| Introduction Agriculture plays an important but highly variable
role in the economies of Mexico, Central America and South America,
contributing as much as 30 percent to the gross national product (GNP)
in the case of Honduras, but less than 6 percent in Venezuela.
Traditionally, agricultural exports have been significant contributors.
A variety of soils and climatic conditions exists in the region,
resulting in a diversity of agricultural products.
In general, fertilizer use is low,
averaging only 23 kg/ha in Honduras and 43 kg/ha in Nicaragua, The
averages are brought down by the low rates of application in subsistence
agriculture and by the fact that only 8 percent of the arable land is
irrigated in Latin America. Also, fertilizer is often not applied in
balanced proportions. In Mexico, for example, in a recent year the
average N:P2O5:K2O ratio used on crops was 1.0:0.3:0.1. The ratio in
Peru was 1.0:0.2:0.1, while that in Brazil was 1.0:1.60:1.55. The ratios
are strongly influenced by economics, soil conditions and cropping
patterns.
The region has varied soil and water
pollution problems...deforestation, erosion, garbage, etc. Land area for
agricultural use is shrinking on a per capita basis. Inflation is
eroding the farmer's purchasing power. At the same time, there is a lack
of current, unbiased scientific data to encourage the proper use of
purchased inputs such as mineral fertilizers. Soil analysis services are
insufficient to permit extension agents and distributors to give sound
technical advice.
Agriculture in the region faces strong
challenges, but it also has multiple opportunities...to feed its people
and support its export market. These guidelines can be useful in
contributing to the development of agricultural systems that are
environmentally and economically sustainable. Farmer profits and
environmental protection must go hand-in-hand. Ecological preservation
is possible only under favorable economic conditions for the farmer.
Both mineral fertilizers and other
nutrient sources (animal manures, plant residues, municipal and
industrial waste, etc.) must be used scientifically in a system of best
agricultural practices if crop needs of the region are to be met. The
accepted principles of crop fertilization covered in these guidelines
can be applied in a general way for all fertilizer sources, mineral and
organic. However, site specific management guides should be developed
for each country, each area within the country...even individual farms
within an area. Specific scientific data should be the basis for the
guides so that balanced fertilization can be practiced.
Objectives of the Guidelines
- To explain how mineral fertilizers
should be used to supplement existing soil fertility levels in
meeting plant nutrient requirements.
- To restore and maintain soil
fertility levels for long-term soil productivity by proper use of
mineral fertilizers.
- To show how nutrient losses, both by
erosion and to groundwater, can be minimized with sound fertilizer
management.
- To promote economic crop production
and environmental protection through the use of best agricultural
practices.
- To create public awareness and to
provide planners, policy-makers and other leadership with an
understanding of the critical role of mineral fertilizers in
sustaining agricultural production.
Crop nutrition and plant nutrient loss
from soils
Crops require an adequate supply of
nutrients. Sufficient fertilizer should be applied to meet crop needs.
Proper balance is also required to produce optimum yields, while
protecting the environment. Some of the benefits of fertilization are:
- Promotion of more extensive root
systems, increasing nutrient and water uptake for higher yields, and
holding the soil in place against wind and water erosion,
- Providing a quicker canopy cover,
exposing more leaf area to sunlight to increase photosynthesis, and
covering the soil surface, reducing the erosive impact of rainfall,
- Supporting the production of more
residue, above and below ground, to build soil organic matter and
nutrient reserves and to stabilize the soil,
- Improving water use efficiency of
the growing crop,
- Increasing crop resistance to
stresses such as drought, pests, heat and cold.
Use of mineral fertilizers to supplement
soil fertility is an ideal method of ensuring that crop nutrient
requirements are met and the environment protected. Mineral fertilizers
are relatively easy and efficient to transport and can be stored without
risk to the environment. Nutrient contents are known and can be balanced
for individual crop needs. Micronutrients can be added as required.
Since they can be easily applied, by hand or with mechanical equipment,
mineral fertilizers can be used near the time the crop needs the
nutrients. This increases efficiency and reduces potential loss.
Nutrients contained in most mineral fertilizers are already in a form
the plant can take up and use. The key is to provide balanced nutrition
at the time the crop can use it most efficiently.
Organic fertilizers, such as animal
manures, are also good nutrient sources and should be utilized where
available. Their value can be increased by composting to improve their
efficiency. The incorporation of crop residues, rather than burning
them...the common practice, should be encouraged. Wherever organic
by-products can be recycled as nutrient sources, they should be used.
However, storage, transfer and application of organic fertilizers are
labor intensive and expensive. Their release rates of nutrients are
unpredictable and difficult to control. Organic materials can normally
supply only part of the nutrient requirements of the crop and need to be
supplemented with mineral fertilizers. Also, most often, organic sources
are imbalanced with respect to plant needs.
Long-term productivity can be improved by
restoring or building soil fertility levels as well as supplying
nutrient requirements of the current crop. Good planning is required to
take best advantage of existing market prices, for fertilizers as well
as crops being grown. Mineral and organic fertilizers can be used to
build fertility...and, thus, productivity. Certain soil and/or climatic
conditions might preclude this practice, however. For example, many
tropical soils are extremely deficient in phosphorus, but may also have
high fixing capacities, making fertilizer phosphorus less available for
crop use. Decisions as to how and where soil fertility levels can be
improved should be based on local climatic, crop, soil and economic
conditions.
It should be understood that some
nutrient loss will occur, regardless of the fertilizer source or method
of application. Certain forms of nitrogen are lost to the air. Other
nutrients can be tied up in organic matter or 'fixed' in unavailable
forms in the soil. Nitrogen and other nutrients might be lost by
leaching or erosion. Many of these same kinds of losses occur when the
soil is not fertilized. Fertilizers do not necessarily increase the
losses. In fact, when balanced fertilization is followed, plant uptake
efficiency is increased, and nutrient losses are often reduced,
improving farmer economics and protecting the environment.
On some marginal lands or where soils
have been excessively mined of nutrients or eroded because of poor
management, a combination of legume cover crops and fertilization can be
used to rehabilitate the land and reduce erosion. Cover crops, both
grass and legume, can also be used to stabilize soils in those areas
where there is a significant period of time between harvest and planting
of the next crop. Legumes grown in rotation with grass crops or
inter-cropped with perennial species can add nitrogen to the soil. They
must be adequately fertilized with other essential nutrients, however,
if they are to be of significant benefit.
The quantities of plant nutrients to be
applied
Plant nutrient requirements depend upon
several factors, including crop to be grown, yield potential and
nutrient-supplying power of the soil (soil fertility level or rating).
- Crop.
Each crop has specific nutrient needs. That is, it takes up and
removes from the soil a certain ratio of nutrients that might be
significantly different from another species. For example, the ratio
of N:P2O5:K2O uptake by rice is
1.0:0.5:1.5, while that for banana is 1.0:1.0:3.8. Grass crops must
be fertilized with nitrogen; legumes produce most or all their
nitrogen requirement.
- Yield.
Total quantities of nutrients needed by a growing crop can be
determined when the uptake per yield unit is known (e.g., kg/tonne)
and when yield level is established.
- Soil fertility level.
Analytical testing methods are available to determine how much
nutrition can be supplied to the crop from the soil. Total
fertilizer requirement can then be calculated when crop, yield and
soil nutrient contribution are established, keeping in mind the need
to build or maintain fertility levels in addition to supporting
nutrient requirements of the current crop.
Setting realistic yield goals is critical
to optimizing nutrient use efficiency by the crop, to profitability, and
to environmental protection. In setting yield goals, production history
should be reviewed. This requires good record keeping on the part of the
farmer. Where there is no yield history for the specific farm, best
averages for the area can be used. Yield goals should be realistic, but
high enough so the farmer can realize an acceptable economic return from
his investment.
It is critical that fertilizer rates not
exceed the economic optimum for the crop, field and management level of
the farmer. When nutrients are applied at optimum economic and balanced
agronomic rates, the efficiency is greatest and potential loss to the
environment is minimal. When such rates are exceeded, farmer profits are
lost and potential damage to the environment increases. The ideal
program in applying plant nutrients is to follow a scientific,
site-specific approach. Where research data are not available, data from
similar areas can be used and adapted to best farmer practices for the
area.
When soils are too acid (low pH), lime
should be applied before fertilization. Lime reduces potential toxicity
from some elements, improves soil microbial activity, and enhances the
efficiency of plant nutrient use.
Timing and method of application
Timing of fertilizer application is most
critical for mobile nutrients such as nitrogen. Nitrogen is very soluble
in water and can be lost in runoff during intense rainfall as well as
leached out of the soil profile. Immobile nutrients such as phosphorus
and potassium can be safely applied at any time if incorporated into
soils where erosion is not a problem. Large amounts of potassium should
not be applied to deep, sandy soils.
For optimum crop use efficiency and
minimum potential for environmental damage, fertilizer nutrients should
be applied as near to the time the crop needs them as is practical.
Further, they should be placed in the soil so the plant root system can
reach them.
Management factors such as planting date,
crop rotation, soil fertility and weather conditions should be
considered in determining fertilizer application methods and timing.
Further, all production inputs...tillage. plant population, variety
selection (where available), crop protection and others...should be
managed at optimal levels to enhance nutrient uptake and use.
Fertilizer application equipment should
be adjusted to assure uniform spread and correct rates. Equipment should
be well maintained. When fertilizer is applied by hand, extreme care
should be taken to distribute nutrients uniformly.
Type of fertilizer to be applied
It is recognized that the availability of
different fertilizer sources might be limited in a given area. Most
mineral fertilizers perform equally well when suited to crop and soil
conditions and when applied at optimal rates and by proper methods.
Where there are choices, the farmer should make the selection based on
the best economic and environmental advantages. For example, it might be
advantageous to use ammonium-based nitrogen sources and nitrification
inhibitors when a high risk for nitrate-nitrogen leaching exists. The
use of nitrate-nitrogen may be advantageous when faster action is
required or for surface application on pastures to reduce
volatilization. Slow release nitrogen fertilizers may be appropriate for
some crops and/or certain soil conditions.
Attention should be given to the physical
quality of fertilizer materials. Care should be taken to maintain good
physical characteristics during transport and storage. Good quality,
homogenous granules are important for the accurate application of
fertilizers. In the case of bulk blends, it is most important that the
different materials be of similar particle size to avoid undue
segregation.
Fertilizers with a high nutrient content
(high analysis) offer substantial savings in transport, handling and
application costs, though primary consideration must be given to the
nutrient requirements of the crop(s) to be grown.
General
These guidelines are, of necessity,
advisory in nature. They present principles and concepts which can be
adjusted and adapted on a site-specific basis. Their intent is to focus
on those management techniques which will result in the use of crop
fertilization in such a way that both the farmer and grower will
benefit. They encourage the use of scientific knowledge.
It is hoped that these guidelines will
further the cause of correct and balanced use of all fertilizer sources,
mineral and organic...so the farmer, consumer and environment will all
benefit.
Further information :
PPI - Potash & Phosphate Institute
655 Engineering Drive, Suite 110 - Norcross, GA 30092-2837 - USA
Tel: (1) (770) 4470335 - Fax: (1) (770) 4480439
ANDA - National Association for the
Promotion of Fertilizers and Lime
Praça Dom José Gaspar 30, 9. Andar - 01047-901 Sao Paulo, SP - Brazil
Tel: (55) (11) 2559277 - Fax: (55) (11) 2142831
First published : June 1994
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