Choose Your Fertilizer Wisely

The fertilizer you have been applying may be the potential source of problems for certain trees and shrubs in your home landscape. Certain acid-loving trees like the Pin oak, Red maple and River birch, often develop yellowing leaves while the leaf veins remain green. We call this interveinal chlorosis. These tree varieties require slightly acidic soil conditions so mineral elements like iron, zinc and manganese are not tied up in the soil and unavailable for root uptake. In many instances, soil pH, or the measurement of the soil’s acidity or alkalinity may indeed be the problem. A soil pH of 6.5 is ideal in most situations. If the soil tends to be alkaline, interveinal chlorosis may develop. The long-term solution is to acidify the soil by applying the appropriate amount of an acidifying agent like sulfur. Chelated products containing chelated iron, zinc or manganese are also helpful If the chelation method used is correct. For example, a bag that contains iron sulfate won’t release the iron in high pH soils. For the best results, look or request a product labeled as using the EDDHA Chelation process. Such a product will be formulated to work effectively in a high pH soil of 8 which is commonly seen here in Dubuque.

Soils high in phosphorus could also cause interveinal chlorosis by out competing iron, zinc, and manganese for uptake by the tree’s root system. High phosphate fertilizers are unnecessary in most of our urban landscape soils. Phosphorus does not leach readily from the soil like nitrogen, and in fact will build up to harmful levels. Phosphorus is rarely deficient in urban soils as compared to agricultural soils where annual crops tend to use more of it.

Let’s look at a few other problems resulting from too much phosphorus. It is a myth that high phosphate fertilizer stimulates root growth. They do not. If the soil was deficient in phosphorus, and phosphorus was added, it may appear that the phosphorus stimulated root growth. If the level of phosphorus was adequate, no increase in root growth would occur even though more phosphorus was added.

Many trees, shrubs and grasses form a partnership with good fungi in the soil called mycorrhizae. This is a very beneficial association for the plant. High phosphate soils are detrimental to mycorrhizal health and development that adversely affect tree health.

Over application of phosphorus that wash away with water runoff, pollute our waterways. Massive algae blooms are the result. As the blooms die off and decay, they deplete the water oxygen supply resulting in large fish kills.

For responsible fertilizer management, start by having a complete soil test done. The test results will allow a knowledgeable ISA Certified Arborist to create a better fertilization plan and/or diagnose fertilizer deficiencies or toxicities.