pasture soils. Studies have shown that it is
normally not possible to increase soil organic matter
by more than 1 percent, but even an increase of this
much can dramatically improve soil fertility.
During the formation of soil organic matter, nutri-
ents such as nitrogen (N), phosphorus (P), and sulfur
(S) are incorporated into the soil structure, allowing
the soil to act as a reservoir of these and other nutri-
ents. The decomposition of soil organic matter releas-
es nutrients, at which point they become available for
plant uptake. Generally, 2 to 5 percent of soil organic
matter decomposes annually. Soil organic matter
VEGETABLE
RESEARCH AND
INFORMATION
CENTER
Organic
Vegetable
Production in
California
Series
Specific information on organic vegetable production practices in California is scarce, and growers need sound information
to guide their management decisions. The Organic Vegetable Production in California Series is made up of publications
written by Farm Advisors and Specialists from the University of California's Division of Agriculture and Natural
Resources. Each publication addresses a key aspect of organic production practices applicable to all vegetable crops.
www.sfc.ucdavis.edu vric.ucdavis.edu
http://anrcatalog.ucdavis.edu
http://www.ams.usda.gov/nop/indexIE.htm
http://anrcatalog.ucdavis.edu/pdf/7248.pdf
http://anrcatalog.ucdavis.edu/pdf/7248.pdf
http://www.sfc.ucdavis.edu
vric.ucdavis.edu
contains a number of fractions that vary in composi-
tion and activity. Humus is the most resistant and
mature fraction of soil organic matter. It is very slow
to decompose and may last for hundreds of years.
Plant residues that are high in carbon (C) and low
in nitrogen, such as straw or cornstalks, decompose
slowly but are efficient producers of humus. Residues
that contain high levels of nitrogen, such as young
cereals and legumes, decompose quickly, producing
less humus. Although the process of organic mat-
ter formation is not well understood, it is clear that
increasing the amount of soil humus improves soil
properties and crop growth.
The decomposition of organic matter in soils can
provide significant quantities of several important
nutrients. A portion of the nitrogen from organic
matter is converted into plant-available mineral
forms such as ammonium (NH4+) and nitrate (NO3–)
through the process of mineralization. However,
the timing and amount of mineralization often do
not coincide with crop need, making in-season fer-
tilization necessary. This lack of synchrony between
nitrogen mineralized from organic matter and crop
nitrogen uptake is a major challenge for fertility man-
agement in organic systems. Organic matter is a good
source of phosphorus; as phosphorus is mineralized
from organic matter it becomes available for plant
growth or becomes bound to soil minerals. Organic
matter is also a significant source of micronutrients
such as iron (FE), copper (Cu) and zinc (Zn).
In addition to supplying nutrients, soil organic
matter improves soil fertility by imparting favorable
chemical and physical attributes to soil. Soil organic
matter can bind nutrients through the process of
cation exchange. Ammonium (NH4+), calcium (Ca),
magnesium (Mg), and potassium (K) are nutrient cat-
ions that are held on cation exchange sites on organic
matter. The cation exchange capacity of soil organic
matter can contribute from 20 to 70 percent of the
total cation exchange capacity of soil. Soil structure
is influenced by the association of soil organic matter
with minerals to form aggregates. Aggregate forma-
tion improves soil structure and water infiltration
and increases water-holding water capacity. These
changes improve root growth and provide habitat
for a diversity of soil organisms. Soil organic mat-
ter enhances nutrient cycling, provides habitat for a
diversity of soil organisms, and creates a favorable
environment for plant growth.
HOW TO DETERMINE NUTRIENT NEEDS
Crop nutrient requirements and the nutrient-supplying
capacity of the soil dictate the management practices
necessary for successful crop production. Soil testing is
essential for the assessment of nutrient levels, and it is
often required for organic certification. Management of
nutrients such as phosphorus, potassium, calcium,
magnesium, and sulfur should be directed toward rais-
ing these nutrients to optimal levels in the soil as deter-
mined by soil testing. Phosphorus availability in soils
with pH greater than 6.0 is assessed by the Olsen bicar-
bonate test; for soils with pH less than 6.0 the Bray test
is used. In most vegetable production areas in California
soil pH is greater than 6.0, so this discussion will focus
on the Olsen bicarbonate soil test. Natural levels of
phosphorus in most California soils were formerly less
than 30 ppm. Over years of fertilization for commercial
vegetable production, fields now routinely have soil
phosphorus greater than 60 ppm along the coast, and
somewhat less in the interior valleys. Phosphorus avail-
ability is reduced at low soil temperatures (i.e.,
or 15.6ºC) and, as a result, crops grown in the cooler
part of the year need higher levels of available soil phos-
phorus for good growth. Approximate soil adequacy
values from the bicarbonate phosphorus test for warm-
and cool-season vegetables are given in table 1.
Table 1. Adequate soil phosphorus levels
(bicarbonate phosphorus test)
Crop Adequate soil P level (ppm)
warm-season vegetables 20–25
cool-season vegetables 50–60
Compost and certain organic fertilizers are good sources
of phosphorus. It is important to monitor soil phos-
phorus levels on a yearly basis, as soil phosphorus
can rapidly build up high to levels when composts
and other organic amendments are used. Excessive soil
phosphorus can result in high phosphorus concentra-
tion in field runoff, which can impair the quality of
surface waters such as rivers, creeks, and lakes.
Soil potassium level is best determined by an
ammonium acetate extraction test. In general, if soil
potassium is greater than 200 ppm, no increase in
yield is likely to be obtained with additional potas-
sium fertilization. However, maintenance applica-
tions of potassium may be helpful in replacing soil
potassium that is removed in the crop. For soils at
less than 150 ppm potassium, fertilization is warrant-
ed. Composts and some organic fertilizers are good
sources of potassium.
Calcium, magnesium, and sulfur are usually pres-
ent in the soil and in irrigation water in sufficient
quantities to adequately supply a crop. In very sandy
soils with low levels of organic matter, sulfur avail-
ability may be limited, but normal organic practices
(application of compost, use of sulfur as a fungi-
cide) typically maintain adequate levels of soil sulfur.
While neither calcium nor magnesium availability is
2 • Organic Certification, Farm Planning, Management, and Marketing
often limiting for crop nutrition, in some fields rela-
tively low soil calcium and/or high magnesium con-
tent can result in poor soil structure and slow water
infiltration. In these circumstances application of gyp-
sum (naturally occurring calcium sulfate) is the most
appropriate remedy.
In organic systems, appropriate nitrogen man-
agement cannot be directly inferred from a simple
soil test. Unlike conventional production, in which
nitrogen management is based on the use of soluble,
readily available nitrogen fertilizers, in organic sys-
tems nitrogen management is based on manipulating
organic sources of nitrogen; organic nitrogen must be
mineralized through the action of soil microbes before
it is available for plant uptake. Although this process
can supply a significant quantity of nitrogen, estimat-
ing the amount and timing of nitrogen mineralization
is complicated because a number of factors affect the
process. The most important of these factors are as
follows.
• Soil temperature: Mineralization is insignificant
below 50ºF (10ºC), but above that temperature,
mineralization increases as soil temperature
increases.
• Soil moisture: Mineralization proceeds rapidly in
moist soils, but is inhibited by either excessively
wet or dry conditions.
• Tillage practices: Soil tillage stimulates a
temporary burst of microbial activity, which
declines over the course of days or weeks.
Despite the complex interactions of these factors, a
rough estimate of mineralization from soil organic
matter can be made based on the amount of organic
nitrogen present in the soil and the percentage of that
nitrogen likely to mineralize over a given period of
time. The following procedure describes a method for
estimating the amount of nitrogen likely to be mineral-
ized from soil organic matter.
The first step is to estimate the amount of organic
nitrogen in the soil. This can be done directly by a spe-
cialized laboratory test, or it can be inferred from the soil
organic matter content. In most agricultural soils, organ-
ic nitrogen constitutes approximately 7 percent of soil
organic matter. The vast majority of nitrogen mineraliza-
tion takes place in the top 1 foot (30 cm) of soil. A stan-
dard estimate of soil weight is 4,000,000 pounds of dry
soil per acre-foot (about 1,816,000 kg /100 cubic meters).
The organic nitrogen content of a soil with 1 percent
organic matter would be
4,000,000 lb soil ✕ 0.01 (percent organic matter) ✕
0.07 (percent N in organic matter) = 2,800 lb organic
N/acre (3,136 kg/ha)
The second step is to estimate the percentage of
soil organic nitrogen likely to mineralize during the
crop cycle. Laboratory incubation studies of dozens
of California soils have shown that, at best, about 2
percent of soil organic nitrogen is mineralized in a
2-month period at 77ºF (25ºC). For a soil with 1 per-
cent organic matter, that would be
2,800 lb organic N/acre ✕ 0.02 (percent of organic
nitrogen that mineralizes) = 56 lb plant-available
N/acre (63 kg/ha)
The 2 percent estimate for nitrogen availability for a
short-term annual crop can be adjusted to fit field-spe-
cific conditions based on the factors previously
described. Fields that are sprinkler-irrigated keep the
entire soil surface moist, while much of the surface soil
in drip-irrigated fields may be very dry. The soil tem-
perature during spring and fall crops is lower than that
for summer crops. Fields in which any form of reduced
tillage is practiced tend to have slower nitrogen miner-
alization. Heavy clay soil is more readily waterlogged
by rain or irrigation, and effective nitrogen mineraliza-
tion may be reduced. Note that this technique for esti-
mating nitrogen mineralization from soil organic matter
does not take into account the nitrogen contribution
from recently incorporated crop residue, compost, or
other organic amendments. These contributions are
described elsewhere in this publication.
Synchronizing nitrogen mineralization from soil
organic matter, cover crop residues, and organic
amendments to maintain adequate nitrogen availability
for crop production is challenging. The generalized
pattern of nitrogen mineralization and crop nitrogen
uptake is presented in figure 1. The rate of nitrogen
mineralization from soil organic matter and recently
incorporated residues and amendments typically peaks
before the crop reaches its maximum rate of nitrogen
uptake. Even in organic systems, nitrogen loss through
leaching or denitrification (conversion of nitrate to gas-
eous nitrogen in wet soil and subsequent loss to the
atmosphere) can be substantial if excessive water from
rain or irrigation is applied to the field in the early
weeks of the growing season.
Short-season crops with low nitrogen requirements
such as leafy greens and radishes (table 2) may pro-
duce well with the nitrogen available from soil organ-
ic matter plus cover crop residues and/or a compost
application. Crops with higher nitrogen requirements
and longer growing seasons often need supplemental
sidedress applications of organic nitrogen fertilizer.
For many vegetable crops, quality is as important as
yield. Product size, color, and uniformity can be criti-
cal, and nitrogen management is often the key to
maximizing these quality attributes.
3 • Organic Certification, Farm Planning, Management, and Marketing
Low total N content
Medium total N content
120–200 lb/acre
High total N content
> 200 lb/acre
baby greens carrot broccoli
beans corn, sweet cabbage
cucumbers garlic cauliflower
radish lettuce celery
spinach melons potato
squashes onion
peppers
tomatoes
Figure 1. Timing of nitrogen mineralization from
soil organic matter, cover crop residue, and organic
fertilizer in relation to crop nitrogen uptake.
NUTRIENT SOURCES
Cover Crops
Cover crops fix and trap nutrients, add organic matter
to soils, and reduce nitrate leaching, nutrient runoff,
and soil erosion. In California, cover crops are widely
used in organic farming systems because the climate is
mild enough to support growth during the fall, winter,
and early spring in most crop production areas.
Nonleguminous cover crops, such as grasses and
Brassica species, are preferred in situations where nutri-
ent availability is high in the fall and where cover crops
can trap nitrate and phosphate that would otherwise
be lost by leaching or runoff. Nonlegumes also tend to
be more tolerant of cooler temperatures than legumes.
Legumes fix atmospheric nitrogen, at least when con-
centrations of mineral nitrogen in the soil are low, and
add to the net availability of nitrogen in the cropping
system. Mixtures of legumes and grasses are a com-
mon strategy because the grass consumes soil nitrogen,
avoiding high soil nitrogen concentrations than might
otherwise inhibit fixation. Mixtures also ensure that the
cover crop is productive under a range of weather con-
dition, due to the different environmental tolerances of
the various plant species.
In California, cover crops typically take up or fix
from 100 to 200 pounds of nitrogen per acre (112 to
224 kg/ha). Cover crops are often tilled into the soil
when the carbon to nitrogen (C:N) ratio is less than
20:1 (e.g., legumes and younger stages of cereals and
mustards) to achieve a net release of nitrogen to the
soil in order to feed subsequent vegetable crops. The
high nitrogen content in cover crops reduces compe-
tition for mineral nitrogen between the subsequent
vegetable crop and the soil microbiota. When cover
crops with a low nitrogen content, such as mature
cereals (i.e., C:N ratio > 20:1) are incorporated into
the soil, subsequent vegetable crops can be tempo-
rally nitrogen deficient because soil microbes use
available soil nitrogen to break down the cover crop
residue. However, these cover crops with a higher
C:N ratio are instrumental in building soil organic
matter, which is advantageous for long-term soil fer-
tility and improvements in soil physical properties.
A longer-term grass or brassica cover crop is there-
fore recommended periodically, as long as cropping
patterns permit a sufficient period without crop pro-
duction for residue decomposition to occur.
Less than half of the amount of nitrogen in a
cover crop typically becomes available to the subse-
quent crop. Much of the cover crop nitrogen remains
in resistant organic forms in soil organic matter,
unavailable to plants. The organic nitrogen in the
readily decomposable fraction of cover crop resi-
dues, however, can be very rapidly mineralized to
plant-available forms of nitrogen in the first few
weeks after incorporation. The rate of mineralization
of available nitrogen from a cover crop with a low C:
N ratio (
following incorporation, and then returns to prein-
corporation levels by weeks 6 to 8 (see fig. 1).
Therefore, a cover crop can be a valuable short-term
source of nitrogen, but longer-season vegetable
crops following a cover crop rotation may require
additional applications of nitrogen later in the sea-
son. When nitrogen from cover crops is mineralized
it can be taken up and used by the crop or lost via
leaching during spring rainy periods. For this rea-
4 • Organic Certification, Farm Planning, Management, and Marketing
Table 2. Nitrogen requirement of vegetable crops based on seasonal nitrogen uptake
Crop Demand
Cover crop
mineralization
Fertilizer
mineralization
Cover crop
incorporation
Soil organi
