ORGANIC AND INORGANIC P CONTENT, MOVEMENT AND MINERALIZATION OF P IN SOIL BENEATH A FEEDLOT

Abstract

Greater amounts of 0.5 M NaHCO3 and water-extractable P were found in soil beneath a cattle feedlot located on an alkaline sandy soil than in soil in an adjacent non-manured field. The 0.5 M NaHCO3-extractable P contents of the feedlot soil samples were greater than for the adjacent field to a depth of 120–150 cm, suggesting that P from the manure had moved to this depth. Water extracted very little P from all field samples and the feedlot samples obtained below 120 cm. Concentration of total P in the feedlot soil was usually greater than in the corresponding field soil. The field soil contained more organic P than the feedlot soil at depths of 0–90 cm. Organic P concentrations at the 0 to 15-cm depths were 268 and 56 ppm for the field and feedlot sites, respectively. The organic C:N:P ratios for the 0 to 15-cm feedlot and field samples were 214:18:1 and 132:8.7:1, respectively. Mineralization of organic P in laboratory experiments was greater in flooded soils than in soils maintained at field capacity. Rates of mineralization were greater for manured than for non-manured samples. Organic and inorganic P moved at about equal rates in soil treated with manure extract. Rates of movement of both decreased with increasing sample depth in the feedlot soil. The feedlot soil below 30 cm and the field soils exhibited a high potential for inorganic and organic P fixation. Organic and inorganic P applied as manure extract moved faster than an equivalent concentration of P as KH2PO4.