Abstract:
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 concentration
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 measured
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 virgin
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 phosphate
(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<0.001) relationships
were found for P uptake vs. Olsen-P, P uptake vs. Pi fractions (Ca2-P, Ca10-P,
Al-P, Ca8-P, Fe-P) and Olsen-P vs. Pi fractions. NaHCO3-extractable P seems to be adequate
for evaluating plant available P in calcareous soils. However, the closer relationship
for the Fe-P fraction vs plant P uptake than for Olsen-P versus plant P uptake indicates
that NaHCO3 may not provide the best estimate of plant available P for calcareous soils.
Using stepwise regression analysis, it was found that the Ca2-P fraction was most predictive
of P uptake (60%), total dry matter (68%), relative yield (74%) and Olsen-P (69%),
followed by the Fe-P fraction.
