When you hear the words arid and semi-arid fifty inches of water a year of precipitation does not come to mind. Those regions typically receive that precipitation, which is hard to believe. While that sounds like a great deal of water, when you consider evaporation and transpiration ratios, the significance begins to diminish, and fifty inches quickly disappears. So what is a farmer to do? They cannot control the weather, but they can control what they plant. In those regions with a high evapotranspiration ratio the ideal answer is cover cropping. While numerous plants function as cover crops these regions require something a little more and that is why legumes are the cover crop of choice.
The benefit of cover cropping is substantial and when that cover crop happens to be a legume the result is even better. Yet, when planning to cover crop your farm it is important to know the average precipitation values, type of soil you are working with, and what affect the cover crop is having on your soil.
In the western United States 80% receives less than 500 mm (roughly 15 inches) of annual precipitation. In Africa 50% of the area has similar precipitation, Australia approximately 70%, Mexico 75% and South America differs between 65% and 75% on an average annual basis (Holechek 2004). The remaining percentages still fall well within the 50 inches of precipitation, with exceptions in areas near the equator. These percentages demonstrate just how prevalent of a problem the lack of precipitation is.
Additionally, climate models predict that average temperatures will increase. Consequently, the increase in temperature will affect the evapotranspiration ratio, primarily the evaporation, making the need for water efficient farming even more important (Ragab 2002).
Cover cropping is a relatively new addition to traditional agriculture but is quickly becoming standard practice. The two methods of cover cropping include the planting of a crop during what would normally be a fallow year, or intercropping, planting a cover crop right alongside the desired crop. Both methods help to provide an increase in organic matter, when the cover crop is a legume the quality of that organic matter increases, due to the additional nitrogen which results in a lower carbon: nitrogen (C:N) ratio. Because of the lower C:N ratio microbial decomposition is more rapid which may result in a decrease in overall organic matter mass (Gleissman 2007).
Dominant soil orders within arid regions are Aridisols or Entisols depending on their maturity as determined by the accumulation of organic matter. Aridisols are the more mature with a very shallow layer of organic matter while Entisols are still in the parent material stage (Brady 2002). Some arid regions can have Mollisols however; the presence of a mollisol suggests that there is sufficient precipitation to create a mollic epipedon that has a high accumulation of organic material. The fact that the dominant soil orders lack organic matter makes it important to have a management plan that provides a large amount of organic matter.
In the arid regions, the amount of precipitation is not conducive to dense vegetation and the unavailability of nutrients provided by organic matter makes it even more difficult to grow a crop. The roots quickly extract those nutrients that are available thus rapidly depleting them (Brady 2002). This makes the use of legumes as cover crop doubly beneficial as they not only provide organic matter, but are also nitrogen fixers, which is one of the most limiting nutrients in arid regions.
Cover crops in general increase primary production for subsequent crops due to the addition of nutrients, organic matter, and reduced water loss. With legumes, the increase in primary production generates additional organic carbon. The nitrogen-fixing microbes, found in conjunction with legumes, use the carbon as an energy source (Tonitto 2006).
A common practice for legumes is to grow them as a winter cover crop; this of course is climate dependent. The subsequent crops after winter cover cropping by a legume have, on average, an increased yield and require less fertilizer, herbicide and pesticide application (Sainju 2003).
Popular legume cover crops include hairy vetch (Vicia villosa), crimson clover (Trifolium incarnatum), bellbean (Vicia faba), red clover (Trifolium pretense), sweet clover (Melilotus officinalis), field pea (Pisum sativum), and alfalfa (Medicago sativa) (Tonitto 2006, Gleissman 2007, and Sainju 2003). The majority of these legumes provide not only substantial organic matter and nitrogen, but they also serve as a foliage crop or pasture.
One legume that is quickly growing popularity is the medic species (Medicago) in Australia. Australian farmers learned, quite by accident, the benefits of using medic as a cover crop during what would have normally been their fallow year. Medic, once considered a pest species, provides a high quality livestock forage and is self-regenerating. As a legume, it also “puts” nitrogen back into the soil for subsequent crops to use. Farmers now use the “ley” system crop rotation pattern of pasture (medic), crop, pasture (medic). This rotation improves soil quality and water use efficiency (Groose 1999).
The genus Medicago predominantly found in Mediterranean climates is uncommon in the arid and semi-arid regions. As a result, the United States does not consider it as one of the popular legumes for cover cropping. Currently researchers are trying to manipulate the genetic make-up of medic so that it can be implemented in our higher elevation climes.
The scarcity of precipitation, quality of soil, and general nutrient deficiency predominantly found in arid and semi-arid regions demands a detailed management program for farming. Implementation of legume cover crops into the management program is easily done and the rewards are well worth it.
Brady, Nyle C., and Ray R. Weil. The Nature and Properties of Soils: Thirteenth Edition. Upper Saddle River, New Jersey: Pearson Prentice Hall, 2002. pp 58, 88-91, 98-100, 103-104, 233-236.
Gleissman, Stephen R. Agroecology: The Ecology of Sustainable Food Systems. Boca Ragon, Fl: Taylor & Francis Group, 2007. pp 108- 109, 208-211,225.
Groose, Robin W. “Lamb chops & rolls on 12 inches of rain.” Casper Star Tribune 22 Aug. 1999.
Haan, R.L. et al. “Evaluation of Annual Medicago for Upper Midwest agroecosystems.” Journal of Agronomy & Crop Science. 188 (2002) 417-425
Holechek, Jerry L., et al. Range Management: Principles and Practices Fifth Edition. Upper Saddle River, New Jersey: Pearson Prentice Hall, 2004. pp 51-55.
Qi, A., et al. “Modelling the effects of Temperature on the rates of seedling emergence and leaf appearance in Legume cover crops.” Exploring Agriculture 35 (1999)327-344.
Ragab, R. and Christel Prudhomme. “Climate Change and Water Resources management in Arid and Semi-arid Regions: Prospective and challenges for the 21st Century.” Biosystems engineering 81.1 (2002) 3-34.
Sainju, Upendra M, Wayne F. Whitehead and Bharat P. Singh. “Agricultural Management Practices to Sustain Crop Yields and Improve Soil and Environmental Qualities.” The Scientif World Journal 3 (2003) 768-789.
Tonitto, C., et al. “Replacing bare fallows with cover crops in fertilizer-intensive cropping systems: A meta-analysis of crop yield and N dynamics.” Agriculture Ecosystems & Environment 112 (2006) 58-72.
Cover Crops: Forage Information System