How to get more phosphorus into soil

Primary nutrients of plants include nitrogen, phosphorus and potassium — they are the ones indicated on each fertilizer package. For example, 5-10-15 on the label means this fertilizer contains 5 percent nitrogen, 10 percent phosphorus and 15 percent potassium.

Many soils are deficient in phosphorus, especially those red Hawaii 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 be near the soil surface.

So, how 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 is a lot of labor when dealing with many trees.

2. Similarly, fertilizer spikes, although expensive, can be placed into the ground around the drip-line. In both cases, roots will gradually gravitate to the higher nutrient content and flourish in the area of the spike.

3. Foliar applications of phosphate fertilizers do not work well. However, frequent applications of 3-4 times per year can potentially meet the trees’ requirement.

4. Perhaps the best alternative for improving phosphate availability and movement into the soil is to apply it in its organic form such as manures, fish meal and blood meal. Likewise, adding organic mulch is beneficial. As organic material breaks down, its end product is humus. Humus retains 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 is an ideal place to apply a phosphate fertilizer.

I planted a clove and mangosteen tree in my yard five years ago. Can you tell me when I can expect fruit on these trees?

A clove tree should begin flowering in its fourth or fifth year and sometimes as late as the eight year. In regards to the mangosteen, under ideal conditions they will bear in five to seven years.

However, during its early years (the juvenile phase) the mangosteen must attain a certain shoot to root growth ratio. If this growth is not attained, then trees may not bear until the 10th to 15th year.

Can you suggest a good resource for learning about plant diseases?

The first step in evaluating a declining plant is to know the various diseases that are common to that plant species. Consequently, I recommend exploring the University of Hawaii College of Tropical and Human Resources publication by S. Nelson and B. Bushe, “Hawaii: Top Crops, Top Diseases.” You can view it here:

By searching out the plant you are interested in and reading a description of the various diseases, you might be able to narrow down what ails your plant.

Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at


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