Optimum Nutrient Concentration and Ion Uptake
The size of a root system as well as the ion concentration in the root zone affect ion uptake. A linear increase in ion uptake follows when the nutrient concentration is increased, but only within limits. As soon as the EC rises above a certain level, the rate of ion uptake decreases.
Although the uptake of all the ions decrease at excessively high EC levels, the uptake of the big, double charged Ca ion is suppressed the most. This explains the higher incidence of Ca-deficiencies such as blossom-end-rot and tip-burn when salts accumulate in root zones due to insufficient drainage or poor irrigation practices.
Even a saline-tolerant crop such as Amaranthus grows slower when the EC of the root zone solution is increased beyond its optimum need. Benoit compared a salt sensitive- to a salt tolerant rop and found that the yield potential of the sensitive crop declined at a rate of at least 10% for each 1 mS cm-1 rise beyond its optimum EC of 2 mS cm-1. The yield of the salt tolerant crop only declined at EC levels higher than 4 mS cm-1.
To improve the taste of ‘Daniela’, one of the first ‘long shelf life’ tomato cultivars grown in South Africa, the nutrient solution’s EC of 2 mS cm-1 was increased to 4 mS cm-1 by adding NaCl at 20 meq L-1 to the solution. The fruit’s total soluble solids (oBrix) increased significantly, but the associated higher sugar percentage was due to less water being taken up (more concentrated fruit juice) and not due to better sugar production.
This was reflected by an associated 30% reduction in yield and fruit size. The optimum nutrient solution EC is also affected by the prevailing growing conditions. Under high light intensities and low humidity levels, plants need more water. Extra care should then be taken not to use high EC nutrient solutions that may restrict the uptake of water.
Similarly, under conditions where water uptake and transpiration rates are limited due to low temperatures, low light intensities or high humidity levels, the uptake of water will be low, limiting nutrient uptake. To improve nutrient uptake, the nutrient solution concentration may then be increased.
As a rule of thumb: During sunny, hot and low humidity conditions (with poor climate control structures) the EC of the solution should be reduced by lowering the nutrient solution concentration by 15 to 25%.
By: Dr NJJ Combrink, SU Horticulture from his book: ‘Nutrient solutions and Greenhouse management’ only available from the Combrink family trust: E-mail: njjc@sun.ac.za