| Need
and Role of Nutrients in Plant Nutrition |
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| Plants
need water, air, light, suitable temperature, and sixteen
nutrients to grow. Plants get carbon, hydrogen and oxygen
from air and water. The other thirteen nutrients come
from the soil. Soil nutrients are divided into two groups
according to the amounts needed by plants. The Macronutrients
are nitrogen, phosphorus, potassium, calcium, magnesium,
and sulfur. The Micronutrients, which are needed in only
trace amounts, are iron, manganese, boron, zinc, copper,
molybdenum and chlorine. |
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| These
nutrients are essential for plant growth. Plants will
grow normally until they run short of one nutrient. Then
growth is limited by the availability of that nutrient.
Occasionally two or more nutrients will run short at the
same time. If the nutrients are deficient, or too abundant,
then plants will be discolored or deformed.The deficiency
symptoms will indicate which nutrient or nutrients are
needed. However, it is much better to supply additional
nutrients before deficiency symptoms appear. A soil test
will tell which nutrients are low before growth is affected.
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| Most
plants in most soils will grow better if additional nutrients
are provided by fertilizing. A soil test will give a complete
and accurate measure of the nutrients in the soil. A general
recommendation is that all soils need more nitrogen. Shallow
rooted plants, such as grass and flowers, need more phosphorus
and potassium. Acid loving plants, such as rhododendrons,
azaleas, camellias, junipers and pin oaks, often need
more iron. Sometimes sandy soils need micronutrients,
but rarely clay soils. Certain micronutrients may be deficient
in certain parts of the country. |
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| Many
fertilizers are available to supply additional nutrients.
Some fertilizers only supply one nutrient. Many supply
N, P and K only. A few fertilizers include all of the
macronutrients and micronutrients. The label on the package
will tell which nutrients are included as well as the
sources of the nutrients. The nutrients are identical
whether they come from organic or synthetic sources, but
the source will affect how fast the nutrients are available
to plants. Ammonia sulfate and water soluble fertilizers
release most of their nitrogen in a few days and may burn
plants if too much is applied. Blood meal releases its
nitrogen over a period of months. Organic fertilizers
and specially treated synthetic fertilizers release slowly
so they last longer and won't burn. Deeper rooting trees
and shrubs can be fertilized once a year, but shallow
rooted plants, such as grass and flowers, will need regular
fertilizing throughout the growing season. |
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| Water
can move nitrogen several inches in the soil. Phosphorus
and potassium hardly move in the soil. |
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There
are 16 nutrient elements that have been proven to
be essential for the growth and reproduction of
plants . Thirteen of these essential elements, which
may be supplied by the soil or supplemented by fertilizers,
are generally divided into two groups. The macronutrients
are nitrogen (N), phosphorus (P), potassium (K),
sulfur (S), calcium (Ca), and magnesium (Mg). The
second group of essential elements is called micronutrients
because those elements are required in small (micro)
amounts by plants. They include manganese (Mn),
iron (Fe), boron (B), zinc (Zn), copper (Cu), molybdenum
(Mo), and chlorine (Cl). Although these elements
are frequently referred to as minor or trace elements,
the term "micronutrient" is preferred.
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Considerable
discussion can be generated over what criteria defines
an essential element. In the context of this presentation,
we caution that the fact that an element is essential
does not mean that it needs to be added to soil
as fertilizer. |
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Since
plants obtain carbon, hydrogen, and oxygen from
the air and water, there is little control over
the availability of these nutrients. For most plants,
the dry tissue is composed of 94 percent or more
of these three elements. The other 13 elements combined
represent less than 6 percent of the plant dry matter.
Yet, crop production is frequently reduced and growth
limited by a deficiency of one or more of these
13 elements. |
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The
availability of nutrients to plants from the soil
is strongly influenced by various mechanisms of
removal, fixation, and release. One of the most
important soil properties that affects the availability
of nutrients is soil pH, a measure of the acidity
or alkalinity of the soil. |
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| Soil
Factors That Affect Micronutrient Availability |
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| Physical
and chemical characteristics of soil affect the availability
and uptake of micronutrients: |
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Soils
low in organic matter (less than 2.0%) may have
lower micronutrient availability. |
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Soils
with higher amounts of clay (fine texture) are less
likely to be low in plant available micronutrients.
Sandy soils (course texture) are more likely to
be low in micronutrients. |
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Soils
that have very high levels of organic matter (greater
than 30% organic matter to a depth of 30 cm) often
have low micronutrient availability. |
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Soil
temperature and moisture are important factors.
Cool, wet soils reduce the rate and amount of micronutrients
that may be taken up by crops. |
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As
soil pH increases the availability of micronutrients
decreases, with the exception of molybdenum. |
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