Organic farming systems are characterized
by the strong regulation of the import of
nutrients into the farming system to replace nutrient
losses via sold products. In the present study mineral
nutrient flows and balances of P, K and magnesium
(Mg) were analysed for a mixed organic cropping
system with dairy husbandry and for a stockless
organic farming system. Also the influence of biogas
digestion of farmyard residues (stable wastes, crop
residues, etc.) as well as the effect of the import of
substrates for biogas digestion on plant mineral
nutrient uptake and farmgate nutrient balances was
analysed. The objectives of the current study were;
(1) to study the effects of anaerobic digestion of cattle
manure and crop residues on plant mineral nutrient
uptake; and (2) to model nutrient flows and balances
related to the input of different kind of substrates for
biogas digestion at the farmgate. Results indicated
that slurry digestion did not influence plant P and K
uptake. Import of single allowed substrates for
digestion would lead to large imbalances in nutrient
inputs compared to withdrawals. Most of the suited
substrates for biogas digestion were associated with
large K surpluses and insufficient P returns in
comparison to mineral nutrient outputs via sold
animal and plant products.