SUMMARY These experiments were undertaken to determine the effects of liming on certain groups of soil bacteria in a typical Wisconsin drift soil. They justified the following general conclusions: 1. Applications of lime up to three tons per acre lead to an increase in the numbers of bacteria developing on "modified synthetic" agar. They also produce an increase in ammonification, nitrification, and in nitrogen fixation when these processes are tested by the beaker method. These increases are in all cases almost proportionate to the amount of lime applied. 2. Natural increases in numbers of bacteria tend to obscure the effects of applications of lime, while natural decreases make them more pronounced. 3. Peptone solutions do not permit of the determining of the largest number of bacteria which will destroy humus with the production of ammonia. 4. The beaker method, with dried blood or cottonseed meal for ammonification, with ammonium sulfate or dried blood for nitrification, and with mannite for nitrogen fixation, is eminently satisfactory. 5. The ammonification of dried blood or of cottonseed meal runs parallel with the numbers of bacteria while there is very little relation between the ammonification of peptone solutions and numbers. 6. Increased nitrification leads to slight accumulations of nitrates in the soil. 7. Natural accumulations of nitrates in the soil tend to obscure the differences due to the lime treatment. 8. The solution method for nitrogen fixation is quite unreliable. 9. Applications of lime increase the yield of oats; onehalf and one ton per acre very slightly, but two and three tons to quite a large extent. 10. Applications of lime up to three tons per acre increase the nitrogen content of the oats crop more rapidly than the yield itself. Some Bacteriological Effects of Liming By Percy Edgar Brown I. INTRODUCTION GENERAL The use of lime in agriculture is exceedingly old. Mention is made of it in the writings of certain Romans many years before the Christian era. At that early date its benefits were clearly recognized but the reason for its beneficial action on most soils was not known. As time went on the relation of lime to crop production became a subject of considerable importance, and almost a century ago Johnston spoke of lime as the "basis of all good husbandry." Since that time multitudinous experiments with liming have been carried out under a great variety of conditions and many facts and theories regarding its action have been evolved. In the earliest investigations calcium was classed among the essential plant food constituents. It was natural, therefore, that the first theory regarding the beneficial action of lime should be that it supplied a lack of calcium. It soon became evident, however, that lime increased the crop yields of soils showing an abundance of calcium. Also, chalk soils and soils of limestone origin yielded larger crops when lime was applied. Hence it became necessary to look further for the explanation of such phenomena. Very soon lime came to be classed as an indirect manure and its action was recognized to be the resultant of several forces working in opposition or in harmony. These forces are now classed as physical, chemical, physiological, and bacteriological. PHYSICAL EFFECTS OF LIMING The physical effects of liming are two-fold, depending on the character of the soil. On heavy clay soils, lime causes a flocculation of the fine particles and thereby materially improves the tilth, increasing aeration and facilitating the circulation of water. The chemical and bacteriological changes in the soil are consequently affected, leading to the presence of larger amounts of soluble plant food and subsequently to increased crop production. On light, sandy soils lime causes a reduction in porosity. This increases the water-holding power, limits oxidation processes and favors bacterial activity. Hence, while the benefits of liming may apparently be physical only, altered physical conditions bear such an important relation to chemical and bacteriological activities that the three factors should be considered together. CHEMICAL EFFECTS OF LIMING The chemical effects of liming are mainly those dealing with the supply of plant food in the soil. Potash, and in some cases phosphoric acid, are the main constituents affected. So great is the action of lime in liberating potash from insoluble compounds that it is regarded in many cases as an indirect potash fertilizer. In soils where phosphoric acid exists as iron or aluminum compounds, lime reacts with them, forming calcium phosphate. Consequently in this case liming may be regarded as an indirect application of phosphoric acid. On the other hand if lime is added to soils containing large amounts of calcium phosphate, the change of phosphorus into a soluble form is retarded. Another effect of liming which may be regarded as chemical is its neutralization of acid substances. These substances are in the soil as the direct result of the growth of crops and also of the decay of humus and in the absence of some base may accumulate so largely as to exert a decidedly depressing effect on crop production. Here again we find a complexity of factors, for such an accumulation of acid substances reduces crops not only because of its chemical action, but also because of its restriction of bacterial activities and consequent reduction of plant food and because of the physiological action on the plants themselves. PHYSIOLOGICAL EFFECTS OF LIMING Plants are materially affected by acid conditions not only because they diminish the supply of plant food, but also because of their direct physiological effect on the plants themselves. This effect naturally varies with the plant as well as with the nature of the acid substance and soil conditions. Lime, then, in neutralizing such acid substances has a direct physiological effect on the plants. Furthermore, it has been shown that the relation of calcium to magnesium in the soil is of vital importance to plants physiologically, and hence applications of lime by altering the normal ratio in soils may benefit or harm plants. BACTERIOLOGICAL EFFECTS OF LIMING Bacteria are very sensitive to any change in reaction in a culture medium and consequently the reaction of a soil largely determines the bacterial flora of that soil. In all the bacterial soil processes which deal with the transformation of plant food, lime plays an important part. It vitally affects the decay bacteria and the increase or retarded destruction of humus leads to changes in carbon dioxide production which in turn affect the solution of potash and phosphoric acid compounds. Furthermore, the transformation of proteins in the humus into nitrates may thus be materially influenced. Lime affects not only the decay bacteria but also those organisms which are active in the other stages of simplification of nitrogenous compounds. The ammonifying power of a soil may be increased or diminished by applications of lime and in nitrification lime is absolutely essential to provide a base to neutralize the nitrous and nitric acids produced. Nitrogen fixation, whether symbiotic or nonsymbiotic, is quite dependent on the presence of lime. In fact, the inoculation of most legumes is impossible without lime. It has also been demonstrated that azotobacter refuse to grow under acid conditions and that for any fixation of nitrogen to occur, lime is necessary. Thus it is evident that lime largely influences the rate of decomposition of plant food in the soil through its influence on bacterial activities, and consequently the bacteriological effects of liming are directly related to the effect on soil fertility. |