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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:
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Correct diagnosis of the problem;
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Understanding the other environmental limitations on
yield;
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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.
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.
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Further
topics on Soil Management:
Soil
management
Soil
structure
Soil
organic matter
Plant
nutrients
Fertilisation
Causes
of nutritional disorders
Diagnosing
nutritional
disorders
Other topics on Crop Management: Land
preparation
Planting
material preparation
Planting
Water
management
Vine
lifting
Integrated
pest management
Harvesting
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).
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