Effects of phosphate buffer capacity on critical levels and relationships between soil tests and labile phosphate in wheat growing soils

Abstract

Thirty-nine soils from northern New South Wales were used to examine the effects of phosphate buffer capacity on (i) the extraction of labile phosphate by four soil tests, (ii) the relationships between the four soil tests, and (iii) the critical level of each soil test required for near-maximum yield of wheat under field conditions.The results confirmed the principle, recently proposed by the author, that the larger the negative effect of buffer capacity on extraction of labile phosphate by a soil test, the higher is the correlation between the soil test and plant yield response to phosphate. The acidic ammonium fluoride extractant of Bray and Kurtz was the most sensitive to buffering in this respect, while the alkaline sodium bicarbonate extractant of Olsen el al. was less sensitive and the modified sodium bicarbonate test of Colwell least sensitive to buffering. Whereas a previous glasshouse study suggested that the ammonium fluoride test was over-sensitive to buffering, and hence underestimated available phosphate in strongly buffered soils, this field study showed that the test is correctly sensitive to buffering. Consequently critical levels for near-maximum wheat yields do not vary for the ammonium fluoride tests, but increase with increasing buffer capacity for the sodium bicarbonate tests. The additional measurement of buffer capacity is therefore required to give precision in the use of the sodium bicarbonate soil test and particularly the Colwell test.The results also suggest that a high correlatiori between two soil tests can only be expected where each test is similarly sensitive to buffering, provided of course that both tests extract phosphate mainly from the labile pool.

Extratores de fósforo em solos da Amazônia tratados com fertilizantes fosfatados

“…Tais explicações não parecem justificar, satisfatoriamente, as diferenças entre os métodos testados. Contudo, Holford (1980) considera que, quanto mais similares forem dois extratores na sua sensibilidade ao tamponamento do solo, maior será a correlação entre ambos.…”

Accuracy of Olsen P to assess plant P uptake in relation to soil properties and P forms

“…Different factors can affect the relationship between P in solid and liquid phases thus affecting the accuracy of estimated threshold levels for each index even in groups of soils of homogeneous properties (Delgado et al 2010). It is generally accepted that the P buffer capacity (PBC), which is the soil solution resistance to a change in P concentration, is a crucial factor affecting equilibrium between solid and liquid phases (Holford 1980;Ehlert et al 2003;Celardin 2003). Factors such as soil P status (Barrow and Debnath 2014), the affinity of sorbent surfaces for P, or the proportion of exchangeable cations can also affect this equilibrium (Delgado et al 2010).…”

Comparative performance of Caucasian clover(Trifolium ambiguumcv. Monaro) after 11 years under low‐input conditions in south‐eastern Australia

“…A detailed account of the phosphorus (P) chemistry of the soil can be found in Dear et al (1992) as site "3F". Before the trial was established in 1981, the soil P content (Bray) was 11.4 ppm [equivalent to an Olsen P of 11.1 for a weakly buffered soil, see Holford (1980)]. Table 1 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 Time (years) rainfall is evenly distributed throughout the year, it is highly variable as shown by a plot of deviation from mean monthly rainfall in the period 1981-93 (Fig.…”