Abstract
Phosphorus was extracted from 32 field soil samples with water (Pw), with 0.5 M NH4-acetate-0.5 M acetic acid at pH 4.65 (PAAAc) and by a method in which freshly precipitated iron hydroxide is used as the sink for P desorbing from the soil (Pi method). Pi is supposed to be the quantity of reversibly adsorbed P in the soil. The results of the three chemical methods were compared with the ones obtained in a bioassay in which four yields of ryegrass were grown in 0.2 dm3 of soil. The grass took up P effectively and the P reserves of the soils were probably the growth limiting factor. The quantities of P taken up corresponded to 5—21 % (median 10 %) of soil inorganic P. The uptake of P by the grass was approximately twice the quantity of Pi (median 25.9 mg/dm3) and several times higher than the quantities of Pw (median 6.3 mg/dm3) and PAAAc (median 6.4 mg/dm3). The results of all three chemical methods predicted P uptake by ryegrass accurately, the correlation coefficients (r) ranging from 0.88*** to 0.93***. However, in soils low in P (PAAAc
Highlights
Assessment of reserves of plant-available P by soil analysis is important for crop production as well as from the environmental point of view
Low P concentrations in the grass as well as the positive correlation between the yield and the indices of soil P status suggest that P supply to the plants has been the growth-limiting factor in the pot experiment
Owing to the apparent P deficiency experienced by the grass, it can be assumed that the reserves of plant-available P were practically used up in the course of the pot experiment
Summary
Assessment of reserves of plant-available P by soil analysis is important for crop production as well as from the environmental point of view. A new method (Pj), which is claimed to give the quantity of reversibly adsorbed P, has recently been introduced (Zee et al 1987, Menon et al 1989) In this method, the P desorbing from the soil is trapped by strips of filter paper which are impregnated with freshly precipitated iron hydroxide. Owing to the high affinity of phosphate for iron hydroxide, the concentration of P in the liquid phase during the extraction is extremely low, which promotes desorption of P from the soil. When this method was tested with soil samples from Finland (Yli-Halla 1989), the mean (31.2 mg/dm 5 ) of Pj was three times higher than the mean of water-extractable P. Some remarks on the relationship between P uptake and the fractions of inorganic P in the soil were made
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