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For The Garden

Soil Secrets

Author: Cathy Cromell
Issue: June, 2014, Page 118

A healthy soil medium helps non-native vegetables flourish in the desert

Native desert soil contains less than one-half of 1-percent organic matter. This surprisingly minimal amount poses no problem for native plants because they have evolved to thrive in what is available. However, most of our favorite vegetables evolved in different soil conditions. To produce abundant harvests, it is important to start with organically rich soil.

Gardeners have varied options to enhance soil, such as digging in compost before each planting season, or planting cover crops and green manures. A cover crop is sown to “cover” the soil to inhibit erosion and weed growth when a bed is left fallow. The goal of “planting” green manure is to add significant nitrogen and organic matter to the soil.

These two terms—cover crops and green manures—are often used interchangeably and both practices offer valuable benefits. They provide organic matter, create and add nitrogen, reduce soil compaction, inhibit weeds, prevent erosion, improve soil tilth and feed beneficial insects that consume pests.

Many cover crops/green manures are legumes, which are plants that produce a bean or pea pod. In a symbiotic relationship with soil bacteria, legumes convert abundant nitrogen in the air to a form that plants can use. This process is called nitrogen fixation. When leguminous cover crops decompose, nitrogen in their root nodules is released to the soil where other plants can absorb it.

Cowpea—also called black-eyed pea (Vigna unguiculata)—is an excellent legume for the warm season, according to Kelly Murray Young, assistant agent, agroecology at University of Arizona Maricopa County Cooperative Extension. “Cowpeas thrive in the hottest weather and will quickly cover the soil and outcompete most weeds, with only two or three irrigations to get them up and running,” says Murray Young. “When incorporated into the soil, they release substantial quantities of nitrogen.”

Cowpeas start flowering 60 to 90 days after sowing. If you plant cowpeas in June or July, they will mature in time to enrich your cool-season garden. To maximize nitrogen and organic matter in the soil, cut back and till or turn under plants while still green but before they set seed. If allowed to go to seed, the plants’ nitrogen will be trapped as protein in the seed and unavailable for other plants to absorb.

After incorporating any cover crop/green manure, wait a month before planting to allow for decomposition. Adding moisture to the soil after crops have been tilled speeds decomposition.
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