Diploma of Environmental Monitoring & Technology
Study module 4
Soil nutrients & fertilisers
Sampling & testing of
soils
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STS Study module 4
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INTRODUCTION
2
Nutrient classification
Nitrogen
Phosphorus
Potassium
Magnesium and Calcium
Sulfur
2
3
5
6
6
7
MICRONUTRIENTS
7
Cations (iron, manganese, zinc and copper)
The others – boron, chlorine, molybdenum
Optimum nutrient levels in soil
7
8
8
FERTILISERS
9
Inorganic fertilisers
Organic fertilisers
Fertiliser use
9
11
12
ASSESSMENT TASK
14
Assessment & submission rules
Problems?
References & resources
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Introduction
Plant nutrients are the species that they require to obtain from outside the plant (air, water,
soil) in order to grow and survive. In this chapter, we will look at the nutrients that plants
gain from contact with the soil. Carbon (as atmospheric carbon dioxide) and water (from the
soil) are not considered nutrients, in that without them the plant would not grow at all.
Nutrients move around in the plant through the course of its growth. Taking the example of
a seasonal plant such as corn, the major point of concentration initially is in the leaves. As
the growing season continues, the nutrients move towards the stalks and cobs. The grain
(fruit) develops last, and has a major requirement for the nutrients. The mobile nutrients
move from the leaves, stalks and cobs into the fruit. This develops deficiency symptoms in
the leaves as they drop below the necessary nutrient content.
Different nutrients have different abilities to move through the plant. Nitrogen is very
mobile, and will move easily to points where a deficiency or need occurs. When a nitrogen
deficiency occurs, it will move from the older growth to the new tissue. The same applies to
phosphorus, potassium and magnesium. Calcium and sulfur are much less mobile, and when
a deficiency occurs, the symptoms will appear in the new growth.
Nutrient classification
Nutrients are classified as macro or micro on the basis of their content in normal plants:
macronutrients have levels of greater than 500 mg/kg. The table below lists the macro and
micronutrients and a brief description of their role in plants.
MACRO
Role in Plants
Nitrogen
Constituents of all proteins, chlorophyll, and in coenzymes and nucleic
acids
Phosphorus
Important in energy transfer as part of adenosine triphosphate.
Constituent of coenzymes, nucleic acids and metabolic substances
Potassium
Little if any role as constituent of plant compounds. Functions as
regulatory mechanisms as photosynthesis, carbohydrate translocation
and protein synthesis
Calcium
Cell wall component. Plays role in the structure and permeability of
membranes.
Magnesium
Constituent of chlorophyll and enzyme activator.
Sulfur
Important constituent of plant proteins.
MICRO
Boron
Somewhat uncertain, but believed important in sugar translocation and
carbohydrate metabolism
Iron
Chlorophyll synthesis and in enzymes for electron transfer
Manganese
Controls several redox systems, formation of O2 in photosynthesis
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Copper
Catalyst for respiration, enzyme constituent
Zinc
In enzyme systems that regulate various metabolic activities
Molybdenum
In nitrogenase needed for nitrogen fixation
Chlorine
Activates system for product of O2 in photosynthesis
Cobalt
Essential for symbiotic nitrogen fixation
Table 4.1 - Macro and micronutrients in plants
Nitrogen
While nitrogen gas (as N2) dominates the atmosphere, plants absorb this nutrient from the
soil. Nitrogen, by amount, is the most important plant nutrient, and is the major limitation
to plant growth. The cycling of nitrogen in the biosphere involves six basic processes as
shown in the figure below.
Figure 4.1 - The nitrogen cycle
Fixation
Although nitrogen gas in the air is soluble in water, it cannot be absorbed directly by any
part of the plant. Nitrogen fixation solves that problem by the conversion of nitrogen to
ammonia by an enzyme called nitrogenase. Not all plants can do this. Those that can are
known as legumes, and form a symbiotic relationship with bacteria called rhizobium. The
bacteria reside in the plant roots and produces absorbable nitrogen for the plant. Other
bacteria reside in the soil and fix nitrogen for uptake by non-fixing plants.
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Mineralisation
At any given time, most nitrogen in the soil is in the form of organic nitrogen, held in organic
matter. This nitrogen is not available to plants. About 2% of the organic nitrogen will
decompose in a year to form inorganic (or mineralised) nitrogen as ammonia (which to
some extent determined by pH will ionise to ammonium, NH4+). Some plants (e.g. rice) are
capable of absorbing ammonium ions, but most prefer a different ionic form, nitrate.
Nitrification
Ammonium ions can be converted by soil bacteria to nitrite (NO2-) and then to the useful
form nitrate (NO3-). This process obviously requires oxygen and thus will not readily occur in
compacted or water-saturated soils. The soil pH should also be greater than 6 to encourage
nitrification. Nitrate is not retained on soil minerals as mentioned in an earlier chapter, but
ammonium is, so it provides a small reserve of nutrient in depleted soils.
Immobilisation
Once the plant takes up the inorganic nitrogen as ammonium or nitrate, it will then utilise
the element in one of the many organic compounds that requires it, most particularly
protein and chlorophyll. The nitrogen is covalently bound and will not be available until the
death of the plant or the metabolism of that compound.
Decomposition
Dead plant matter becomes available as food for organisms such as worms, and microorganisms such as bacteria in the soil. This provides a release for the nutrients such as
nitrogen bound up in the plant. The C:N ratio of decaying organic matter is important in
terms of the rate of its decomposition. Dry, woody material, such as straw, has a much
higher ratio (80) than can be readily consumed by bacteria. In general, C:N ratios of 20-30
are more readily used by bacteria, which in the absence of this source of N, will use the soil
reserves, possibly causing a depletion in N available to plants.
Denitrification
Nitrogen fixed from the atmosphere is not permanently lost to the atmosphere (otherwise
we would see a major decrease in atmospheric levels!). Other bacteria are capable of
reversing the process. They convert nitrate to nitrogen gas or nitrogen oxides. In a natural
ecosystem, there is a balance between nitrogen gained by fixation and lost by
denitrification, which prevents significant runoff into groundwater. However, man’s impact
on the biosphere has been to add more N (as fertiliser) than is lost. The consequence is that
some of this nitrogen ends up where it is not wanted: in the waterways, producing algal
blooms and eutrophication.
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In the plant
Low levels of nitrogen reduce growth, and produce yellowed leaves, as the green
chlorophyll which requires nitrogen is not present in sufficiently high quantities. Excess
levels of N cause rapid growth and dark leaves, but in flowering or fruit plants, this growth
may be at the expense of the crop.
Phosphorus
The rest of the macro- and micronutrients come from the soil. Phosphorus is present at
around 0.1% levels in the earth’s crust. The principal source of “new” phosphorus in its cycle
is from some rock minerals. Once freed by weathering from the minerals, it is found in the
form of phosphate (or more accurately, hydrogen phosphate, H2PO4- in alkaline soils and
dihydrogen phosphate, H2PO4- in acidic soils) and as organic phosphorus in soil organic
matter, after conversion from phosphate by micro-organisms. The figure below shows the
phosphorus cycle.
Phosphorus, as phosphate, is not found in soil solution to any great extent. This is due to
formation of very insoluble compounds with calcium, iron or aluminium, as well as
adsorption onto clay, processes which are not irreversible, but very slow in the opposite
direction. Uptake by the plant is therefore a necessarily efficient process.
Native plants in Australia have adapted to soils that are relatively low in phosphorus in any
form. Introduced species, including the grain, fruit and vegetable crops, needed addition of
phosphate in the form of fertiliser. This increases the soil solution level temporarily, but
much is lost to the processes above.
Testing soil for phosphorus can give misleading results if the purpose of the test is not made
clear. Total phosphorus is very different to available phosphorus, so various extracting
solutions have been devised to simulate the availability of the element.
Figure 4.2 - The phosphorus cycle
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In the plant
Phosphorus is converted to organic form in the plant as a “fuel” for all biochemical activity
in cells, as part of the exothermic conversion from adenosine triphosphate (ATP) to
adenosine diphosphate (ADP). Symptoms of phosphorus deficiency are reduced growth,
purpling of green leaves and death of older leaves.
Potassium
Potassium is present in the crust at around 2.6%. The potassium cycle is much simpler,
because it cannot form organic compounds, as shown in the figure below.
Figure 4.3 - The potassium cycle
In most soils, the potassium is present in sufficient quantities of both solution (around 3% of
total K) and adsorbed (97%) forms. As discussed in an earlier chapter, the adsorbed cations
provide a reserve when soil solution levels are low. Some soils, particularly those rich in
organic matter, may be low in potassium simply because the minerals from where it comes
are present at lower levels. Some clay minerals are very good at taking the adsorbed
potassium into the spaces between the alumina and silica layers, where it becomes much
less available.
In the plant
Plants require potassium for synthesis and transport of carbohydrates like cellulose, used in
cell walls. Potassium deficiency can been seen in plants where the stalks are relatively weak
and break easily. In the older leaves, yellowing and death occurs around the edges. Excess
potassium in the soil can lead to the plant taking up too much, at the expense of calcium
and magnesium.
Magnesium and Calcium
The descriptions for the sources and cycling of potassium in the soil apply equally well to
calcium and magnesium. The adsorption of Ca and Mg to cation sites is greater than of K; in
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fact 90% of adsorbed cations in neutral or alkaline soils will be these elements, especially
calcium. Therefore, the reserve supplies are likely to be good. Furthermore, the plant’s need
for these elements is much less than that for potassium, so deficiencies are less common.
In the plant
Magnesium is present in chlorophyll, the light-absorbing green compound in
photosynthesis. Magnesium deficiency leads to yellowing of leaves, in the areas surrounding
the veins.
Calcium deficiency is very rare, and the plant is likely to have had other major problems
before it runs short of calcium.
Sulfur
Sulfur is present to a limited extent in minerals, and is more likely to be added to the soil by
the decomposition of organic matter, where the sulfur is an essential component in
proteins. It can also come from the atmosphere in the form of sulfate, which has arisen from
SO2 emissions (natural or man-made). The sulfur cycle is similar to the nitrogen cycle, except
that sulfur in the atmosphere doesn’t need to be fixed by soil micro-organisms, S only
occurs in one inorganic form – sulfate – and is more likely to adsorb onto clay minerals.
In the plant
Plants use sulfur primarily in the production of amino acids which are used by proteins and
enzymes. Deficiency symptoms are generally displayed in the same way as nitrogen.
Micronutrients
Cations (iron, manganese, zinc and copper)
Each of these elements is released to the soil by mineral weathering, with iron being the
most prominent. The soil solution levels (especially for iron and zinc) are very affected by
pH, with alkaline soils more likely to suffer deficiencies because of the precipitation of
hydroxide salts.
Given that a majority of soils are alkaline, plants have developed methods for overcoming
this problem: root production of acid and also ligands to bind to iron. Each will make more
the elements soluble and available. Copper readily forms complexes with organic
compounds in soil, and are is likely to be pH affected. Available zinc levels can be reduced by
excess phosphate fertilising.
Deficiencies of iron and manganese give similar visible signs to that of magnesium: yellowing
of the leaf tissue between the veins: this is known as chlorosis. It can be difficult to tell
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which element is deficient. Copper deficiency, rare as it is, is shown by at the leaf tips, which
whiten and become twisted. Zinc deficiency exhibits various leaf discolouration effects, as
well as reduction in the size of fruits and leaves.
The others – boron, chlorine, molybdenum
Boron
Boron is released into soils from one major mineral in the form of boric acid, which
unusually is the form in which it is taken up by plants. In more alkaline soils, it becomes
ionised, and borate ion is strongly adsorbed onto clay minerals, causing a decrease in
availability. Boron deficiency can produce physiological diseases in plants, such as internal
rotting of fruit.
Chlorine
Chlorine, as chloride, is the most common anion in soils, and given that plants have a very
low requirement for the element, there is never a situation where deficiency occurs.
However, when water tables rise, the high salt levels (sodium and chloride) make it
impossible for most plants to survive.
Molybdenum
Molybdenum is a metallic element, but is present in soils as the molybdate ion, MoO 42-. It
becomes unavailable in acidic soils. It is used by legumes in the fixation of nitrogen, so these
plants, if suffering a Mo deficiency, will exhibit nitrogen-deficiency symptoms. Other plants
display a range if symptoms, including unusual leaf shapes. Very low levels of molybdenum
are needed by plants, though an excess is not a problem and may be taken up into the
leaves. It is, however, toxic to animals.
Optimum nutrient levels in soil
This would seem to be a very obvious and necessary section, but in reality, it is not as easy
as it might seem to provide a guide to how much of the various species should be in the soil.
There are a variety of reasons why this should be:
◗ the capacity of different soils to hold different nutrients varies widely
◗ the need of different plants for different nutrients varies greatly
◗ growing conditions differ across the country and the world
◗ the test methods used and the way the data is reported
Therefore, all that can be provided is a guide to the nutrient needs of grain crops such as
wheat. Grain crops are considered to be midrange in terms of demand for nutrients,
grasses being lower and flowering and fruit and vegetable crops being higher.
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Nutrient
Nitrogen (N) %
Grains
1.7-3
Phosphorous (P) %
0.3-0.5
Potassium (K) %
1.5-3.0
Sulphur (S) %
0.15-0.4
Calcium (Ca) %
0.2-1
Magnesium (Mg) %
0.1-0.5
Zinc (Zn) ppm
15-70
Copper (Cu) ppm
3-25
Iron (Fe) ppm
20-250
Manganese (Mn) ppm
15-100
Boron (B) ppm
Molybdenum (Mo) ppm
5-25
0.03-5
Table 4.2 - Nutrient levels for grain crops (Source: Canadian Dept of Agriculture)
Fertilisers
A fertiliser is any material, organic or inorganic, natural or synthetic, that is added to soil to
supply one or more nutrient elements. Because of intensive cropping, soil fertility can only
be maintained by the addition of fertiliser.
Fertiliser can be in a number of different forms. The more important division is between not
natural versus man-made, but organic and inorganic. Organic fertilisers have the
macronutrients – especially the key three N, P and K – but also organic matter which
enriches the soil. Inorganic fertilisers, which are generally more processed, only provide the
macro- and micronutrients.
Inorganic fertilisers
For most inorganic fertilisers, which are a mixture of salts of the various nutrients, the N:P:K
grade is an important measure. This is simply the %w/w of the three elements (with P
expressed as P2O5 and K expressed as K2O). For example, a fertiliser with a grade of 10-6-8 is
composed of 10% N, the equivalent of 6% P2O5 and the equivalent of 8% K2O.
The fertiliser does not actually contain these forms of P and K; they are simply the standard
way of reporting their levels. It is like the reporting of water hardness as mg CaCO 3/L even
though there is no calcium carbonate as such in the water.
To convert between the actual (total) and reported (grade) for levels of P and K, use the
following factors.
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Figure 4.4 - Conversion factors for P & K
Element
Nitrogen (as ammonium)
General purpose
5.0
Slow release
10.5
Soluble
3.6
-
7.5
8.8
Total nitrogen
5.0
18.0
15.0
Phosphorous (water soluble)
4.3
4.3
4.0
Phosphorous (citrate soluble)
0.9
0.5
-
Phosphorous (citrate insoluble)
0.3
-
-
Total phosphorous
5.5
4.8
4.0
Total potassium (as potassium
chloride)
4.1
9.1
26.0
Sulfur (as sulfates)
11.5
4.0
-
Calcium (as superphosphates)
12.2
0.96
-
Nitrogen (as nitrate)
Table 4.3 – Typical ratios of compounds in fertilisers.
The levels of the nutrients vary greatly depending on the intended use for the fertiliser.
General purpose fertilisers often contain only the macronutrients (and most will not have
magnesium). There are many different fertilisers designed for particular applications:
Complete
This type has the full set of macro and micronutrients.
Trace element mixture
This type of fertiliser only has micronutrients as the active constituents.
Specific plants
These are formulations for plants requiring nutrients in different proportions to normal, eg
rose, citrus, azalea & camellia).
Slow release
This type of fertiliser has a special organic coating on the fertiliser granules. The coating
slowly decomposes, allowing a gradual release of the fertiliser over a number of months.
This reduces the need for careful attention by the gardener, but importantly provides a
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steady flow of nutrient to the soil, rather than a major burst, then nothing.
Soluble
In this type of fertiliser, all components are soluble, providing a quick tonic for plants, but
one that is readily lost by leaching; ideal for pot plants and flowering plants; these tend to
be high in nitrogen (above 25%) to give the extra boost in growth.
The distinction between the different forms of the particular nutrients – ammonium vs
nitrate for N, water soluble vs citrate soluble/insoluble for P – is important because of the
different availability of the various forms. Citrate solubility for phosphorous is a measure of
the solubility in a solution of neutral ammonium citrate, which presumably is supposedly to
be a simplified representation of soil solution.
Sources of inorganic nitrogen
Nitrogen industrially is fixed by a chemical reaction from N 2 to ammonia. From there, it is
converted into ammonium by acidification, usually with nitric acid, to give ammonium
nitrate, which has the two useful forms of nitrogen for plants. The sulfate and phosphate
forms are also used in fertilisers. All ammonium salts are very soluble. One other form of
nitrogen in fertilisers is urea (NH2CONH2), which slowly decomposes in the soil to form
ammonium. This of course is generally converted to nitrate before uptake.
Phosphorous is primary available from large deposits of phosphate rock, which is essentially
mineralised bird and bat faeces. Some South Pacific islands, like Nauru, are almost
completely composed of this rock. The mineral phosphorous is very insoluble, and
essentially useless as a fertiliser. It is processed by treatment with phosphoric acid into
triple superphosphate, which is calcium dihydrogenphosphate. This is water-soluble, with a
fertiliser grade of 0-45-0. Ammonium phosphate, formed by the reaction of ammonia with
phosphoric acid has a grade of 10-34-0.
Potassium is present in fertilisers as the chloride salt, which is produced either from wood
ash treated with hydrochloric acid, or from mineralised salt deposits.
Large scale fertilising of farms is often done by application of single nutrient fertilisers, while
mixed fertilisers, which are more expensive, are more commonly used on smaller-scale
plots. Mixed fertilisers are normally granulated with clay during processing to keep them
from absorbing water and from clumping together.
Organic fertilisers
Natural material, principally manure, is both mulch (i.e. a slowly decomposing organic cover
over the soil to retain moisture) and a source of nutrients. The grade of fresh animal manure
is typically around 10:5:10, but after drying, this will increase somewhat. Poultry manure
tends to be much higher in nitrogen than cow manure, as shown in the figure below. The
addition of organic matter to the topsoil is probably as important in manure fertilising as the
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provision of elemental nutrients. It is something that the synthetic inorganic nutrients
cannot do.
It is common for a natural fertiliser to be processed to some extent before use. This is often
for reasons of safety – the risk of disease being spread by animal waste – or contamination –
especially by weed seeds. Two important examples are “Dynamic Lifter” (granulated chicken
poo) and “Blood and Bone” (as the name implies).
Wood ash is often used for fertilising soil. It contains a high proportion of potash –
potassium carbonate – and is therefore a good source of that element, but is strongly
alkaline, and may cause problems with overuse.
20
18
16
Fertiliser grade
14
12
N
10
P
8
K
6
4
2
0
Horse
Cow
Pig
Sheep
Poultry
Figure 4.5 - Fertiliser grades for animal manures
Fertiliser use
Overuse of fertilisers by commercial farming activities has led to pollution of groundwater
and surface water through leaching of excess nutrients. The major problems are N and P,
which have led to algal blooms, toxic algae and eutrophication. In a small scale garden, the
levels recommended on the fertiliser packaging are simple enough to follow, and unlikely to
cause problems, but the application of fertiliser to large farming areas must be planned
more carefully.
Soil sampling and testing is an obvious need, especially where fertility problems are
suspected or obvious. The problem of available versus bound levels of nutrients, especially
with phosphorous, makes the results of testing difficult to convert into kg of fertiliser per
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hectare, and the type of crop also needs to be taken into account. Computer programs exist
to take all possible factors into account.
Timing of the fertiliser application is also important. This is particularly the case for nitrogen,
which is readily lost if not immediately used. The most appropriate time for nitrogen
application is the plant moves into its main growth cycle. However, if the plant is moving
towards flowering of fruiting, then nitrogen is the last thing it needs, because it will only
encourage leaf growth.
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Assessment task
This section provides formative assessment of the theory. Answer all questions by typing
the answer in the boxes provided. Speak to your teacher if you are having technical
problems with this document.
◗ Type brief answers to each of the questions posed below.
◗ All answers should come from the theory found in this document only unless the
question specifies other.
◗ Marks shown next to the question should act as a guide as to the relative length or
complexity of your answer.
1. What is the difference between a macro and micro nutrient? [1mk]
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2. List and describe the role of three macronutrients. [4mk]
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3. List and describe the role of three macronutrients. [4mk]
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4. What happens to nitrogen when it is ‘fixed’? [2mk]
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5. What happens to nitrogen when it ‘denitrifies’? [2mk]
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6. What happens to nitrogen when it ‘nitrifies’? [2mk]
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7. What happens to nitrogen when it ‘nitrifies’? [2mk]
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8. What happens to nitrogen when it ‘mineralises’? [2mk]
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9. What happens to nitrogen when it ‘immobilises’? [2mk]
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10. What happens to nitrogen when it ‘decomposes’? [2mk]
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11. What is the key species of phosphate in soil? [2mk]
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12. Identify the two methods that plants employ to increase the solubility of minerals such
as iron. [2mk]
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13. What is a fertiliser? What is the difference between a nutrient and a fertiliser? [2mk]
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14. A fertiliser has a grade of 10.2-8.7-9.3. What are the actual levels of N, P and K? [4mk]
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Assessor feedback
15. Provide an example of inorganic and organic fertilizers. [2mk]
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16. Read the ‘Soil Test methods’ chapter (from the website) and identify which analytical
procedures could be used to determine the concentrations of following nutrients in
soils? [8mk]
a. Nitrogen as N
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b. Nitrogen as NH3
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c. Nitrogen as NO3-
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d. Phosphorus as P
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e. Phosphorus as PO43-
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f. Sulfur as SO42-
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g. Cations such as Na+, K+
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h. Organic content
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Use the documents ‘Save As…’ function to save the document to your computer using the
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References & resources
The Physical Geography website given in Study module 1 is a marvellous resource for all
facets of the environment.
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