Proper Feeding For Tomatoes

Editor’s Note: Reiners is an associate professor in the Department of Horticulture at Cornell University.


It is no secret that to have a high-yielding, healthy tomato crop, proper plant nutrition plays a crucial role. The nutrients that seem to cause the most problems in tomatoes are nitrogen (N), potassium (K), magnesium (Mg), calcium (CA), phosphorus (P) and to a lesser extent, boron (B).

What follows are brief descriptions of these nutrient issues in tomatoes and a plan of action.

Phosphorus levels in many soils in the Northeast and Mid-Atlantic region are already high due to the over application for many years of fertilizer containing this nutrient. If the soil is low in phosphorus, though, growers will see a deficiency early in the season when soils are cool and the nutrient is not yet easily available from the huge soil reservoir.

Tomato plants suffering from a phosphorus deficiency will appear as dark green and on closer inspection the leaf undersides are often red or purple. Growers can easily get around this early deficiency on high phosphorus soils by using a starter solution high in phosphorus or planting into mulches that raise soil temperatures.

Nitrogen And Magnesium
With a nitrogen deficiency, the older leaves on the bottom of the plant start with a lighter green color which, if not corrected, will turn yellow. The color change is uniform across the entire leaf with both veins and interveinal areas showing the same color.

Contrast that to a magnesium deficiency, which also affects older leaves. This deficiency starts out as a blotchy yellow color between the veins, with veins remaining dark green. At that stage, the blotchy appearance may look like a virus symptom.

With a virus, only a few plants may show symptoms while the entire field will show the magnesium deficiency. Eventually, the yellow appearance can turn brown and necrotic and look more like a fungal disease. Almost every field or tunnel of tomatoes will show this deficiency symptom later in the season unless magnesium is applied with the other fertilizers.

Potassium can be difficult, as a severe deficiency will appear as a browning of the leaf edges, which may appear as if the plants have been burned. Of more concern, however, is that potassium plays a role in overall fruit quality. As a result, a potassium deficiency is of great consequence to growers. Low soil levels can lead to blotchy ripening, internal yellowing, and other color disorders, along with tasteless fruit.

The ratio of potassium to other nutrients also plays a role. It has been shown that a high level of magnesium and a low level of potassium can lead to fruit quality issues. This ratio, called the Hartz Ratio, is a good predictor of color disorders. Tomato growers may want to view the Hartz ratio Calculator ( developed at Ohio State and enter their soil test results.

Calcium And Blossom End Rot
Of course, calcium deficiency gives us the classic case of blossom-end rot (BER), with the bottom of the fruit turning dark brown. BER is always a dry rot, which makes it easy to tell the difference from fruit rots caused by disease, which tend to result in soft rots.

In general, it is thought that low levels of calcium in the soil cause less than 5% of all BER. The vast majority is caused by dry soil conditions that lead to a decrease in calcium uptake. Calcium is taken up with water through the roots so if anything reduces water uptake, it reduces calcium uptake.

Since calcium moves with the water stream to areas of greatest transpiration, i.e. leaves, a grower can see that one of the last places calcium will be deposited is the lower half of a fruit.

Boron deficiency in tomatoes is getting a lot of attention recently as it, along with calcium, seems to play a role in reducing shoulder check. This condition results in surface roughness of the top part of the fruit that reduces marketability and storage life. Researchers at Michigan State found that foliar sprays of calcium and especially boron reduced this defect significantly.