Primary nutrients of plants include nitrogen, phosphorus and potassium; they are the ones indicated on each fertilizer package. For example, 5-10-15 printed on the label signifies that the fertilizer contains 5% nitrogen, 10% phosphorus and 15% potassium.
Many soils are deficient in phosphorus, especially those red Hawaiian soils high in iron and aluminum oxides. When growing vegetables, or any annual crop, phosphorus can be incorporated into the soil before the crop is planted. It can also be incorporated before trees and shrubs are planted. The difficulty comes when attempting to apply a phosphate fertilizer to an established tree or shrub with existing roots already deep in the soil.
In contrast to nitrogen fertilizers which are water soluble and move well in the soil, phosphate fertilizers do not move well. In fact, years after a surface application of a phosphate fertilizer, the majority of the phosphorus will still remain near the soil surface. How then can a phosphate fertilizer be applied and penetrate down to the roots of mature trees? Here are some better ways of applying a phosphate fertilizer to existing plants.
1. Remove a core of soil at the drip line of the tree, at the four corners, 1 – 2 feet deep. Cores can also be removed going around the drip line of the tree in a circular manner. Apply a measured amount of phosphate fertilizer to these holes, either dry and mixed with soil, or in a water solution. Ammonium polyphosphate is especially good. Admittedly, this involves a good deal of labor when treating many trees.
2. Similarly, fertilizer spikes, although expensive, can be placed in the ground around the drip line. In both cases, roots will gradually gravitate to the higher nutrient content and flourish.
3. Foliar applications of a phosphate fertilizers do not work well. Frequent applications of 3-4 times per year, however, can potentially meet the trees requirement.
4. Perhaps the best alternative in improving phosphate availability and movement into the soil is to apply it in its organic form; for example, manures, fish meal and blood meal. Likewise, adding organic mulch is beneficial. As the organic material breaks down, the end product is humus. Humus is loaded with negatively charges sites that are able to hold onto positively charged plant nutrients such as potassium, calcium, magnesium, manganese and iron. Keeping the tree under a bed of mulch will supply a small but constant supply of various essential nutrients. Roots will eventually grow into the mulched area and proliferate, picking up essential nutrients. In addition, because of the abundance of new surface roots in this area, it now becomes an ideal place to apply a phosphate fertilizer.