Lucerne plants were inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus, Glomus caledonius and Rhizobium meliloti and grown in pots in voli~re. Treatments were 0, 1 and 2mg P added to 100g of a soil with low P-fertility. Plants were harvested after 6, 9, 13 and 18 weeks, allowing shoots to regrow between harvests. VAM-infection was determined after 6 and 18 weeks, dry weight, concentration and uptake of P and concentrations of N, K, Ca, Mg, Cu and Zn were determined at all harvests.VAM-inoculation increased growth of shoots, P-concentration and P-uptake at all soil-P levels and the increase was most pronounced at the lowest soil-P level. P-fertilization also increased growth, P-concentration and P-uptake. VAM-inoculation showed tendency to increase N-and K-uptake and decreased Ca-, Mg-and Zn-concentrations in shoots. The uptake ratio of fertilizer-P was increased by VAM~
Results of a laboratory investigation showed that anaong soils in Denmark it are chieIly those rich in clay minerals that will fix and release potassium and ammonium. The two cations were fixed in nearly equivalent proportions, but ammonium was fixed preferentially to potassium when added together to the soil.The maximum fixation reached 20 to 30 per cent of the addition in top soil layer but 50 to 80 in subsoil.Soils from a permanent Iertilizer experiment showed the strongest fixation and least release of potassium where no potassie fertilizer had been applied for more than 70 years. In soils irom another field experiment on residual e•iect of various nitrogenous fertilizers there was a small but signiIicant increase in content of inorganically fixed ammonium where urea had been applied.
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