The visual symptoms of tomato plant nutrient deficiencies often point to a number of potential causes. Yellowing leaves, for example, may suggest an iron deficiency, mid-stage nitrogen deficiency or early stage potassium deficiency. When you understand how these nutrient deficiencies progress, then you move closer to making an accurate diagnosis.
All plants, including tomatoes, need micronutrients and macronutrients. Macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorus and potash. Plants get their carbon, hydrogen and oxygen from air and water. The other macronutrients come from the soil or from fertilisers. Micronutrients include iron, manganese, boron, zinc and copper. Plants also need calcium, magnesium and sulphur in amounts between the other two. When plants can't get enough of one or more nutrients, they become deficient in them.
When tomatoes are phosphorus-deficient, they do not grow quickly and may be stunted. Eventually, the leaves turn bluish-green and wilt. In places where there is abundant natural phosphorus in the soil, you may be putting the plants out too early. Cold soil, where phosphorus is plentiful, is one of the causes. Try waiting until the soil warms up more. You can speed up the warming process by covering the tomato plants in black plastic. If phosphorus is lacking in your soil, then use an organic fertiliser that is high in phosphorus.
Tomato leaves turn from their normal deep green to a pale green in the early stages of nitrogen deficiency. Then, the leaves gradually turn yellow. As this happens, the veins in the leaves stay green for a while before also turning yellow. The undersides of the leaves on some plants might get red or purple. If you do not correct the situation, then the older leaves will show signs of stress quite quickly when the plant does not get enough water. Tomato plants recover quickly when you apply some nitrogen in a foliar feeding. Follow up by adding nitrogen to the soil.
Leaves turn yellow (chlorosis) and then brown, or scorched-looking. The edges curl and lose colour. You can't reverse the chlorosis by adding potassium. When the deficiency is high, you might only see it on new leaves. Soils with high clay content tend to bind up the potassium and keep it from the plant, especially when dry. The approach here is a defensive one. Make sure your soil is loose, has balanced nutrients and has a good mix of composted materials.
This deficiency affects the fruit of the tomato plant by causing blossom end rot. The base of the tomato forms a rotting spot while the fruit is still young. The plant's leaves turn brown and curled from the stem to their lower parts. In extreme cases, the new leaves die before unfolding, resulting in just a stump on the plant. Do a foliar feeding of calcium and concentrate on the new leaves to correct the soil deficiency.