Greenhouse Evaluation of Four Phosphorus Soil Tests in Relation to Phosphate Buffering and Labile Phosphate in Soils

Abstract

Four soil P tests — Olsen, Colwell, Bray, and Mehlich — were evaluated in a greenhouse experiment in relation to plant uptake, relative yield, labile P, and buffer capacity on 30 soils varying in pH from 5.4 to 8.1. The Olsen and Colwell tests were most highly correlated with plant uptake, and the Olsen and Bray tests with relative yield. When the soils were stratified into weakly, moderately, and strongly buffered groups, it was found that extraction of labile P was depressed by increasing buffer capacity most in the Bray test, followed by the Olsen test, and least in the Colwell test. The Bray test tended to overcompensate for the effects of buffering on plant uptake and therefore underestimated the amount of available P in strongly buffered soils, whereas the Colwell method tended to overestimate it in strongly buffered soils. The Olsen test appeared to be correctly sensitive to buffering and therefore gave the highest correlations with both plant parameters. The Mehlich test extracted large amounts of nonlabile P in soils with pH >6.0 and was therefore weakly correlated with plant uptake and relative yield. The results showed that in a successful soil test, increasing buffer capacity will depress the extraction of labile P in the same way as it depresses uptake by plants.

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

“…It was also shown by Holford and Mattingly (1979) that the more sensitive a soil test is to buffer capacity, the better the correlation between the soil test values and phosphorus uptake by ryegrass. However, in a subsequent study, using white clover on a much more variable group of soils, Holford (1980) found that the relative efficacy of a soil test may be affected by its over-sensitivity as well as undersensitivity to buffering. A soil test which is over-sensitive will underestimate available phosphate on strongly buffered soils, while a soil test which is under-sensitive will overestimate it.…”

“…It has been shown recently that the amounts of labile phosphate extracted by some soil tests tend to decrease with inoreasing buffer capacity of the soils (Holford and Mattingly 1979). Because the magnitude of this effect, and hence the relative sensitivity to buffer capacity, varies between soil tests, it is reasonable to suppose that buffer capacity is an important soil property influencing inter-relationships between soil tests, particularly those that extract phosphate only from the labile pool (Holford 1980). It was also shown by Holford and Mattingly (1979) that the more sensitive a soil test is to buffer capacity, the better the correlation between the soil test values and phosphorus uptake by ryegrass.…”

“…The aims were to examine the effects of buffer capacity on (i) the extraction of labile phosphate by four soil tests which were thought to remove phosphate mainly from the labile pool, (ii) the relationships between the four soil tests, and (iii) the critical level of each soil test for near-maximum yield of wheat under field conditions. The same soil tests were used as in the previous glasshouse study (Holford 1980), except for the Mehlich test which was found to extract large quantities of non-labile phosphate.…”

Pig effluent-P application can increase the risk of P transport: two case studies

“…Some bicarbonate extract solutions were also analysed by inductively coupled plasma techniques (ICP) to determine total P in the extracts (P TB ; detection limit 1 mg/ kg; using a Thermo Jarrell Ash Iris/HR Duo). Bicarbonate-extractable P is related to the quantity of labile P (Dalal and Hallsworth 1977;Holford 1980), and was used in this study as an estimate of labile P. Particle size analysis of the soil samples was carried out using the pipette method described in Gee and Bauder (1986), including sample pre-treatment for removal of soluble salts and organic matter (where necessary).…”

Removal of Lithium Citrate from H3A for Determination of Plant Available P

“…Few currently available soil extractants are capable of multi-nutrient extraction without sacrificing accuracy for one compound or another [1]. Plant-available soil P has been difficult to accurately assess across the naturally occurring pH range of soils (calcareous to acid) with a single extractant because soil pH and P solubility are highly interrelated [2] [3] and soil pH has a strong influence on soil-solution chemistry [4].…”