Effect of phosphate topdressing on a soil from Andesitic Volcanic Ash

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Abstract
A comparison has been made within a single soil type, New Plymouth black loam, of soils collected from farms that differed in their phosphate topdressing management: (a) not topdressed; (b) topdressed for 25 years at 20–30 lb P/acre/year; (c) topdressed for the last seven years at 60–80 lb P/acre/year. Total, organic, apatite and Fe- and Al-bound phosphorus has been determined in these soils and also phosphorus soluble in dilute acids and neutral ammonium fluoride. Phosphate topdressing has had little effect below 2 inches depth. In the 0–2 in. layer, it has resulted in large increases in the amount of Fe- and Al-bound phosphorus, phosphorus soluble in dilute acids and neutral ammonium fluoride, exchangeable calcium, and available phosphorus. There has been a small rise in the levels of apatite phosphorus and cation-exchange capacity, and the topdressing has had little or no effect on the soil pH and the level of organic matter and organic phosphorus. There has been a decrease in the amount of iron and aluminium extracted by sodium hydrosulphite. In each instance the change has been greater in the Heavy-P than in the Mod.-P soils, although the total amount of phosphorus added is similar for both soils. It is considered that most of the accumulated phosphate has been Al-bound and that the very strong phosphate-retaining power of this soil is due to the high content of the amorphous clay allophane and associated amorphous oxides of iron and aluminium, and their very high specific-surface areas.