Living mulch

Cover crop grown with a main crop as mulch

In agriculture, a living mulch is a cover crop interplanted or undersown with a main crop, and intended to serve the purposes of a mulch, such as weed suppression and regulation of soil temperature. Living mulches grow for a long time with the main crops, whereas cover crops are incorporated into the soil or killed with herbicides. Other benefits of mulches are slowing the growth of weeds, and protecting soil from water and wind erosion. Some living mulches were found to increase populations of the natural enemies of crop pests. Legumes used as living mulches also provide nitrogen fixation, reducing the need for fertilizer.

When cover crops are turned over into the soil, they contribute nutrients to the main crop so that less chemical fertilizer is required. The amount of the contribution depends on the biomass, which varies over time and depends on rainfall and other factors. The greater the biomass, the greater the nutrient turnover of land. Legume cover crops turn over nitrogen fixed from the atmosphere. Reports indicate that legumes in general have higher foliar nitrogen contents, from 20 to 45 mg g 1. Bare soil resulting from intensive tillage can lead to soil erosion, nutrient losses, and offsite movement of pesticides. In addition, weeds can germinate and grow without competition. Living mulches can reduce water runoff and erosion, and protect waterways from pollution. Living mulches have also been shown to increase the population of organisms which are natural enemies of some crop pests. In some situations, the allelopathic properties of living mulches can be used to control weeds. For example, the allelopathic properties of winter rye (Secale cereale), ryegrasses (Lolium spp), and subterrain clover (Trifolium subterraneum) can be used to control weeds in sweet corn (Zea mays var "rugosa") and snap beans (Phaseolus vulgaris). Populations of ground dwelling predators were greater in a corn and soybean rotation with alfalfa and kura clover living mulches than without a living mulch. This situation was due in part to a change in the composition of vegetation in the agricultural system.

Unfortunately, living mulches compete for nutrients and water with the main crop, and this can reduce yields. For example, Elkins et al. (1983) examined the use of tall fescue (Festuca arundinacea), smooth bromegrass (Bromus inermis), and orchargrass (Dactylis glomerata) as living mulches. They found that herbicides killed 50% to 70% of the mulches but corn yield was reduced 5% to 10% at the end of the harvest. Although leguminous cover crops have large biomass production and turnover, they are not likely to increase soil organic matter. This is because legumes used as living mulches have greater N contents and a low C to N ratio. coconut and rubber. In Mexico, legumes used traditionally as living mulches were tested as nematode and weed suppressors. The mulches included velvetbean (Mucuna pruriens) jackbean (Canavalia ensiformis), jumbiebean (Leucaena leucocephala) and wild tamarindo (Lysiloma latisiliquum). Aqueous extract of Velvetbean reduced the gall index of Meloidogyne incognita in the roots of tomato, but suppressed tomato rooting as well. In addition, Velvetbean suppressed the radical growth of the local weeds Alegria (Amaranthus hypochondriacus) by 66% and Barnyardgrass (Echinochloa crus galli) by 26.5%. Nicholson and Wien (1983) suggested the use of short turfgrasses and clovers as living mulches to improve the resistance soil compaction. These authors established Smooth Meadow grass (Poa pratensis) and white clover (Trifolium repens) as living mulches since they did not cause reduction of yield corn (the accompanying main crop). In one study, chewing fescue or red fescue (Festuca rubra) and ladino clover (Trifolium repens) were effective living mulches for controlling weed growth. It is important to judiciously select the appropriate herbicide rate for burning down a living mulch. In 1989, Echtenkamp and Moomaw found that herbicide rates were inadequate to suppress all the living mulches. This grass is also well adapted to dry and poor soils.

Legume cover crops have important positive effects on the nutrient cycling of tree crops. In one experiment, water runoff and soil loss on a 14% slope was compared for rototilled (RT), no till with corn stover mulch (NT CSM), no till in CSM+ birdsfoot trefoil living mulch (NT BFT) and no till in CSM and crownvetch living mulch (NT CV). The results indicated that the water runoff was 6,350 L ha 1 for NT BFT, 6,350 L ha 1 NT CSM, 5,925 L ha 1 for NT CV, and 145,000 L ha 1 for RT. The soil loss for the RT was 14.22 t ha 1 while with the other treatments it was less than 0.5 tons ha 1. The least soil loss was obtained with NT CV 0.02 tons ha 1. The reduction of water runoff and erosion is one of the greatest advantages of having a cover crop. Soil can be easily eroded with no vegetative ground cover or plant residue. Ideally soil erosion should be less than 4 to 5 tons/ha/year.

Living Mulch by Dr. Mary Peet, NCSU