This 7-page document by Jeff Mitchell of the University of California discusses aspects of irrigation use and cover crops that can help improve productivity. Excerpts:

Using irrigation to increase crop production

Historically, irrigation has been an important agricultural technology with strong impact on crop productivity. On average, irrigated crop yields tend to be higher than those from unirrigated land (Lascano and Sojka, 2007), but there can also be unintended negative consequences of irrigation if it is not managed well including river and groundwater depletion and salinization.

Irrigation is useful in regions where the process of evaporation (E) and traspiration (T) (water loss from crop plants) exceed precipitation or rainfall.

Using cover crops to increase crop productivity

Cover crops are legumes, cereals, or an appropriate mixture grown specifically to protect the soil against erosion; ameliorate soil structure; enhance soil fertility; and suppress pests, invading weeds, insects, and pathogens. When cover crops are converted to a surface residue mulch, they can also serve to conserve soil water.

Because low soil fertility has been a major constraint hampering the productivity of Kenyan small holder farms, and because legumes can play a major role in improving farm productivity, the Legume Research Network Project (LRNP) has done a lot of work in Kenya over the past 15 years to introduce legume cover crops throughout the country.

When cover crops are converted to surface mulches, or when crop residues are maintained on the soil, they may serve to reduce soil water evaporation and thereby conserve water for crop transpiration. Thus, by generating and maintaining residues, a farmer, tries to “take the ‘E’ out of ‘ET’, to increase crop water use efficiency. Estimates of water savings due to reduced tillage and surface residues have been determined to be upwards of 10 cm during a crop season in the US.

When cover crops are converted to surface mulches, or when crop residues are maintained on the soil, they may serve to reduce soil water evaporation and thereby conserve water for crop transpiration. Thus, by generating and maintaining residues, a farmer, tries to “take the ‘E’ out of ‘ET’, to increase crop water use efficiency. Estimates of water savings due to reduced tillage and surface residues have been determined to be upwards of 10 cm during a crop season in the United States.

How much Nitrogen can cover crops provide? Vigorous cover crops can be produced by following existing recommendations for species and variety selection,planting date, seeding rate, inoculation procedures and termination date. The amount of nitrogen a cover crop produces depends entirely on the cultural practices that are used.