Effect of phosphate topdressing on a soil from Andesitic Volcanic Ash

Abstract

SummaryA 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 Ib P/acre/year; (c) topdressed for the last seven years at 60-80 Ib 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 !ittle effect below 2 inches depth. In the 0-2 in. layer, it has resulted in large increases in the amount of Fe-and AI-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 AI-bound and that the very strong phosphate-retaining power of this soil is due to the high content of the amorphous c1ay allophane and associated amorphous oxides of iron and aluminium, and their very high specific-surface areas.

Long-term effect of superphosphate on accumulation of soil phosphorus and exchangeable cations on a grazed, irrigated pasture site

“…AI-P is often observed to be a major fraction in which P~ accumulates in fertilised pasture soils (Batten et al, 1979;Saunders, 1959), but such a marked accumulation of P as Ca-P is less typical. However, Condron and Gob (1989) suggested that liming the site in 1962 would have tended to favour the formation of Soil P fractions in grazed pasture 131 calcium phosphate compounds as reaction products following dissolution of superphosphate whilst the use of superphosphate with an unusually high proportion of unreacted apatite during the 1970's would have favoured accumulation of residual apatite.…”

Residual effect of phosphate top-dressing on a yellow-brown loam from andesitic ash

“…Because this soil is also potassium-deficient regular potassium top-dressing is needed, and phosphate and potassium are usually added as potassic super. Saunders and Metson (1959) have suggested that the best utilisation of soluble potassium fertilisers is obtained by split dressings. It therefore becomes desirable to add potassium separately from phosphate except in late summer.…”