Effect of Nitration of Organic Materials on Nitrogen Availability and Yield of Tomato in Soilless Culture

In the cultivation of horticultural plants in greenhouses the most commonly used substrate is rockwool, while organic substrates are used in an unprocessed state. Cultivation in organic substrates, after they have been suitably processed, also makes it possible to obtain high yields at levels similar to those in rockwool cultures. Fibrous organic substrates, due to the wide C:N ratio, are characterized by high biological sorption towards nitrogen, causing N deficiency during the initial period of plant growth. In 2007-2009, a study was conducted, the aim of which was to determine the effect of nitration of fibrous organic substrates (sawdust, rye straw, flax shives) with nitric acid on nitrogen availability and the yields produced by tomato plants. The process of nitration of the organic substrates was carried out with the use of 10% HNO₃. The appropriate pH of the substrates after the process of nitration was obtained by washing the substrates with water. As a result of nitration the levels of nitrogen fixed in lignin compounds and simpler polysaccharides increased to about 2%. Nitration of the organic substrates increased nitrogen availability to plants in the first five weeks of culture. Subsequently, the N-NO₃ content in the nutrient solution collected from the nitrated substrates was at the same level as for the nonnitrated substrates. The yields produced by tomato plants depended on both the nature of the substrate and whether or not it had been nitrated with nitric acid. The most useful organic substrate for soilless culture, retaining good structure during the growing period, was the substrate of sawdust. The substrate of straw was found to undergo quick biodegradation, therefore its usefulness for soilless culture was low. The highest yield of tomato was obtained from the cultures on rockwool and on the nitrated sawdust substrate; the lowest yield was produced on the straw substrate.