Cover crops are fast growing crops such as winter rye and clovers that are planted between periods of regular crop cultivation. By covering the soil surface, they protect the soil from erosion, and if leguminous, they fix nitrogen. Later, when ploughed under, they provide humus and carbon to the soil, as well as nitrogen for the subsequent crop.
Cover cropping is an effective method of reducing emissions of CO2. These crops grow over entire land areas or in localized spots such as grassed waterways, field margins, and shelterbelts. Compared to leaving fields fallow, they reduce emissions and can sequester carbon during periods when primary crops are not grown. Cover crops are usually an option on surplus agricultural land or on cropland of marginal productivity.
Cover crops are grown all over the world. However, adoption is limited because of the many concerns of growers and the specificity of profitable cropping systems as discussed in the next section.
Lack of knowledge, incorrect choice of cover crop, and the economic costs of planting and terminating cover crops are all concerns of growers, and they have led to the slow adoption of this practice. If land is fallow for portions of the year, cover crops should be considered. However, they need to be selected on the basis of the growing season, protection capacity, nitrogen fixing capability, and economic feasibility. They vary from region to region, cropping system to cropping system, and crop season to crop season. Therefore, local research must be conducted in order to obtain the knowledge needed to use this practice reliably.
- 1. A primary advantage is that by increasing plant residues and roots, cover crops can sequester carbon during times when the soil surface would normally be bare and emitting carbon due to soil respiration.
- Cover crops can alleviate nutrient deficiencies and reduce artificial fertiliser use by nitrogen fixing, if leguminous. This will save fossil fuel used in fertiliser manufacture, although more nitrogen in the soil can increase N2O emissions.
- Cover crops reduce soil erosion as well as rainfall runoff by improving water infiltration and water adsorption in the soil matrix.
- Cover crops can also reduce use of pesticides and herbicides for the associated cash crop by suppressing weed growth and providing a substantial habitat for beneficial arthropods.
- There are costs associated with planting and terminating cover crops.
- If not terminated properly, cover crops may act like weeds and compete with the following cash crops for light, nutrients and water.
- The residues from cover crops can potentially interfere with post-emergence herbicides, which results in the escape of weeds.
- In some cases, the additional water requirement of the cover crops may make this practice economically and environmentally less viable.
Several examples have been presented showing that the growing of cover crops is profitable. In one experiment hairy vetch was grown during the off-season for a main crop of corn. The costs of fertiliser and of hairy vetch seed required for the no-till zero-tillage cover crop systems were $117.08 and $16.62 ha-1 yr-1, respectively, while the cost of fertiliser for conventional no tillage system was $174.97 ha-1 yr-1. The cover crop system produced average corn yield of 7.86 Mt ha-1 in a no-tillage conventional system. The average gross margin (profit) was $238.28 ha-1 yr-1 in cover crop system and $233.27 ha-1 yr-1 in conventional no tillage system.
Cover crops can also increase soil carbon sequestration. Lal (1998) lists carbon sequestration rates from 0.28 to 2.60 Mg ha-1 yr-1 from growing cover crops on an eroded Alfisol in western Nigeria.
There are costs associated with planting and terminating cover crops. The economic costs of planting and terminating cover crops are all concerns of growers, and they have led to the slow adoption of this practice.
Lal, R. (1998). Land use and soil management effects on soil organic matter dynamics on Alfisols in Western Nigeria. In Lal, R, Kimble JM, Follett RF, and Stewart BA. Soil Processes and the Carbon Cycle. CRC Press LLC. 109-126.