1 - Holes filled with compost, sulfur, and an iron source placed just outside the dripline of trees can help overcome iron chlorosis (deficiency) symptoms in the trees.

Yard and Garden December 6, 2014

Q.

I live about a mile north of Tome Hill off south El Cerro Loop on an old farm field. The caliche level is 12-18" deep. I have numerous trees that are chlorotic. I plan to use water soluble sulfur pellets in 2" holes drilled around the drip line of the trees. In addition I will spread the pellets around the surface. Is this a good idea? Will the sulfur in the holes be too concentrated? How far apart should I drill the holes? How deeply? How much sulfur in each hole?

A.

Sulfur can indeed help acidify the soil to release iron that has been chemically bound to alkaline soil. A common recommendation is to auger several holes around the tree outside the dripline of the tree. These holes should be about 2 inches in diameter (however, diameter is not critical) and about 1 foot deep. The hole can be made with a post hole digger, hand auger, power auger, or water from the end of a water hose eroding a hole as the hose is pressed into the ground. Moist soil will excavate much more easily than dry soil. These holes should be placed about every 2 to 3 feet apart in a circle around the tree. This will create a zone of acidified soil with iron available to the tree roots. You will need to do this again in a few years when the symptoms of chlorosis redevelop. At that time move out a little farther from the tree and place the holes in locations between the previous holes.

We often recommend refilling the hole with a mixture of compost, iron sulfate or iron chelate, and sulfur. This mixture provides several benefits, and is less likely to damage roots than if the holes were filled with just sulfur pellets. In either case this is a temporary measure because the high level of calcium and other minerals in a caliche layer will ultimately overcome the effects of the sulfur and other acidifying compounds. Compost is slightly acidifying, but helps provide bulk for refilling the holes for safety, for holding moisture, and slowly releasing nutrients. Compost also creates an environment that encourages proliferation of roots in the area of the hole. The iron sulfate or iron chelate material provides iron to overcome the iron chlorosis you have described in your trees. Iron sulfate will acidify the soil, but iron chelate will provide a longer lasting supply of iron to the roots of the trees. An iron chelate compound (a substance that chemically binds the iron and slowly releases it to the soil) using the chelating agent EDDHA have proven to be the best chelating compounds for our alkaline soils. Addition of sulfur further acidifies the soil around the hole, prolonging the availability of the iron.

These holes filled with the mixture or just sulfur pellets will allow water to more easily reach the roots of trees. As with most of the above information the proportions of these materials, like the depth and diameter of the holes, is not critical. However, a rough guide to proportions is about 6 parts compost, 2 parts iron sulfate/chelate, and 2 parts sulfur by volume. So, 60% compost, 20% iron compound, and 20% sulfur.

The soil around the holes will more readily provide iron to overcome iron chlorosis symptoms (especially if an iron supplying material is added). This zone of moist, less alkaline soil will foster growth of numerous, small absorptive roots that will collect the nutrients needed for tree health. In areas that are flood irrigated the holes may not be necessary, the sulfur (and iron compounds) can be surface applied and leached into the soil with irrigation. In clay soils that are difficult to moisten or when drip or sprinkler irrigation is used, the holes with acidifying iron mixture will be more effective.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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