Comparison of soil quality and nutrient budgets between organic and conventional kiwifruit orchards

Three long-term (>10 years) systems of kiwifruit production were compared at 36 sites with respect to simple input/output nutrient budgets, extractable soil nutrient levels, soil organic matter status, the size and activity of the soil microbial biomass, earthworm numbers and key soil physical properties. These systems were (i) conventional production of the green-fleshed variety ‘Hayward’ (Green), (ii) organic production of ‘Hayward’ (Organic) and (iii) conventional production of the yellow/gold-fleshed variety ‘Hort 16A’ (Gold). Crop yields and nutrient removals were least for Organic and greatest for Gold, with Green being intermediate. The major nutrients removed in the harvested crop were K and N. Simple input/output nutrient budgets showed that inputs greatly exceeded removals in the harvested crop for all nutrients considered (i.e. N, P, S, K, Mg, Ca) in all three systems, suggesting nutrient inputs could be reduced. Soil organic C and total N content were greater under Organic and Gold than Green whilst extractable P was least under Organic. Soluble C, basal respiration and metabolic quotient were unaffected by production system whilst microbial biomass C and N were greatest under Organic. Within systems, organic C, total N, microbial biomass C and N and mineralisable N were greater between plant rows than below the vine canopies whilst the reverse was the case for metabolic quotient and extractable P. Soil bulk density was least and water content at field capacity and earthworm numbers were greatest under the organic systems. It was concluded that long-term soil fertility can be maintained adequately under organic management and added benefits are increased organic matter content, a larger microbial biomass and improved soil physical condition. Although Organic orchards generally produce less fruit than their Green counterparts, mainly because of fertiliser differences and the absence of synthetic growth regulators, comparatively good returns and surpluses can still be achieved.