Yield and quality response of hydroponically grown tomatoes (Lycopersicon esculentum Mill.) to nitrogen source and growth medium

[摘要] ENGLISH ABSTRACT: Pine sawdust-shavings (Pinus spp.) is at present a very popular soilless substrate inSouth African greenhouses. Growers use fresh pine sawdust-shavings as a substrate,which is biologically highly unstable. The greenhouse industry is looking atalternative organic substrates such as coco peat, which already went through adecomposition process and is more stable. A biological inactive substrate such as sandwas included to compare microorganism activity with organic substrates. The mainobjective of this study was to compare the growth, yield and quality of hydroponicallygrown tomatoes in response to different growth mediums in combination withnitrogen source, irrigation frequency, period of substrate use and liming. In generalthe drainage water pH declined with an increase in NlLt+-N in the nutrient solution.Low pH values in the drainage water, especially when coco peat was used, had adetrimental effect on marketable yield. The drainage water pH of pine sawdustshavingsincreased during the growing season when 100 % N03--N was used. Due tothe higher cation exchange capacity of coco peat, the drainage water electricalconductivity tends to increase more rapidly than with pine sawdust-shavings, duringconditions with high temperatures and when insufficient irrigation volumes perirrigation cycle is applied. As expected the drainage water N03--N content decreasedas the NlLt+-N content increased in the nutrient solution. Pine sawdust-shavingsrecorded a much lower N03--N and NlLt+-N content than sand and coco peat and thussupports the hypothesis that microbiological activity is higher in pine sawdustshavings,especially in the second season of substrate use. Coco peat produced thehighest number of marketable fruit and yield per plant, followed by pine sawdustshavingsand sand in the first season of substrate use. The number of marketable fruitand yield decreased with an increase in NlLt+-N content in the nutrient solution duringproduction in warmer, summer conditions. Contrary to these fmdings, production incooler, winter conditions recorded high yields when only N03--N or 80% N03--N :20% NRt +-N was applied. The unmarketable yield increased with an increase inNlLt+-N in the nutrient solution. Visual evaluations showed that blossom-end rot(BER) was the main contributor to unmarketable yield. Increasing levels ofN03--N asnitrogen source in the nutrient solution, reduced weight loss and increased the loss offruit firmness of tomatoes during storage. Increasing levels of N03 --N also increasedfruit pH and reduced total titratable acidity. Coco peat produced fruit with a higher pHthan pine sawdust-shavings. An increase in irrigation frequency affected fruit firmnessnegatively when coco peat was used as substrate. Different irrigation and fertigationpractices are needed for different growth mediums and management needs to beadapted according to the growing season (winter vs. summer).

[发布机构] Stellenbosch University