Soil strength dynamics under different management systems

The dynamic behavior of Yolo loam soil in terms of changes in its mechanical strength was analyzed for three management systems (conventional, low input, and organic) which have been established as experimental plots for the last 11 years. The above analysis was conducted for dry beans during the summer months of 1999 and required three determinations at different vegetative stages (post-emergence, flowering, and harvest). Measurements consisted of weekly monitoring of soil moisture content as well as three determinations of Soil Cone Index (CI), and the tractive force required to pull a vertical, 30-cm-deep chisel (TCI) through the soil. All the measurements were done on both trafficked and non-trafficked furrows, and they consisted of nine sampling points for the CI per plot and continous measurement of TCI by means of GPS-aided instrumentation. The results for an integrated layer of 0-30 cm depth show that the most significant differences occurred at the time of highest demand of water from the crop. As time passed, soil strength decreased to values close to the initial ones without significant differentiation among management systems. Closer inspection of the data suggested that integrated values for the 0-30 cm. stratum at the beginning and at the end of the cropping cycle seemed to be quite similar, but there were important variations in soil hardness in sub-layers within this stratum. Throughout the season, management systems were clearly differentiated in regard to the water-holding capacity of the soil; this was reflected in the inverse relationship between the applied chemical inputs and the moisture content.