Effects of ameliorating exposed subsoil prior to sowing on the water relations and productivity of pasture during an irrigation cycle

Abstract

Adding organic matter or replacing topsoil have been shown to improve the growth of pasture on areas where subsoil is exposed during landforming. Intensive experiments were carried out in each of the 1985-86 and 1986-87 irrigation seasons to examine the basis of improvements in the productivity of pastures after such amelioration. Measurements were collected during the period between two irrigations which were separated by approximately 70 mm of cumulative Class A pan evaporation. Treatments included (i) high rates of nitrogen and phosphorus (NP); (ii) NP + organic matter (OM); (iii) NP + topsoil (TS); and (iv) NP +Well watered (WW).Compared to NP, pasture production in OM and TS treatments was improved in the first year, but in the second year only the TS treatment gave significant improvement. These increases in dry matter yields were reflected in rates of leaf elongation, canopy conductance, evapotranspiration and photosynthesis that were up to 50% greater than those in WW during the first half of an irrigation cycle, but there were no differences in leaf water potential. There was a strong relationship between canopy conductance and photosynthesis, and it appears that the plants in the TS and OM treatments were responding to an improved root perception of soil water availability. It is hypothesised that this allowed them to maintain canopy conductance and therefore photosynthesis at higher levels than in NP and WW. The improved performance of the TS and OM treatments represents an increase in the productive potential of exposed subsoil areas that cannot be achieved by improving fertilizer and irrigation management alone.