While the soil system limits the amount of phosphorus in the soil solution at any one time and limits its re-supply, the plant root also has its problems. The concentration of roots in the soil volume is relatively small. It has been calculated that roots contact only about one percent of the soil volume. Phosphorus enters the root primarily by diffusion (90-98 percent), which can occur only if the phosphorus is very close to the root. Very little phosphorus enters the root by mass flow in the water (one percent). Root growth is essential for adequate phosphorus uptake or the soil solution needs to be replenished frequently. Actually since roots contact such a small amount of the soil, the soil solution in the areas of root contact must be replenished more often than twice a day or phosphorus deficiencies will occur. This makes the labile forms (those weakly bound to compounds or clay) very important in soil phosphorus supply.
Research in recent years has developed valuable models which predict phosphorus plant uptake and the factors that influence it. One of the most commonly known models has been developed by Dr. Barber at Purdue University. His model indicates phosphorus uptake is largely a function of size and nature of the root system, rate of water absorption, amount of phosphorus in the soil, and ability of the soil to supply phosphorus to the soil solution. His model includes seven plant factors and three soil factors. While phosphorus models are helpful from a research view for understanding phosphorus uptake by plants, relatively little on-farm applied use has been made of models.