SOIL BACTERIA.

Otago Witness, Issue 3149, 22 July 1914, Page 16

SOIL BACTERIA.

1 In the light of present knowledge accumulated as the result of scientific investigation, the soil can no longer be regarded as an inert mass of material yielding or ber stores for crop-production in a purely mechanical or chemical fashion. Rather must it now be considered as the home ot infinite numbers of microscopic organisms, by whose adSRm its character and composition are being constantly changed and much of its reserve food supply . rendered available for plant growth From this "biological point of view, soil study assumes a new aspect, and in so far as cropproduction is affected, becomes of more than passing interest to the farmer. The subject of soil bacteria has not yet been adequately explored by scientists, but sufficient is already known to definitely prove that many of the changes in the material of which the soil is composed, and which are of importance as affecting plant nutrition, are associated with groups of bacteria. Soihe of these changes are of a beneficial nature, resulting in an increase in the supply of available plant food, while crthers are the reverse, being destructive to the food supplies in the soil. The point of view, then, for the farmer is how best to encourage the development of the bacteria of the useful type, and so increase the fertility of the soil. Fortunately the very conditions for this purpose are those which reduce the number of noxious organisms to a minimum, and may be summed up as good cultivation, humus, liming, and effective drainage. The soil bacteria concerning which most knowledge has been accumulated are those associated with the plant’s nitrogen supply. In this connection is should be noted that although nitrogen forms four-fifths of the bulk of the atmosphere, plants cannot make use of. it as a source of supply'. They are dependent for their nitrogen on the nitric acid produced by the action of certain bacteria on the organic matter of the soil, so that the maintenance of the humus supply and the conditions favourable to its decomposition is really the key to the problem of soil fertility. If these favourable .conditions do not obtain, the action of other noxious organisms predominates, with the result that the nitrogenous compounds instead of being rendered available to plants, are converted into free’ nitrogen, and as -Such, of course, lost to the soil. In addition to. these two groups another type of bacteria are capable of utilising the free nitrogen of the soil air and com- ; pounding it into a form available as a plant food. These latter organisms usually form an association for mutual -benefit with leguminous plants, deriving from them the carbohydrates necessary to maintain their energy, while • in return providing nitrogen in a form which the plants can use. It is to this symbiotic arrangement with i these bacteria that leguminous plants owe ' their value as soil renovators, and find such favour as green manure crops. Which of the changes associated with the former i group of bacteria will predominate on the ; soil is determined by physical conditions — | aeration, temperature, moisture, and humus [ content. The bacteria concerned in the 1 production of nitric acid or in the process ! of nitrification are favoured by a free ! supply of fresh air, sufficient moisture ■ without excess, a certain degree of warmth, j and the presence of some basic substance like lime to neutralise the acid produced. | It will be seen that these favourable con- \ ditions are just such as are established i by good cultivation, liming, and drainage, i Carelessness, however, in regard to these ! details promotes the action if those organisms which have a destructive effect on the i food supply in the soil. Since nitrogen is ; the most expensive ingredient to purchase, ■ combined as a fertiliser it is clearly sound i economy to utilise such a cheap source 1 of supply as is afforded by leguminous ! plants, and to guard against the loss of i any supply which the soil may already ; contain. In addition it should be remembered that the conditions favouring the action of destructive or denitrifying ! bacteria are distinctly unfavourable to the healthy growth of plants. In proportion therefore as the farmer plays his part in the cultivation operations, so will the soil respond in yielding of her stores.