Internal properties of soil aggregates from the market gardens of Pukekohe

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Abstract
Porosities of small aggregates of Patumahoe clay loam were considerably less at wilting point than at field capacity. At similar water contents in the wetter part of this range, porosities of aggregates from market gardens cultivated for more than 15 years were significantly less than porosities of aggregates from gardens more recently developed from pasture. The internal air contents of aggregates from new gardens were at least 6 percent of aggregate volume at the wilting point, and some aggregates from new gardens contained air at field capacity. Aggregates from some sites in old gardens contained only 2 to 3 percent of air by volume at the wilting point and are calculated to have anaerobic centres if they exceed 1.1 to 1.3 cm in diameter. Such centres could exclude roots. There were significant positive correlations (1) between the overall dry bulk densities of lower topsoils during the growing season and the dry bulk densities of the aggregates composing them; (2) between the air capacities of lower topsoils and the air contents of the aggregates composing them. The aggregates containing least air were found where the lower topsoils had air capacities marginal or inadequate for the optimum growth of vegetables.

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