Fertilizing Vegetables

1. Q. My tomatoes are always small and the plants won't grow very large. I think that maybe I should add more fertilizer - - even more than you recommend. Can I fertilize my plants too much?

A. Excesses of anything can cause problems. Too much water can kill trees as well as gardens - - most of us experienced the "too much" of water this spring. Too much fertilizer can also cause problems and plant death because FERTILIZER IS SALT. Why are salts toxic to many plants and most of our crops species? There are several reasons. First is that when salts are dissolved in the soil solution the plant cannot absorb and use the water it needs to survive. This is because the potential of the plant to pass water out of and into the root system must be lower than that of the soil's holding capacity instead of the other way around. Plants can wilt when given a heavy dose of fertilizer salts. In order for the plant to adjust to salinity it must absorb and accumulate salt inside or manufacture organic solutes (sugars, organic acids, amino acids, etc.), so that the concentrations are high enough and water again can be taken up into the roots. Both of those responses require energy and take time. That's one reason why plant growth actually slows when salts build up because of excess fertilization.

A second reason is that many salts are themselves toxic to the plant because they poison enzyme systems or block biochemical pathways. Sodium, chloride, boron, and bicarbonate are examples of specific ionic toxins which can accumulate under saline conditions.

Roots may also be injured by salts. Salt injury often makes plant roots susceptible to a wide range of soil diseases. Extreme injury may also interfere with water uptake and result in excessive wilting.

The net result of all of this is that the plant response to salt stress can be a combination of slow growth rate, and often apparent or incipient nutrient deficiency or toxicity brought on by biochemical interference. Plant injury resulting from too much salt may first be observed as yellowing of the foliage and, later, browning of leaf tips and margins.

As you can see, too much fertilizer may be fatal. The problems which you are experiencing could be the result of too much shade or variety selection.

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