Regarding soil conservation the term ‘cover crops’ is almost symonynous to the ‘green manure’ crops. The cover crops are primarily grown as a measure to control or reduce the soil erosion. These are grown either during off-season or when ground protection is required.
In United States these are grown at winter annuals, and after harvesting they are ploughed for green manuring. The largest use of cover crops is recommended in those areas, where mild winter leaves the land unprotected and subject to erosion from rains, followed by autumn and early spring rains.
To create protection such areas should be covered by those crops, which have longer growing seasons; allow a considerable plant growth and also enhance the magnitude of cover for green manuring as well as erosion protection. The typical crops used in USA are the rye, oats, lucern, Australian winter pea etc. The sowing of rye, using broadcasting requires about 120-130 kg/ha seed. The wind tunnel studies indicate that the winter rye sown at recommended density can prevent the blowing of soil in significant amount at wind speed of 21 m/s.
The cover crops are seldom used in U.S. Northern states following inter tilled grain crop maize with the crops of shorter growing period. The vegetable crops are also grown as cover crops in U.S. for protecting the soil against erosion, and also to add organic matter in the soil, following short season crops. The vegetable lands often have coarse textured soil, which are more susceptible to wind and water erosion. The growing of vegetables as cover crop on such soils, provides a good effect on control of soil erosion.
The ground cover is also developed by growing the tree crops to protect the soil from splashing effect. In this type of ground cover when rainfall takes place, the intercepted water in the form of large size water droplets falls over the ground surface through the plant leaves, which impacting force is much high, as result there is generated erosion problem.
Normally, the covers are not established with the tall crops such as rubber, because the height of fall of water droplets is very much to cause the falling of drops at terminal velocity. The most common crops used in tropical regions are Pueraria phaseoloides, Calopogonium mucunoides and Centrosema pubescens.
The function of cover crop is not only to create protection to the soil from erosion, but also to serve the following other purposes:
i. To add organic matter into the soil through the roots and ploughed down of crop biomass.
ii. To enrich the soil by nitrogen, by growing the legume crops to develop ground cover.
iii. To improve physical conditions of the soil.
iv. To check the water and wind erosions.
v. Also, to conserve the plant nutrients by taking up of soluble elements, which might be otherwise lost by leaching action.
Vegetations or plant covers cause the erosion protection in following ways:
i. It acts as cushion over the ground surface, by which the beating action of the raindrops gets dissipated.
ii. Offers resistance to the moving surface water, and thus slows down the flow velocity, causing the water gets additional time to go down into the lower soil profiles.
iii. The root system of the crop holds the soil particles very tightly at their original place, which is more pronounced in reduction of soil erosion.
iv. The roots and residues of the crop improve the soil structure, by virtue of which the soil becomes more porous and better capable to absorb the rain water.
The effectiveness of plants and their cover to protect the soil from erosion, depends not only on the density of plant population, but also on their growth stages. The greatest protection is resulted from dense and close-growing grasses or legumes or in combination of both. However, rows or inter-tilled crops also make protection from erosion but in less magnitude than the grasses or legume crops.
In general, regardless of crop kinds the greatest soil protection is achieved, when a crop has following characteristics:
ii. Fast growing; and
iii. High yielding
The high yielding grain crops return greater amount of residues to the soil for example – a maize crop adds about 3 tonnes or more per acre as dry matter in the form of stalks, and leaves into the soil. In spite, it also adds another tone of dry matter from its root system to the soil.
The roots are mixed in the soil and enrich the humus content of the soil, which result is observed on soil structure improvement. The well-managed soils have grannular structure, referred as good tilth. Such soils are characterized by porous in nature and to remain open for absorption of rain water.
Soil Cover Rating:
In order to evaluate the soil covers for their suitability to reduce the soil detachment, a term referred as ‘soil cover rating’ is normally used, which denotes the effectiveness of cover in preventing the detachment of soil particles due to rain drop impact during storm occurrence. The following formula can be used to evaluate the soil cover rating –
SCR = soil cover rating (%)
Wb = weight of soil splashed from the bare land
wc = weight of soil splashed under cover.
In accordance with this formula, the worst condition for erosion is concerned, would be the bare soil, which would have the value of ‘soil cover rating’ as zero. By evaluating the SCR of different covers, the effectiveness of them to check the soil detachment can be determined. The ‘soil cover rating’ of different cover crops at different growth stages can also be evaluated, which is a valuable information for choosing the cover crops for better control of erosion problem of a specific area.
Effects of Plant Cover:
Plant cover is very effective to prevent the soil erosion by dissipating the kinetic energy of falling raindrops, and also creating obstructions in flow path of overland flow. For significant result the planning of crop growing should be in such a way that the cover could get develop fully during the periods when rainfall is at frequent intervals.
The plant cover slows down the overland flow or sheet flow, and also keeps the soil surface porous. The plant cover is one of the most important factors for affecting the soil erosion. It has been reported that if the plant cover of a field is reduced from 100% to 0% then the soil erosion gets increase from 1 to more than 1000 tones.
During major rainstorms the angle of incidence of raindrops is generally less than 25°, except certain cases when it is as much as 45°. This phenomenon also depends on the architecture of the plant structure, i.e. the foliage height above soil surface; and whether the plants are funnel-like as in the case of pineapple and maize in which the water gets concentrate; or umbrella-like as in cassava in which the water gets scatter.
Very little works have been done on the dynamics of cover development under different crops; and also there is no reliable technique for measuring different types of plant covers. However, the common parameters being used for measuring the plant cover of different vegetations on erosion plots have been described by Roose (1973), outlined in Table 11.10.
The grasses normally protect the soil better than the pulses or cassava. The soil protection capacity of crop can be improved to a large extent by early planting, keeping in view the probability of heaviest rainstorm time. Few plants are called soil-degrading plants, because they cover the soil at very slow rate. The pineapple and cassava are the soil degrading plants because their rate of development of soil cover varies from 10 to 20% per month. On the other hand, the plants which rate of soil cover is very high are called protective plant.
The grasses are the protective crops. Few plants such as groundnut, maize and other cereals cover the soil very poorly in the first two months; hardly reaching 80% at the end of third month.
Also, their cycle is fairly short (4 months), as result for the remainder months of the year, the soil is left under bare condition; and is exposed to the erosion unless there has been germinated weeds to cover the soil and absorb the raindrop energy. Since, the percentage area covered by the degrading plants is very less; therefore, there is more soil erosion.
In case of soil degrading crops, this problem can be checked by mulching the areas of poor canopy cover. Mulching effect on erosion control is very significant, even without covering the whole ground.
Experimentally it has reported that as little as 20% area covered under mulch reduces the soil erosion by about 40%; on 40% mulch coverage the erosion is reduced up to 60%, and on 80% area coverage the reduction in erosion goes up to 90% as compared to the soil erosion from the field under bare condition.
Fig. 11.3 shows the combine effect of mulch and canopy cover on soil erosion. In absence of crop canopy there is very high rate of soil erosion. As per figure, with only mulch covering of 20% the erosion is about 60%, but when there is 100% canopy cover along with mulch cover, then erosion is not more than 30%.
Types of Plant Cover and their Effects:
In broad sense the following three types of plant covers are very common:
1. Full cover year-round
2. Incomplete cover for the part of the year; and
3. Fallows providing little cover during most aggressive months.
These are described as under:
1. Year-Round Full Cover:
This category of plant cover is related to the close forests, but also to the secondary shrub forest, un-burnt tree savannah, natural fallow, grasslands with grasses more than one year old, and shrubs with cover plants or mulching are also included in this type of plant cover. From such plant covers the erosion susceptibility of soil gets significantly reduced (about to negligible). The rate of soil erosion and runoff is very less under forest covers.
In the forest lands the tree canopy foliage, the bushes and litters of dead leaves provide year-round cover to the soil, and protect the soil from erosion caused by the energy of falling raindrops. Also, in such lands the mesofauna (termites and earthworms) keep the soil porous, as result the infiltration rate remains very high throughout the rainy season. This also causes reduction in amount and rate of sheet flow; and soil erosion, thereby.
Since, in the forest areas the overland flow travels between the soil surface and the litter, and is constantly slowed down by uneven soil topography and surface roughness, and also trapped by the holes formed by the rotted roots and mesofauna. In this condition the amount and concentration of rainwater gets significantly reduced, which is very conducive to check the soil erosion. Table 11.11 illustrates the effect of plant cover on runoff and soil erosion.
2. Incomplete Cover for the Part of Year:
Few food crops or forage crops are planted late and have incomplete soil cover for the part of year. In this type of plant cover the degree of soil erosion control depends very much on the planting date, crop spacing, soil slope and chopping techniques. In general, the food crops provide least soil protection as compared to the creeping crops/grasses.
For example – the erosion under cassava cover amounts to 22 to 93 t/ha/y on 7% land slope, whereas under maize and groundnut cover it is between 35 and 131 t/ha/y. Since, in any case 80% cover develops after 2 to 5 months, i.e. after heaviest rains; and the erosion gets increase, accordingly.
3. Fallows Providing Little Cover during Most Aggressive Months:
At the steeper slope and more aggressive climate the soil erosion is more. In principle, the soil should never be left uncovered during rainy season, must be under suitable crop cover. The soils without crop cover behave like bare soil, on which the rate of soil erosion is very high, because of exposure to the direct hitting of rain drops and accelerated overland flow. On an average, the rate of soil erosion from a late-sown plot is about 80% of the bare plot.
The erosion is not only dependent on the plant cover, but also on the plant height above the ground. For example – if plant cover is 100% but height is 4m, then soil erosion is about 75% of the bare plot; at 2m canopy height it is about 50%; and at 0.50 m height the erosion is about 18% of bare field soil erosion. However, if there is used mulch, then erosion is up to 3% only.
The canopy height basically affects the velocity of falling water drops (intercepted water). From greater canopy height there is possibility of falling of water droplets at terminal velocity, which results into greater detachment of soil particles from the soil. Fig. 11.4 illustrates the view of canopy height effects on soil erosion.
The plant architecture affects the soil erosion. Especially, the trees in which the leaves are in the shape of channel (funnel), they direct the rainwater towards trunk, which gets collect at the base of trunk. The collected water ultimately starts flow towards lower side. This flow of water develops a kind of rills. And if this action is not checked properly then there is formation of big size rills.
This occurrence is very common in pineapple plantations, and to a lesser extent in the maize, also. The other type of plant architecture is the umbrella-like appearance. In this type of plant architecture the water drops move outwards; and thus scattering their energy. The banana and cassava are the examples of this type of plant canopy architecture.
Under plant architecture the influence of root system is also taken into consideration regarding effect on soil erosion. The roots hold the soil; and affect the soil detachment. The taproots grow in volume; occupy the soil’s macro pores; and thus reduce the infiltration rate of the soil.