Variations in microbial population and nitrogen cycle microorganisms have been studied over one crop cycle in an acid Ranker over granite, under grass fertilized with cattle slurry and in the unfertilized soil. Cattle-slurry treatment brought about a great rise in total microbial population but affected different groups differently, though without changing the order of their population sizes. Bacteria were favoured, while the numbers of actinomycetes, fungi and algae were reduced, the latter especially. Slurry treatment increased the capacity for degradation and mineralization of nitrogen compounds by increasing the populations of proteolytics, ammonifiers and nitrifiers, though the latter group survived only briefly, their numbersfalling sharply again after a short time. The populations of denitrifiers and anaerobic free-nitrogen fixers also increased when slurry was applied, though much less strikingly than the groups mentioned above, whereas aerobic free-nitrogen fixers were depressed. Proteolytics, ammonifiers and denitrifiers were affected more by the first application of slurry than by the second, whereas the reverse was true of nitrifiers and anaerobic free-nitrogen fixers. All the groups studied except the algae tended to return to their initial population levels after some time.

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