[cantharis]

I thought I might post a link discussing this common ailment. All green plants produce sugars, which are used to make cellulose, lignin, and other materials, by a process called photosynthesis. Light acts on CO2 and water to produce sugar - a substance vital to this process is chlorophyll, it is chlorophyll which actually makes plants green. Each molecule of chlorophyll contains one atom of magnesium, so magnesium is a vital trace element for plant growth.
Many composts and ferts contain NO magnesium, and magnesium deficiency is a common problem; symptoms are lack of growth and yellowing of leaves (lack of green chlorophyll, OK?). I should state also that there are other causes of leaf yellowing - lack of light and nitrogen deficiency are two - but these are rarer as we routinely make sure we have enough light and give nitrogenous ferts. Pests and viruses can also have similar symptoms.
Fortunately Mg deficiency is easily remedied - about once or twice in the growing season give a little Epsom salts, magnesium sulphate MgSO4. About a teaspoonful in a gallon of water - a little goes a long way.
Epsom salts are so called as they occur naturally in the spring waters of Epsom, England.