Restorative crops for the amelioration of degraded soil conditions in New Zealand

Abstract

The effects of restorative crops on the amelioration of a degraded soil were investigated in a 6-year field experiment. Treatments included perennial pastures, annual pastures, and arable crops. Improvements in some aspects of chemical, biological, and physical fertility were related to the amount of herbage dry matter returned to the soil and root production. Beneficial effects associated with returned organic matter were partly negated by the degradative effect of tillage. Treatments that returned most organic material to the soil showed the greatest increase in aggregate stability and supported the largest earthworm populations, especially without annual tillage. Differences between treatments in soil organic C content were not generally significant until the sixth year. In contrast, differences between treatments in microbial biomass C were apparent by the third year. Compaction by sheep during grazing appeared to result in a loss of soil macroporosity. In the sixth year, soil macroporosity was greatest in the annually cultivated, ungrazed treatments. The grazed perennial ryegrass and ryegrass/white clover treatments were the most effective in ameliorating degraded soil conditions. The rate of soil amelioration declined with depth, and was mainly confined to the top 10 cm of soil. The rate of amelioration was relatively slow, with, for example, 3 years needed for most of the increase in aggregate stability at 0–5 cm depth.