Nitrogen, weeds and water as yield-limiting factors in conventional, low-input, and organic tomato systems

The importance of nitrogen (N), weeds, and water as yield-limiting factors was evaluated over a 4-year period in tomato crapping systems under convent ional, low-input, and organic management. The cropping systems studied were part of the Sustainable Agriculture Farming Systems (SAFS) Project at the University of California, Davis, a comparison of conventional and alternati ve farming systems in California's Sacramento Valley. Water applied, soil N levels, plant N uptake, weed abundance, and tomato yield were measured and compared among treatments. Tomato yields ranged from just under 55 to over 90 t ha(-1) and significant treatment differences were observed in 2 of th e 4 years. Multivariate analyses, used to sort out the effects of N, weeds, and water, indicated all three factors influenced yields in this study but their relative importance was dependent upon the management system. Result s indicated that N availability was most important in limiting yields in th e organic system and water availability was more important under convention al management. Although weed abundance was relatively high in the organic s ystem in 2 years of the study, weed competition for N was not evident. Inst ead, relative N input levels and N immobilization by soil microflora appear ed to explain N uptake and tomato yield variation. The findings indicate th at organic and low-input tomato systems in this region can produce yields s imilar to those of conventional systems but that the factors limiting yield may be more difficult to manage. (C) 1999 Elsevier Science B.V. All rights reserved.