K:N ratio
Nitrogen is the element which, more than any other, controls the growth rate of the plant. Up to a certain point, the more N supplied, the more rapidly the plant grows. The ratio between potassium and nitrogen is also important--the higher the ratio, the slower the growth. The optimal ratio of potassium to nitrogen varies with growth stage (Adams, 1999). When the first truss is in flower, the K:N ratio should be 1.2:1, the same K:N requirement as in most plants during the vegetative stage. However, this ratio increases as the fruit load on the plant increases, since about 70% of the potassium absorbed moves into the fruit. By the time the ninth cluster flowers open, the ratio should be 2.5:1. Low potassium during times of high fruit load reduces tomato quality, and especially flavor. Other nutrients are generally supplied in constant ratio to each other.
Interactions and nitrogen forms
The form of nitrogen
provided is important (Adams, 1999).
Too much ammonium-nitrogen (NH4-H) will reduce the calcium
content of the crops substantially, may reduce growth, and can result
in BER. Ammonium-nitrogen is particularly likely to harm the plant early
in the season when conversion of NH4-N to NO3-N
is slow. Later, up to 10% of the nitrogen requirement can be supplied
in the ammonium form, but more than 20% results in BER (Adams,
1999).
Excessive fertilization can also create an imbalance. For example, high potassium levels will reduce calcium and magnesium uptake. In general, nitrogen and phosphorus have antagonistic effects and induce or accentuate potassium deficiency. Other conditions that reduce calcium uptake include the presence of high concentration of Na and Mg. Calcium (and to a smaller extent magnesium) antagonize potassium uptake. Ammonium greatly decreases the rate of potassium uptake. Potassium deficiency tends to induce or accentuate iron deficiency.