During cool and wet conditions when plant growth is slow and the root system is shallow, band placement of P fertilizers becomes extremely important. Consequently, water solubility is of little importance where P fertilizers are broadcast. Phosphorus availability in the band is generally improved by the addition of N to the P starter and by increasing the granule size. Large granules contact less soil per unit of P than small granules. Thus, initial P fixation is lower, and availability is improved.
Most potassium fertilizers are highly water soluble.
Potassium stays relatively close to the initial point of application. Leaching of K is not generally a problem, except on very sandy or gravelly soils, due to insufficient cation exchange bonds are much weaker. Because K is not subject to the same fixation reactions as P, water solubility is not considered important. Consequently, banding K is only important where soil tests for K are extremely low. Table 1 contains the chemical analysis of these and other fertilizer materials.