contribution of inorganic phosphorus fractions to plant nutrition in alkaline-calcareous soils

to evaluate the relationships between soil inorganic phosphorus p (pi) fractions, the soil p test and plant parameters such as plant p uptake, dry matter yield, tissue p concentra-tion and relative yield, glasshouse experiments and chemical analyses were conducted on 13 calcareous soils from six agricultural and seven adjacent bushland (virgin soil) sites. four rates of p (0, 15, 30, 60 mg/kg soil) were applied as reagent-grade kh2po4 to the soils in a randomised complete block design with three replications. perennial ryegrass (lolium perenne cv. roper) was grown and forage was harvested five times over a period of 210 days. successive harvesting resulted in the depletion of plant available p as meas-ured by nahco3-extractable p, which coincided with the decrease in the plant dry matter yield and p uptake. after five harvests, the order of reduction in pi fractions induced by cropping without added p was ca10-p>al-p>ca2-p>ca8-p>occluded-p>fe-p for the vir-gin soils and ca2-p>al-p>ca10-p>ca8-p>fe-p>occluded-p for the agricultural soils. the order of abundance of pi fractions for p treated-soils was non-occluded al and fe phos-phate (al-p+fe-p)>secondary ca-bound p (ca2-p+ca8-p)>acid-extractable p (ca10-p)>occluded-p for both virgin and agricultural soils. although a marked proportion of added p was transformed into less soluble al and fe phosphates, successive harvesting had depleted considerable percentages of these fractions. highly significant (p