Comparison of a bioassay and three chemical methods for determination of plant-available P in cultivated soils of Finland

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 (PAAA