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Managing for High Quality Soils: Organic
Matter, Soil Physical Condition, Nutrient Availability
Because organic matter is lost from the soil
through decay, washing, and leaching, and because large amounts
are
required every year for crop production, the necessity of maintaining
the active organic-matter
content of the soil, to say nothing of the desirability
of increasing it on many depleted soils, is a
difficult problem.
A. F. Gustafson, 1941
Building high-quality soils takes a lot of thought
and action over many years. Of course, there are things that can
be done right off -- plant a cover crop this fall or just make a
New Year's resolution not to work soils that really aren't ready
in the spring (and then stick with it). Other changes take more
time. You need to study carefully before drastically changing crop
rotations, for example. How will the new crops be marketed and are
the necessary labor and machinery available?
There are three different general management approaches
to enhancing soil health. First, various practices to build up and
maintain high levels of soil organic matter are key. Second, developing
and maintaining the best possible soil physical condition often
requires other types of practices, in addition to those that directly
impact soil organic matter. Paying better attention to soil tilth
and compaction is more important than ever, because of the use of
very heavy field machinery. Lastly, although good organic matter
management goes a long way toward providing good plant nutrition
in an environmentally sound way, good nutrient management involves
additional practices.
ORGANIC MATTER MANAGEMENT
It is difficult to be sure exactly why problems develop
when organic matter is depleted in a particular soil. However, even
in the early 20th century, agricultural scientists proclaimed, "Whatever
the cause of soil unthriftiness, there is no dispute as to the remedial
measures. Doctors may disagree as to what causes the disease, but
agree as to the medicine. Crop rotation! The use of barnyard and
green manuring! Humus maintenance! These are the fundamental needs"
(Hills, Jones, and Cutler, 1908). Close to a century later, these
are still some of the major remedies available to us.
There seems to be a contradiction in our view of soil
organic matter. On one hand, we want crop residues, dead microorganisms,
and manures to decompose. If soil organic matter doesn't decompose,
then no nutrients are made available to plants, no glue to bind
particles is manufactured, and no humus is produced to hold on to
plant nutrients as water leaches through the soil. On the other
hand, numerous problems develop when soil organic matter is significantly
depleted through decomposition. This dilemma of wanting organic
matter to decompose, but not wanting to lose too much, means that
organic materials must be continually added to the soil. A supply
of active organic matter must be maintained so that humus can continually
accumulate. This does not mean that organic materials must be added
to each field every year. However, it does mean that a field cannot
go without additions of organic residues for many years without
paying the consequences.
Do you remember that plowing a soil is similar to
opening up the air intake on a wood stove? What we really want in
soil is a slow, steady burn of the organic matter. You get that
in a wood stove by adding wood every so often and making sure the
air intake is on a medium setting. In soil, you get a steady burn
by adding organic residues regularly and by not disturbing the soil
too often.
Soil Organic Matter Management
Strategies
Increase Additions of organic residues to
soils.
Use varied sources of organic materials.
Decrease losses of organic matter from soils.
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There are three general management strategies for organic matter
management. First, use crop residues more effectively and find new
sources of residues to add to soils. New residues can include those
you grow on the farm, such as cover crops, or those available from
various local sources. Second, be sure to use a number of different
types of materials crop residues, manures, composts, cover crops,
leaves, etc. It is important to provide varied residue sources to
help develop and maintain a diverse group of soil organisms. Third,
implement practices that decrease the loss of organic matter from
soils because of accelerated decomposition or erosion.
Soil Organic Matter Levels
Raising and maintaining soil organic matter levels.
It is not easy to dramatically increase the organic matter content
of soils or even to maintain good levels once they are reached.
Improving organic matter content requires a sustained effort that
includes a number of approaches to return organic materials to soils
and minimize soil organic matter losses. It is especially difficult
to raise the organic matter content of soils that are very well
aerated, such as coarse sands, because added materials are decomposed
so rapidly. Soil organic matter levels can be maintained with less
organic residue in high clay-content soils with restricted aeration
than in coarse-textured soils.
All practices that help to build organic matter levels
do at least one of two things -- add more organic materials than
was done in the past or decrease the rate of organic matter loss
from soils (table 8.1). Those practices that do both may be especially
useful. Practices that reduce losses of organic matter either slow
down the rate of decomposition or decrease the amount of erosion.
Soil erosion must be controlled to keep organic matter-enriched
topsoil in place. In addition, organic matter added to a soil must
either match or exceed the rate of loss by decomposition. These
additions can come from manures and composts brought from off the
field, crop residues and mulches remaining following harvest, or
cover crops. Reduced tillage lessens the rate of organic matter
decomposition and also may result in less erosion. When reduced
tillage increases crop growth and residues returned to soil, it
is usually a result of better water infiltration and storage and
less surface evaporation. It is not possible in this book to give
specific soil organic matter management recommendations for all
situations. In chapters 9 through 15, we will evaluate management
options and issues associated with their use.
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How much organic matter is enough? Unlike the case with plant
nutrients or pH levels, there are no accepted guidelines for organic
matter content. We do know some general guidelines. For example,
2 percent organic matter in a sandy soil is very good, but in a
clay soil, 2 percent indicates a greatly depleted situation. The
complexity of soil organic matter composition, including biological
diversity of organisms as well as the actual organic chemicals present,
means that there is no simple interpretation for total soil organic
matter tests.
Using Organic Materials
Crop residues. Crop residues are usually the
largest source of organic materials available to farmers. The amount
of crop residue left after harvest varies depending on the crop.
Soybeans, potatoes, lettuce and corn silage leave little residue.
Small grains, on the other hand, leave more residue, while sorghum
and corn harvested for grain leave the most. A ton or more of crop
residues per acre may sound like a lot of organic material being
returned to the soil. However, keep in mind that after residues
are decomposed by soil organisms only about 10 to 20 percent of
the original amount is converted into stable humus.
The amount of roots remaining after harvest also can
range from very low to fairly high. For a crop of corn, roots may
account for over a ton of dry weight per acre (thus more than 4
½ tons of surface residues plus roots about 60 percent of the
total plant remain following a Midwest grain harvest of about 120
bu. per acre). The estimated root residues (from Prince Edward Island
in Canada) give some idea of the differences that you might find
(table 8.2).
Some farmers remove above ground residues from the
field for use as animal bedding or to make compost. Later, these
residues return to contribute to soil fertility as manures or composts.
Sometimes, residues are removed from fields, to be used by other
farmers or to make another product. There is renewed interest in
using crop residues as a wood substitute to make a variety of products,
such as particleboard. This activity could cause considerable harm
because residues are not returned to soils.
Crop
Residues
The amount of residue left in the field after
harvest depends on the type of crop and its yield. The table
on the left contains the amounts of residues found in California's
highly productive, irrigated San Joaquin Valley. These residue
amounts are higher than would be found on most farms, but
the relative amounts for the various crops are interesting.
|
Crop Residue in the
San Joaquin Valley (California) |
Residues of Common Crops in the
Midwest and Great Planins
|
Crop |
TONS/ACRE |
Corn (grain) |
5 |
Broccoli |
3 |
Cotton |
2.5 |
Wheat (grain) |
2.5 |
Sugarbeets |
2 |
Safflower |
1.5 |
Tomatoes |
1.5 |
Lettuce |
1 |
Corn (silage) |
.5 |
Garlic |
.5 |
Wheat (after baling) |
.25 |
Onions |
.25 |
-- Mitchell et. al., 1999 |
|
Crop |
TONS/ACRE |
Corn (120 bu.) |
3.5 |
Sorghum (80 bu.) |
2.5 |
Wheat (35 bu.) |
2 |
Soybeans (35 bu.) |
less than 1 |
-- From various sources |
|
Burning of wheat, rice, and other crop residues in
the field is a common practice in parts of the United States as
well as in other countries. Residue is usually burned to help control
insects or diseases or to make next year's fieldwork easier. Residue
burning may be so widespread in a given area that it causes a local
air pollution problem. Burning also diminishes the amount of organic
matter returned to the soil and the amount of protection against
raindrop impact.
Sometimes, important needs for crop residues and manures
may prevent their use in maintaining or building soil organic matter.
For example, straw may be removed from a grain field to serve as
mulch in a strawberry field. These trade-offs of organic materials
can sometimes cause a severe soil-fertility problem if allowed to
continue for a long time. This issue is of much more widespread
importance in developing countries where resources are scarce. There,
crop residues and manures frequently serve as fuel for cooking or
heating when gas, coal, oil, or wood are not available. In addition,
straw may be used in making bricks or used as thatch for housing
or to make fences. Although it is completely understandable that
people in resource-poor regions use residues for such purposes,
the negative effects of these uses on soil productivity can be substantial.
An important way to increase agricultural productivity in developing
countries is to find alternative sources for fuel and building materials
to replace the crop residues and manures traditionally used.
Using residues as mulches. Crop residues or
composts can be used as a mulch on the soil surface. This occurs
routinely in some reduced tillage systems when high residue-yielding
crops are grown or when killed cover crops remain on the surface.
In some small-scale vegetable and berry farming, mulching is done
by applying straw from off-site. Strawberries grown in the colder
northern parts of the country are routinely mulched with straw for
protection from winter heaving. The straw is blown on in late fall
and is then moved into the interrows in the spring, providing a
surface mulch during the growing season.
Mulching has numerous benefits, including:
- enhanced water availability to crops (better infiltration
into the soil and less evaporation from the soil);
- weed control;
- less extreme changes in soil temperature;
- reduced splashing of soil onto leaves and fruits and vegetables
(making them look better as well as reducing diseases); and
- reduced infestations of certain pests (Colorado potato beetle
on potatoes is less severe when potatoes are grown in a mulch
system).
On the other hand, residue mulches in cold climates
can delay soil warming in the spring, reduce early season growth,
and increase problems with slugs during wet periods. Of course,
one of the reasons for the use of plastic mulches (clear and black)
for crops like tomatoes and melons is to help warm the soil.
Effects of Residue Characteristics on Soil
Decomposition rates and effects on aggregation.
Residues of various crops and manures have different properties
and, therefore, have different effects on soil organic matter. Materials
with low amounts of hard-to-degrade hemicellulose and lignin, such
as cover crops when still very green and soybean residue, decompose
rapidly (figure 8.1) and have a shorter-term effect on soil organic
matter levels than residues with high levels of these chemicals
(for example, corn and wheat). Manures, especially those that contain
lots of bedding (high in hemicellulose and lignin), are decomposed
more slowly and tend to have more long-lasting effects on total
soil organic matter than crop residues or manures without bedding.
Also, cows because they eat a diet containing lots of forages, which
they do not completely decompose have manure with longer lasting
effects on soils than non-ruminants, such as chickens and hogs,
that are fed exclusively a high-grain/low-fiber diet. Composts contribute
little active organic matter to soils, but add a lot of well decomposed
materials (figure 8.1).
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Figure 8.1 Different types
of residues have varying effects on soils (thicker lines indicate
more material, dashed line indicates very small percent of that
type). Modified from Oshins, 1999. |
In general, residues containing a lot of cellulose
and other easy-to-decompose materials will have a greater effect
on soil aggregation than compost, which has already undergone decomposition.
Because aggregates are formed from by-products of decomposition
by soil organisms, organic additions like manures, cover crops,
and straw will enhance aggregation more than compost. (However,
adding compost does improve soils in many ways, including increasing
the water holding capacity.)
Although it's important to have adequate amounts of
organic matter in soil, that isn't enough. A variety of residues
is needed to provide food to a diverse population of organisms,
nutrients to plants, and to furnish materials that promote aggregation.
Residues low in hemicellulose and lignin usually have very high
levels of plant nutrients. On the other hand, straw or sawdust (containing
a lot of lignin) can be used to build up organic matter, but a severe
nitrogen deficiency and an imbalance in soil microbial populations
will occur unless a readily available source of nitrogen is added
at the same time (see discussion of C:N ratios below). In addition,
when insufficient N is present, less of the organic material added
to soils actually ends up as humus.
C:N Ratio of organic materials and nitrogen availability.
The ratio of the amount of a residue's carbon to the amount of nitrogen
influences nutrient availability and the rate of decomposition.
The ratio, usually referred to as the C:N ratio, may vary from around
15:1 for young plants, to between 50 to 80:1 for the old straw of
crop plants, to over 100:1 for sawdust. For comparison, the C:N
ratio of soil organic matter is usually in the range of about 10
to 12:1 and the C:N of soil microorganisms is around 7:1.
The C:N ratio of residues is really just another way
of looking at the percentage of nitrogen (figure 8.2). A high C:N
residue has a low percentage of nitrogen. Low C:N residues have
relatively high percentages of nitrogen. Crop residues are usually
pretty close to 40 to 45 percent carbon, and this figure doesn't
change much from plant to plant. On the other hand, nitrogen content
varies greatly depending on the type of plant and its stage of growth.
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Figure 8.2 Nitrogen release and immobilization
with changing nitrogen content. Based on data of Vigil and Kissel,
1991. |
If you want crops growing immediately following the
application of organic materials, care must be taken to make nitrogen
available.
Nitrogen availability from residues varies considerably.
Some residues, such as fresh, young, and very green plants, decompose
rapidly in the soil and, in the process, may readily release plant
nutrients. This could be compared to the effect of sugar eaten by
humans, which results in a quick burst of energy. Some of the substances
in older plants and in the woody portion of trees, such as lignin,
decompose very slowly in soils. Materials, such as sawdust and straw,
mentioned above, contain little nitrogen. Well-composted organic
residues also decompose slowly in the soil because they are fairly
stable, having already undergone a significant amount of decomposition.
Mature plant stalks and sawdust that have C:N over
40:1 (table 8.3) may cause temporary problems for plants. Microorganisms
using materials containing 1 percent nitrogen (or less) need extra
nitrogen for their growth and reproduction. They will take the needed
nitrogen from the surrounding soil, diminishing the amount of nitrate
and ammonium available for crop use. This reduction of soil nitrate
and ammonium by microorganisms decomposing high C:N residues is
called immobilization of nitrogen.
When microorganisms and plants compete for scarce
nutrients, the microorganisms usually win, because they are so well
distributed in the soil. Plant roots are in contact with only 1
to 2 percent of the entire soil volume whereas microorganisms populate
almost the entire soil. The length of time during which the nitrogen
nutrition of plants is adversely affected by immobilization depends
on the quantity of residues applied, their C:N ratio, and other
factors influencing microorganisms, such as fertilization practices,
temperature, and moisture conditions. If the C:N ratio of residues
is in the teens or low 20s, corresponding to greater than 2 percent
nitrogen, then there is more nitrogen present than the microorganisms
need for residue decomposition. When this happens, extra nitrogen
becomes available to plants fairly quickly. Green manure crops and
animal manures are in this group of residues. Residues with C:N
in the mid-20s to low 30s, corresponding to about 1 to 2 percent
nitrogen, will not have much effect on short-term nitrogen immobilization
or release.
Sewage sludge on your fields? In theory, the
use of sewage sludges on agricultural lands makes sense as a way
to resolve problems related to people living in cities, far removed
from the land that grows their food. However, there are some troublesome
issues associated with agricultural use of sludges.By far, the most
important problem is that they frequently contain contaminants from
industry and from various products used around the home. Although
many of these metal contaminants naturally occur at low levels in
soils and plants, their high concentrations in some sludges create
a potential hazard. The U.S. standards for toxic materials in sludges
are much more lenient than those in other industrialized countries
and they permit higher loading of potentially toxic metals. So,
although you are allowed to use many sludges, you should carefully
examine a sludge's contents before applying it to your land.
Another issue is that sludges are produced by varied
processes and, therefore, have different properties. Most sludges
are around neutral pH, but, when added to soils, cause some degree
of acidification, as do most nitrogen fertilizers. Because many
of the problem metals are more soluble under acidic conditions,
the pH of soils receiving these materials should be monitored and
maintained at around 6.8 or above. On the other hand, lime (calcium
hydroxide and ground limestone are used together) is added to some
sludges to raise the pH and kill disease bacteria. The resulting
"lime-stabilized" sludge has extremely high levels of
calcium, relative to potassium and magnesium. This type of sludge
should be used primarily as a liming source and levels of magnesium
and potassium in the soil need to be carefully monitored to be sure
they are present in reasonable amounts, compared with the high levels
of added calcium.
The use of "clean" sludges those containing
low levels of metal and organic contaminants for agronomic crops
is certainly an acceptable practice. Sludges should not be applied
to soils when growing crops for direct human consumption, unless
it can be demonstrated that, in addition to low levels of potentially
toxic materials, organisms dangerous to humans are absent.
C:N
Ratio of Active Organic Matter
As residues are decomposed by soil organisms,
carbon is lost as CO2, while nitrogen is mostly
conserved. This causes the C:N ratio of decomposing residues
to decrease. Although the C:N ratio for most agricultural
soils is in the range of 10 to 12:1, the different types of
organic matterwithin a soil have different C:N ratios. The
larger particles of soil organic matter have higher C:N ratios,
indicating that they are less decomposed than smaller fractions.
Microscopic evidence also indicates that the larger fractions
are less decomposed than the smaller particles.
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|
Figure 8.3 C:N ratio of different size fractions
of organic matter.
Magdoff, F., unpublished data, average for three soils. |
Application rates for organic materials. The
amount of residue added to a soil is often determined by the cropping
system. The crop residues can be left on the surface or incorporated
by tillage. Different amounts of residue will remain under different
crops, rotations, or harvest practices. For example, three or more
tons per acre of leaf, stalk, and cob residues remain in the field
when corn is harvested for grain. If the entire plant is harvested
to make silage, there is little left except the roots.
When "imported" organic materials are brought
to the field, you need to decide how much and when to apply them.
In general, application rates of these residues will be based on
their probable contribution to the nitrogen nutrition of plants.
We don't want to apply too much available nitrogen because it will
be wasted. Nitrate from excessive applications of organic sources
of fertility may leach into ground- water just as easily as nitrate
originating from purchased synthetic fertilizers. In addition, excess
nitrate in plants may cause health problems for humans and animals.
Sometimes the fertility contribution of phosphorus
may be the main factor governing application rates of organic material.
Excess phosphorus entering lakes can cause an increase in the growth
of algae and other aquatic weeds, decreasing water quality for drinking
and recreation. In these locations, farmers must be careful to avoid
loading the soil with too much phosphorus, from either commercial
fertilizers or organic sources.
Effects of residue and manure accumulations.
When any organic material is added to soil, it decomposes relatively
rapidly at first. Later, when only resistant parts (for example,
straw stems high in lignin) are left, the rate of decomposition
decreases greatly. This means that although nutrient availability
diminishes each year after adding a residue to the soil, there are
still long-term benefits from adding organic materials. This can
be expressed by using a "decay series." For example, 50,
15, 5, and 2 percent of the amount of nitrogen added in manure may
be released in the first, second, third, and fourth years following
addition to soils. In other words, crops in a regularly manured
field get some nitrogen from manure that was applied in past years.
So, if you are starting to manure a field, somewhat more manure
will be needed in the first year than will be needed in years 2,
3, and 4 to supply the same total amount of nitrogen to a crop each
year. After some years, you may need only half of the amount used
to supply all the nitrogen needs in the first year.
Organic Matter Management on Different Types of Farms
Animal-based farms. It is certainly easier
to maintain soil organic matter in animal-based agricultural systems.
Manure is a valuable by-product of having animals. Animals also
can use sod-type grasses and legumes as pasture, hay, and haylage
(hay stored under air-tight conditions so that some fermentation
occurs). It is easier to justify putting land into perennial forage
crops for part of a rotation when there is an economic use for the
crops. Animals need not be on the farm to have positive effects
on soil fertility. A farmer may grow hay to sell to a neighbor and
trade for some animal manure from the neighbor's farm, for example.
Occasionally, formal agreements between dairy farmers and vegetable
growers lead to cooperation on crop rotations and manure application.
Systems without animals. It is more challenging,
although not impossible, to maintain or increase soil organic matter
on non-livestock farms. It can be done by using reduced tillage,
cover crops, intercropping, living mulches, rotations that include
crops with high amounts of residue left after harvest, and attention
to other erosion-control practices. Organic residues, such as leaves
or clean sewage sludges, can sometimes be obtained from nearby cities
and towns. Straw or grass clippings used as mulch also add organic
matter when they later become incorporated into the soil by plowing
or by the activity of soil organisms. Some vegetable farmers use
a "mow-and-blow" system where crops are grown on strips
for the purpose of chopping them and spraying the residues onto
an adjacent strip.
Maintaining
Organic Matter in Small Gardens
There are a number of different ways that home
gardeners can maintain soil organic matter. One of the easiest
is using lawn grass clippings for mulch during the growing
season. The mulch can then be worked into the soil or left
on the surface to decompose until the next spring. Also, leaves
can be raked up in the fall and applied to the garden. Cover
crops can also be used on small size gardens. Of course, manures,
composts, or mulch straw can also be purchased.
There are a growing number of small-scale market
gardeners, many with insufficient land to rotate into a sod
type crop. They also may have crops in the ground late into
the fall, making cover cropping a challenge. One possibility
is to establish cover crops by over-seeding after the last
crop of the year is well established. Another source of organic
materials grass clippings are probably in short supply compared
with the needs of cropped areas, but are still useful. It
might also be possible to obtain leaves from a nearby town.
These can either be directly applied and worked into the soil
or composted first. As with home gardeners, market gardeners
can purchase manures, composts, and straw mulch, but should
get volume discounts on the amounts needed for an acre or
two. |
Maintaining Soil Biodiversity
The role of diversity is critical to maintaining a
well functioning and stable agriculture. Where many different types
of organisms coexist, there are fewer disease, insect, and nematode
problems. There is more competition for food and more possibility
that many types of predators will be found. This means that no single
pest organism will be able to reach a population high enough to
cause a major decrease in crop yield. We can promote a diversity
of plant species growing on the land by using cover crops, intercropping,
and crop rotations. However, don't forget that diversity below the
soil surface is as important as diversity above ground. Growing
cover crops and using crop rotations help maintain the diversity
below ground, but adding manures and composts and making sure that
crop residues are returned to the soil are also critical for promoting
soil organism diversity.
Managing Soils and Crops
to Minimize Pest Problems
Many of the practices discussed in this chapter
and the other chapters in Part 2 help to reduce the severity
of crop pests. It is now known that plants have very sophisticated
defense mechanisms against insects and diseases. When plants
are under environmental stresses caused by compact soils,
droughty conditions, or excess nitrogen, they are less able
to combat pests and may be even more attractive to them. On
the other hand, healthy plants growing on soils with good
biological diversity are able to mount a strong defense against
many pests. For example, when attacked by insects they may
emit chemicals that attract beneficial insects that are predators
of the pest. In addition, good soil management decreases levels
of pests that live in the soil.
It is well established and known by most farmers
that crop rotation can decrease disease, insect, nematode,
and weed pressures. A few other examples are given below.
- Insect damage can be reduced by avoiding
excess nitrogen levels in soils through better nitrogen
management.
- Root rots and severity of leaf diseases
can be reduced with composts that contain low levels of
available nitrogen, but still have some active organic matter.
- Fungal diseases of roots and insect damage
are decreased by lessening soil compaction. 3 Many pests
are kept under control by competition for resources or direct
antagonism (including the beneficials feeding on them).
Good quantities of a variety of organic materials help maintain
a diverse group of soil organisms.
- Root surfaces are protected from fungal
and nematode attack by high rates of beneficial mycorrhizal
fungi. Most cover crops help keep mycorrhizal fungi spore
counts high and promote higher rates of infection by the
beneficial fungi.
- Parasitic nematodes can be suppressed by
cover crops.
- Residues of some cover crops, such as winter
rye, reduce weed seed germination.
- Weed seed numbers are reduced in soils with
a lot of biological activity, with both microorganisms and
insects helping the process.
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Managing Soil Physical Conditions:
Developing and maintaining an optimum physical
environment. Plants thrive in a physical environment that allows
roots to actively explore a large area, gets all the oxygen and
water needed, and maintains a healthy mix of organisms. Although
the soil's physical environment is strongly influenced by organic
matter, the practices and equipment used from tillage to planting
to cultivation to harvest have a major impact. If a soil is too
wet whether it has poor internal drainage or it receives too much
water some remedies are needed to grow high yielding and healthy
crops. Also, erosion whether by wind or water is an environmental
hazard that needs to be kept as low as possible. Erosion is most
likely when the surface of a soil is bare and doesn't contain sufficient
medium- to large-size water-stable aggregates. Practices for management
of soil physical properties are discussed in chapters 13 to 15.
Nutrient Management
Many of the practices that build up and maintain soil
organic matter also help enrich the soil with nutrients or make
it easier to manage nutrients in ways that satisfy crop needs and
are also environmentally sound. For example, a legume cover crop
increases a soil's active organic matter and reduces erosion, but
it also adds nitrogen that can be used by the next crop. Cover crops
and deep-rooted rotation crops help to cycle nitrate, potassium,
calcium, and magnesium that might be lost to leaching below crop
roots. Importing mulches or manures onto the farm also adds nutrients
along with the organic materials. However, specific nutrient management
practices are needed, such as testing manure and checking its nutrient
content before applying additional nutrient sources. Other examples
of nutrient management practices not directly related to organic
matter management include applying nutrients timed to plant needs,
liming acidic soils, and interpreting soil tests to decide on the
appropriate amounts of nutrients to apply (see chapters 16 to 19).
Development of farm nutrient management plans and watershed partnerships
also improve soil while protecting the local environment.
Sources
Barber, S.A. 1998. Chemistry of soil-nutrient interactions and future
agricultural sustainability. In Future Prospects for Soil Chemistry
(P.M. Huang, D.L. Sparks, and S.A. Boyd, eds.). SSSA Special Publication
No. 55. Soil Science Society of America. Madison, WI.
Brady, N.C., and R.R. Weil. 1999. The Nature and
Properties of Soils. 12th ed. Macmillan Publishing Co. New York,
NY.
Cavigelli, M.A., S.R. Deming, L.K. Probyn, and R.R.
Harwood (eds.). 1998. Michigan Field Crop Ecology: Managing
Biological Processes for Productivity and Environmental Quality.
Michigan State University Extension Bulletin E-2646. East Lansing,
MI.
Mitchell, J., T. Hartz, S. Pettygrove, D. Munk, D.
May, F. Menezes, J. Diener, and T. O'Neill. 1999. Organic matter
recycling varies with crops grown. California Agriculture
53(4):3740.
Oshins. C. An Introduction to Soil Health. 1999. A
slide set available at the Northeast Region SARE website: http://www.uvm.edu/~nesare/slides/index.htm
Topp, G.C., K.C. Wires, D.A. Angers, M.R. Carter,
J.L.B. Culley, D.A. Holmstrom, B.D. Kay, G. P. Lafond, D.R. Langille,
R.A. McBride, G.T. Patterson, E. Perfect, V. Rasiah, A.V. Rodd,
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