STS Study module 4Soil nutrients & fertilisers
Diploma of Environmental Monitoring & Technology
Study module 4
Soil nutrients & fertilisers
Sampling & testing of soils
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Introduction
Nutrient classification
Nitrogen
Phosphorus
Potassium
Magnesium and Calcium
Sulfur
Micronutrients
Cations (iron, manganese, zinc and copper)
The others – boron, chlorine, molybdenum
Optimum nutrient levels in soil
Fertilisers
Inorganic fertilisers
Organic fertilisers
Fertiliser use
Assessment task
Assessment & submission rules
Problems?
References & resources
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 PlantsNitrogen / 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
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.
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 micro-organisms 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.
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
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 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 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, MoO42-. 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.
Nutrient / GrainsNitrogen (N) % / 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 / 5-25
Molybdenum (Mo) ppm / 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 CaCO3/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.
Figure 4.4 - Conversion factors for P & K
Element / General purpose / Slow release / SolubleNitrogen (as ammonium) / 5.0 / 10.5 / 3.6
Nitrogen (as nitrate) / - / 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 / -
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 hasmicronutrients 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 steady flow of nutrient to the soil, rather than a major burst, then nothing.