NITROGEN-FIXATION BY A SUBCLOVER-GRASS MIXTURE GROWN ON 2 ACID SOILS TREATED WITH LIME

The effect Of liming on symbiotic N2 fixation by subclover (Trifolium subterraneum) grown with ryegrass (Lolium multiflorum) was measured under glasshouse conditions using the N-15 dilution technique. Two acid soils, a sand and a loam, were treated with two rates of lime or left unamended. In general, lime addition to both soils increased total dry matter yields, but in the loam, grass made up most of the increase, whereas in the sand, subclover made up all of the increase in dry matter. N2 fixation in both untreated soils was negligible. Lime addition increased the percentage N in the total herbage, had little effect on the proportion (P(atm)) and amount of N fixed by subclover grown on the loam, but dramatically increased both P(atm) and the amount of N fixed by subclover growing in the sand. Subclover grown in the loam was poorly nodulated, despite the addition of large amounts of nodulaid (rhizobium survival in the soil was not a factor), but was well nodulated in the sand, thus explaining the lack of fixation in the loam. Higher amounts of inorganic N produced in the loam shortly after germination have the potential to inhibit nodulation in this soil. Also the loam, after liming but before planting, contained higher amounts of nitrate-N and total inorganic N than the sand, and the total inorganic N mineralized by the loam (518 versus 347 mg per pot by harvest) would also have had some effect on N2 fixation had nodulation taken place at a later date. Levels of critical nutrients (B, Ca, Cu, K, Mg, Mo and P) were measured in the clover tops, but no element appeared to be in deficit or excess, except Al which could not be excluded as being at a toxic level in the limed loam in the present trial.