Fertility Advice From The Greenhouse Tomato Short Course

Joe Kemple

The 22nd annual Greenhouse Tomato Short Course has now come and gone. Participation was excellent this year, with growers from coast to coast in the U.S., as well as all the way from Brazil, the Bahamas, Trinidad,  Tobago, and Ghana!  

If you have never come to Mississippi for this national conference designed for greenhouse tomato growers, check the web site for information:

While there was too much presented during two days of seminars for an article like this, I’d like to focus on some nutrition and fertility information presented by Dr. Joe Kemble of Auburn University.

Mobile Elements

First of all, an understanding of how mobile elements are is important to trying to figure out which nutrient deficiencies your tomato plants may have.

Mobile elements — those that move easily within the plant from older growth to newer growth — include nitrogen, potassium, and magnesium.

Why is this important? If there is not enough of one of these elements in
the fertilizer solution, the older leaves will show deficiency symptoms first. Older leave are those at the bottoms of plants, as well as those half way or so up the plant.

On the other hand, iron is considered to be an immobile element. It will not move very much from older to newer leaves. Symptoms of not enough iron will show up first in the newer leaves, i.e. at the tops of the plants.

Calcium is the problem child. It is mobile — sort of. If the youngest parts of the root system are active and healthy, they will take up calcium and it will travel throughout the plant. If, however, there are any stressors on the plant that slow down these young roots from growth, calcium uptake is limited and deficiency symptoms will be evident. Calcium is taken in mostly through the very young roots and not much by older roots.
Look For Symptoms
With nitrogen, a general light green or greenish yellow coloration indicates deficiency. This will start with the older leaves, but can progress upward to include the whole plant if it continues unchecked. Deficiency also produces “leggy” plants, with thinner stems and longer internodes, and some purpling may show up on the undersides of leaves.

Low potassium will show up as necrotic (dead, brown) spots on the older leaves. The spots are interveinal — between the veins; veins remain green.

These will progress inward toward the main stem, and also upward toward new growth. Potassium symptoms may also show up as yellowing margins (edges) of the leaves. Fruit symptoms include whitish, unripe areas inside the fruit, and softening of fruit tissue.

With lack of magnesium, a pronounced interveinal chlorosis (yellowing) of older leaves is very evident. While veins remain green, the yellowed tissue between veins becomes very prominent. Sometimes the margins remain green. Often, this occurs about half way up the plant shortly after the fourth cluster has set fruit. This is probably the most common deficiency
in tomatoes grown in the greenhouse.

Since iron is immobile, a limitation will cause it to remain in the old growth and cause symptoms at the tops of plants. This occurs as interveinal chlorosis in the young leaves. Unique with iron is that the yellowing starts at the bottom of leaflets, and then progresses upward until the whole leaflets will be yellow or even whitish in severe cases. Veins normally remain green.

This occurs when iron in the fertilizer is too low, but can also happen if the growing medium is kept too wet or even waterlogged.

Prevention, Diagnosing Problems

A tissue analysis is probably the best single thing a grower can do to detect nutritional problems even before there are symptoms, or to diagnose a problem after it occurs. While there is a fee for testing, a biweekly schedule is advisable and not too expensive to keep plants healthy. Either a testing lab at your state university or a commercial analytical lab can be used. Test results will help growers decide how to make changes in the fertilizer program. 

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