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

The effects of restorative crops on the amelioration of a degraded soil wer e investigated in a 6-year field experiment. Treatments included perennial pastures, annual pastures, and arable crops. Improvements in some aspects o f chemical, biological, and physical fertility were related to the amount o f herbage dry matter returned to the soil and root production. Beneficial e ffects associated with returned organic matter were partly negated by the d egradative effect of tillage. Treatments that returned most organic materia l to the soil showed the greatest increase in aggregate stability and suppo rted 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 treatme nts in microbial biomass C were apparent by the third year. Compaction by s heep 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 clov er treatments were the most effective in ameliorating degraded soil conditi ons. The rate of soil amelioration declined with depth, and was mainly conf ined 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 s tability at 0-5 cm depth.