Geoscience Reference
In-Depth Information
or in this case, 'clover sickness', is likely to occur on soils where the particular crop
plant has been grown continuously for many years. Although some build-up of root
nodule bacteria would have taken place as a result of continuous cropping of the host
plant, rapid multiplication of the appropriate bacteriophage could eliminate eventu-
ally the
Rhizobium
bacteria. In absence of the vital nitrogen-fixing nodules or without
suitable fertilizer treatment, the clover crop would decline.
Microbial populations in soil are affected also by various physical factors, such
as aeration, temperature and water content. For instance, if there is sufficient organ-
ic matter in the soil, prolonged water-logging will not only impair aeration of the
root zones of some plants but the anaerobic conditions may favour development of
undesirable microorganisms, like the sulphate-reducing bacteria. The latter produce
toxic hydrogen sulphide which is poisonous to aerobic microorganisms and to many
plants and animals. Later, when the water eventually drains away, the soil again be-
comes aerated, and free oxygen then, in turn, suppresses undesirable anaerobic bac-
teria and the normal soil microflora takes over.
All microorganisms require moisture for normal growth and excessive dryness
can seriously affect microbial populations. The spores or resting forms of fungi, ac-
tinomycetes and spore-forming bacteria and the cysts of amoeba, as mentioned earli-
er, can withstand dry conditions. Non-sporing bacteria can also sometimes survive
dry conditions in soil. It is supposed that clay minerals or soil colloids can protect
adsorbed microorganisms in some way. Nevertheless, generally speaking, when soils
become very dry during hot weather many microorganisms and small delicate animals
succumb and, in due course, their dead biomass becomes transformed into nutrients
for other living organisms when favourable conditions again prevail.
Microorganisms can be killed in other ways than by desiccation; for instance, by
heat treatment as in steam sterilization of soil, or by toxic chemicals, as in soil fumig-
mass of killed organisms will, in turn, be utilized by surviving microorganisms and
eventually will be completely mineralized. This provides inorganic nutrients for crop
plants and so there is some stimulation of plant growth, just as in the case of repeated
drying and re-wetting of soil. Much of the carbon in the dead biomass is lost as car-
bon dioxide, but most of the nitrogen will remain in the ammonium form, which can
be transformed into nitrate by nitrification. A small proportion of the nitrogen may
remain in combined form in the soil humus or in soil organic matter. Trace elements
and other minerals are mostly retained in the soil.
F
UNGI, YEASTS, ACTINOMYCETES AND ALGAE
