Cropping Systems for Intensive Desert Vegetable Production

View the project final report

Commodities

• Agronomic: peas (field, cowpeas), sorghum sudangrass
• Fruits: melons
• Vegetables: greens (lettuces)

Practices

• Crop Production: conservation tillage
• Education and Training: demonstration, extension
• Farm Business Management: new enterprise development, budgets/cost and returns, whole farm planning
• Pest Management: cultural control, integrated pest management, mulches - killed, physical control, mulching - vegetative, weed ecology
• Production Systems: agroecosystems, holistic management
• Soil Management: green manures, organic matter, nutrient mineralization, soil quality/health
• Sustainable Communities: sustainability measures

There has been an exponential increase in acres planted to organic vegetables in Arizona and California. The low desert area has many advantages that producers can use to exploit unique market niches. But the soils are exceptionally low in organic matter. We quantified what is often called "the organic effect", i.e. the positive changes that result from the transition to organic production practices. The experimental fields in Thermal, California and Yuma, Arizona are in the major vegetable production area of the lower Colorado River desert. Two cover crops (cowpea and sudangrass) were compared to traditional summer fallow. Sudangrass was incorporated into the soil and cowpea was either incorporated or used as mulch (reduced tillage system). Cowpea mulch increased daily minimum soil temperature and decreased maximum soil temperatures, which may allow growers to extend their season into more profitable markets. In Year 1, yields for both melons and lettuce were lower when lettuce or melons were managed organically. In Yuma, AZ, lettuce and melon yields were always less for the organic treatments compared to the conventional treatments. For lettuce both insects and soil fertility limited yields in the organic plots. For melons the limiting factor in the organic plots was always insects.
However, organic yields equaled those of the conventional management system in Year 2 at Thermal, CA. Insect populations in Thermal, CA were generally lower across all years of study than in Yuma, AZ. Weed populations in Year 2 were reduced under organic management. Cowpea cover crops also decreased weed species diversity, with fewer weed species present following cowpea cover crops. No increase in insect pest populations were observed in the cover crop plots, indicating that they do not serve as an alternate host for pests. Cowpea cover crop significantly increased yield of fall planted lettuce and winter/spring planted cantaloupe. Cowpea cover crops require little water and no fertilizer, produce abundant biomass and nitrogen, and can reduce weeds and nematodes. Incorporating summer cowpea residues into the soil significantly increased yield for both conventional and organic management system. Growers have followed up on our results by increasing the acreage planted to cowpea cover crops from nearly zero in 1995 to several thousand acres and increasing.

Sudangrass increased the yield of cantaloupe, but not lettuce. Reduced lettuce yield following sudangrass was possibly due to nutrient immobilization or allelopathy from the cover crop residues. Our results strongly support the use of cowpea cover crops prior to fall planted lettuce in the desert. Additional investigations are needed on the benefits of sudangrass cover crop in a rotation where it precedes lettuce. A reduced tillage system using cowpea mulch reduced weed populations and provided nutrients to the fall crop. However, due to rapid residue decomposition in the desert, those benefits were not carried over to the spring cantaloupe crop.

1) Evaluate the effectiveness of cover crops in reduced tillage (surface mulch) and conventional tillage (incorporated) production .

2) Develop cost studies for desert for ICM and organic based vegetable production systems and compare those costs to conventional production systems.

3) Extend current knowledge of cover crop systems to vegetable growers, Soil Conservation Service, and other interested parties.