I received an interesting phone call from a district conservationist a while back. The DC and staff during lunch were discussing "Why do producers continue to apply Phosphorus fertilizers even though soil tests are in the high and very high ranges?" The simple answer to this question is: I don't know. There is no economic benefit from yield response to applying fertilizer above the optimum ranges. A producer may apply fertilizer because of the uncertainty associated with testing for Phosphorus. Phosphorus is highly variable across a field. In visiting with Antonio Mallarino at Iowa State, he commented that because of variability and the recent high yields producers may choose to maintain the soil test P in the high range. There is nothing wrong with this because of the variability. As for maintaining in the very high range, there is no reason for this if a producer is purchasing phosphate fertilizers. However, if a producer is applying manures for the nitrogen, they are getting the P along with the manure. In the case of applying manures the P-Index needs to run to assess the risk of P movement from the field.
What does a soil test tell us?
The test is an estimate of plant available P — not the total P in the soil.
The test results serve more as an indicator of P available than an actual quantity of P in the soil.
The test measures P availability in both the solution and active pools.
Chemical analysis of the soil sample is calibrated to the response of crop yield from P fertilization. So as soil test P increases, the response in yield from fertilization decreases.
So a test result in the very low 0-8 ppm range would have an excellent response to P fertilization.
Optimum 16-20 ppm would have a neutral response to fertilization.
Very high 31+ ppm would have no response in yield from fertilizing with P.
To simply explain ---- P in soil is contained in three separate pools or portions.
Solution P or Available P pool is where plants can absorb P. This is always the smallest of the P pools.
Labile P or Active P pool is the weakly retained P by the soil. (P adsorbed to mineral surfaces or soluble inorganic or organic compounds.) The portion of P that when solution P decreases labile will fairly rapidly replace the solution P.
Stable P, Nonlabile or Fixed P pool is the largest portion to total P in the soil. The weathering process is very slow in converting this to available P for plant growth.
All of this is why it does not take very long to build up soil test P when rates are applied above crop removal. However to bring soil test P down to say the optimum range from an extremely high test of over 100 ppm may take decades to accomplish.
Phosphorus is taken up by plants via three mechanisms:
Diffusion is the movement of molecules through the soil. The size of soil particles and moisture level determine how long it takes nutrients to reach the roots. Higher water content and smaller soil particle sizes provide a more direct path to the root surface. The distance P travels by diffusion in soils is extremely small. Therefore, an active and large root system is important. Phosphorus is supplied to roots primarily by diffusion and root interception.
Mass flow/bulk flow is the movement of nutrients to root surfaces through soil water movement. Mass flow to roots is driven by plant transpiration; however, mass flow is not a major pathway of P movement to plants.
Root interception is the growth of root structures into new soil that contacts plant-available P. Root growth is important because it provides additional root surface area for P uptake.