Organic crops are frequently already lacking in nitrogen shortly after they emerge. This is the conclusion of the thesis with which Arnout van Delden obtained his doctorate at the University of Wageningen on 26 November 2001. Van Delden studied the effects of nitrogen fertilisation on crop development and weed seed production in the organic farming sector. His research involved field tests and simulation models with potato and wheat, varying the levels and application time of nitrogen fertilisation. Van Delden is the first to scientifically prove the importance of giving organic crops a fertiliser in the spring that is rich in directly absorbable nitrogen.

The fertilisation with directly absorbable nitrogen can be in the form of urine. The remaining nitrogen fertiliser can be applied as a normal compost or slurry. The latter is best administered in the spring in order to prevent the nitrogen being flushed away during the winter.

Van Delden discovered that a high nitrogen dosage in wheat can have the disadvantage of stimulating seed production of the important late-germinating weed, chickweed. This occurs due to the fact that the soil nitrogen content in wheat is so low without the extra nitrogen that the growth of chickweed is restricted. As one would expect, a nitrogen dose also stimulates other late-germinating weeds – weeds that germinate after the last mechanised weed control of the crop. Although these weeds do not threaten the yield, they can lead to an excess of seedlings in subsequent crops such as carrot and onion, reducing growth. The manual removal of these seedlings is expensive in terms of labour costs.

Comparison with tests carried out by colleagues showed that early-germinating chickweed that has survived the weed control in wheat forms far more seeds than the late-germinating. For organic farmers it is therefore more important to devote extra time to a good early mechanised weed control than bother about the effect of nitrogen on the seed production of late-germinating weeds.