| Reducing tillage and fallow periods may improve agricultural sustainability, addressing issues such as soil degradation, water and air pollution, and rising energy costs. From 1999 to 2012 we compared conservation (CT) and standard tillage (ST) with (CC) and without (NO) cover crops for a cotton-tomato rotation in a Panoche clay loam soil in Five Points, CA. CT considered a reduction of tractor passes of 50% and 40% for tomato and cotton respectively. Soil samples from 0-15 cm and 15-30 cm were analyzed for total N and C, nitrate, Olsen phosphorus, interchangeable potassium, pH, electrical conductivity, and organic matter. After 13 years of rotation, both conservation tillage and cover cropping increased soil C and N. In CTCC, total C and N increased approximately 19 and 1.1 t ha-1 respectively in the 0- to 30 cm depth. In STNO the C was around 8 t ha-1 higher, while total N was approximately 0.2 t ha-1 lower than in 1999. Electrical conductivity increased in all treatments, primarily in the 15- to 30- cm depth, as a consequence of salts accumulation from irrigation water and fertilizers. The EC at that depth was significantly higher in ST than CT; reaching approximately 1.53 mmhos cm-1 compared to 1.23 mmhos cm-1 in CT. CTCC showed the highest values for extractable P and K. Neither pH nor nitrate content changed significantly in any of the treatments during the duration of the study. Total C, N, P and K were higher in CT and as a result, some agronomical features of the soil were improved. However, these nutrients were concentrated in the surface layer of soil, away from where the vast majority of roots are, which may limit their uptake by crops and thus other mechanisms would be needed to make them available. Under irrigated Mediterranean conditions, both CT and CC appear as feasible alternatives to conventional agriculture. If profitability is maintained, and water constraints are managed, these practices may become increasingly attractive for cotton and tomato farmers in the San Joaquin Valley. |
