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VEGETABLE

RESEARCH AND

INFORMATION


CENTER


Organic

Vegetable


Production in

California


Series


Small Farm

Program


vric.ucdavis.eduwww.sfc.ucdavis.edu


University of California • Division of Agriculture and Natural Resources

Publication 7248


Soil is a fundamental resource base for agricultural pro-

duction systems. Besides being the main medium for

crop growth, soil functions to sustain crop productivity,

maintain environmental quality, and provide for plant,

animal, and human health. The terms soil quality and

soil health describe the soil’s ability to perform these crit-

ical functions. Soil quality or health is generally seen as

the foundation of successful organic vegetable crop

production systems. Sustaining and improving soil

quality over the long term are frequently identified by

organic farmers as their primary management goals.

What follows is a summary of the major factors that

contribute to soil quality and the ways a grower can

enhance soil quality in an organic production system.


SOIL QUALITY ASSESSMENT


Between 1990 and 2000, our ability to assess soil health

and to measure the impacts of management practices

aimed at improving it have been the topics of consider-

able discussion in agricultural circles. Clearly, what is

considered good soil quality in one farming context

may not be so good in another, and this makes quanti-

tative assessment difficult. There is, however, a grow-


ing recognition that much like air or water quality, the

quality of soil has a profound impact on the health and

productivity of a given agroecosystem and on the

ecosystems that interface with it. Fairly definitive stan-

dards have been defined for air and water quality, but

the definition and assessment of soil quality is more

problematic. Soil is not directly consumed by humans

and animals, and it is difficult to relate measurable soil

quality indicator properties to specific soil functions or

management goals.


The assessment of soil quality or health has been

likened to a routine medical examination for a human

being, when a doctor measures a number of key para-

meters as basic indicators of overall system function.

Because soils perform many simultaneous functions,

however, the goal of relating indicator properties to

specific functions or processes is very difficult; some

would say impossible. Over the last several years,

researchers and farmers alike have tried to establish

what are now widely called minimum data sets of physi-

cal, chemical, and biological properties that can be used

as quantitative indicators in soil health assessments.

Indicator properties that are frequently identified in

these sets are listed in Table 1.


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.


SOIL MANAGEMENT

AND SOIL QUALITY


FOR ORGANIC CROPS

JEFF MITCHELL, UC Cooperative Extension Vegetable Crops Specialist,

Kearney Agricultural Center, Parlier; MARK GASKELL, UCCE Farm


Advisor, Santa Barbara and San Luis Obispo Counties; RICHARD SMITH,

UCCE Farm Advisor, Monterey and Santa Cruz Counties; CALVIN


FOUCHE, UCCE Farm Advisor, San Joaquin County; and STEVEN T.

KOIKE, UCCE Farm Advisor, Monterey and Santa Cruz Counties


Table 1. Soil quality indicator properties


Physical property Chemical property Biological property


bulk density pH microbial biomass carbon

rooting depth electrical conductivity microbial biomass nitrogen

water infiltration rate cation-exchange capacity earthworms

water-holding capacity organic matter enzymes

aggregate stability mineralizable nitrogen disease suppressiveness


exchangeable potassium

exchangeable calcium


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Characterization of soil health using these indicators

can be quite time-consuming and expensive, and is not

feasible as a general practice for every farm. In an effort

to better enable farmers to conduct ongoing assess-

ments of soil health and to compare management

impacts on soil health, the USDA Natural Resources

Conservation Service (NRCS) is developing soil quality

test kits that provide a relatively inexpensive way to

measure a number of these indicator properties. The

test kits are now being evaluated at several locations

throughout the country and may become useful and

accessible tools to help farmers with routine assess-

ments of soil quality. For further information on these

kits and their potential usefulness in soil health assess-

ments at your farm, contact your nearest USDA-NRCS

field office.


A number of soil health “scorecards” have also been

developed as qualitative tools for characterizing soil

health. These scorecards are typically available in book-

let form, and have been designed as a farmer-based

field tool for assessing soil health. A typical scorecard

lists several primarily sensory or descriptive indicator

properties that a grower can routinely evaluate for a

given soil. Their usefulness and validity are being eval-

uated at a number of cropping system comparison tri-

als throughout the United States. Initial testing has

shown that the information they generate may hold

promise as a means for monitoring individual descrip-

tive soil quality indicators in the field. Sample score-

cards are available through some local University of

California Cooperative Extension and USDA-NRCS

offices.


Soil organic matter (SOM) content is frequently iden-

tified as a primary attribute of soil quality assessment.

SOM influences many soil properties including infiltra-

tion rate, bulk density, aggregate stability, cation-

exchange capacity, and biological activity, all of which

are related to a number of key soil functions. SOM

serves as a slow-release reservoir for plant macronutri-

ents (especially nitrogen) and also aids in plant

micronutrient nutrition. It facilitates the infiltration of

water and air into the soil, increases water retention by

the soil, and is important in maintaining soil tilth. Over

time, increases in SOM can lead to a larger and more

diverse population of soil organisms and may thus

enhance the biological control of pests and plant dis-

eases. Large quantities of fresh organic matter that are

added to the soil, however, may stimulate plant patho-

genic organisms and seed and seedling pests such as

cabbage maggots and wireworms, which can cause

serious losses.


In organic production systems, soil fertility is often

augmented through applications of materials such as

compost and manure and by the use of cover crops (see


Soil Fertility Management for Organic Crops, ANR

Publication 7249). Organically managed soils that rou-

tinely receive these deliberate inputs typically differ

substantively in fertility as well as a number of other

soil quality properties when compared to conventional-

ly managed soils.


Two major projects are currently comparing soil

quality indicator properties under different manage-

ment systems, including organic. These projects, the

Sustainable Agriculture Farming Systems (SAFS)

Project (Davis, California) and the Biologically

Integrated Farming Systems (BIFS) Project (Five Points,

California), have found that organic soil management

can result in fundamental differences in a number of

soil health indicator properties including water infiltra-

tion rate, microbial biomass carbon and nitrogen, and

disease suppressiveness in the case of the SAFS com-

parison, and SOM and microbial biomass carbon and

nitrogen in several BIFS comparisons. Determination of

the practical significance of these management-induced

differences is the focus of intense ongoing research.

Preliminary analyses from the SAFS comparison pro-

ject, however, suggest that while organic systems might

be “leakier” in terms of nitrogen losses during the tran-

sition period when relatively high nitrogen loading is

common, these systems may eventually cycle nitrogen

more efficiently and thereby result in greater nutrient

conservation.


PROVIDING FOR SOIL HEALTH


Growers use a wide variety of practices to maintain or

improve soil health in organic vegetable production

systems in California. These practices generally are part

of long-term, site-specific management programs that

aim at developing fertile and biologically active soils

that readily capture and store water and nutrients, have

good tilth, and suppress plant disease. Deliberate and

routine carbon inputs are essential to achieving this

goal in organic production environments. Special care

is needed to select organic carbon sources that will

ensure short-term productivity while building long-

term soil quality.


Rotations


Judicious crop rotation may be a useful strategy for

increasing short-term SOM and for establishing

healthy, fertile and productive soils. Amounts of

postharvest crop residues in California organic veg-

etable production systems vary widely depending on

the crop and the intensity with which it is harvested.

Rotations that include small grain crops such as wheat,

barley, oats, rye, or triticale that are harvested for seed


Soil Management and Soil Quality for Organic Crops • 2


typically add 8,000 to 10,000 pounds of dry matter per

acre to the soil after harvest. By including these crops in

a vegetable rotation, a grower can also lessen the inci-

dence of several potentially devastating vegetable crop

soil diseases and help with nematode problems (See

Integrated Pest Management for Small Grains, ANR

Publication 3333). Field residues from broccoli harvest

may typically provide nearly 7,000 pounds dry matter

per acre, and residues from other vegetables such as

tomato, lettuce, onions, and garlic may respectively add

on average 2,500, 1,200, 700, and 500 pounds of dry

matter per acre.


Organic Amendments


While composts and manures are frequently consid-

ered to be mainstays of fertility management programs

in organic systems, these amendments often vary wide-

ly in nutritive value and thus are increasingly being

applied as a basic carbon source to enhance overall and

long-term soil health. The carbon content of these mate-

rials is also quite variable, though it generally ranges

from 20 to 40 percent on a dry-weight basis. Annual

applications of composts and manures at rates of 3 to 5

tons per acre are common in organic vegetable systems

in California. Such organic amendments add significant

amounts of carbon to the soil and are generally associ-

ated with improved tilth, lower bulk density, and

increased water infiltration. Very few studies, however,

have been conducted to monitor changes in key soil

quality indicator properties or processes that may result

from the application of these amendments over the

diverse range of organic vegetable systems in

California.


Cover Crops


Cover cropping (also called green manuring) is widely

seen as an important part of soil quality management in

organic production systems in California. Cover crops

can provide a practical and economical means for sup-

plying organic matter, enhancing soil fertility, suppress-

ing weed growth, attracting beneficial insects, spiders,

and predatory mites, and reducing nitrate leaching

losses to the groundwater during periods between

crops. Cover crops also may seriously limit a grower’s

options for planting and harvesting alternative main

cash crops, and, depending on the specific cropping sit-

uation, the use of cover crops may also result in poten-

tially adverse consequences such as soil moisture deple-

tion, temporary immobilization of plant nutrients,

increased pest problems, and increased management

and associated costs.


While cover cropping has not been an integral com-

ponent of many annual crop production systems in


California over recent decades, there is now growing

interest in the use of cover crops to store carbon and

improve resource use efficiencies in these systems.

Recent research in the Sacramento Valley suggests that

cover crop–based production systems may exert a more

favorable influence on annual water balances than has

previously been thought. Intensifying concerns about

increasing atmospheric CO2 levels, global warming,

and the potential roles that carbon sequestration in

plants and the soil may play in mitigating the green-

house effect also may result in more widespread use of

cover crops. The key to the effective and profitable use

of cover crops lies in the creative design of manage-

ment options to take advantage of those windows of

opportunity when they can be grown to maximum

advantage within vegetable crop rotations without

missing opportunities for income as a result of not uti-

lizing the land for cash crops.


A considerable body of information currently exists

within California’s main organic vegetable production

regions on how to select, grow, and work in cover

crops. This information is readily available through UC

Cooperative Extension county farm advisors and can

be used as a point of departure for establishing small-

scale on-farm evaluations of cover crops for achieving

particular management goals (see Cover Crops for

California Agriculture, ANR Publication 21471). While

much of this information has been developed for winter

cover crop species, several recent studies have also

evaluated late summer or other nonconventional cover

crop growing windows. For example, while October-

planted, March-incorporated rye and vetch cover crops

respectively produce about 9,000 and 5,000 pounds of

dry matter per acre in the Central Valley, an August-

planted, November-incorporated Sudangrass crop may

provide twice the biomass per acre.


Cover crop species mixtures are gaining wider adop-

tion throughout California because they may provide

multiple benefits to a production system and may serve

as insurance against conditions that are unfavorable to

a single species. Recent research conducted by the

Sustainable Agriculture Farming Systems Project in

Davis, California, suggests that legume cover crops

may be particularly important factors in the develop-

ment of key humic acid fractions that typically distin-

guish organic soils from conventionally managed soils

and that may be significant indices of soil quality

improvement.


CONSERVATION TILLAGE


Although increasing SOM is widely recognized as a

primary goal of soil management programs for organic

vegetable producers, tillage has a negative effect on

SOM. While moderate tillage may provide more favor-


Soil Management and Soil Quality for Organic Crops • 3


able soil conditions for crop growth and development

and weed control over the short term, intensive tillage

of agricultural soils has historically led to substantial

losses of soil carbon, ranging from 30 to 50 percent.


Conventional tillage practices disrupt soil aggre-

gates, exposing more organic matter to microbial degra-

dation and oxidation, and are among the primary caus-

es of tilth deterioration over the long term. Micro and

macro channels within the soil, created by natural

processes such as the decay of roots and worm activity,

may also be destroyed by tillage. Deep tillage, a cus-

tomary “soil preparation operation,” is also costly and

requires high energy and increased subsequent efforts

to prepare seed beds. A recent survey documented a 40

percent decline in SOM since intensive tillage practices

began in the Salinas Valley. This survey confirms the

conclusion drawn from other long-term crop rotation

studies as the Morrow Plots at the University of Illinois,

the Sanborn Field Plots in Columbia, Missouri, and the

Columbia Plateau Plots near Pendleton, Oregon: that

intensive tillage typically leads to decreased soil carbon

in virtually all crop production systems.


Recent studies involving a variety of tillage methods

indicate that there are major gaseous losses of carbon

immediately following tillage, but point to the potential

to reduce soil carbon losses and enhance soil carbon

management through the use of conservation tillage

(CT) crop production practices. Though these practices

have been developed over the past several decades pri-

marily for erosion control in other parts of the United

States, recent concerns regarding the need to sustain

soil quality in areas such as California (where CT is vir-

tually nonexistent) as well as potential global climate

changes have reemphasized the importance of CT and

its potential for implementation on a broader scale to

help reduce soil carbon losses, improve soil quality, and

sustain agricultural productivity.


The term conservation tillage describes production

systems in which at least 30 percent of the soil surface is

covered by residues from previous crops. Traditionally,

organic farmers have avoided production systems that

leave a lot of surface residues because such systems

make mechanical weed control difficult. As Dr. Ronald

Morse, a Professor of Horticulture at Virginia Tech and

a long-time no-till vegetable pioneer, has pointed out,

this creates a true paradox for organic growers: primary

tillage and weed cultivation as used extensively on

organic farms incorporate surface residues, excessively

aerate the soil, and reduce soil organic matter content

and soil quality, the very opposite of the goals organic


vegetable producers want to achieve. The development

of high-residue CT systems that enable adequate weed

control is a challenge facing innovative organic veg-

etable producers.


Studies are underway in California on reduced-

tillage crop production systems. The basic feature of

these systems is the use of surface organic mulches that

are derived from cover crops grown in the off season.

Winter annual cover crops such as rye and vetch, for

example, have been used successfully both as cover

crops and as mulches in a variety of CT systems. As

cover crops, these species recycle nutrients, reduce soil

erosion, add organic matter to the soil, and (in the case

of vetch or other legumes) fix nitrogen. When mowed

and converted to a mulch, they reduce weed emer-

gence, lower soil temperatures during the hot summer

months, reduce water loss from the soil, and act as a

slow-release fertilizer. There may be problems associat-

ed with using cover crops in this way: the cover-

cropped land is put out of production, the land’s soil

moisture may be depleted more by the cover crop than

it would be by a winter fallow, early summer soil tem-

peratures may be cooler with a surface mulch than bare

soil, and cover crops used as surface mulches may not

release nutrients until incorporated into the soil. There

may also be problems related to the management of

cover crops and cover crop residues that can be phyto-

toxic to certain crops that follow a green manure mulch.


A major limitation to conservation tillage and cover

crop mulch systems in organic crop production is the

need to manage the cover crop so that it does not com-

pete with or reduce the growth of the cash crop that is

planted into it. Various management options to ensure

this condition are currently under evaluation. Timing

the growth of the cover crop so it will reach maturity

and complete its life cycle at the time it is scheduled to

be mowed, rolled, or planted into may be a useful strat-

egy in certain contexts. This has been done successfully

with rye. However, this approach generally assumes

that the vegetable crop will not be planted or trans-

planted into the cover crop mulch until late in the

spring. A grower can also use irrigation cut-off or rely

on increasing temperatures to kill off certain cover

crops to an extent that prevents them from effectively

competing with the vegetable crop.

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