The vertical distribution of total and dilute acid-soluble phosphorus in twelve Iowa soil profiles

Twelve soil profiles representing 11 soil series which occur in Iowa were selected for a study of the vertical distribution of total and dilute acid-soluble phosphorus. Seven of the soils studied belong to the Prairie, three to the Planosol, and two to the Gray-Brown Podsolic groups. The profiles were sampled in layers that were horizons or subdivisions of horizons not exceeding 3 inches, except the Grundy silt loam which was sampled in part at 4-inch intervals. Two of the Prairie soils were developed from glacial till and one from alluvial material. The other nine soils were formed from loess. In all the profiles studied total phosphorus was found to decrease with depth to a minimum between the lower A and upper C horizons. Below this zone, in 11 of the 12 soils, the amounts increased rapidly with depth to the bottom of the profile. In eight of the soils the concentration of phosphorus in the C horizon was more than double that in the lower A to upper B horizons. The soils developed from till contained much smaller quantities of total phosphorus throughout the profile than those formed from loess. The former contained an average of about 300 p.p.m. of phosphorus as compared to 500 to 700 p.p.m. in the latter. In general, the distribution of total phosphorus in the Planosol soils resembled that in the Prairie soils. The distribution in the Gray-Brown Podsolic soils differed considerably from that in the other groups, particularly in that the zone of minimum phosphorus content occurred nearer the surface. Dilute acid-soluble phosphorus also decreased with depth to a minimum in the lower A or upper B horizon, and then increased markedly in subsequent layers to a maximum in the C horizon. The content and distribution of dilute acid-soluble phosphorus in the Planosol soils is quite similar to that in the normal Prairie soils developed from loess. In the Gray-Brown Podsolic soils the increase with depth begins at a point nearer the surface and is more gradual than in the Prairie soils formed from loess. Much larger quantities of easily soluble phosphorus were found in the C horizons of the Prairie soils developed from loess than in those formed from glacial till; the maximum amounts present in the former were generally higher than 200 p.p.m. as compared with less than 60 p.p.m. in the latter. No consistent relation was found between pH and the dilute acid-soluble phosphorus content of the soils. In seven of the 12 soils studied more than 25% of the total phosphorus present in the lower layers was soluble in dilute acid, and in one soil 55% was soluble, as compared with 0.94 to 3.63% soluble in the surface layers.