Diagnosing nutritional disorders

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Plants respond to a deficiency of any nutrient firstly by decreasing their growth rate. Specific symptoms which may allow the deficient nutrient to be identified, usually occur only at relatively severe levels of deficiency. Nevertheless, such symptoms are often the first thing to alert the grower that there is a problem, and they are very useful for diagnosis. Since plant species differ in sensitivity to particular nutrient stresses, observation of symptoms on other crops in the same area should indicate to producers and advisers that the problem may also be affecting sweetpotato, even if symptoms are not evident on sweetpotato crops. Chemical analysis of the plant tissue provides a second diagnostic tool, which, when available, can be very valuable for confirming a tentative diagnosis based on visible symptoms, or identifying a suspected problem when symptoms are not visible.

If a soil is low in several nutrients, the plant usually shows deficiency symptoms of the element which is most limiting to growth. If that nutrient is supplied, the growth rate will increase until it is limited by the next most scarce nutrient, and a new set of symptoms may develop. It is difficult to establish from plant symptoms or tissue composition which nutrients are likely to be deficient other than the one most limiting. Soil tests may provide some guidance, but they need to be calibrated for each crop and soil type before they can be interpreted with confidence.

One of the simplest and most reliable methods of discovering all the nutrients which are deficient in a soil is through small pot trials. In nutrient omission pot trials, each pot receives an adequate supply of all the mineral nutrients (control, or “All” treatment), or all minus one. Plants are grown for a short period (usually 4-6 weeks), and the dry weight of tops is compared with that of the “All” treatment. Any nutrient whose absence results in a significant decrease in weight is regarded as deficient. Usually an indicator plant such as maize is used, which offers rapid and uniform establishment from seed, and well characterised responses to nutrient deficiencies. However, nutrient levels which are deficient for one species may be adequate for another. sweetpotato tip cuttings have been used successfully in nutrient omission pot trials.

If a nutrient deficiency is suspected, the best way of confirming it is by observing a positive response to fertiliser containing that nutrient. When testing a fertiliser response, it is important to have an untreated area for comparison. Usually a narrow test strip in the middle of the field is treated with fertiliser. Alternatively, fertiliser may be applied to the whole field, with the exception of a central strip, which would remain unfertilised. Fertilisers often contain more than one nutrient, and it is important to establish which nutrient is causing the response. For example, N and S deficiencies both produce similar symptoms in the crop. They can be differentiated by applying sulfate of ammonia (containing both N and S) to one strip and urea (containing only N) to another. If the crop greens up in both strips, the problem is N deficiency, but if it only responds to the sulfate of ammonia, it is S deficiency.

Another useful technique for confirming deficiencies of micronutrients is leaf painting. By carefully painting half of an affected leaf with a dilute solution of the suspected nutrient, the response can be seen by comparing the colour or expansion of the painted half with the unpainted half. Leaf painting has been used successfully to diagnose deficiencies of Fe, Mn and Zn in sweetpotato.

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.

Diagnosing nutritional disorders - further topics:

Visible symptoms

Tissue analysis

Soil analysis

Further topics on Soil Management:

Causes of nutritional disorders

Correcting nutritional disorders