Agri Kultuur March/ Maart 2016 - Page 20

Uptake (meq L-1) Crop Substrate(max) (meq L-1) Na+ Cl- Na+ Cl- 1 1.2 10 15 Cucumber 1.2 2 8 10 Pepper 0.8 1 8 12 Lettuce 1.5 2 8 15 Chrysanthemum 0.3 0.9 6 8 Rose 0.3 1 6 8 Gerbera 0.4 1.5 10 10 Tomato Table 1: The uptake concentrations of Na+ and Cl- by some of the most common greenhouse crops as well as the maximum concentrations allowed to maintain optimal growth and crop yield. High Zn-levels can often be found in water gathered from galvanised roof surfaces or fed trough galvanised pipes. Manganese toxicity problems may develop on lettuce (open or loose tulip shaped heads) where seedlings are raised on sphagnum peat, due to high levels of Mn in this substrate. Both iron (Fe) and manganese (Mn) may also cause production problems by blocking irrigation drippers. The pH of the water to be used as feeding water for the nutrient solutions should also be indicated on the water analysis. Most soils are well-buffered which means that their pH values do not change easi- ly. However, in soil-less production systems this is not the case and special precautions are needed to control the pH since the pH will affect the availability of nutrients in solution. The recommended pH level for soilless systems is normally between 5.8 and 6.5. Nutrients are absorbed by plant roots as ions (NO3-, NH4+, H2PO4-, Ca2+) but at high pH values many of these ions, including the micronutrients Fe and Zn, can precipitate as insoluble s