Agri Kultuur March/ Maart 2016 - Page 19

Figure 3 and 4: A typical pump room where the feeding water will be monitored for pH and EC, adjusted and nutrients added before being delivered to the plants in the greenhouse. A full chemical analysis of the water is however always recommended since the EC alone will not give an indication of specific ions in the water. If the moderate to high EC reading of the feeding water is the result of high concentrations of essential nutrients such as calcium (Ca2+), magnesium (Mg2+) and sulphate (SO42-) the potential quality is essentially quite good. If however the dissolved ions are mostly sodium (Na+) and chloride (Cl-) it might not be suitable for use in a hydroponic system, especially not in a systems where the nutrient solution will be re-used. Most crop plants are salt excluders and take up only small quantities of Na+ and Cl-. High levels of Na+ can reduce the uptake of other cations, especially Ca2+ while high Cl- levels will inhibit the uptake of anions, such as nitrate (NO3-). Crops also differ in their tolerance to root-zone salinity and norm values have been developed for maximum Na+ and Cl- levels allowed in soilless systems. Some of these are indicated in Table 1. If there is no alternative to using high EC water or feeding water with a high quantity of Na+ and Cl-, the best management practise is to ensure that the leaching fraction remains above the crop demand to limit the build-up of salts in the rootzone. Micronutrients are usually present at very low concentrations and even Figure 5: Strawberries in hydroponics are sensitive to levels of boron(B) deemed adequate for other crops such as tomatoes. high concentrations will not be detected by looking at the EC reading alone. In some cases, low EC feeding water may contain micronutrients at phytotoxic levels. Micronutrient-sensitivity also differs between crops. For example, strawberries need relatively low B levels due to physiological problems with B at >0.32 mg L-1. Tomatoes can however tolerate B at levels up to 1.1 mg L-1, almost three times higher than its recommended rate. The recommend Zn concentration for substrate-grown tomatoes is 0.33 mg L-1 but at only twice this concentration, toxicity can be expected. Figure 6: Typical leaf burn symptoms on cucumber plants in a soilless systems associated with a high EC in the rootzone.