Fertilization strategies to increase nitrogen content in organic apple trees in the flowering period

The major sources of nutrients to organic grown apple trees are fertilizers made from manure, compost, bone meal, etc. Depending on humidity and temperature in soil and air, the nutrients are dissolved or mineralized and made available to the trees during the growing season. In conventional apple growing, the trees are given mineral fertilizers in early spring to improve the nitrogen status in the trees during flowering for better fruit set. Is it possible in an organic production system to increase the plant available nitrogen in the flowering period by application of liquid N-fertilizers? The standard fertilizer in Norwegian organic fruit growing is dried and pelleted chicken manure with bone meal and vinasse (Marihone plus; NPK 8-4-5). In these experiments, a liquid fertilizer (Pioner Hi-fruit; NPK 4-1-5) based on vegetable matter plus potassium-vinasse was compared to the standard fertilizer. The liquid fertilizer was applied to the soil as fertigation from 2 weeks before the estimated start of flowering. The dry product was applied 2 weeks prior to flowering. To incorporate the fertilizers into the soil, a mechanical hoer (Orizzonti, Italy) was run in all plots after the application of dry fertilizer. The nitrogen and mineral contents in soil, leaves and fruit were analyzed. The liquid fertilization applied on the soil in the spring gave higher N-contents in soil and trees compared to the dried manure product. However, the increase in N-content was not very strong in the leaf samples. Apples from trees given high doses of liquid fertilizers were greener with less cover colour and higher IAD-indexes. Still they were softer and had less starch than fruit from other treatments.