Transcript Sulfur Dynamics in Soils

NPK Management in
Soils
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Introduction
Soil Nitrogen (N) Dynamics
Soil Phosphorus (P) Dynamics
Soil Potassium (K) Dynamics
Introduction
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NPK as the three principal fertilizer elements
have the most widespread influence on both
natural and agricultural ecosystems.
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Plants that are deficient in any of NPK are often
doomed because the elements are components of
essential plant compounds which deficiency
cannot be easily overcome.
The importance of NPK in crop production and
environmental management depends on the
ecosystem in question –natural, agricultural, or
polluted ecosystems.
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1. Nitrogen Dynamics of
Soils
1.1 Introduction
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Nitrogen is an essential plant nutrient
It is mainly in organic forms in soil
In it’s ionic form (NO3-, NH4+), it is very
mobile in soils and plants
It is also responsible for some
environmental problems.
1.1. Role of nitrogen in plants
Component of essential plant compounds
Amino acids ~ building blocks of proteins,
enzymes
 nucleic acid ~ hereditary control
 Chlorophyll ~ photosynthesis
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Plants respond to good available N by having
deep green color of leaves
 Increased protein content
 increased plumpness of grains
 Increased plant productivity in general
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1.2. Deficiency of nitrogen in
plants
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Nitrogen is quite mobile (easily translocated) within
plants, so available N is sent to newest foliage first
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Deficiency exhibits the following:
Pale yellowish green color (chlorosis) in older leaves
 Have stunted growth and thin stems (low shoot:root
ratio)
 Plants mature more quickly than healthy plants
 Protein content is low and sugar content is high
 There is reduced productivity in general
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1.3. Oversupply of nitrogen
in plants
When too much N is applied, the following
may occur:
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Excessive vegetative growth (but weak plant
stems) that lead to lodging with rain or wind
Delays maturity and cause plants to be prone
to diseases
General decline of plant product quality
Environmental aspects (build up of nitrates)
1.4. Distribution of Nitrogen
(The Nitrogen Cycle)
1.5. Management of Soil
Nitrogen
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Objectives of good N management
1. Maintain adequate N supply
2. Regulate the soluble forms of N to ensure enough is
readily available
3. Minimizing leakage from soil-plant system
Strategies for Achieving the Objectives:
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Taking into account N contribution from other
sources so as not to oversupply N
Improving efficiency with which fertilizer is applied
Improving crop response knowledge
Avoiding overly optimistic goals of meeting crop
needs that are higher than possible
2. Soil Phosphorus
2.1. Phosphorus and the environment
1.
Total concentration of phosphorus in soils is usually low (compared to nitrogen
and potassium).
2.
Compounds in soil that are the primary source of phosphorus –phosphates are
not very soluble, resulting in phosphorus being unavailable enough for plants.
3.
When available forms of the element is applied to soil in forms of fertilizer or
manure, they can be easily fixed by soil colloids.
4.
Too little or too much phosphorus can have negative impacts on the
environment.
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When there is too little P –land degradation
P poor soils provide little vegetative cover to prevent land surface wash by erosion, and eroded materials
sediment down stream.
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When there is too much P –eutrophication
P enters streams from point and nonpoint sources.
Timber harvest, livestock grazing, fertilizers and application of animal wastes, and soil tillage.
2.1. Phosphorus and Plant
Growth
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Adequate P enhances fundamental processes in
plants such as photosynthesis, nitrogen fixation,
flowering, fruiting, and maturation.
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P is very important for the overall improvement of
crop quality whether in forages or vegetables
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It Stimulates root growth, particularly development
of lateral roots and fibrous rootlets.
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P is a component of the energy transformation
compounds (adenosine diphosphate, ADP;
adenosine triphosphate, ATP; nicotinamide adenine
dinucleotide phosphate, NADP+).
2.2. Deficiency of Phosphorus in
Plants
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A P deficient plant is usually stunted and
thin stemmed, but retains a dark color.
Not as simple to detect as is the case of N
deficiency.
When severe cases of deficiency occur,
there is yellowing and senescence of
leaves.
But these occurs first in older leaves.
2.3. The Phosphorus Cycle
2.4. Managing Soil
Phosphorus
1.
Proper management of soil pH (liming or acidification)
to a pH between 6-7
2.
Since P is not very mobile, little is leached and
therefore little reaches subsurface soil. This may
require some incorporation
3.
Banding application is generally better than broadcast.
Significant amounts of P can be transported with
sediment to surface waters were it can cause
eutrophication.
4.
Therefore, to minimize P contamination, every effort to
minimize erosion should be made.
3.1. Potassium
Dynamics in Soils
It is present in soil solution as positively
charged cation, K+
It does not form any gases that is lost to air
Its behavior in soil is influenced more by cation
exchange reactions than by microbiological
processes
It does not cause off site environmental
problems
It is not toxic and does not cause eutrophication
problems
3.2. K Problem in Soil
Fertility
3.3. Potassium in Plant and
Animal Nutrition
Potassium activates many enzymes in plants and animals that
are responsible for energy metabolism, photosynthesis and
other processes
As a component of the cytoplasm of plant cells, it helps to
lower cellular osmotic water potential thereby increasing the
ability of root cells to take up water.
K is important for nitrogen fixation in legumes
Good K nutrition has been shown to help plants adapt to
environmental stress
In animals including humans, K is important in regulating the
nervous system and maintenance of good blood vessels.
3.4. Deficiency Symptoms of
Potassium
Reduced ability to adapt to environmental
stress, e.g., drought, lodging, etc
Tips and edges of leaves become yellow
(chlorosis) and then die (necrosis).
In some crops, K deficiency produces
white necrotic spots that looks like insect
damage.
3.5. The Potassium Cycle
3.6. Potassium Management
The main problem with managing soil K is
that of converting the unavailable forms of
the element to available forms
 Available K is usually supplemented by
fertilization
Another problem is removing a lot of crops
from the soil without returning the crop
residues
 Attempts should be made to return as much
residue as possible for the natural plant-soil
cycling of K to continue
Potassium Management contd
Growing high K content plants places demand on
the soil supply of potassium
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To have high yields of such crops, e.g., alfalfa, it needs to be
planned in advance to supply soil with enough K to last the
cropping season –fertilization
Light and frequent application of fertilizer is
better than heavy and infrequent application to
reduce luxury consumption
Treating soils with lime have been found to
increase K retention in soils.