supply of a mineral nutrient to the plant is determined by:
total abundance in the soil,
proportion of the total which is in an available form at any time, and
plantís ability to capture this available component.
only a small proportion of the total nutrient content of soil is available for
uptake. Some elements are part of the fabric of the soil particles, and are only
released slowly as a result of weathering. Others are bound more or less
strongly to the surface of those particles.
Availability of many essential nutrients is affected by soil
pH. At high pH (alkaline soil), the solubility of P, and of many micronutrients
(eg. Fe, Mn, Zn and Cu) is greatly reduced, and the crop may experience
deficiencies of these nutrients. At low pH (acid soil), the solubility of some
elements, particularly P and Mo, is reduced, while that of Al and Mn may be
increased to toxic levels.
Other examples of nutrient deficiencies induced by low
availability include so-called P-fixing soils, which require very high P
fertiliser rates to saturate the soilís binding capacity. Peaty soil can bind
Cu, and organic matter with a high ratio of carbon to nitrogen will tend to Ďfixí
soil nitrogen, as the soil microbes use all the available N for their own
nutrition, leaving little for plant roots to access
Disorders such as Al toxicity, which inhibit root
development, reduce the plantís ability to capture nutrients and water, and
may induce symptoms of secondary disorders such as Mg deficiency or water
stress. Also, some nutrients compete with each other for uptake by the plant. An
example is competition between Ca, Mg and K. A very high level of one of these
may induce deficiency of another. It is important to recognise the primary agent
in such cases.
Deficiencies of macronutrients, particularly N, P and K, are
often associated with fertility decline following sequential cropping. These
elements are taken up in such large quantities by the crop that the soilís
reserves become depleted after a number of crops have been produced. Increasing
cropping intensity results in an increasing dependence on external supplies of
these nutrients. In low-intensity agriculture, supplies may be replenished
during a bush fallow, by allowing time for weathering of mineral particles, by
redistributing nutrients into the cropís root zone from below, and by capture
of wind-borne nutrients from sea spray. Nutrients accumulated by plants during
the fallow become available to the crop as the plant material decomposes, or
when it is burnt. Burning makes many nutrients immediately available to the
following crop, but it also makes those nutrients easily lost by leaching.
Burning also decreases the soilís ability to continue supplying nutrients, as
there is less organic material to decompose. Some nutrients, particularly N and
S, are lost into the atmosphere during burning
Deficiencies of micronutrients are usually associated with
low natural abundance in the soil, or unfavourable soil conditions causing
insolubility of these nutrients. In the case of low abundance, correction
usually requires the application of only a few kilograms per hectare of the
deficient nutrient, which may be effective for a number of years. Such inputs
are likely to be cost-effective, even when the application of N or P fertilisers
is not. The management is more difficult when the deficiency is due to adverse
soil conditions, such as the very high pH in coralline soils. Applying the
nutrient as a foliar spray is one way to avoid it being fixed in such soils.
Increasing the organic matter content of the soil is also beneficial.
J.N., Asher, C.J. and Blamey, F.P.C. (1997) Nutrient Disorders of Sweet Potato.
ACIAR Monograph No. 48, Australian Centre for International Agricultural
Research, Canberra, 136 p.
topics on Soil Management:
Other topics on Crop Management: