Changes in Phosphorus Fractions in Soils under Intensive Plant Growth

Abstract

The total quantity of P and plant‐available P often differ greatly in soils of the tropics, which typically range in weathering intensity. Assessing available P is fundamental to managing P in many of these soils. Phosphorus availability in some soils has been inferred from the Hedley sequential extraction assuming that each P fraction reflects similar plant availability in different soils. However, experimental measurements of plant P availability were either of short duration or involved multiple P applications, which complicates assessment of the plant availability of P fractions. The objectives of this study were to examine the changes in P fractions under exhaustive cropping on diverse soils and to discern the differences in plant availability among P fractions. Eight soils ranging in weathering from Vertisols and Mollisols to Ultisols and Oxisols were amended with Ca(H2PO4)·H2O to raise soil solution P to 0.2 mg L−1 and planted for 14 crops to remove available P. The results indicated that the Fe‐impregnated strip–P and inorganic NaHCO3–P (NaHCO3–Pi) decreased the most in response to plant P withdrawal in all soils. The inorganic NaOH‐P (NaOH‐Pi) also declined with plant P uptake in all soils. The HCl‐P and residual P seemed to act as a buffer for the strip‐P and the NaHCO3–Pi in the slightly weathered soils, whereas NaOH‐Pi seemed to act as a buffering pool for strip‐P and NaHCO3–Pi in the highly weathered soils. Residual P in the slightly weathered soils was plant‐available on a relatively short time scale. In contrast, residual P in the highly weathered soils accumulated in the presence of intensive plant P removal, indicating that it was unavailable to plants. Organic P (NaHCO3‐ and NaOH‐Po) fractions were not significant contributors to available P in these soils that received high levels of inorganic P. Phosphorus fractions separated by the same sequential method were not of equal availability to plants in all soils.

Bone char effects on soil: sequential fractionations and XANES spectroscopy

“…The results indicated that the largest P proportion was found in the NaOH fraction, reflecting P fixed to Fe and Al oxides, followed by the NaHCO 3 -P fraction assigned to P weakly absorbed on crystalline Fe and Al oxides or surface of minerals. Guo et al (2000) reported that the NaOH-P fraction may support the labile NaHCO 3 -P fraction as a buffering P SOIL, 4, 23-35, 2018 www.soil-journal.net/4/23/2018/ M. Morshedizad et al: Bone char effects on soil: sequential fractionations and XANES spectroscopy 31 pool in highly weathered and acidic soils. According to soil pH values (4.7 and 5.2), the larger proportions of NaHCO 3 -P even than HCl-P can be explained by the abundance and surface loadings of Fe and Al oxides that support the electrostatic binding of phosphate ions and a scarcity of Ca and Mg minerals or soluble ions.…”

Consequences of Ontario P index recommendations for reduced manure and fertilizer phosphorus applications on corn yields and soil phosphorus

“…The relatively less labile NaOH-Pi fraction was not significantly influenced by P source, rate, or P source × rate interaction (Table 3). This likely reflects the low loading rate of applied P and (or) the relatively minor potential contribution of this pool to plant P nutrition in a moderately weathered soil with adequate labile Pi content to meet crop requirements (Guo et al 2000).…”

Fósforo orgânico em solos sob florestas montanas, pastagens e eucalipto no Norte Fluminense

“…Em solos mais intemperizados, a solubilidade de formas secundárias de Pi é muito pequena, e, por essa razão, a disponibilidade de P seria regulada pela mineralização de Po. Guo et al (2000) mostraram aumento na proporção de Po lábil com o avanço do intemperismo do solo. Assim, mais de 80 % da variabilidade em P disponível pode ser explicada pela variação no Po lábil (Tiessen et al, 1984;Guerra et al, 1996).…”

“…Portanto, em condições tropicais úmidas, o P orgânico (Po) assume relevante importância na conservação do P disponível às plantas, por reduzir os efeitos do processo de adsorção de P inorgânico (Pi), pela fase mineral do solo. Nesse aspecto, o P biologicamente ativo, representado, em parte, pelo Po lábil, tem sua participação aumentada no compartimento de P total deste, conforme o grau de intemperismo do solo (Guo et al, 2000), passando de 5 % na ordem Entissol para 35 % na ordem Oxissol (Cross & Schlesinger, 1995).…”