Healthy plants need a variety of minerals for good growth, the most important being nitrogen, phosphorus and potassium. What might appear to be symptoms of fertilizer deficiency might be due to disease, insect damage to roots, poor soil drainage, cold soil, or excess fertilizer. The best way to know if your soil is lacking minerals is to have its pH tested. Many state universities have agricultural extension services that can tell you how to get that done
Older, lower leaves turn yellow from the tips going toward the stem. The plant turns yellowish-green from the ground up, becoming stunted and spindly with small stems and leaves. In time, it will die. Nitrogen deficiency is often found in plants growing in heavy clay soils, in heavily-watered sandy soils, and in soils rich in poorly-composed organic matter.
Phosphorus deficiency can cause nitrates to accumulate in the plant, giving it a dull, dark-green look. The bottoms of older leaves sometimes turn purple. The roots grow slowly, causing delayed maturity. In annuals, a lack of phosphorous can cause poor flowering. Soils heavy in iron can cause phosphorus "fixation," delaying the spread of phosphorous to plant tissue.
After nitrogen, potassium is the second nutrient mineral found in plant tissue, although it can be higher in some plants. The symptoms start with leaf scorch, small spots on the margins of the leaves. This begins on the lower, older leaves. The spots get larger, eventually coming together. High levels of calcium in the soil can cause potassium deficiency.
Newer leaves turn yellow. The veins remain green at first, while the areas between the veins turn light green, then a greenish yellow. The veins then lose their color, and the leaf turns yellow or even white. In time, the leaves can turn brown and die. Weed killers can produce symptoms that look like a deficiency of iron. Iron deficiency can be caused by oil that is cold and wet or that contains too much manganese or where the pH is too high.
Older leaves turn yellow between the veins, starting at the bottom leaves and working its way upward. Plants need magnesium, which is generally obtained in soil. It has to be added to the solutions of plants grown hydroponically.
Calcium deficiency affects those tissues of the plant where growth occurs. The tips of the roots and the shoots die first. If calcium is not added, the stem will begin to die, followed by the entire plant. As with magnesium deficiency, the lack of calcium is rare in plants grown in soil.
A lack of boron looks like calcium deficiency; it affects the root tips and shoots. The difference is that a plant lacking boron will continue to send out shoots that are sometimes called "witches' broom." Deficiencies of boron are most often experienced by commercial growers.
The symptoms of manganese deficiency are similar to those produced by a lack of iron; the space between the veins of newer leaves turns yellow. The difference is that when manganese is deficient, the finest veins remain green. The fine green veins on a yellow background make the leaves look like lace.