Nitrogen fixation by lucerne (Medicago sativa) was measured on ten agricultural soils in pot culture. There was a three-fold variation in nitrogen fixation, smallest quantities being fixed on a red brown earth, a euchrozem, and a wiesenboden. Both lucerne yield and nitrogen fixation were significantly correlated with soil pH over a range of 6.2 to 8.6 when phosphorus and sulphur were applied. These two nutrients caused significant increases in lucerne yields and nitrogen fixation on all soils. Fixed nitrogen originating solely from the underground parts of the lucerne plants caused significant increases in the nitrogen yield of wheat grown subsequently on the same soils.