Organic fertilisers and nitrogen availability

Liquid organic fertilisers allow growers to abandon the use of conventional de novo (mined or synthesised) fertilisers without the need for major technological adaptions in the greenhouse. It was decided to run a cultivation experiment to find practical information for producers and growers when using liquid organic fertilisers in cultivation. The objective was to compare crop production between plants grown with liquid organic fertilisers and plants grown with conventional de novo fertilisers. Cucumbers were propagated in coir and peat and planted on slabs of respectively coir and peat. The plants on the two rooting media received either liquid organic or chemical fertilisers, resulting in four treatments in total. Plants fed with liquid chemical fertilisers were shown to initially produce 10-20% more fresh weight per unit time than plants fed with liquid organic fertilisers. When organic fertiliser was used during cultivation, ammonium accumulated to levels of 10-20 mmol L-1, while nitrate levels dropped to below 1 mmol L-1. To explain the results, the conversion of nitrogen was studied. Conversion of organic nitrogen into nitrate requires enzymatic hydrolysis to ammonium, followed by bacterial oxidation to nitrate. This microbiological process depends on two physiological groups of naturally occurring aerobic soil bacteria; ammonia oxidising bacteria (AOB) and nitrite oxidizing bacteria (NOB). In conclusion nitrate concentration rather than the concentrations of organic nitrogen or ammonium, were growth rate determining. The low conversion rate of ammonium into nitrate by bacteria present in substrate limited the leaf area growth of cucumbers. Suggestions to overcome the problem are made.