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
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:
topics on Soil Management:
of nutritional disorders
Other topics on Crop Management: