Micronutrients (manganese, iron, boron, zinc, copper, molybdenum, cobalt, and chlorine) are essential for plant growth, but they are needed in very small quantities. They are usually added only when a deficiency is noted. They are seldom truly deficient in soil, but are sometimes unavailable to plants, especially in alkaline soils. Iron and manganese deficiencies are seen in the West almost as commonly as nitrogen deficiency. Alkaline soils cause these elements to form insoluble compounds. Adding iron or manganese alone seldom helps, since the added iron is immediately bound into an insoluble form in the soil.
These nutrients can be added as chelates, a word derived from the Greek word for claws. Chelates are organic ring-structure compounds that combine with and protect a nutrient, such as iron, making it difficult for the nutrient to react chemically with soil particles so that it remains available to plants.
But chelates are expensive and must be added at regular intervals. A more permanent solution is to make the soil more acid, thus freeing the iron and manganese already present for plant use. This can be done by applying acidic materials, such as soil sulfur or iron sulfate.
A combination of these methods is most effective: spray the leaves and treat the soil with a liquid iron fertilizer to treat the immediate problem, and add sulfur to make the soil more acid to keep the problem from returning.
Soil acidity can be maintained by using fertilizers that have an acidifying effect on the soil. Ammonium sulfate is often used for this purpose. Also, several fertilizers are blended especially for plants that need acid soil. These fertilizers contain chelated iron as well as having an acid reaction on the soil.