Role of cover crops and their effects
Soil erosion control -Establishing cover crops with high biomass production can effectively
protect soil against erosion, especially in the part of the season when the soil is bare. Plant
cover crops splice soil particles with their roots, while the above-ground biomass prevents
fine particles of soil surface from being flushed away by water or carried off by wind. Dense
perennial crops improve the retention and absorption of water by the soil, and reduce runoff, whereas the above-ground parts of the plants reduce the impact of the rain drops. After
the destruction of the cover crops, the dry above-ground biomass forms mulch on the soil
surface, which also provides the protection against soil erosion. Grasses and grass mixtures
provide the best protection against erosion, and winter grains or winter brassicas are suitable for covering the soil during the winter.
Increasing the organic matter content - Cover crops leave plant residues thus causing the
increase of organic matter in the soil. The increase depends on the amount of the biomass
formed in a cover crop and the ratio of carbon between nitrogen in plant residues(C:N ratio,
Table 5).
This ratio depends on plant species and the time of the year when the cover crops
are destroyed. Plant residues of cover crop with a low ratio of C:N ‒ legumes (Fabaceae),
are decomposed faster than those with a high ratio ‒ grasses (Poaceae). Thanks to their
powerful roots, plants from the grass family accumulate nutrients, especially nitrogen, that
remains in the soil after the harvest. However, these nutrients become available to plants
rather slowly, so that grass cover crops have more effect on the increase of organic matter
in the soil. On the other hand, plant residues of legumes are rapidly degraded in the soil due
to their lower C:N ratio, therefore less increasing the content of organic matter in the soil
than grass cover crops. For the degradation of plant residues in which the C:N ratio is above
30, the microorganisms consume additional nitrogen (nitrogen immobilization), creating a
temporary shortage of this element for the next crop. The pairing of different plant species
from different families (grasses and legumes) in cover crops can reduce the C:N ratio, and
thus accelerate decomposition of the total biomass produced. Knowing the individual characteristics of the species in cover crops, the processes in the soil can be directed towards the
set goal, whether we need organic matter or available nitrogen for the next crop.
Table 5. Carbon and nitrogen ration (C:N) in plant residues
Organic matter C:N ratio
Young rye plants 14:1
Mature rye plant 40.1
Maize stems 60:1
Hairy vetch 10:1 to 15:1
Crimson clover 15:1
Sawdust 200:1
Enhancement of soil structure –Plant roots in cover crops bind soil particles, during their
decomposition in the soil, organic molecules are released, and, also, fungal networks are
formed. The best effects to enhance soil structure are achieved by the establishment of cover
crops that form a strong hairy root system, such as grass species. The roots of these species
bind and can also penetrate into the soil. Species with spindly, parsnip-like root (brassicas)
penetrate into deeper layers of the soil, thus making it possible for the following crop roots
to easily penetrate into those deeper layers, where they make use of water and nutrients.
Plant species with strong root systems can also be used in mitigating soil compaction. It was
noted that crops such as corn, sorghum, Sudan grass, common milletrelieve the effects of
compaction. Cover crops that grow in winter are suitable for breaking compact layers in the
soil, because they can penetrate through those layers when they are softened with plenty of
water during this period (which is less likely in summer).
In the course of decomposition, cover crops with a low C:N ratio will release large amounts
of organic molecules, such as polysaccharides, which enhance soil structure. That is the reason why the soil is soft in the spring after a crop of legumes. This effect is expected to last
only as long as there are biodegradable residues. Cover crops with a high C:N ratio will have
a slower release of polysaccharides, which brings about slower soil structure enhancement
than in cover crops with a low C:N ratio.
In a symbiotic relationship with the majority of grown plant species (excluding brassicas) live
mycorrhizal fungi. The plants provide these fungi with energy, and the fungi extend the zone
of the root system, can help the absorption of water and nutrients such as phosphorus. Mycorrhizas settle the cells of root tops, so, as the root is growing, they are dying, on which occasion, glomalin is being released. It is this rejected glycoprotein that contributes to a better
soil structure. The practices that stimulate the creation of mycorrhizas, and hence,
the creation of glomalin itself, include: the implementation of conservation cultivation, continuous
occupation of soil with the living root systems where mycorrhizas can settle, as well as the
avoidance of excessive phosphorus fertilization. Therefore, the establishment of cover crops
at the time of the year when soil is without the main crop positively affects the formation of
mycorrhiza populations.
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Fixation of atmospheric nitrogen -It is well known that a large number of plant species live
in different symbiotic relationships with microorganisms. This group of plants belongs to all
annual and perennial legumes, as well as many other species (mostly grains), with nitrogen
fixing microorganisms (bacteria or fungi) developed on the roots. Soil nitrogen fixing bacteria
(Rhizobiaceae) are located on the roots of legumes (Fabaceae), which bind atmospheric nitrogen into compounds available to plants, hence reducing the need for additional nutrition
of crops with mineral nitrogen nutrients. The intensity of nitrogen fixation depends on the
agroecological and soil conditions. In the soils with low nitrogen content, nitrogen fixation is
significantly higher. To ensure the fixation of nitrogen, plant seeds are treated with biological
products contain in gmicroorganism cultures in order to start symbiotic processes quickly.
These symbiotic communities are particularly useful in a cover crop. Red clover, white clover,
peas and sainfoin are especially suitable as cover crops in spring and summer, with the accent on importance of several annual lupinus species. In the more arid regions (the majority
of Balkan peninsula) white lupinus as the drought tolerant, is the most appropriate. For winter sowing are appropriate Crimson clover, hairy vetch and winter peas. The hairy vetch from
early March onwards, secured 2 kg of nitrogen per day, and was proved to increase the yield
of corn to such an extent that it justified the use of cover crops. Winter peas, vetch and alfalfa
in a cover crop can provide up to 100% of nitrogen for the next crop of potato. In intercrops,
legumes transfer significant amounts of nitrogen to the main crop and these values range
from 30 to 50% of the total fixation. Also, the adoption of cover crops can reduce losses of
nitrogen in the soil and thus preserve it for the next crop. As legume plant residues decompose quickly in the soil and release large amounts of nitrogen, in wet springs, it can cause its
leaching into the deeper layers of the ground. This should be considered in the management
of such cover crops. These occurrences can be prevented by establishing cover crops with
mixed legumes and grasses, which allows a slower decomposition of plant remains.
Soil moisture management – Cover crops consume water, and after their decomposition
prevent evapotranspiration of soil moisture. Also, biomass of the plants and the shredded
plant residues prevent surface water run-off and improve its absorption into the soil. These
positive effects of the utilization of cover crops are expressed fully only if crop residues are
left on the soil surface as mulch, while the effect is minor when the crop residues are incorporated in the soil. Mulch is particularly needed in the main crops if the previous crop has
not left any residues (corn silage). Cover crop stubble may retain the snow, especially if it is
left in different altitudinal zones.
To avoid most of the problems with the lack of water in spring, cover crops should be destroyed at least two weeks before planting of the main crop, which will prevent water loss.
On the other hand on flooded soils, mulch formed from cover crops residues prevent soil
from drying and makes production of the main crop difficult. In such situations, the destruction of the cover crop should be delayed, as living cover crops consume water, and may result
in the decrease of moisture excess in the soil. Good examples of cover crops are: grass cover
crops, rye, which increases organic matter and conserve moisture, sorghum and Sudan grass,
whose roots penetrate deeply into the ground and favorably affect the physical properties of
the soil, as well as ryegrass, which, in the case of over moist soil, stabilizes inter row space in
crops, perennial plantations, on the headlands of agricultural lots and non-agricultural areas.
Fodder radish forms a large root, and thus allows the easy infiltration of water into the soil.
Xerophyte leguminous plants, yellow lentils and alfalfa utilize water efficiently, and, in cover
crops, they conserve soil moisture better than on bare uncultivated soil.
Control of weeds and harmful organisms -Cover crops suppress weeds, reduce damages
from diseases, insects and nemathodes, with different mechanisms. Cover crops suppress
weeds physically or due to their allelopathic effect. The physical activity is about competitive
relations which can manifest as plant shading, resulting in a change in light frequency crops
which prevents weed seeds from germinating, or as a mechanical suppression of weed by
high-coverage crops (crops of small grains, buckwheat and brassicas). Allelopathic effect is
linked with root secretions, which may have a natural herbicidal effect on the weeds. These
natural processes can have a negative effect on the main crop (rye, corn), and therefore be
taken into consideration.
The introduction of bioactive plants - “plant friends” - can be successfully performed in the
biocontrol of cropping systems. They can repel harmful insects (pyrethrum, yarrow), or attract beneficial predators and parasitoids(fennel, cumin, dill, mint, buckwheat), and, in this
way, the number of insect pests can be reduced below a level of harmfulness. Cover crops
can create a negative environment for diseases, especially those based on mustard or phacelia which are capable of producing compounds that reduce pest populations of nematodes.
Selection and management of cover crops
The reasons and goals for cover crop establishment must be determined prior to the establishment. Manifestation of the use of cover crops and the strength of their effects depends
on the site and the season, but a properly set cover crop will almost always achieve at least
two or three positive effects. To start with the advantages which certain cover crops allow,
the goals should be reduced to one or two primary and several secondary goals. Goals could
be: to provide nitrogen, to manage organic matter, to enhance soil structure, to reduce soil
erosion, to conserve soil moisture, to provide weed control, to establish biocontrol of harmful organisms and others.
Apart from their benefits, cover crops bear some potential disadvantages (increased seed
costs, the increase in the amount of labor, the competition with main crops for soil moisture
and nutrients, potential attraction to certain harmful organisms, etc.),
which further limits
their applicability. When choosing and setting up a cover crop management plan, one should
start from setting crop rotations for a longer term, and, for each lot, possible rotational crops
should be pointed out, as well as the terms of the sowing and the harvesting of the main
crop. It is also necessary to take into consideration some issues such as: local agroecological
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conditions (the amount of rainfall, the occurrence of early autumn and late spring frosts
etc.), main crop needs, system of production (organic or integral, irrigation or no irrigation,
conservation or classic processing) etc. In the scope of the abovementioned facts, it is necessary to find time and space for production of cover crops, to determine plant species, as well
as the technological model that will meet the objectives.
