On-farm assessment of organic matter and tillage management on vegetable yield, soil, weeds, pests, and economics in California

In intensive vegetable production, low organic matter (OM) inputs and leaching of nitrate (NO3−-N) decrease soil quality with time. Four management regimes were compared for their effects on soils and on production issues in a cooperative research project with a commercial vegetable grower in the Salinas Valley, California, USA, on an 8.3 ha field: minimum tillage with OM (+OM) inputs; minimum tillage with no OM (−OM) inputs; conventional tillage +OM inputs; and conventional tillage −OM inputs. Minimum tillage retained the same raised beds for the 2-year study (four crop cycles), and tilled to approximately 20 cm depth. Conventional tillage used many passes for surface and subsoil tillage, and disturbed the soil to approximately 50 cm depth. In +OM, compost was added two times per year, with a rye (Secale cereale) cover crop in the fall or winter, whereas −OM treatments followed the typical practice of only incorporating crop residues. Addition of cover crops and compost increased microbial biomass C (MBC) and N (MBN), reduced bulk density, and decreased the NO3−-N pools in the 0–90 cm profile, so that leaching potential was lower compared to −OM treatments. Tillage practices had generally similar effects on soils except that surface soil moisture and NO3−-N in the deep profile were consistently lower with minimum tillage. Minimum tillage tended to decrease lettuce (Lactuca sativa) and broccoli (Brassica oleracea) yields, but was not associated with increased pest problems. Weed density of shepherd’s purse (Capsella bursa-pastoris) and burning nettle (Urtica urens) were occasionally lower in the +OM treatments. Disease and pest severity on lettuce was slight in all treatments, but for one date, corky root disease (caused by Rhizomonas suberifaciens) was lower in the +OM treatments. The Pea Leafminer, Liriomyza huidobrensis, was unaffected by management treatments. Economic analysis of the three lettuce crops showed that net financial returns were highest with minimum tillage −OM inputs, despite lower yields. Various tradeoffs suggest that farmers should alternate between conventional and minimum tillage, with frequent additions of OM, to enhance several aspects of soil quality, and reduce disease and yield problems that can occur with continuous minimum tillage.