. 2008. Nutrient budgets of Ontario organic dairy farms. Can. J. Soil Sci. 88: 107Á114. The sustainability of organic dairying in Canada, with respect to nutrients, remains unexamined. To assess how management affects nutrient status, we documented whole farm nutrient (NPK) budgets over 2 yr (2003 to 2005) and soil (0 to 15 cm) P and K status on 15 long-term Ontario organic dairy farms. Farm size, livestock density and herd productivity averaged 110 ha, 1.00 livestock units ha(1 and 5656 kg milk cow (1 yr (1 , respectively. Annual farm nutrient surpluses of 75 (N), 1 (P) and 11 (K) kg ha(1 yr (1 were lower than those reported for confinement-based dairy farms in the United States, pointing to possible environmental benefits from reduced off farm impacts on air and water quality. Weighted average soil test P levels were low (B10 mg kg ) on approximately 50% of farms, while exchangeable K levels were moderate to high (76 to 160 mg kg(1 ) on all farms. Four farms adopting a ''self-sufficient'' approach, producing most feed on-farm, imported little P as feed (1.37 to 1.90 kg P ha(1 yr(1 ) and had negative aveage farm P balances (avg. (1.54 kg P ha(1 yr(1). An integrated nutrient management approach, along with a flexible feed import strategy, fosters the sustainability of organic dairying systems. Les exce´dents annuels d'e´le´ments nutritifs a`la ferme de 75 (N), de 1 (P) et de 11 (K) kg par hectare annuellement sont infe´rieurs a`ceux rapporte´s dans les e´levages de bovins laitiers en claustration des É .-U., signe que les plus faibles re´percussions des ope´rations hors ferme sur la qualite´de l'eau et de l'air pourraient eˆtre be´ne´fiques pour l'environnement. La concentration moyenne ponde´re´e du P dans le sol e´tait faible ( B10 mg par kg) dans environ la moitie´des exploitations, alors que celle d'ions K e´changeables e´tait de moyenne a`e´leve´e partout (de 76 a`160 mg par kg). Quatre exploitations avaient instaure´une approche d'autosuffisance, la majeure partie des aliments du be´tail e´tant produite sur place avec peu d'achats de P (1,37 a`1,90 kg de P par hectare annuellement), ce qui a eu une incidence ne´gative sur le bilan du P a`la ferme (en moyenne (1,54 kg de P par hectare annuellement). Une approche inte´gre´e a`la gestion des e´le´ments nutritifs assortie d'une strate´gie suffisamment souple pour l'achat des aliments du be´tail favorise la durabilite´des e´levages biologiques de bovins laitiers.
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