Jp. Schmidt et al., SOIL-PHOSPHORUS DYNAMICS DURING 17 YEARS OF CONTINUOUS CULTIVATION - FRACTIONATION ANALYSES, Soil Science Society of America journal, 60(4), 1996, pp. 1168-1172
Excluding fertilizer P, a finite quantity of soil P exists for plant u
ptake. To improve our understanding of sinks and sources of long-term
plant-available P, soil P fractions to a depth of 30 cm were determine
d in soils under a continuous cropping system prior to fertilization (
1975), after 10 yr of P fertilization (1985), and 6 yr after the last
P fertilization (1992). Soil types for two study sites were Norfolk lo
amy sand (fine-loamy, siliceous, thermic Typic Kandiudult) and Davidso
n clay loam (clayey, kaolinitic, thermic Rhodic Paleudult). Superphosp
hate was applied from 1975 to 1986 at rates of 0, 10, 20, and 40 kg P
ha(-1) yr(-1). Average removal of P, via corn (Zea mays L.) and soybea
n [Glycine mar (L.) Merr.] cropping, was between 10 and 20 kg P ha(-1)
yr(-1) from 1975 to 1992. Resin-extractable soil P increased in the N
orfolk soil with annual P applications that were in excess of crop rem
oval and decreased with annual P applications that were less than crop
P removal. Besin-extractable soil P decreased to below 3 mg P kg(-1)
in the Davidson soil regardless of P application or removal rate. Inor
ganic soil P extracted with NaHCO3 and NaOH increased with excess P ad
ditions and decreased with deficient P additions for both soils. Organ
ic soil P in the Norfolk soil extracted with NaOH represented a P sink
at the 40 kg P ha(-1) yr(-1) treatment in 1985, but subsequently mine
ralized by 1992. In both soils, inorganic P extracted in the resin, Na
HCO3, and NaOH fractions, and organic P in the NaOH fraction of the No
rfolk soil, represented the biologically dynamic P.