Soil fertility status and nutrient input–output flows of specialised organic cropping systems: a review

Abstract

The management of specialised organic crops for vegetable and fruit production require the use of imported fertilizers. A wide range of fertilizers is currently available to organic farmers. These include bulky organic materials with a relatively low nutrient concentration commonly used as base dressing (e.g. composts, solid animal manures) and complementary commercial organic fertilizers with relatively high nutrient concentrations to adjust nutrient supply to crop requirements (e.g. feather meal, hoof and horn meal, vinasse, meat and bone meal, etc.). Nutrient imbalances are a major threat affecting the long term sustainability of horticultural and fruit cultivation systems. Major reasons for these imbalances are the biased element composition of base as well as complementary fertilizers in relation to the nutrient offtakes via harvested products. Gaseous nitrogen losses during manure management and gaseous as well as leaching nitrogen losses after application are major reasons for such nutrient imbalances, as they lead to a relative increase in the concentration of other elements. Conceptual weaknesses in the fertilizer approach in organic farming exist, namely the preferred application of slow release nitrogen fertilizers often rich in phosphorus. This review suggests that the current soil fertility approaches based on application of solid fertilizers and simultaneously a low rate of N inputs via N2 fixation do not foster balanced nutrient levels. The key challenge is to design cropping systems with a higher share of N inputs via biological N2 fixation, and to find fertilizers with a nutrient stoichiometry better suited to match the overall specific offtakes of fertilized crops.