84Organic agriculture: a global perspectiveThe purpose of tillage always needs to be to achieve an economic balance between the short-term need for seasonal crop productivity and the long-term need for sustaining production capacity, specifically, maintenance or enhancement of active soil organic matter (SOM) and its attendant (emergent) soil quality attributes (Schjnning et al. 2004). Coer crops �their uses in organic agriculture Production and appropriate management of high-residue cover crops are considered essential for profitable organic cropping systems. Cover crops, whether grass, legume or mixtures, are non-cash crops used to perform or achieve different objectives. When sown soon after cash crops, cover crops minimise soil erosion and absorb (capture) residual nutrients and water. Thus, cover crops perform as catch or scavenger plants to protect the environment from pollution and improve soil quality by providing important food reserves and habitat for beneficial organisms (Magdoff and Weil 2004). Many cover crops are commonly referred to as smother crops, since they are established in high-density plantings, grow rapidly and inhibit (smother) weed growth. Generally, cover crops are grown and killed before planting cash crops. In this usage mode, they can be incorporated as green manure or killed and used as a dead mulch in no-till systems (Barker and Bhowmik 2001). Cover crops that are interseeded and coexist with cash crops during all or a portion of the growing season are called living mulches. For organic growers, an ideal living mulch does not interfere with growth and yield of the cash crop, is low-growing, easily established and maintained by non-chemical means, and serves as a good habitat for beneficial insects. Living mulches are particularly effective with fast-growing cash crops that are supplied supplemental water and nutrients through in-row drip irrigation systems (Brandsaeter et al. 1998). Specialised flowering and high nectar cover crop mixtures are grown as companion or farmscape plantings in field margins or are arranged in rows or patches at regular intervals throughout the field. Ideally, farmscape plantings contain perennial and/or reseeding annual species that smother weeds, protect the environment, build soil quality and serve as habitat and refuge for beneficial insects (Dufour 2000).Organic no tillage �oxymoron or opportunityEffective integration of high-residue cover-cropping practices and no-till systems have many well acknowledged advantages: 1 2 3 4 5 conservation of soil and water; moderation of soil temperatures; suppression of weed and pest growth; improved efficiency of nutrient use, especially nitrogen; and preservation and build-up of active SOM, which is the principle soil constituent responsible for generating soil quality (Magdoff and Weil 2004).In concept, integration of organic cropping systems with no-till practices is a logical step or progression toward improved crop yields and soil quality (Brberi 2002, Schjnning et al. 2004). At one extreme, some growers and researchers believe organic no tillage is an oxymoron, maintaining that chemical herbicides are essential to kill cover crops and control weed growth (Lessiter 2003). However, others have shown that agronomic crops (Derpsch and Grooms 2002) and vegetable crops (Creamer et al. 1996, Morse 2000, Ngouajio et al. 2003, Madden et al. 2004) can be successfully grown using high-residue cover crops that were effectively killed by mechanical methods such as flail mowing or rolling. Those involved with organic no-till
