Causes of nutritional disorders

The supply of a mineral nutrient to the plant is determined by:

• its total abundance in the soil,

• the proportion of the total which is in an available form at any time, and

• the plant’s ability to capture this available component.

Usually 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 soil’s binding capacity. Peaty soil can bind Cu, and organic matter with a high ratio of carbon to nitrogen will tend to ‘fix’ soil nitrogen, as the soil microbes use all the available N for their own nutrition, leaving little for plant roots to access

Uptake

Disorders such as Al toxicity, which inhibit root development, reduce the plant’s 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 soil’s 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 crop’s 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 soil’s 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.

Source: O’Sullivan, 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.

Further topics on Soil Management:

Soil management

Soil structure

Soil organic matter

Plant nutrients

Fertilisation

Diagnosing nutritional disorders

Correcting nutritional disorders

Other topics on Crop Management:

Land preparation

Planting material preparation

Planting

Water management

Vine lifting

Integrated pest management

Harvesting

Postharvest practices