Correcting nutritional disorders

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Once a nutritional problem has been correctly diagnosed, it is usually possible to rectify it, if not for the present crop, then at least for future crops at the same site. However, improving soil fertility costs money (eg. for fertilisers), labour (eg. for gathering manure) and/or time (for fallowing or growing green manure). The best solution will depend on which of these is most limiting, and whether the likely gain justifies the cost. Thus good soil fertility management requires:

  • Correct diagnosis of the problem;

  • Understanding the other environmental limitations on yield;

  • Understanding the socioeconomic situation and the resource base of the farmer.

Appropriate solutions

Nutrient deficiencies are alleviated by increasing the supply of the deficient nutrient to the crop. Applying inorganic fertilisers is one way of doing this. Another may be to add organic material such as animal manure, if it contains an appropriate balance of the required nutrients. Nitrogen may be supplied through biological nitrogen fixation, by planting legume species in rotation or as intercrops with other crops. However, this only provides N to the other crops if the legume material is allowed to stay on the field and be incorporated into the soil.

Other approaches aim to change the soil properties, in order to increase the availability of nutrients already present or to reduce the supply of elements causing a toxicity. The pH of acid soils may be increased by adding lime (CaCO3) or dolomite (CaCO3.MgCO3). Gypsum (CaSO4.2H2O) may be used to correct problems of soil surface crusting and poor permeability in saline and sodic soils. In soils prone to waterlogging, improved drainage may be necessary to reduce denitrification, to reduce the production of toxic forms of Mn, or simply to ensure that the roots receive enough oxygen to function well. Sweet potato roots are very sensitive to low oxygen supply.

Organic matter

Soil fertility decline under intensive cropping is often associated with decline of soil organic matter. Mechanised agriculture tends to ignore this resource, substituting its services with more fertilisers and irrigation. However, changing the farming system to promote soil organic matter can make it more efficient and more sustainable.

Increasing the organic matter content of the soil has a number of beneficial effects. The gradual decomposition of this material provides a steady supply of plant-available nutrients. The organic particles may also provide a suitable substrate on which soil nutrients can be held in an available form. Organic matter increases the soil’s ability to resist acidification. It also increases water retention so that the soil takes longer to dry out, and gives the soil an open texture so that more air can get to the roots.

Organic matter is increased by leaving crop or fallow residue on the field, without burning, or by bringing plant material from another site. If the need for field sanitation prevents leaving the crop residue, then options may be to compost it and return it later, or to use it to mulch another crop which is not at risk from the pest. Often sweet potato vines are removed and used to feed stock. Returning the animal manure to the field replaces some of the nutrients, but it will not maintain the organic matter content of the soil unless supplemented by plant material from fallow or green manure crops.

Time of nutrient application

Farmers are most familiar with fertilizing before planting.  However, some nutrients are easily lost from the soil through leaching or conversion to unavailable forms.  Nitrogen is particularly prone to such losses.  It is advisable to delay nitrogen applications until the crop is established, so that the roots are ready to take it up.  Often two or three applications will be better than one, to ensure that nitrogen supply is adequate throughout crop development.

The faster the growth of a plant, the greater its need for nutrients. Sweetpotato vines grow fastest during the intermediate phase of storage root initiation, which is between four and eight weeks after planting. During this period, all nutrients should be available in balanced concentrations in the ground water. Particularly potassium should not be deficient during this period, since it plays an important role in determining the number of young thick roots to become storage roots. It is common to split potassium applications, with some at planting and some at 4-6 weeks.

Organic fertilizers release nutrients slowly, and therefore should be applied as a basal fertilizer. The supply of nitrogen from organic fertilizer and natural sources is normally enough to support the vine growth during the initial and intermediate phases of crop establishment and storage root initiation.


Soil amendments such as lime or gypsum need to be mixed through the soil to be effective.  This is done during land preparation before ridging.  Phosphorus, on the other hand, is at risk of being bound up if mixed through the soil.  It should be banded in the ridges during their formation.  Within the band, it will saturate the soil's binding capacity, leaving most of the fertilizer available to plants.

Adapted cultivars

Plant species, and even cultivars within species, differ in their sensitivity to low nutrient levels in the soil, and to toxic levels of elements. They may vary in their nutrient uptake or in tissue requirements for a particular nutrient. For some crop species, agricultural scientists have developed cultivars resistant to particular nutrient problems, such as sunflower cultivars tolerant of low B supply, and barley cultivars resistant to B toxicity. In sweet potato, a great diversity of cultivars exist, and there is evidence of regional adaptations conferring tolerance to low N, and possibly to low B. Researchers have identified lines which are tolerant of Al or of salinity and B toxicity. It is likely that variation exists in the requirements for other nutrients also, which may be used in the future for selection and breeding of cultivars to overcome particular nutritional problems.

However, adapted varieties can’t perform miracles. They can’t create nutrients where there are none, and they can’t produce high yields of nutritious food for humans without those essential ingredients. They are most useful on problem soils or low input systems where a moderate yield is better than none. They are no substitute for a fertile soil.

Socioeconomic context

The approach taken to crop nutrition will depend on the grower’s context. Where fertilisers and water are readily available and relatively inexpensive, growers may aim to maximise the crop’s potential by eliminating any nutritional stress. Where fertilisers are unavailable or too expensive, the aim may be to optimise use of resources in the agroecosystem, in order to gain an adequate and sustainable reward for the grower’s labour. At whatever level of operation, it is important to recognise the limitations of the resource base. A traditional cropping system may become unsustainable through intensification, whether by increasing the number of crop cycles between fallows, or shortening the fallow period. However, intensification is frequently paralleled by a shift from subsistence to cash cropping. At some point in this progression, the purchase of inputs, including fertilisers, may become profitable. Grower advisers should remain aware of the options, even if some are not currently cost-effective.


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


Causes of nutritional disorders

Diagnosing nutritional disorders

Other topics on Crop Management:

Land preparation

Planting material preparation


Water management

Vine lifting

Integrated pest management


Postharvest practices


Fertiliser application before planting in the Philippines (F. Villamayor).

Nitrogen fertilizer being banded and buried during re-ridging, at four weeks after planting (J. O'Sullivan).