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What Do Organic Growers Do About Nutrient Deficiencies?
A number of diseases are caused by plants being deficient in certain necessary nutrients or minerals. Magnesium deficiency in tomatoes and chocolate spot on broad beans caused by lack of potassium are common examples. Such problems should not be so frequent for organic gardeners using natural means of fertility. However, the nutrient balance in compost heaps can be variable depending on the materials available to the gardener.
Symptoms
It's easy to confuse the symptoms of deficiencies with other plant diseases, so consider other possibilities first. Discolouration of the leaves of plants, a common indication of deficiencies, can also be caused by pests and other diseases.
Having the correct nutrients and minerals in the soil is not the entire answer to preventing deficiencies. The acidity or alkalinity of a soil can influence their availability. Calcium deficiency frequently occurs when the pH is too low, and the trace elements iron and boron become less soluble when the pH is high.
Potash
Potassium deficiency can be a problem with tomatoes. The edges of the leaves develop a light brown colour and the fruit ripens unevenly with green and yellow areas mixed in with the red. It's also common with gooseberries and beans (Chocolate Spot) where the leaves develop the same scorched look.
Comfrey liquid is the answer to potash deficiency. You need to leave a pound of leaves in three gallons of water for about a month so it's best to have some ready to use as a foliar feed in an emergency. Alternatively you can mulch your plants with comfrey at planting time. With a mulch of comfrey leaves or alternatively a sprinkling of wood ash around the base of your plants, you might avoid the problem altogether.
Magnesium
Magnesium deficiency is probably the most common deficiency, particularly with tomatoes. (Tomatoes are great for learning about diseases !) The lower, older leaves turn yellow while the veins remain green. If the plants are at the fruiting stage 'green back' occurs. However these symptoms can also be caused by overwatering or by the practise of flooding the tomato beds to reduce watering time. Lazy gardeners will have some experience of this!
The traditional remedy is to apply Epsom salts (magnesium sulphate) as a spray, at the rate of half an ounce to a gallon of water. If the problem is a frequent one then as well as ensuring you are applying enough organic matter, it would be worthwhile to apply dolomite to the soil.
Phosphorus
Phosphorus deficiency is mainly a problem on acid soils. It affects roots such as carrots, stunting the leaves and causing the foliage to turn a shade of purple. The same symptoms occur on lettuces and cabbages. Again a foliar feed, such as seaweed liquid, can solve the problem. However if your soil is particularly deficient in phosphorus then as a long term solution apply bonemeal or poultry manure to the soil.
Nitrogen
Insufficient nitrogen can be a problem in gardens where artificial fertilisers are not used. Nettles are high in nitrogen, so soak a bucketful for a week in water and apply to your plants.
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