One of the side benefits obtained from using bulky manures and composts is that they provide a great variety of plant foods, not just the three major chemicals nitrogen, phosphorus and potassium. It is true that the other chemicals are usually present in sufficient quantity but this is not always so. When they are deficient there can be some spectacular results. Shortage of iron may cause leaves to go bright yellow; lack of magnesium or manganese can have a similar effect or magnesium deficiency may produce brown patches between the veins of the leaves. These particular shortages are most likely to occur when the soil contains a lot of chalk or lime and are due to chemical reactions in the soil which do not remove the iron or magnesium but make them insoluble so that plants cannot use them. If sulphate of iron or sulphate of magnesium (Epsom salts) is applied there may be an immediate improvement but the treatment may have to be repeated over and over again since the fresh supplies also get locked up through chemical interaction in the soil.
To overcome this difficulty, special forms of iron manganese and magnesium have been prepared, known as scquestrols. They are of varying efficiency and are fairly expensive, so if such deficiencies are suspected it is best to get expert advice on the spot before doing anything about it. Regular use of good animal manure or vegetable compost will help to prevent these difficulties both by returning such elements to the soil and by using up surplus lime or chalk and so preventing the conditions in which these deficiencies are most likely to occur.