BULK DENSITY CRITICAL LIMITS FOR NORMAL ROOT GROWTH OF COVER CROPS

Soil compaction is a serious problem. for the soil quality and development of sustainable agriculture, because compaction, affects the flow of soil water and air and reduces crop yields. One of the possibilities to tackle this problem is the use of plants with a deep and vigorous its rooting system. The objective of this study was to evaluate the physical quality of a Typic Hapludalf in, a no-tillage system, after the use of cover crops, and to measure the critical limit of bulk density. In. the autumn winter of 1999/00 and 2000/01 the site was cultivated with black oats (Avena strigosa) with vetch (Vicia sativa) and in. 2001/02, with forage turnip (Raphanus sativus). Corn was sown, (Zea mays) in the summer, and in. the end of the cycle, four summer cover crops were sown (Crotalaria juncea, Cajanus cajan, Stilozobium cinereum and Canavalia ensiformes) and compared to fallow, soil. Soil balk density and penetration resistance were measured. The rooting system was evaluated by a root profile method during the cycle of forage turnip, corn and summer cover crops. No tillage increased soil balk density to levels considered critical for plant growth. The root growth of all crops used in. the rotations suffered some degree of restriction. There were no differences among species in. their root growth potential in compacted soil. The cover crop root growth, was normal up to a balk density of 1.75 Mg m(-3), while values between . 75 and 1.85 Mg m(-3) were restrictive, with visible deformation of the root Morphology. Above 1.85 Mg m(-3) these deformations were more pronounced, with increased root thickening, deviations in. the vertical root growth direction and concentration of roots in. the topmost soil layer. The evaluated species can. be used in compacted soils, bid at soil balk density > 1.85 Mg m(-3) soil mobilization, through mechanical methods may be necessary to alleviate the effects on. root and plant growth.