High bicarbonate-soluble phosphorus (P) in some Utah subsoils may affect the validity of fertilizer recommendations based on plow layer samples. The primary objective of this experiment was to determine the proportion of plant P derived from subsoils at typical Utah soil P levels and soil temperatures. A calcareous soil very low in bicarbonate-soluble P was treated with various amounts of P and packed into deep pots to provide 24 centimeters of "topsoil" and 27 centimeters of "subsoil". Water baths in a growth chamber were used to control soil temperatures at 14 centigrade and 20 centigrade. Light, air temperature and humidity were varied. The treated soils were analyzed for NaHCO3-P before and after cropping, and changes were related to plant P uptake. At a topsoil NaHCO3-P level of 17 parts per million, subsoil apparently contributed no more than ten percent of the plant P in the harvested tops unless subsoil P was greater than five parts per million. At lower levels of topsoil P, subsoil P became more important. When topsoil P was five parts per million or less, 20 to 67 percent of the plant P came from the subsoils.