effective fertilization of cereal crops.pdf - Rice Knowledge Bank

Effective fertilization of cereal crops
Fertilizer requirements of cereal crops depend
mainly on two things: the fertility status of the
soil (after harvesting the previous crop) and the
yield targets of the farmer. Soils of low fertility
status require more fertilizer as do higher yield
targets. At each harvest, nutrients taken up from
the soil by the growing crop are exported from
the field in the form of grain or stover (see Table
1), further depleting the productive capacity of
the soil. Thus, arable soils require external
nutrient replenishment in the form of fertilizers
(organic and mineral). Nutrient mining and
insufficient supply of plant nutrients are some of
the major problems contributing to declining
crop yields. Maize and wheat, like most field
crops, require a good supply of primary
nutrients (i.e. nitrogen ‐ N, phosphorus ‐ P, and
potassium ‐ K) with the addition of other
essential elements (e.g. magnesium ‐ Mg,
sulfur – S, zinc – Zn, and boron ‐ B) in smaller
quantities where necessary.
How to choose the fertilizer
Fertilizer use in cereal systems aims at providing
the crop with balanced proportions of essential
nutrients when required. Crop growth is
dependent on a sufficient supply of each
nutrient, and yield is limited by the nutrient in
shortest supply. To determine fertilizer needs
for your crop, it is essential to know which
nutrients are required for the desired target
yield. Nitrogen is the most important nutrient
in plant growth and is involved in all major
processes of plant development and yield
formation. Loss of green color is the most
typical hunger symptom of N. Yellow
discoloration of leaves from midrib backwards
and premature death of lower leaves are also
indicators of N deficiency. In the early stages of
a plant growth, P is important in laying down
the foundation for its productivity. P deficiency
is accompanied by a marked reduction in plant
growth and leaves develop a dark bluish‐green
and purple color. A K deficiency is accompanied
by a weakening of the stalk, resulting in lodging
and crop loss. Symptoms of K deficiency include
drying along the tips and edges of lower leaves.
Figure 1. Careful fertilizer application is essential to prevent
overuse and reduce the overall fertilizer costs.
The recommended fertilizer application rate
depends on a number of factors:
• Crop variety and yield potential: Varieties
differ in their response to fertilizers
depending on their yield potential.
Improved cereal varieties with high yield
potential will require more nutrients in
order to achieve their yield potential. Their
yields will be significantly reduced if
fertilizer rates are low.
• Crop and fertilizer management: Efficient
use of applied nutrients depends on soil and
crop management. Fertilizer application
methods and timing affect nutrient
availability. Timely weeding reduces
competition for nutrients by weeds.
Application of irrigation water (in terms of
both amount and timing) influences nutrient
movement in the soil and the uptake by
crop.
• Soil type: Soil fertility is determined by soil
type, which in turn is based upon its depth,
organic matter content and texture. N, P and
S reserves in inherently poor soils with low
organic matter content, coarse texture and a
history of continuous cropping for many

years, are usually very low while there may
be sufficient K for 50‐75% of crop needs.
• Balance of other fertilizer elements: A
sufficient supply of all nutrients (including
micronutrients) is important for a good,
healthy crop and the efficient use of each
applied nutrient. If the nutrient is present in
soil, but insufficient in quantity, the plant
growth and uptake of other nutrients is
limited.
• Weather conditions: Response of rainfed
crops to fertilizer also depends on soil
moisture. Highly variable rainfall patterns
make optimum yield and fertilizer
requirements difficult to predict, so fertilizer
use is a risky investment. When drought
risk is high, split applications of nitrogen
fertilizers may be advisable, with
adjustments throughout the season based
upon evolving weather conditions.
Generally P applied in the previous season
retains some residual availability for the
next crop since it is not easily leached.
Blanket fertilizer recommendation packages for
different agro‐ecological zones based on a
‘normal’ season’s yield potential are available
from most national agricultural research
institutions. However, many of these
recommendations do not take into account the
variations in soil type, different crop
management practices, and weather conditions.
Timing and methods of fertilizer application
One of the most important aspects of fertilizer
usage is to know when and how fertilizers
should be applied. It depends primarily on the
type of crop grown and the mobility of the
particular nutrient applied to soil. With
nutrients that are stored efficiently in soil (i.e. P,
K, S), fertilizers can be broadcast annually at or
before planting, or banded below the seed. P is
immobile in soil and it should therefore be
incorporated into the root zone (e.g. during
sowing). N application should be timed to
coincide with periods of peak demand and rates
adjusted according to rainfall received during
the season via split application (particularly in
sandy soils) to reduce leaching losses.
Without a means of anticipating the nature of
the upcoming season, the best a farmer can do is
to apply fertilizer tailored for a typical season.
To minimize losses, farmers can employ
‘response farming’ strategies where applications
are conditional upon rainfall and are adjusted
according to the evolution of the season.
Application of nitrogen shall be split to at least
two (heavy textured soils) or three (sandy soils)
doses in a season. Nitrogen should not be
applied if there is severe wilting.
Figure 2. Maintaining crop residues in the field contributes to
natural cycle of nutrients and reduces the need for fertilizer.
Nutrient Maize, grain (1 t) Maize, stover (1 t) Wheat, grain (1 t) Wheat, straw (1 t)
Nitrogen 16 kg 7.3 kg 26.8 kg 6.4 kg
P2O5 6.8 kg 2.6 kg 10.7 kg 1.5 kg
K2O 4.8 kg 18 kg 6.1 kg 10.9 kg
Magnesium 1.6 kg 2.3 kg 2.7 kg 0.9 kg
Sulfur 1.4 kg 1.4 kg 1.8 kg 1.3 kg
Table 1. Nutrient removal rates in maize and wheat ‐ adapted from International Plant Nutrition Institute
(http://www.ppi‐far.org/ppiweb/usanc.nsf/$webindex/E71D7CA9BD24A18D86257060007A8EB3)
This fact sheet was prepared as part of the IRRI‐CIMMYT alliance Cereal Knowledge Bank
(knowledgebank.cimmyt.org) • Developed with input from F. Mtambanengwe, P.Kosina, J.Jones • 2007