Fixation of ammonium in Finnish soils

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The ammonium fixing capacity of Finnish soils was studied by analysing a material of 139 samples from surface soils and 127 samples of deeper layers collected mainly from cultivated soils from various parts of the country. The pH-values of these samples measured in 0.02 N CaCl2-suspension ranged from 3.3 to 7.5, the content of organic C from 0 to 10.1 per cent, and the content of clay (< 2 μ) from 0 to 96 per cent. The ammonium fixing capacity was determined under moist conditions by treating the samples for 24 hours with N NH4CI solution corresponding to 1000 m.e. of NH4-N per 100 g of soil, and removing the easily exchangeable ions by washing with CaCl2 -solutions. The difference in the nitrogen content of treated and untreated samples determined by digesting in concentrated sulfuric acid, was taken to indicate the amount fixed under these conditions. The results varied from 0 to 4.0 m.e./100 g of soil in the surface samples, and from 0 to 15.9 m.e./100 g of the soils from the deeper layers. The corresponding mean values were 1.0 and 3.8 m.e. per 100 g of soil, respectively. The association of the ammonium fixing capacity (1) with the clay content (2), pH (3), and the content of organic C (4) of the samples could be characterized by the following partial linear correlation coefficients; r12;34 = 0.472*** r13;24 = 0.177 r14;23 = –0.313** The total linear correlation coefficient between the ammonium fixing capacity and the fixation of potassium under moist conditions (2.5 m.e. of K added to 100 g of soil) was r = 0.829***. No correlation existed between the ammonium fixing capacity and the content of exchangeable potassium in these samples. Some of the results point to the possibility that in certain soils the coarser fractions, from 2 to 6 μ, or even from 6 to 20 μ, may play an important role in the fixing of ammonium in difficultly exchangeable form. In spite of the fact that under laboratory conditions the ammonium fixing capacity of Finnish soils may be fairly high, even in the surface soils, the conclusion was drawn that usually under the field conditions, the fixation of ammonium ions in difficultly exchangeable form might not reduce the effect of ammonium nitrogen fertilizers to any marked degree.