Nutrient and water use of tomato (Solanum Lycopersicum) in soilless production systems

[摘要] ENGLISH ABSTRACT: Soilless production of crops relies on the addition of high concentrations of nutrients with the irrigationwater. The drained nutrient solution should be re-used to reduce the risk of pollution and to increasethe water- and nutrient use efficiency of the system. Besides the risk of pathogen build-up, one of themain impediments of a wider application of this method is the frequent analysis required to maintainoptimum nutrient concentrations and ratios in the rootzone. Yield reductions may be caused by anunbalanced nutrient solution.Alternatively the addition level of nutrients can be calculated through the use of nutrient uptake modelsthat simulate the change in the re-circulated nutrient solution. To simulate crop water and nutrientdemand necessary for model based regulation it was necessary to quantify the key factors affectingnutrient uptake by plants.The nutrient solution concentration and ratios between the macro-nutrients affected the uptake ofwater and nutrients. The total nutrient uptake per root dry weight increased and more specifically thenitrate (NO3-), phosphate (H2PO4-), potassium (K+) and sulphate (SO42-) uptake increased with anincrease in nutrient solution electrical conductivity (EC) from 0.8 to 4.0 mS cm-1 while water uptakedecreased. Except for Ca2+ uptake there was no correlation between nutrient and water uptake.Nutrient uptake can thus not be calculated based on water uptake. Instead a mechanistic high-affinityMichaelis-Menten based model can be used to estimate macro-nutrient uptake (Un, mg m-2 hr-1).Water and nutrient uptake was also affected by the solar radiation levels. Since nutrient uptake isrelated to the growth rate, solar radiation levels can be expected to influence nutrient uptake. Theuptake of all ions increased with an increase in the solar radiation levels and for NO3-, K+ and H2PO4-the uptake rate was higher at higher nutrient solution concentrations. The Michaelis-Menten basedmodel was adjusted to incorporate the effect of solar radiation levels on nutrient uptake. Water uptake(Wu, L m-2 day-1) was simulated as a function of crop transpiration and crop leaf area using a linearregression model, but since leaf area development was affected by solar radiation levels this wasadditionally incorporated into the estimation of the leaf area index (LAI).The composition of the nutrient solution also affected the biomass allocation of the crop which can again affect nutrient use as well as the fruit yield. There was also a direct effect of nutrient solution composition on fruit yield and quality with higher EC's resulting in smaller fruit but an increase in fruitdry matter %, total soluble solids (TSS), titratable acidity (TA) and lycopene content.The results in this thesis make a valuable contribution to our understanding of the effect of nutrientavailability (concentration and ratios) and nutrient requirement for growth (solar radiation levels) onnutrient uptake. Incorporating these into nutrient uptake models resulted in the development of ahandy tool to simulate changes in composition of re-circulating nutrient solutions ultimately resulting inan improvement of the water and nutrient use efficiency of soilless systems.

[发布机构] Stellenbosch University