Phosphorus and potassium are essential elements needed through the entire season. As crops transition from vegetative growth to reproductive growth, root growth slows and new regions of the root zone are not explored for nutrients like P and K. Nutrient supply becomes even more dependent upon diffusion of nutrients to the roots. Crop vigor may slow without nutrient supplementation.
For the above reasons and more, fertigation with potassium and phosphorus is common. However, fertigation has limitations such as how quickly a root can take up nutrients after a threshold level is met. Increasing nutrient supply in the root zone does not result in faster or greater uptake. The plant adjusts its physiological processes when it cannot meet all of its simultaneous metabolic demands. This adjustment may result in crop responses such as deceleration of fruit sizing, fruit drop, slower sugar accumulation, and delayed coloring. Some research has shown a crop may require foliar nutrition at key timings to reach the yield or quality levels required by current economic conditions.
Foliar applications of phosphate and potassium compounds can be significant in keeping leaf photosynthetic apparatus functioning for 7-10 additional days. These applications provide energy for growth, sugar production and carbon supply for metabolic functions. For example, foliar potassium delayed leaf senescence by 7-10 days in cotton which allowed for an added 10-15% yield to be produced. Significant responses may still be observed from foliar P and K applications on crops where tissue test values are above the deficient level, but below the optimum leaf levels for phosphorus and potassium. Formulations are available that allow for significant quantities of P and K to be safely applied to foliage without visible, or even unseen damage to the plant. Having the proper formulation and surfactant choice enables effective applications after leaves have developed thick waxy cuticles.