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Some Recent Researches on the Soil Organic Cycle

A. C. S. Wright,
Soil Bureau, Department of Scientific and Industrial Research, Wellington.

Modern soil textbooks make a point of impressing the reader with the need to bear in mind that “the soil is a living body.” Such warning is hardly intended for the biologist who, no matter what his special interest, cannot fail to be aware of soil as an integral part of the plant and animal environment, and further he will know that many organisms have it in their power to modify their immediate habitat, making it conform more nearly to their needs. Soil, therefore, has many of the attributes of a living body solely because it supports so many living organisms. These range in size from bacteria to kauri trees. The by-products of their life in the soil and the residues left in the soil as a result of their occupation together constitute the soil organic cycle.

The earliest members of the organic cycle are those micro-organisms, mainly soil bacteria, which derive their energy from the multi-valent elements like sulphur, iron or manganese, and build up carbon compounds from carbon dioxide which may be present in either gaseous or dissolved form. Organic compounds built up in the bodies of these pioneer organisms then become a source of energy for more specialised micro-organisms which have the chemical tools required for breaking down complex carbohydrates.

The organic cycle commences at a very early stage in soil formation, often simultaneously with normal soil weathering processes. In many cases it can be shown that organic acids released by soil microorganisms are an important factor in the weathering of mineral particles.

The biotics division of the Soil Bureau is a research team recently set up to enquire into the nature of the organic cycle in New Zealand soils. So far it has done mainly exploratory work to see whether our soil classification (based upon the concept that soil types can be grouped by considering the action of a common set of soil forming factors upon the various kinds of parent materials) means anything to the organisms that live in the soil. J. D. Stout has classified the protozoan members of the soil fauna into cosmopolitan and indigenous elements and made the important discovery that, amongst the latter, there are some species that appear consistently in certain soil types or are restricted to a group of closely related soil types. He has shown that the protozoan fauna of the soils above the bush line on Mount Egmont is almost identical with that in the closely related soils of the sub-antarctic islands. One protozoan species, known only from the leached yellow brown earth developing under Nothofagus truncata page 101 in the Nelson district, was not encountered again until a similar soil associated with N. solandri and N. cliffortioides in the Taupo region was examined.

K. E. Lee, who is making a survey of the indigenous earthworm species associated with the different soil types has made some valuable observations on the competitive element amongst native worm populations and the latter's reaction to invasion by the more aggressive introduced species. Worm distribution, unlike that of the soil protozoa, does not seem to be so closely related to soil type, probably because worms are better able to change uncongenial conditions in their immediate surroundings than smaller soil organisms. By using thin slices of an undisturbed soil profile, and by pot experiments, Lee is studying the extent to which an earthworm can modify the soil environment, thereby incidentally altering soil fertility—a property of the soil that is closely regarded by the farmer. R. H. Thornton has commenced a survey of fungi and actinomycetes in New Zealand soils but the problems of identification are too difficult to be resolved by a solitary worker in this field and he is at present studying under Professor Chesters at Nottingham University. P. J. Culliford has commenced a survey of the conditioning influence of the different native tree species upon soil development. This is going to be an important line of research, for the great majority of our soil types are developing mainly through the process of leaching, and differences in the organic cycle (of which the tree is usually the strongest component) can readily cause major differences in the rate and kind of leaching and so greatly modify the kind of soil formed. This is well illustrated by the mosaic pattern which is often noticeable in freshly ploughed land that has formerly been covered with forest. The lighter gray patches of soil are areas once occupied by those trees whose litter was, for various specific reasons, prone to accumulate, forming a layer of peat in which slow decomposition was carried out by a predominantly fungal micro-population. The resulting increase in acidity has accelerated normal leaching and caused the “bleached” appearance of the soil.

As we learn more of the activities of the higher plants and animals, and microorganisms, we find that their metabolism is often controlled by specific soil conditions. Out of this grows the technique of bio-assay, whereby the growth of an organism can be used as an indicator for a particular soil condition. The soil fungus, Aspergillus niger, requires a small amount of copper to complete its life cycle. The ramifying hyphae can extract copper from a mineral soil in about the same degree as the roots of higher plants. Thus the fungus can be grown in agar cultures with a small volume of soil added to supply copper, and the colour of the spores at maturity can be used as an index of the amount of copper in the soil which is available to higher plants. This technique is being employed by D. Breen to make a survey of the available copper in New Zealand soil types. H. W. Johnston has page 102 carried this bio-assay idea further, using a local strain of the fungi Trichoderma sp. for boron bio-assay work, and Cunninghamella sp. for zinc, while Aspergillus niger can also be used to estimate magnesium, manganese and phosphate.

The higher plants are themselves employed for evaluating availability of certain trace elements by E. J. S. Gridley of the plant indicator research unit. This section grows indicator plants, such as mustard, virginia stock, lettuce, etc., in pots and studies the effect of a standard range of trace elements on plant growth in different soil types. We have been able to demonstrate that hitherto unsuspected cases of boron, copper, zinc and manganese deficiencies are present in a number of important soil types.

Considering their importance for our well-being, it is remarkable how little is known about the working parts of the organic cycle in soil. The biotic surveys were expected to find many new and unexpected facts about soil life but so far it has been the bio-assay work (originally started as a service to supplement the work of the chemistry division of the bureau) which has stirred up the more thought-provoking problems. While using Aspergillus niger for phosphate bio-assay, Breen discovered that on some phosphate deficient soils the fungus is quite unable to grow unless supplied with either citric acid or a small amount of soluble phosphate as a “starter” in the medium. It is well known that A. niger and many other soil fungi secrete organic acids but Johnston has shown that many of these organic acids readily dissolve calcium and magnesium phosphates and even attack iron and aluminium phosphates. Johnston considers that there perhaps lies the explanation of such problems as the phosphate nutrition of lichens growing on exposed fresh rock surfaces, the beneficial results of mycorrhizal association in the roots of higher plants, and the tooth-destroying powers of the organisms responsible for dental caries. Theoretically it should be possible to culture powerful phosphate solubilizing micro-organisms in phosphate deficient soils and, as a result, make available to the higher plants phosphate from sources which are normally not available to plant roots. Pot trials with sour milk (nourishing lactic acid bacteria) and Aspergillus broth have gone some way to confirming the hypothesis.

Another unexpected idea came from our trace element experiments. It is customary to add most trace elements as sulphates, and both copper and zinc sulphates have been fairly consistent in promoting increased plant growth. It would be unwise, without further experiment, to conclude that this indicates a general copper or zinc deficiency in the New Zealand soils, for the increased plant growth strongly resembles that which normally occurs when nitrogen is added to the soil. It may yet be shown that some trace elements, when added to the soil, have their main effect upon the growth and metabolism of the soil micro-organisms rather than the higher plant, and that at least a part page 103 of the improvement in plant growth is a secondary result of this response shown by soil micro-organisms. Or again, the effect of copper sulphate may be largely a simple sterilization effect, the dead bacterial cells and hyphae providing a source of extra nitrogen for the plant. Preliminary experiments in which we studied the uptake of copper by pasture growing in sand and peat, tended to confirm this latter hypothesis. Eventually, we may have to regard soil micro-organisms in the same light as animal research workers are coming to look upon the bacteria and fungi which live in the alimentary tract of animals—an essential factor in the normal nutrition of the higher organism. Our ultimate objective, after biotic surveys have told us enough about the natural system with which we are working, will be to find out how we can manipulate and control the soil population so that we can modify the soil organic cycle for the benefit of its greediest component—the human race.