Deterioration of soil structure in the market gardens of the Pukekohe District, New Zealand

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Abstract
In the Pukekohe area, which lies about 30 miles (50 km) south of Auckland, vegetables for market have been grown for at least 30 years in some gardens on Patumahoe clay loam, a naturally well structured soil derived from volcanic ash. In some areas this soil now shows signs of structural deterioration. Samples taken from the surface 8 in. of gardens of various ages show that the content of organic carbon of the soil has declined from 5.5 percent by weight to a little over 2 percent in 30 years of market gardening. Water-stable aggregates larger than 2 mm in diameter have almost disappeared over the same period. However, these have broken down to smaller aggregates, not to the ultimate soil particles, and soil compaction due to the breakdown of aggregates on wetting does not appear to be severe. In certain gardens, of various ages, the soil at a depth of 3 to 8 in. contained as little as 4 to 8 percent by volume of pores drained by a tension of 50 cm of water. The occurrence of these compacted zones is not related to the water-stability of the soil aggregates, but is attributed to compression by cultivation implements. It is considered that if ways are found of keeping compaction by cultivation implements to a minimum, the Patumahoe clay loam will be capable of growing vegetables for many more years without serious reduction of yields.