Cover Crops to Improve Arizona Soil

Cover crops fall into two categories: legumes and grasses. Legumes include peas, beans, clover, alfalfa, vetch, trefoil, and medic. Legumes can be annuals, biennials, or perennials and have the added advantage of being able to fix nitrogen (converting nitrogen in the atmosphere to plant available nitrogen in the soil). Some legumes can fix as much as 200 lbs of nitrogen per acre per year. To ensure an adequate level of nitrogen fixation, legumes must be inoculated with the appropriate strain of Rhizobacteria. These inoculants are inexpensive and the process is simple.

Annual legumes are ideal for short-term plantings in fall or early spring. Biennials grow much more vigorously in the second year and are best planted on areas that will not be cultivated for one year. Perennial legumes, such as alfalfa, grow deep taproots that are very good at improving compacted soil conditions but can be difficult to eliminate from the garden once they have served their purpose.

Grasses do not fix nitrogen, but have a fine textured, fibrous root system that is efficient at stabilizing soils and is easily decomposed to add organic matter to the soil. Some common annual grass cover crops are barley, oats, rye, and wheat. Perennial grasses are usually used in orchards, vineyards, and other areas where tillage is unlikely to occur. These include fescues, orchardgrass, bluegrass, ryegrass.

Many varieties of cover crop seeds may be purchased from organic gardening suppliers through catalogs or the Internet. A simple mix of annual grass (rye or barley) and clover is a great place to get started with cover crops. Annual grasses germinate quickly and act as a nurse crop for the legumes. As time goes on, the grasses provide a scaffold for the legumes to grow upward and spread. Plant about one ounce of annual grass seed mixed with one half ounce of clover seed per 100 square feet. Inoculate clover seed (and other legumes) or purchase freshly inoculated seed. Broadcast onto raked soil, and then cover the seed to a depth of 2-3 times the width of the seed and firm the soil with a rake. Irrigate 1-3 times after planting to establish the cover before cold weather sets in.

Before the cover crop has started to form seed, it should be tilled into the soil as green manure. Green manure adds organic matter, nitrogen and other nutrients that were contained in the leaves, stems and roots of the cover crops. The green manure should be allowed to decompose for at least three weeks before planting. Legumes add nitrogen and decompose easily while grass roots add easily decomposed organic matter. The grass leaves are less easily decomposed and will contribute organic matter over a longer period.

Most cover crops are not allowed to mature and then tilled into the soil as “green manure” prior to summer planting. Once incorporated, the cover crop decomposes in three to four weeks releasing the organic matter and nutrients over the summer growing season.

Cover crop plants can be a single species or a combination of species suited to your climate and gardening objectives. Legumes are almost always used as green manure cover crops because of their ability to convert unavailable nitrogen in the atmosphere into plant-available nitrogen in the soil. Legumes are plants such as alfalfa, peas, beans, clover, vetch, and their relatives (including mesquite and palo verde trees). Many gardeners are aware of this phenomenon, but for those that aren’t, the process is called nitrogen fixation and carried out by bacteria (Rhizobium) that live in the roots of legumes. This is a symbiotic relationship where the legume receives nitrogen from the bacteria and the bacteria receive sugars from the plant.

Quite often, cover crops also include grasses to increase soil organic matter. Grasses have fibrous root systems that utilize nitrogen released by the legumes and decompose readily to contribute organic matter. When tilled in, the tops of the grass plants also contribute nutrients and organic matter.

Legume cover crops can add up to 300 lbs per acre of nitrogen. Some cool season annual legumes suitable for cover crops in north central Arizona include common vetch, hairy vetch, sweet clover, red clover, medic, and fava beans. Alfalfa and other perennial legumes are sometimes used as cover crops. These plants penetrate more deeply in the soil, but may become persistent and difficult to control. All legume seeds should be inoculated with the proper strain of Rhizobium bacteria to ensure successful nitrogen fixation. This information is available in seed catalogs that sell cover crop seeds.

I seeded half of my vegetable garden this fall with a mixture of hairy vetch and cereal rye. I used an inoculant purchased from the supplier that was recommended for hairy vetch. After digging up a vetch seedling this morning, I found that it had nodulated. My plans are to allow this cover crop to grow until 50% of the flowers are in bloom before I mow it and incorporate it into the soil as green manure. The usage of cover crops and green manure is an important sustainable agriculture practice that adds nitrogen and organic matter to the soil and reduces the need for chemical fertilizers.

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