Maintaining soil organic matter is a significant challenge in the production of annual vegetable crops. There are two reasons for this:
Carrots and onions are prime examples of low-residue crops that leave little organic matter to replenish the soil after harvest. Potatoes produce high amounts of organic matter per acre, but most is removed in the tubers. Radishes and leafy crops like lettuce and spinach have short growing seasons and return little residue even with multi-cropping. To compound the challenge of maintaining SOM, frequent and intense cultivation are generally used in the production of these crops.
Methods discussed in the previous section for increasing SOM—reducing tillage, improving soil structure, and recycling excess plant nutrients—are all suitable for vegetable crops. Application of manures, composts, biosolids, and other organic wastes are good ways to add organic matter. If biosolids are used, they should be low trace-metal, "clean" sludges that can be applied to food crops with no restrictions.
Soil preparation and management operations such as preparing seedbeds, controlling and cultivating weeds, burying crop residue (e.g., for disease control), and building raised beds are important in vegetable production. These operations improve certain soil conditions that otherwise limit crop growth and development. However, each operation also accelerates the loss of SOM by mixing and aerating the soil. And excessive tillage severely degrades soil structure. Loss of structure makes soil prone to compaction and related problems like reduced water infiltration and poor root growth. Tillage operations should be selected carefully, because in some situations growers must weigh short-term benefits to the current crop against long-term reductions in soil quality.
Growing cover crops and preceding vegetables with high-residue rotation crops, such as small grains and forages, can increase SOM. In addition, many cover crops suppress weeds; legume cover crops add nitrogen; and cereals and other grasses scavenge residual nutrients and keep them from moving off-site. Buckwheat, sorghum-sudangrass, millets, and annual ryegrass are useful summer cover crops that can follow early-season vegetables or precede fall crops.
Rye and millet produce large amounts of vegetative growth, adding potentially large amounts of organic matter. It is important to fertilize rye cover crops or the subsequent cash crop adequately to avoid nitrogen tie-up during decomposition. Common winter cover crops include small grains, such as cereal rye, wheat, and oats. Oats often winter-kill and produce much less dry matter but are a good cover crop for wet soils because they are less likely to delay field operations in the spring. Hairy vetch and red clover are common legume cover crops. They can be grown alone or in grass/legume mixtures.
Many vegetable crops have relatively shallow, sparse root systems but are well fertilized because of their high value. This combination of small root systems and high fertilizer rates may lead to another potential soil-quality problem—the possible movement of excess nutrients to surface water or groundwater. Following a good soil-testing program, banding fertilizer, splitting nitrogen applications, using fertigation, and avoiding excess irrigation can minimize unwanted nutrient movement.
No-till alternatives have not received as much attention in vegetable crops as in agronomic crops, especially in temperate climates with cold winters. Yet, cover crops are used in successful no-till vegetable systems. Transplanting into killed cover crops (e.g., tomato into hairy vetch) has produced the best results to date. Various practices to overcome the challenges of no-till vegetable production are being tested. For example, seeding or transplanting into living mulches like short life-cycle medics may allow for more timely planting and still offer the benefits of reduced tillage. These and other practices may soon become viable alternatives, especially in northern growing regions.
| Loss of prime farmland to non-agricultural use is a major concern in many areas. As the amount of land available for crop production and rotation is reduced by development, it is even more important to maintain our remaining agricultural soils at their highest possible level of quality and productive capacity. |