Fertilizer trials involving application of phosphorus (P) as both rock phosphate and superphosphate were conducted on adjacent organic and conventional farms in SE-Australia in 1991 and 1992. Seed P concentration and seed P content (P concentration multiplied by seed mass) were higher on the organic farm in 1991 when rainfall was below average, but were higher on the conventional farm in 1992, a wetter year. Addition of superphosphate increased seed P content and concentration on both farms in 1991 and on the organic farm in 1992. These results indicated that seed produced on a conventional farm with addition of fertilizers containing soluble P is likely to have a significantly greater seed P content than seed produced on an organic farm with application of rock phosphate or with no fertilizer addition.