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CHAPTER II
REVIEW OF RELATED LITERATURE AND STUDIES
Plants are important to human life in various ways. Without plants, animal life on
earth would be almost impossible. Plants are the primary producers on our planet. All
other consumers depend on plants for their food source. Even tiny organism that
decompose the remains of other organism need plants to produce food first. By producing
oxygen as a by-product of photosynthesis, plants give other organism the main tool
needed to burn food calories. Without oxygen, no food energy could be released to
nourish other life. Plants of all sizes with root system stabilizes the soil. Without them,
severe erosion can occur and destroy the ability of land to support major plant growth
(Teal, 2007)
Although plants are the primary producer on our planet, plant themselves need
some elements for them to grow. Basically, plants need soil, water, sunlight, proper space
and other mineral nutrients.
Plants need soil. Water and minerals are taken from the soil through the roots. Soil
also provides support for plant and an anchor for the roots to grow. Decaying plants and
animals leave behind minerals in the soil that are essential for a healthy growth.
Plants need sunlight in order to grow properly. They also use sunlight in the food
production process called photosynthesis. Photosynthesis occurs when green plants use
the energy from sunlight to put together foodstuffs from carbon dioxide and water. (Plant
Nutrition, n.d)
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Plants need water. Water is necessary to all life on earth. No known organism can
survive without water. Water is essential for the germination of seeds and growth of
plants. Water serves as the medium in which plants absorb soluble nutrients from the soil.
Also, water serves as a medium for transport of chemicals to and from cells. (Khanna,
2011)
One of the mineral elements that plants need to grow is Nitrogen. Plants are
surrounded the nitrogen in our planet. Every acre of the earth’s atmosphere is covered by
thousands of pounds of essential nutrient. The plant world may literally be said to be
submerged in a sea of nitrogen, yet nitrogen is not directly available to the plants that
need it to grow, develop and reproduce. Despite nitrogen being one of the most abundant
elements on earth, nitrogen deficiency is probably the most common nutritional problem
affecting the plants worldwide.
Healthy plants often contain 3-4% nitrogen in their above ground tissues. These are
much higher concentration than those of any other element except carbon, hydrogen and
oxygen nutrients, not of soil fertility management concern in many situations. (Eckert,
n.d) Nitrogen’s most recognized role in plant is its presence in the structure of the protein
molecule. In addition, Nitrogen is found in such important molecules as purines,
pyrimidines, poryphrins and coenzymes. Purines and pyrimidines are found in the nucleic
acids RNA and DNA essential for protein synthesis. The poryphrin structure is found in
such metabolically important compounds as a chlorophyll pigments and the cytochromes
essential in photosynthesis and respiration. Coenzymes are essential to the function of
many enzymes. The most easily observed symptom of nitrogen defieciency is the
yellowing of leaves due to a loss in chlorophyll. We usually notice this first symptom in
the more mature leaves and last in the upper, more actively growing leaves. The nitrogen
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deficiency symptoms appear last in the younger because of the high mobility of nitrogen
in the plant. The younger leaves retain their nitrogen and, in addition, obtain nitrogen
translocated from older leaves. Under severe conditions of nitrogen deficiency, the
lowermost leaves on plants such as tobacco or bean will be dry and yellow and, in many
cases will abscise. Under these conditions, the topmost leaves are generally pale green in
colour.
One interesting characteristic of nitrogen deficiency found in many plants is the
production of pigment other than chlorophyll when nitrogen is lacking. In tomato plants,
a purple colouring of the leaf petioles and veins can be caused by anthocyanin formation.
(Devlin & Witham, 1983)
Adequate supply of nitrogen is essential for a healthy plant growth. Many people
use some forms of nitrogen fertilizer to enhance the nitrogen level in their soil. To
enhance the nitrogen level in the soil, many people are tempted to add more and more
nitrogen into the soil. But there are costs of using too much nitrogen fertilizer. Excessive
fertilizer can burn plants and damage leaves by increasing the mineral salt concentration
into the soil. Also, extra nitrogen not used by plants may be leached into the groundwater,
in the form of nitrate, a common pollutant. (Andrews, 1998)
Nitrogen fertilizers are said to impact positively on crop production. The
increasing cost of fertilizer, however, continues to be a major problem by farmers. Today,
producers complain that fertilizer costs in the Philippines are far higher than those in
Malaysia and Thailand. High fertilizer price translates to higher production cost. (Sevilla,
2006)
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One way of acquiring nitrogen for the soil is through nitrogen fixation. Nitrogen
fixation is the natural process, either biological or abiotic, by which nitrogen (N2) in the
atmosphere is converted into ammonia (NH3) (Nitrogen Fixation, n.d).
There are ways of fixating atmospheric nitrogen into the soil. One way is by
lightning. The energy from lightning causes nitrogen and water to combine to form
ammonia and nitrates. Precipitation carries the ammonia and nitrates to the ground, where
they can be assimilated by plants. (Helmenstine, n.d)
Another way is by using legumes. Legumes are notable for their ability to fix
atmospheric nitrogen because of its mutualistic symbiotic relationship with the bacteria
Rhizobia found in the root nodules of this plant. The ability to form this mutualism
reduces fertilizer cost for farmers and allows legume to be used in crop rotation to
replenish the soil which has been depleted of nitrogen. (Nitrogen Fixation by Legumes,
n.d)
According to a study in New Mexico State University in 2008, legume nitrogen
fixation starts with the formation of a root nodule. A common soil bacterium Rhizobia
invades the root and multiplies within the cortex cells. The plants supply all necessary
nutrients for the bacteria. Within a week after infection, small nodules are visible with the
naked eye. In the field, small nodules can be seen 2-3 weeks after planting, depending on
the legume species and germination conditions. When nodules are young and not yet
fixing nitrogen, they are usually white or gray inside. As nodule grow in size, they
gradually turn pink or reddish in color, indicating nitrogen fixation has started. The pink
or red color is caused by leghemoglobin (similar to haemoglobin in blood) that controls
oxygen flow to the bacteria. Nodules on many perennial legumes such as alfalfa and
clover, are finger-like in shape. Mature nodules may actually resemble a hand with a
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center mass (palm) and protruding portions (fingers), although the entire nodule is
generally less than ½ inch in diameter. Nodules on perennials are long-lived and will fix
nitrogen through the entire growing season, as long as conditions are favourable. Most of
the nodules (10-50 per large alfalfa plant) will be centered on the tap root. Nodules on
annual legumes, such as beans, peanuts, soybean, are round and can reach the size of a
large pea. Nodules on annual legumes are short-lived and will be replaced constantly
throughout the growing season. At the time of pod fill, nodules on annual legumes
generally lose their ability to fix nitrogen, because the plant feeds the developing seed
rather than the nodules. Beans will generally have less than 100 nodules per plant,
soybeans will have several hundred per plant and peanuts may have 1000 or more
nodules on a well developed plant. Legume nodules that are no longer fixing nitrogen
usually turn green and may actually be discarded by the plant. Pink or red nodules should
predominate on a legume in the middle of the growing season. If white, grey, green
nodules predominate, little nitrogen fixation is occurring as a result of an inefficient
Rhizobium strain, poor plant nutrition, pod filling or other plant stress. The nitrogen fixed
is not free. The plant must contribute a significant amount of energy in the form of
photosynthate (photosynthesis derived sugars) and other nutritional factors for the
bacteria. A soybean plant may divert 20-30% of its photosynthate to the nodule instead of
the other plant functions when the nodules if actively fixing nitrogen. Any stress
(especially phosphorus, potassium, zinc, iron, molybdenum and cobalt) can be corrected
with fertilizers. When a nutritional stress is corrected, the legumes responds directly to
the nutrient and indirectly to the increased nitrogen nutrition resulting from enhanced
nitrogen fixation. Poor nitrogen fixation in the field can be easily be corrected by
inoculation, fertilization, irrigation or other management practices.
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Nitrogen can be lost from the field through three principal pathways:
denitrification, leaching and surface volatilization. The form of N2 a farmer chooses
should depend on how serious a problem he has with the N2 losses.
Denitrification occurs when nitrate (NO3-) is present in a soil and not enough
oxygen (O2) is present to supply the needs of the bacteria and microorganisms in the soil.
If O2 levels are low, microorganisms strip the oxygen from the nitrate, producing N2 or
nitrous oxide (N2O), which volatilizes from the soil. Three conditions that create an
environment that promotes denitrification are wet soils, compaction and warm
temperatures.
Leaching occurs when soils have more incoming water (rain or irrigation) than the
soil can hold. As water moves through the soil, the nitrate (NO3-) that is in the soil
solution moves along with water. Ammonium (NH4+) forms of N have a positive charge
and are held by the negative sites on the clay in the soil; therefore, NH4+ forms of N leach
very little in clay. Ammonium forms of N can leach in a coarse-textured sands and some
muck soils. These are the only soils where ammonium leaching may be significant. One
way to minimize N leaching and denitrification is to minimize the time the N2 is in the
soil before plant uptake. This cuts down on the time when conditions are favourable for
losses.
Surface Volatilization of N2 occurs when urea forms of N2 break down and form
ammonia gases and where there is a little soil water to absorb them. This condition occurs
when urea forms of N are placed in the field but not in direct contact with the soil. The
rate of surface volatilization depends on moisture level, temperature and the surface pH
of the soil. If the soil surface is moist, the water evaporates in the air. Ammonia released
from the urea is picked up in the water vapour and lost. On dry soil surface, less urea N is
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lost. Temperature greater than 50 F and a pH greater than 6.5 significantly increase the
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rate of urea conversion to ammonia gases. (Vitosh & Johnson, 1995)
A perennial or forage legume crop only adds significant nitrogen for the
following crop if the entire biomass ( stems, leaves, roots) is incorporated into the soil. If
forage is cut and removed from the field, most of the nitrogen fixed by the forage is
removed. Roots and crowns add little soil nitrogen compared with the aboveground
biomass. (Lindermann & Glover, 2008)
The best way to prevent nitrogen losses from agricultural lands is through good
soil and water management practices. The first step in reducing potential nitrogen losses
is to have soil tested.
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