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Soils contain a complex mixture of chemicals including all
the essential nutrients. However, the abundance or availability of any nutrient
may not be sufficient to meet the crop’s needs. Fertilizers, manures and
composts are used to supplement the soil’s nutrient supplies. Chemical
fertilizers contain only one or a few nutrients, in high concentration. Manures
and composts, which are derived from plant and animal wastes, generally contain all essential
nutrients, but in lower concentrations and not necessarily in appropriate
ratios.
The most commonly deficient nutrients are N, P and K. These
elements are taken up in sufficient quantities that the soil becomes depleted
after a number of crops have been removed. In addition, N is readily lost to the atmosphere, and P
is often tightly bound by soils, making it only sparingly available to plants.
This is why these nutrients are often routinely applied.
However, in many situations other nutrients are also
deficient but the deficiency may not be easily recognized by the farmer. When
this is the case, fertilization with NPK fertilizers will not greatly increase
yields unless the deficient nutrient is also applied.
It is also important to recognize that supplying a nutrient
in excess of the crop’s requirement is of no benefit to the crop. It is an
unnecessary expense, and may cause environmental pollution when excess nutrients
leach into streams and groundwater supplies.
The following table summarizes the uptake and fertilization
method for each nutrient. Fertilizers should only be applied when they are known
to be deficient, and in the minimum quantity required to correct the deficiency.
This depends not only by the crop’s uptake, but also by losses due to
leaching and conversion to unavailable forms in the soil or air.
|
Nutrient |
Uptake
(per
tonne of storage root yield) |
Fertilizer
quantity
(kg/ha
of nutrient) |
Common
Sources |
Fertilizer Strategy |
|
Nitrogen:
N |
3-5 kg |
30 – 100 |
Urea (45% N),
Ammonium nitrate (35% N),
Animal manures (0.4 – 6 % N) |
Manure buried in top of ridge before planting;
Urea as split application, part at planting plus one or two
side-dressings. |
|
Phosphorus:
P |
0.5-1 kg |
20 – 200 |
Triple Superphosphate (TSP) (24% P),
Single Superphosphate (10% P) |
In a furrow on top of ridge (mixed with manure if used) and covered
with soil before planting. |
|
Potassium:
K |
5-8 kg |
50 – 200 |
Potassium Chloride (= muriate of potash) (50% K),
Plant mulches and composts (0.1 – 4% K) |
Composts incorporated in soil; KCl half in basal application and half
as side dressing (once or twice, 30 – 60 days after planting). |
|
Calcium:
Ca |
1-2 kg |
100 - 2000 |
Lime (40% Ca) –only on acid soils,
Gypsum (22% Ca),
Superphosphate (23% Ca) |
Broadcast and incorporated in soil before forming ridges. |
|
Magnesium:
Mg |
0.5-1 kg |
10 – 50 |
Magnesium Sulfate or Kieserite on neutral soils,
Magnesium Oxide or Dolomitic Lime – on acid soils |
Broadcast and incorporated in soil before forming ridges. Magnesium
sulfate can be sprayed on foliage. |
|
Sulfur:
S |
0.2-0.5 kg |
10 – 50 |
Gypsum (17% S),
Ammonium Sulfate (24% S),
Superphosphate (11% S) |
Gypsum broadcast and incorporated in soil; other fertilizers banded. |
|
Iron:
Fe |
10-20 g |
2 – 10 |
Chelated iron, ammonium ferric sulfate,
pieces of rusted scrap iron |
Soluble iron may be sprayed on foliage or banded. Scrap iron buried
under hills. |
|
Boron:
B |
5-10 g |
1 – 4 |
Borax, Solubor (13% B) |
Solution may be sprayed on foliage or banded. |
|
Manganese:
Mn |
10-30 g |
10 - 20 |
Manganese Sulfate or Oxide |
Solution may be sprayed on foliage or banded. |
|
Zinc:
Zn |
5-10 g |
3 - 10 |
Zinc Sulfate (hydrated) (23% Zn) |
Solution may be sprayed on foliage or banded. |
|
Copper:
Cu |
1-5 g |
1 – 7 |
Copper Sulfate |
Solution may be sprayed on foliage or banded. |
|
Molybdenum:
Mo |
0.2-1 g |
0.2 – 0.3 |
Sodium- or Ammonium molybdate (8% Mo),
Lime (to raise pH and availability of Mo) |
Solution may be sprayed on foliage or banded.
Lime broadcast and incorporated. |
Contributed
by: Jane O'Sullivan
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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
Correcting
nutritional
disorders
Other topics on Crop Management: Land
preparation
Planting
material preparation
Planting
Water
management
Vine
lifting
Integrated
pest management
Harvesting
Postharvest
practices
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