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BUILDING SOIL FERTILITY
AND TILTH WITH COVER CROPS
by Marianne Sarrantonio
Soil is an incredibly complex substance. It has physical and chemical
properties that allow it to sustain living organisms—not just
plant roots and earthworms, but hundreds of thousands of different
insects, wormlike creatures and microorganisms. When these organisms
are in balance, your soil cycles nutrients efficiently, stores water
and drains the excess, and maintains an environment in which plants
can thrive.
To recognize that a soil can be healthy, one has only to think
of the soil as a living entity. It breathes, it transports and transforms
nutrients, it interacts with its environment, and it can even purify
itself and grow over time. If you view soil as a dynamic part of
your farming system, unsustainable crop management practices amount
to soil neglect. That neglect could worsen as the soil sickens and
loses its life functions one by one.
Regardless of how healthy or alive your soil is right now, cover
crops can play a vital role in ensuring that your soil provides
a strong foundation for your farming system. While the most common
reasons for including cover crops in a farming system may relate
to the immediate short-term need, the continued practice of cover
cropping becomes an investment in building healthy soil over the
long term.
Cover crops improve soil in a number of ways. Protection against
soil loss from erosion is perhaps the most obvious soil benefit
of cover crops, but providing organic matter is a more long-term
and equally important goal. Cover crops contribute indirectly to
overall soil health by catching nutrients before they can leach
out of the soil profile or, in the case of legumes, by adding nitrogen
to the soil. Their roots can even help unlock some nutrients, converting
them to more available forms. Cover crops provide habitat or a food
source for some important soil organisms, break up compacted layers
in the soil and help dry out wet soils.
EROSION PROTECTION
Erosion of topsoil occurs on many farms, depriving fields of the
most fertile portion, that containing the highest percentage of
organic matter and nutrients. Cover crops can play a major role
in fighting soil erosion.
A raindrop falling at high speed can dislodge soil particles and
cause them to move as far as 6 feet (42).
Once a soil particle is loose, it is much more vulnerable to being
carried away by running water. Any aboveground soil cover can take
some of the punch out of a heavy rainfall simply by acting as a
cushion for raindrops.
A cover crop also can:
 Slow
the action of moving water, thus reducing its soil-carrying capacity,
by creating an obstacle course of leaves, stems and roots through
which the water must maneuver on its way downhill
Increase
the soil’s ability to absorb and hold water, through improvement
in pore structure, thereby preventing large quantities of water
from moving across the soil surface
Help
stabilize soil particles in the cover crop root system
The reduction in soil erosion due to cover cropping will be roughly
proportional to the amount of cover on the soil. The Revised Universal
Soil Loss Equation developed by the Natural Resources Conservation
Service predicts that a soil cover of just 40 percent when winter
arrives can reduce erosion substantially until spring.
It’s worthwhile to get covers established early, to ensure
that maximum soil cover develops before winter rains. Consider overseeding
covers at layby cultivation, aerial seeding or hand spreading before
harvest, or planting as soon as possible after harvest. It’s
always a good idea to maintain year-round soil cover whenever possible.
ORGANIC MATTER ADDITIONS
The benefits of organic matter include improved soil structure,
increased infiltration and water-holding capacity,
increased cation exchange capacity (the ability
of the soil to act as a short-term storage bank for positively charged
plant nutrients) and more efficient long-term storage
of nutrients. Without organic matter, you have no soil to speak
of, only a dead mixture of ground-up and weathered rocks.
Organic matter includes thousands of different substances derived
from decayed leaves, roots, microorganisms, manure and even groundhogs
that died in their burrows. These substances function in different
ways to build healthy soil. Different plants leave behind different
kinds of organic matter as they decompose, so your choice of cover
crop will largely determine which soil benefits you will receive.
Soil scientists may argue over how to classify the various soil
organic components. Most will agree, however, that there is a portion
that can be called the “active” fraction,
and one that might be called the “stable” fraction,
which is roughly equivalent to humus. There are
many categories in between the active and stable fractions.
The active fraction represents the most easily decomposed parts
of soil organic matter. It tends to be rich in simple sugars and
proteins and consists largely of recently added fresh residues,
microbial cells and the simpler waste products from microbial decay.
Because microorganisms, like human organisms, crave sweet stuff,
compounds containing simple sugars disappear quickly. Proteins also
are selected quickly from the menu of edible soil goodies. When
these compounds are digested, many of the nutrients that they contain
are released into the soil. Proteins are nitrogen-rich, so the active
fraction is responsible for the release of most N,
as well as some K, P and other nutrients, from organic matter into
the soil. The easily decomposed proteins and sugars burn up
almost completely as energy sources, and don’t leave much
behind to contribute to organic matter building.
After the microorganisms have devoured the portions of the active
fraction that are easiest to digest, a more dedicated subset of
these microorganisms will start munching on the more complex and
tough material, such as celluloses and lignins, the structural materials
of plants. Since cellulose is tougher than simple sugars, and lignin
breaks down very slowly, they contribute more to the humus or stable
fraction. Humus is responsible for giving the soil that rich, dark,
spongy feeling and for properties such as water retention and cation
exchange capacity.
Plant materials that are succulent and rich in
proteins and sugars will release nutrients rapidly but leave behind
little long-term organic matter. Plant materials that are
woodier or more fibrous will release nutrients much more
slowly, perhaps even tie up nutrients temporarily (see Tillage,
No Tillage and N Cycling), but will promote more stable organic
matter, or humus, leading to better soil physical conditions, increased
nutrient-holding capacity and higher cation exchange capacity.
In general, annual legumes are succulent. They release nitrogen
and other nutrients quickly through the active fraction, but are
not very effective at building up humus. Long-term use of annual
legumes can increase soil humus, however, some research suggests
(429).
Grains and other grasses and nonlegumes will contribute to humus
production, but won’t release nutrients very rapidly or in
large quantities if incorporated as they approach maturity. Perennial
legumes such as white and red clover may fall in both categories—their
leaves will break down quickly, but their stems and root systems
may become tough and fibrous and can contribute to humus accumulation.
Cover Crops Help “Glue” Soil
As soil microorganisms digest plant material, they produce some
compounds in addition to the active and stable fractions of the
organic matter. One group of these by-products is known as polysaccharides.
These are complex sugars that act as glues in the soil to cement
small soil particles into clusters or aggregates.
Many farmers use the term “crumb” to
describe soil clusters about the size of a grain of rice. A well-aggregated
or “crumby” soil—not to be confused with crummy
or depleted soil—has good aeration. It allows better infiltration
and retention of water.
Cover crops can promote good aggregation in the soil through increased
production of these and other microbial glues. See Cover
Crops Can Stabilize Your Soil. Well-aggregated soils also are
less prone to compaction, which has been shown to reduce yields
of vegetables such as snap beans, cabbage and cucumber by 50 percent
or more (451).
As they decompose, leguminous cover crops seem to be better than
grasses for production of polysaccharides (9).
However, polysaccharides will decompose in a matter of months, so
their aggregation effect is likely to last only the season after
the use of the cover crop.
Grass species also promote good aggregation, but by a different
mechanism. Grasses have a ‘fibrous’ root system—made
of numerous fine roots spreading out from the base of the plant.
These roots may release compounds that help aggregate the soil between
roots.
Organic matter builds up very slowly in the soil. A soil with 3
percent organic matter might only increase to 4 percent after a
decade or more of soil building. The benefits of increased organic
matter, however, are likely to be apparent long before increased
quantities are detectable. Some, such as enhanced aggregation, water
infiltration rates and nutrient release, will be apparent the first
season; others may take several years to become noticeable (429).
Your tillage method is an important consideration when using cover
crops to build soil, because tillage will affect the rate of organic
matter accumulation. It is difficult to build up organic matter
under conventional tillage regimes. Tillage speeds up organic
matter decomposition by exposing more surface area to oxygen, warming
and drying the soil, and breaking residue into smaller pieces with
more surfaces that can be attacked by decomposers. Like fanning
a fire, tillage rapidly “burns up” or “oxidizes”
the fuel, which in this case is organic matter. The resulting loss
of organic matter causes the breakdown of soil aggregates and the
poor soil structure often seen in overtilled soil.
When adding cover crops to a system, minimize tillage to maximize
the long-term soil benefits. Many of the cover crops discussed in
this book are ones you can seed into growing crops or no-till plant
into crop residues. Otherwise, the gain in organic matter may be
counteracted by higher decomposition rates.
TIGHTENING THE NUTRIENT LOOP
In addition to reducing topsoil erosion and improving soil structure,
cover crops enhance nutrient cycling in your farming system by taking
up nutrients that otherwise might leach out of the soil profile.
These excess nutrients have the potential to pollute groundwater
or local streams and ponds, not to mention impoverishing the soil
they came from.
Of the common plant nutrients, nitrogen in the nitrate form is
the most water-soluble and therefore the most vulnerable to leaching.
Anytime soil is bare and appreciable rain falls, nitrates are on
the move. Nitrate can be present in the soil at the end of a cropping
season if the crop did not use all the N applied. Decomposing organic
matter (including plant residues, compost and animal manures) also
can supply nitrate-N, as long as the soil temperature is above freezing.
Even in a field where the yearly application of N is well-suited
to crop needs, nitrates can accumulate after crops are harvested
and leach when it rains.
Cover crops reduce nitrate leaching in two ways. They soak up available
nitrate for their own needs. They also use some soil moisture, reducing
the amount of water available to leach nutrients.
The best cover crops to use for nitrate conservation are nonlegumes
that form deep, extensive root systems quickly after cash crops
are harvested. For much of the continental U.S., cereal rye is the
best choice for catching nutrients after a summer crop. Its cold
tolerance is a big advantage that allows rye to continue to grow
in late fall and put down roots to a depth of three feet or more.
Where winters are mild, rye can grow through the winter months.
Cover Crops Can Stabilize Your Soil
The more you use cover crops, the better your
soil tilth, research continues to show. One reason is that
cover crops, especially legumes, encourage populations of
beneficial fungi and other microorganisms that help bind soil
aggregates.
The fungi, called mycorrhizae, produce a water-insoluble
protein known as glomalin, which catches
and glues together particles of organic matter, plant cells,
bacteria and other fungi (453).
Glomalin may be one of the most important substances in promoting
and stabilizing soil aggregates.
Most plant roots, not just those of cover crops,
develop beneficial mycorrhizal relationships. The fungi send
out rootlike extensions called hyphae, which
take up water and soil nutrients to help feed plants. In low-phosphorus
soils, for example, the hyphae can increase the amount of
phosphorus that plants obtain. In return, the fungi receive
energy in the form of sugars that plants produce in their
leaves and send down to the roots.
Growing a cover crop increases the abundance
of mycorrhizal spores. Legumes in particular can contribute
to mycorrhizal diversity and abundance, because their roots
tend to develop large populations of these beneficial fungi.
By having their own mycorrhizal fungi and by
promoting mycorrhizal relationships in subsequent crops, cover
crops therefore can play a key role in improving soil tilth.
The overall increase in glomalin production also could help
explain why cover crops can improve water infiltration into
soil and enhance storage of water and soil nutrients, even
when there has been no detectable increase in the amount of
soil organic matter. |
Research with soil high in residual N in the mid-Atlantic’s
coastal plain showed that cereal rye took up more than 70 lb. N/A
in fall when planted by October 1. Other grasses, including wheat,
oats, barley and ryegrass, were only able to take up about half
that amount in fall. Legumes were practically useless for this purpose
in the Chesapeake Bay study (46).
Legumes tend to establish slowly in fall and are mediocre N scavengers,
as they can fix much of their own N.
To maximize N uptake and prevent leaching, plant nonlegumes as
early as possible. In the above study, rye took up only 15 lb. N/A
when planting was delayed until November. It is important to give
cover crops the same respect as any other crop in the rotation and
plant them in a timely manner.
Not Just Nitrogen Cycling
Cover crops help bring other nutrients back into the upper soil
profile from deep soil layers. Calcium and potassium are two macronutrients
with a tendency to travel with water, though not generally on the
express route with N. These nutrients can be brought up from deeper
soil layers by any deep-rooted cover crop. The nutrients are then
released back into the active organic matter when the cover crop
dies and decomposes.
Although phosphorus (P) doesn’t generally leach, as it is
only slightly water-soluble, cover crops may play a role in increasing
its availability in the soil. Some covers, such as buckwheat and
lupins, are thought to secrete acids into the soil that put P into
a more soluble, plant-usable form.
Some cover crops enhance P availability in another manner. The
roots of many common cover crops, particularly legumes, house beneficial
fungi known as mycorrhizae. The mycorrhizal fungi
have evolved efficient means of absorbing P from the soil, which
they pass on to their plant host. The filaments (hyphae) of these
fungi effectively extend the root system and help the plants tap
more soil P.
Keeping phosphorus in an organic form is the most
efficient way to keep it cycling in the soil. So the return of any
plant or animal residue to the soil helps maintain P availability.
Cover crops also help retain P in your fields by reducing erosion.
Adding Nitrogen
One of nature’s most gracious gifts to plants and soil is
the way that legumes, with the help of rhizobial bacteria, can add
N to enrich your soil. If you are not familiar with how this remarkable
process works, see Nodulation:
Match Inoculant to Maximize N.
The nitrogen provided by N-fixation is used efficiently in natural
ecosystems, thanks to the soil’s complex web of interacting
physical, chemical and biological processes. In an agricultural
system, however, soil and crop management factors often interfere
with nature’s ultra -efficient use of organic or inorganic
N. Learning a bit about the factors affecting N-use efficiency from
legume plants will help build the most sustainable cropping system
possible within your constraints.
How Much N is Fixed?
A number of factors determine how much of the N in your legume
came from “free” N, fixed from N2 gas:
Is
the symbiosis (the interdependence of the rhizobia
and the plant roots) effective? See Nodulation:
Match Inoculant to Maximize N. Use the correct rhizobial inoculant
for the legume you’re growing. Make sure it’s fresh,
was stored properly, and that you apply it with an effective sticking
agent. Otherwise, there will be few nodules and N-fixation will
be low.
Is
the soil fertile? N-fixation requires molybdenum, iron, potassium,
sulfur and zinc to function properly. Soils depleted of these
micronutrients will not support efficient fixation. Tissue testing
your cash crops can help you decide if you need to adjust micronutrient
levels.
Is
the soil getting enough air? N-fixation requires that N-rich air
get to the legume roots. Waterlogging or compaction hampers the
movement of air into the soil. Deep-rooted cover crops can help
alleviate subsoil compaction (451).
Is
the pH adequate? Rhizobia generally will not live long in soils
below pH 5.
Does
the legume/rhizobial pair have high fixation potential? Not all
legumes were created equal—some are genetically challenged
when it comes to fixation. Beans (Phaseolus spp.) are
notoriously incapable of a good symbiotic relationship and are
rarely able to fix much more than 40 lb. N/A in a whole season.
Cowpeas (Vigna unguiculata) and vetches (Vicia
spp.), on the other hand, are generally capable of high fixation
rates. Check Chart 2 Performances
and Roles and the sections on individual cover crops for information
about their N-fixation potential.
Even under the best of conditions, legumes rarely fix more than
80 percent of the nitrogen they need to grow, and may only fix as
much as 40 or 50 percent. The legume removes the rest of what it
needs from the soil like any other plant. Legumes have to feed the
bacteria to get them to work, so if there is ample nitrate already
available in the soil, a legume will remove much of that first before
expending the energy to get N-fixation going. In soils with high
N fertility, legumes may fix little or no nitrogen. See How
Much N?.
While it is tempting to think of legume nodules as little fertilizer
factories pumping N into the surrounding soil, that isn’t
what happens. The fixed N is almost immediately shunted up into
the stems and leaves of the growing legume to form proteins, chlorophyll
and other N-containing compounds. The fixed nitrogen will not become
available to the next crop until the legume decomposes. Consequently,
if the aboveground part of the legume is removed for hay, the majority
of the fixed nitrogen also leaves the field.
What about the legume roots? Under conditions favoring optimal
N fixation, a good rule of thumb is to think of the nitrogen left
in the plant roots (15 to 30 percent of plant N) as being roughly
equivalent to the amount the legume removed directly from the soil,
and the amount in the stems and leaves as being equivalent to what
was fixed.
Annual legumes that are allowed to flower and mature will transport
a large portion of their biomass nitrogen into the seeds or beans.
Also, once the legume has stopped actively growing, it will shut
down the N-fixing symbiosis. In annual legumes this occurs at the
time of flowering; no additional N gain will occur after that point.
Unless you want a legume to reseed itself, it’s generally
a good idea to kill a legume cover crop in the early- to mid-blossom
stage. You’ll have obtained maximum legume N and need not
delay planting of the following cash crop any further, aside from
any period you may want for residue decomposition as part of your
seedbed preparation.
How Nitrogen is Released
How much N will soil really acquire from a legume cover crop? Let’s
take it from the point of a freshly killed, annual legume, cut down
in its prime at mid-bloom. The management and climatic events following
the death of that legume will greatly affect the amount and timing
of N release from the legume to the soil.
Most soil bacteria will feast on and rapidly decompose green manures
such as annual legumes, which contain many simple sugars and proteins
as energy sources. Soil bacteria love to party and when there is
lots to eat, they do something that no party guest you’ve
ever invited can do—they reproduce themselves, rapidly and
repeatedly, doubling their population in as little as seven days
under field conditions (306).
Even a relatively inactive soil can come to life quickly with addition
of a delectable green manure.
The result can be a very rapid and large release of nitrate into
the soil within a week of the green manure’s demise. This
N release is more rapid when covers are plowed down than when left
on the surface. As much as 140 lb. N/A has been measured 7 to 10
days after plowdown of hairy vetch (363).
Green manures that are less protein- rich (N-rich) will take longer
to release N. Those that are old and fibrous or woody are generally
left for hard-working but somewhat sluggish fungi to convert slowly
to humus over the years, gradually releasing small amounts of nutrients.
Other factors contribute significantly to how quickly a green manure
releases its N. Weather has a huge influence. The soil organisms
responsible for decomposition work best at warm temperatures and
are less energetic during cool spring months.
Soil moisture also has a dramatic effect. Research shows that soil
microbial activity peaks when 60 percent of the soil pores are filled
with water, and declines significantly when moisture levels are
higher or lower (244). This
60 percent water-filled pore space roughly corresponds to field
capacity, or the amount of water left in the soil when
it is allowed to drain for 24 hours after a good soaking rain.
Microbes are sensitive to soil chemistry as well. Most soil bacteria
need a pH of between 6 and 8 to perform at peak; fungi (the slow
decomposers) are still active at very low pH. Soil microorganisms
also need most of the same nutrients that plants require, so low-fertility
soils support smaller populations of primary decomposers, compared
with high-fertility soils. Don’t expect N-release rates or
fertilizer replacement values for a given cover crop to be identical
in fields of different fertility.
Many of these environmental factors are out of your direct control
in the near term. Management factors such as fertilization, liming
and tillage, however, also influence production and availability
of legume N.
Tillage, No-Tillage and N-Cycling
Tillage affects decomposition of plant residues in a number of
ways. First, any tillage increases soil contact with residues and
increases the microbes’ access to them. The plow layer is
a hospitable environment for microbes, as they’re sheltered
from extremes of temperature and moisture. Second, tillage breaks
the residue into smaller pieces, providing more edges for microbes
to munch. Third, tillage will temporarily decrease the density of
the soil, generally allowing it to drain and therefore warm up more
quickly. All told, residues incorporated into the soil tend to decompose
and release nutrients much faster than those left on the surface,
as in a no-till system. That ’s not necessarily good news,
however.
A real challenge of farming efficiently is to keep as much of the
N as possible in a stable, storable form until it’s needed
by the crop. The best storage form of N is the organic form: the
undecomposed residue, the humus or the microorganisms themselves.
| How Much N?
To find out if you might need more N than your
green manure will supply, you need to estimate the amount
of N in your cover crop. To do this, assess the total yield
of the green manure and the percentage of N in the plants
just before they die.
To estimate yield, take cuttings from several
areas in the field, dry and weigh them. Use a yardstick or
metal frame of known dimensions (1 ft. x 2 ft., which equals
2 square feet works well) and clip the plants at ground level
within the known area. Dry them out in the sun for a few consecutive
days, or use an oven at about 140° F for 24 to 48 hours
until they are “crunchy dry.” Use the following
equation to determine per-acre yield of dry matter:
Yield (lb.)/Acre = Total weight of dried samples (lb.)/ #
square feet you sampled X 43,560 sq. ft./1 Acre
While actually sampling is more accurate, you
can estimate your yield from the height of your green manure
crop and its percent groundcover. Use these estimators:
At 100 percent groundcover and 6-inch height*,
most nonwoody legumes will contain roughly 2,000 lb./A of
dry matter. For each additional inch, add 150 lb. So, a legume
that is 18 inches tall and 100 percent groundcover will weigh
roughly:
Inches >6: 18 in.– 6 in. = 12 in.
x 150 lb./in.: 12 in. x 150 lb./in. = 1,800 lb.
Add 2,000 lb.: 2,000 lb. + 1,800 lb. = 3,800 lb.
If the stand has less than 100 percent groundcover,
multiply by (the percent ground cover / 100). In this example,
for 60 percent groundcover, you would obtain:
3,800 x (60/100) = 2,280 lb.
Keep in mind that these are rough estimates
to give you a quick guide for the productivity of your green
manure. To know the exact percent N in your plant tissue,
you would have to send it to a lab for analysis. Even with
a delay for processing, the results could be helpful for the
crop if you use split applications of N. Testing is always
a good idea, as it can help you refine your N estimates for
subsequent growing seasons.
The following rules of thumb may help here:
Annual legumes typically have between 3.5 and 4 percent
N in their aboveground parts prior to flowering (for young
material, use the higher end of the range), and 3 to 3.5
percent at flowering. After flowering, N in the leaves decreases
quickly as it accumulates in the growing seeds.
For cereal rye, the height relationship is a bit different.
Cereal rye weighs approximately 2,000 lb./A of dry matter
at an 8-inch height and 100 percent groundcover. For each
additional inch, add 150 lb., as before, and multiply by
(percent groundcover/100). For most small grains and other
annual grasses, start with 2,000 lb./A at 6 inches and 100
percent ground cover. Add 300 lb. for each additional inch
and multiply by (percent groundcover/100).
For perennial legumes that have a significant number of
thick, fibrous or woody stems, reduce these estimates by
1 percent.
Most cover crop grasses contain 2 to 3 percent N before
flowering and 1.5 to 2.5 percent after flowering.
Other covers, such as brassicas and buckwheat, will generally
be similar to, or slightly below, grasses in their N content.
To put it all together:
Total N in green manure (lb./A) = yield (lb./A)
x % N/100
To estimate what will be available to
your crop this year, divide this quantity of N by:
2, if the green manure will be conventionally tilled;
4, if it will be left on the surface in a no-till system
in Northern climates;
2, if it will be left on the surface in a no-till system
in Southern climates.
Bear in mind that in cold climates, N will mineralize
more slowly than in warm climates, as discussed above. So
these are gross estimates and a bit on the conservative
side.
Of course, cover crops will not be the only
N sources for your crops. Your soil will release between 10
and 40 lb. N/A for each 1 percent organic matter. Cold, wet
clays will be at the low end of the scale and warm, well-drained
soils will be at the high end. You also may receive benefits
from last year’s manure, green manure or compost application.
Other tools could help you refine your nitrogen
needs. On-farm test strips of cover crops receiving different
N rates would be an example. Refer to Appendix A, Testing
Cover Crops on Your Farm for some tips on designing an
on-farm trial. In some regions, a pre-sidedress N test in
spring could help you estimate if supplemental N will be cost-effective.
Bear in mind that pre-sidedress testing does not work well
when fresh plant residues have been turned in—too much
microbial interference relating to N tie-up may give misleading
results.
For more information on determining your N from
green manures and other amendments, see the Northeast
Cover Crop Handbook (361).
—Marianne Sarrantonio, Ph.D. |
Let’s consider the N contained in the microbes. Nitrogen
is a nutrient the microbes need for building proteins and other
compounds. Carbon-containing compounds such as sugars are mainly
energy sources, which the microorganisms use as fuel to live. The
process of burning this fuel sends most of the carbon back into
the atmosphere as carbon dioxide, or CO2.
Suppose a lot of new food is suddenly put into the soil system,
as when a green manure is plowed down. Bacteria will expand their
populations quickly to tap the carbon-based energy that’s
available. All the new bacteria, though, will need some N, as well
as other nutrients, for body building before they can even begin
to eat. So any newly released or existing mineral N in soil gets
scavenged by new bacteria.
Materials with a high carbon to nitrogen (C:N) ratio, such as mature
grass cover crops, straw or any fibrous, woody residue, have a low
N content. They can “tie up” soil N, keeping it immobilized
(and unavailable) to crops until the carbon “fuel supply”
starts depleting. Tie-up may last for several weeks in the early
part of the growing season, and crop plants may show the yellowing
characteristic of N deficiencies. That is why it often makes sense
to wait one to three weeks after killing a low-N cover before planting
the next crop, or to supplement with a more readily available N
source when a delay is not practical.
Annual legumes have low C:N ratios, such as 10:1 or 15:1. When
pure stands of annual legumes are plowed down, the N tie-up may
be so brief you will never know it occurred.
Mixed materials, such as legume-grass mixtures, may cause a short
tie-up, depending on the C:N ratio of the mixture. Some N storage
in the microbial population may be advantageous in keeping excess
N tied up when no crop roots are there to absorb it.
Fall-planted mixtures are more effective in mopping up excess soil
N than pure legumes and, as stated earlier, the N is mineralized
more rapidly from mixtures than from pure grass. A fall-seeded mixture
will adjust to residual soil N levels. When the N levels are high,
the grass will dominate and when N levels are low, the legume will
dominate the mixtures. This can be an effective management tool
to reduce leaching while making the N more available to the next
crop.
Potential Losses
a common misunderstanding about using green manure crops is that
the N is used more efficiently because it’s from a plant source.
This is not necessarily true. Nitrogen can be lost from a green
manure system almost as easily as from chemical fertilizers, and
in comparable amounts. The reason is that the legume organic N may
be converted to ammonium (NH4), then to ammonia (NH3) or nitrate
(NO3) before plants can take it up. Under no-till systems where
killed cover crops remain on the surface, some ammonia (NH3) gas
can be lost right back into the atmosphere.
Nitrate is the form of N that most plants prefer. Unfortunately,
it is also the most water-soluble form of N. Whenever there is more
nitrate than plant roots can absorb, the excess may leach with heavy
rain or irrigation water.
As noted earlier, nitrates in excess of 140 lb./A may be released
into warm, moist soil within as little as seven to 10 days after
plowing down a high-N legume, such as a hairy vetch stand. Since
the following crop is unlikely to have much of a root system at
that point, the N has a ticket for Leachville. Consider also that
the green manure may have been plowed down to as deep as 12 inches—much
deeper than anyone would consider applying chemical fertilizer.
Moreover, green manures sometimes continue to decompose after the
cash crop no longer needs N. This N also is prone to leaching.
To summarize, conventional plowing and aggressive disking can cause
a rapid decomposition of green manures, which could provide too
much N too soon in the cropping season. No-till systems will have
a reduced and more gradual release of N, but some of that N may
be vulnerable to gaseous loss, either by ammonia volatilization
or by denitrification, which occurs when NO3-converts to gases under
low O2 (flooded) conditions. Thus, depending on management, soil
and weather situations, N from legume cover crops may not be more
efficiently used than N from fertilizer.
Some possible solutions to this cover crop nitrogen-cycling dilemma
:
a
shallow incorporation of the green manure, as with a disk, may
reduce the risk of gaseous loss.
It
may be feasible to no-till plant or transplant into the green
manure, then mow or incorporate it between the rows 10-14 days
later, when cash crop roots are more developed and able to take
up N. This has some risk, especially when soil moisture is limiting,
but can provide satisfactory results if seedling survival is assured.
Residue
from a grass/legume mix will have a higher C:N than the legume
alone, slowing the release of N so it’s not as vulnerable
to loss.
Consider also that some portion of the N in the green manure will
be conserved in the soil in an organic form for gradual release
in a number of subsequent growing seasons.
OTHER SOIL-IMPROVING BENEFITS
Cover crops can be very useful as living plows to penetrate and
break up compacted layers in the soil. Some of the covers discussed
in this book, such as sweetclover and forage radish, have roots
that reach as deep as three feet in the soil within one cropping
season. The action of numerous pointy little taproots with the hydraulic
force of a determined plant behind them can penetrate soil where
plowshares fear to go. Grasses, with their tremendously extensive
root systems, may relieve compacted surface soil layers. Sorghum-sudangrass
can be managed to powerfully fracture subsoil. See Summer
Covers Relieve Compaction.
One of the less appreciated soil benefits of cover crops is an
increase in the total numbers and diversity of soil organisms. As
discussed earlier, diversity is the key to a healthy, well-functioning
soil. Living covers help supply year-round food for organisms that
feed off root by-products or that need the habitat provided on a
residue-littered soil surface. Dead covers supply a more varied
and increased soil diet for many organisms.
Of course, unwanted pests may be lured to the field. Effective
crop rotations that include cover crops, however, tend to reduce
rather than increase pest concerns. Pest-management considerations
due to the presence of a cover crop are discussed in the next chapter,
Managing Pests
with Cover Crops.
Finally, cover crops may have an added advantage of drying out
and therefore warming soils during a cold, wet season. The flip
side of this is that they may dry the soil out too much and rob
the following crop of needed moisture.
There are no over-the-counter elixirs for renewing soil. A long-term
farm plan that includes cover crops, however, can help ensure your
soil’s health and productivity for as long as you farm.
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