Two case-study farms with negative P balances maintained acceptable productivity without fertilisers, apparently by ‘mining’ ‘available’ P reserves in surface and subsoil. The question for these organic farms is ‘how long before fertiliser is needed?’ With six farms on lower-fertility, weakly acidic to alkaline soils and modest rainfall (380-580 mm/yr), low productivity was related to P deficiency despite positive P balances from using allowable fertilisers. Useful supplies of compost or manure were unavailable. Until effective allowable fertilisers or microbial inoculants have been developed, there is a case for using soluble forms of P fertiliser on soils where soil-solution P is low and soil P-sorption is high, so that additions of soluble P ‘feed the soil, not the plant’
