Guideline Index

Chapter 3: Plant Nutrient Requirements

3.4 The major nutrients or macronutrients

The macronutrients (nitrogen, phosphorus, potassium, sulphur, calcium and magnesium) are required in relatively large quantities by plants; measured by either a percentage or mg/kg. Plant growth may be retarded because:

  • These nutrients are lacking in the soil.
  • They become available too slowly.
  • They are not adequately balanced.

3.4.1 Nitrogen

Nitrogen (N) is needed for all growth processes, as it is the major component of amino acids, which are the building blocks of proteins, enzymes and the green pigment chlorophyll. Chlorophyll converts sunlight energy into plant energy in the form of sugars and carbohydrates. Nitrogen deficiency symptoms

Nitrogen deficiency symptoms include:

  • Stunted growth.
  • Yellowing or light-green colour in pastures (very occasionally orange and red pigments may dominate).
  • Low protein content of grasses and crops.
  • A lack of nodules or very small whitish nodules on clovers and other legumes.

Nitrogen is mobile in plants, so deficiencies show up in the oldest plant tissue first – see Chapter Nitrogen deficiency in legumes and grasses has similar symptoms to a sulphur deficiency. However, sulphur is immobile in most plant species, so sulphur deficiencies typically show up in the youngest plant tissues first. An exception is in subterranean clover where sulphur is more mobile, so the deficiency shows up in young and old plant tissue. The nitrogen cycle

Nitrogen is present in the soil in many different forms (Figure 3.2), including as a gas (N2); as various oxides of nitrogen, such as nitrate (NO3) and nitrite (NO2); and as ammonia (NH3), amines (formed from ammonia), or ammonium (NH4). Organic matter is a major storage area for nitrogen. In fact, in most soils, more than 95% of the nitrogen is present in the organic matter.

Figure 3.2  The nitrogen cycle of a dairy pasture
Figure 3.2 The nitrogen cycle of a dairy pasture

Plants can only use two of the many forms of nitrogen, namely, nitrate (NO3) and ammonium (NH4). Therefore, other forms of nitrogen need to be converted to either nitrate or ammonium before the plant can use them.

The conversion process is carried out by various soil micro-organisms, such as fungi and bacteria, and by chemical reactions in the soil – See Chapter 5.

Major losses of nitrogen occur through leaching, denitrification (breakdown of nitrogen compounds to less available forms), volatilisation (conversion of nitrogen to gaseous forms, which are lost to the atmosphere), and the removal of animal products and fodder (See Chapter 12.3)

Nitrogen is returned to the soil with varying levels of efficiency via animal manure and urine, bought-in feeds, nitrogenous fertilisers, and legumes (See the nitrogen cycle animation). Where do legumes fit in?

The atmosphere is about 80% N, but plants such as legumes are able to use nitrogen from the air. They are able to do this by the development of small growths on their root system called nodules. These nodules contain bacteria called rhizobia, which can ‘fix’, or convert, nitrogen from the air into a plant-available form. This fixed nitrogen then becomes part of the pasture nitrogen cycle (see Figure 3.2). The nitrogen becomes available to grasses when the nodules or legume plants (roots, stems and leaves) die or are eaten by an animal then returned as dung or urine. The legume root nodules have a life span of up to 6 weeks, and new ones are constantly developing. The nodules are a pinkish colour when actively fixing nitrogen; however, they may be white (N deficient), green (when nodules become older) or brown (decomposing) if growing in suboptimal conditions.

Figure 3.3  Nodules sliced in half, showing the pinkish colour (leghaemoglobin) indicative of healthy nodules in the centre, with old decomposing nodules on the left and ineffective white nodules on the right.   Source:
Figure 3.3 Nodules sliced in half, showing the pinkish colour (leghaemoglobin) indicative of healthy nodules in the centre, with old decomposing nodules on the left and ineffective white nodules on the right.

In a ryegrass/clover pasture, 50 to 250 kg N/ha/year can be fixed by the clover, depending on such factors as the clover content of the pasture, soil fertility, and moisture availability. This is equivalent to applying urea (which is 46% nitrogen) at a rate of 109 to 543 kg of urea/ha/year. At a price of $500/tonne spread for urea, this is equivalent to about $55 to $270/ha/year. However, the amount of N fixed by clover in Australian dairy pastures is typically 50 kg N/ha/year or less due to the low legume content – see Chapter 12.2.1.

The rhizobia bacteria supply nitrogen compounds to the legume, and the legume supplies carbohydrates (energy) to the nitrogen-fixing rhizobia bacteria. If the soil environment is not ideal (for example, high acidity, lack of other nutrients, dry soils or salinity), these bacteria are adversely affected, which results in reduced nitrogen fixation and thus reduced pasture growth.

The various legume species often require ‘inoculation’ of the seed (mixing the seed with rhizobia bacteria) at sowing. Specific strains of the rhizobia bacteria are required for each of the major legume groups. For example, sub clovers require inoculant strain C, and white clover requires inoculant strain B.

It is essential to inoculate legume seeds when sowing into virgin, recently flooded, or newly cleared land because the soil will not have enough of the required rhizobia naturally present. Although it may not always be necessary to inoculate when resowing an old pasture, it is advisable.

Lime coating of the legume seed ensures that the soil environment surrounding the seed is more favourable (less acidic) for the rhizobia bacteria and young legume roots. In addition, several proprietary forms of coating (e.g. Prillcote® and Agricote®) contain ingredients to ensure longer survival of the inoculant if sowing is likely to be delayed. Insecticides can also be included in the coating to provide pasture plants with a degree of protection against some insect pests after germination (e.g. lucerne flea or red-legged earth mite).

3.4.2 Phosphorus

Phosphorus (P) helps run the ‘power station’ inside every plant cell and has a key role in energy storage and transfer. Phosphorus is necessary for all growth processes and for the nodulation of rhizobia bacteria and nitrogen fixation. Phosphorus deficiency symptoms

Phosphorus deficiency symptoms include:

  • Stunted growth, weak roots and shoots, fewer tillers.
  • Depressed yields.
  • Purple tints on small leaves.
  • Small, dark green leaves on mature clover plants.

Growth of new pastures can be severely restricted when the soil is deficient in phosphorus. As animals derive their phosphorus requirements from pastures, animal production may also be affected by low phosphorus levels.

Phosphorus is a mobile nutrient within the plant and is moved to the actively growing tissue, such as root tips and growing points in the tops of plants – see Chapter Therefore, deficiency symptoms occur first in the older leaves.

It is important that plants have an adequate supply of phosphorus to ensure recovery and regrowth after grazing. Likewise, newly sown pastures benefit from a supply of readily available phosphorus close to the germinating seed to help quickly develop a large root system. The phosphorus cycle

When phosphorus fertiliser (for example, superphosphate) is applied to a pasture, the phosphorus enters a phosphorus cycle. As can be seen from Figure 3.4, the phosphorus can move around the system, as well as be lost from the system, via many different pathways. The P cycle is very complex, involving a great deal of interaction and chemical reactions in the soil.

The phosphorus in the soil can be taken up by plants, then consumed by animals and returned to the soil in ruminant dung. The phosphorus can also move about in the soil, changing in its chemical form and in its availability to plants.

Being hygroscopic (moisture-attracting) in nature, superphosphate granules attract moisture from the atmosphere, leading to the granule releasing P even in very dry conditions. Despite the movement shown in Figure 3.4, phosphorus in the soil is relatively immobile. Many chemical reactions take place when phosphorus is applied to the soil, and only a small proportion remains in solution and readily available to the plants (see Chapter 9.2.5). The remainder is ‘bonded’ (or ‘fixed’ or ‘sorbed’) in a less available form to the surface of the soil clay particles and organic matter. A proportion of this fixed phosphorus does become available over a period of time and is referred to as the soil phosphorus reserve – See Section ‘Soil sorption’.

Figure 3.4  The phosphorus cycle of a dairy pasture
Figure 3.4 The phosphorus cycle of a dairy pasture Losses of phosphorus

Phosphorus; supplied either as fertiliser applications or naturally from the soil, undergoes losses by various mechanisms. These losses occur by: Removal of phosphorus in plant and animal products

Phosphorus is lost from the pasture in plant and animal products (milk and meat). Cutting hay or silage on a paddock and not feeding it back on the same paddock can very quickly ‘mine’ the paddock’s fertility. Product removal off farm will result in a certain amount of phosphorus leaving the farm. Milk production results in much higher removal rate of phosphorus than does beef or wool production. Redistribution of faeces

Large quantities of phosphorus can be removed or relocated within the growing pasture through the behaviour and management of the dairy herd. Cows graze pasture from all over the paddock but deposit a greater proportion of dung around gateways, stock camps, feedpads, shelter belts, water troughs, and other places where cattle gather. Dung dropped on the dairy yard and laneways can account for approximately 10% of total dung. The amount will vary according to how long the animals have been off pasture and their level of harmony in the dairy shed and yard. Cattle will deposit more dung and urine in the laneways, shed and yard if they are continually upset by dogs or operators. The nutrients contained in dung, urine, milk and those retained are listed in Table 3.2.

Table 3.2  Fate of nutrients consumed by lactating dairy cows.
Table 3.2 Fate of nutrients consumed by lactating dairy cows.
Source: During (1984).

Proportionally more of the phosphorus taken up by dairy cows when they graze pasture is retained by the cows and lost from the grazing area than is the case for potassium and nitrogen. Conversely, most of the potassium ends up in the urine. Table 3.2 illustrates just how little of what is eaten by a lactating dairy cow is actually retained. For information on cycling and losses of a wider range of nutrients see the article by Michael Russelle (2012). Soil losses

Leaching: Despite the solubility of a single superphosphate granule in water, the phosphate ion is generally not leached (washed through the soil profile), as it is rapidly tied up in various forms soon after application. The amount of leaching that occurs in soils varies widely according to the type of nutrient, soil type, and amount of rainfall. Leaching is related to the amount of organic matter or the amount and types of clay minerals to which the phosphorus can adsorb (attach). Leaching is more of a problem in the sandy soil types (since they contain low amounts of organic matter and clay minerals), in areas of high rainfall, or when phosphorus is lost in surface runoff when fertiliser is applied just before a heavy rainfall event.

Phosphorus may also move down the profile in soils that are prone to cracking and in soils that have reached saturated levels of phosphorus.

We do not have accurate figures for this loss in most Australian soils under a pasture situation, but leaching of phosphorus is known to be relatively low in most soils types. Sulphur, boron and nitrogen are much more prone to leaching than phosphorus is.

Surface runoff on irrigated farms: As much as 11% of applied phosphorus can be carried away in irrigation water. See Chapter 10.5.2 for information on ways to reduce these losses.

Surface runoff on dryland farms: A small amount of applied phosphorus may be lost via surface water carrying away minute amounts of dissolved phosphorus in the short term; especially if highly water soluble fertiliser forms e.g. superphosphate are used. Most phosphorus lost in runoff is phosphorus present in pasture residues and other soil biota. This amount of dissolved phosphorus is much higher if a heavy rainfall occurs soon after applying the fertiliser. Research has shown that, on clay loam soils, the loss of P in surface runoff is reduced by 50% if rain falls 4 days after application and by 75% if rain occurs after 7 days, compared to when rain falls immediately after application.

The major effect of this loss is that phosphorus is carried to dams and lakes and, in combination with the plentiful supply of nitrogen in these areas, allows blue-green algal blooms to occur. Losses from phosphorus dissolved in soil moisture, especially following heavy rain, are dependent on the time since application of fertiliser, soil type, rainfall intensity, slope, etc. This is an area that has attracted much research with the Phosphorus Environmental Risk Index (PERI) now providing an indication as to the amount of phosphorus held in the soil that is adequate for pasture or crop production, with anything above this being excessive and at risk of leaching into waterways. See Chapter 10.5.2 for tips on how to reduce these losses.

Sorption by soil: Phosphorus tends to undergo sequential reactions that produce phosphorus-containing compounds of lower and lower solubility when applied to both acidic and alkaline soils. Therefore, the longer the P remains in the soil, the less soluble it is and the less available it becomes for plant uptake.

The primary factors governing the sorption of P in the soil profile include:

  • Soil pH; the more acid the greater the retention,
  • Distribution of soil particle sizes; soil texture with sandy soils having greater leaching ability,
  • Presence of reactive iron (Fe) and aluminium (Al),
  • Organic matter incorporation,
  • Nature of adsorbed cations as well as anion adsorption.

When phosphorus is first applied to soil, a rapid reaction removes the soluble P from soil solution. Slower reactions then continue to gradually reduce P solubility for months or even years as the phosphate compounds age.

Imagine a fountain cascading down a series of steps with the steps indicating time and decreasing availability of P to plants. The soluble P, applied at the top, becomes much less available to plants over time due to an ever-increasing strength of P sorption by soil compounds such as iron, aluminium and manganese phosphate, from the top of the cascade to the bottom.

Soils high in organic matter or clay content have a stronger phosphorus-fixing capacity than do sandy soils. Some clay soil types (for example, krasnozems, or red soils) adsorb more phosphorus than other clay soils because of the type of clay mineral in the topsoil. Most of this adsorbed phosphorus is not available to the plant, although some may become available over time.

Soils with high aluminium and iron levels, such as red soils of volcanic origin, usually have a very high phosphorus-fixing (or sorbing) capacity. In these soils, the phosphorus reacts with the aluminium or iron to form relatively insoluble chemical compounds, which results in a higher proportion of applied phosphorus being locked up and unavailable to plants.

Soils vary widely in the amount of phosphorus (and other nutrients) ‘fixed’ in soils. The Phosphorus Buffering Index (PBI) has been developed and has been widely adopted across many states and industries to help with differentiating soil P-fixing ability – see Chapters and 15.5.2.

Erosion of soil particles: Since phosphorus binds quickly to soil particles (in other words, becomes particulate P), it is obvious that soil erosion can result in phosphorus losses. Such erosion losses can occur along stream banks; via tunnel, gully or sheet erosion; from newly renovated or laser-graded irrigation areas; and from severely pugged pastures or sacrifice paddocks. The quantity of P lost by erosion is usually low but may be a significant contributor to the environmental problem of eutrophication (high levels of nutrients) causing unwanted and large growth of water weeds or an algal bloom of, say, blue-green algae.

Recent research has provided some indication of the extent of this loss and is providing some guidelines for reduction of P losses. Obviously, any management that reduces loose soil particles entering waterways will achieve this. Accurate figures for this loss of phosphorus are difficult to assess for individual farms. It is small but should be reduced or completely stopped, if possible.

3.4.3 Potassium

Potassium (K) is needed for a wide range of important processes within the plant, including cell wall development, flowering and seed set. Potassium has a key role in regulating water uptake and the flow of nutrients in the sap stream of the plant. It helps legumes fix nitrogen and also helps the plant to resist stress from weather, insects and diseases. Potassium deficiency symptoms

Potassium deficiency symptoms include:

  • Reduced growth (possibly up to a 50% drop in yield of some crops before deficiency symptoms appear).
  • Yellowing and whitish spots along the outer margin of clover and lucerne leaves, which subsequently develop a necrosis, or deadening, of the outer leaf margins – see Figure 3.5
  • In grass, a pale-green colour, which may be followed by a pronounced yellowing to browning off, beginning with the tips of older leaves (called chlorosis, or tip burn). These symptoms are not sufficiently different from nitrogen deficiency or frost effect to allow them to be used alone to identify a K deficiency in grass.
  • Excess salinity may also cause brown, necrotic leaf margins, but this occurs mostly in the younger leaves.


Figure 3.5  Lucerne leaves showing varying degrees of potassium deficiency.  Source:  David Hall.
Figure 3.5 Lucerne leaves showing varying degrees of potassium deficiency. Source: David Hall.

Potassium is very mobile in the plant, and deficiency symptoms initially occur in the older leaves – see Chapter Deficiencies are most obvious at times of peak potassium demand; in other words, Spring. Potassium deficiencies may not appear if a combination of nutrient deficiencies, such as phosphorus and potassium together, are limiting growth.

Grasses tend to be more deeply rooted and have more fibrous roots than clovers and therefore can compete more strongly for potassium. A symptom of potassium deficiency is a grass-dominant pasture that often has an abundance of weeds. Older urine patches may show good grass or clover growth; if clover is present in the pasture, as 80% to 90% of the potassium in pasture consumed by stock is excreted in urine.

Deficiencies of potassium are most likely to occur on lighter sandy soils and regular ‘day’ paddocks and particularly in paddocks that have been repeatedly cut for hay or silage. The potassium cycle

Potassium in the soil-pasture system (Figure 3.6) is cycled in a similar way to phosphorus. Animals grazing pastures recycle most of the potassium they take in as urine. However, they concentrate this potassium return around water troughs, stock camps and yards. Hay-making and silage-making are the major ways that potassium reserves are removed or redistributed – See Appendix H for nutrient contents of feeds.


Figure 3.6  The potassium cycle of a dairy pasture
Figure 3.6 The potassium cycle of a dairy pasture

Unlike phosphorus, when potassium is applied as a fertiliser it does not react with the soil to form insoluble compounds. However, like phosphorus, potassium does not form any gases that could be lost to the atmosphere like nitrogen does. The soil’s cation exchange properties and mineral weathering influence its behaviour in the soil. Potassium, unlike P and N, causes no off-site environmental problems, such as eutrophication, when it leaves the soil system.

Potassium can be temporarily held in clay particles as exchangeable potassium and becomes available for plant uptake when it moves back into the soil solution. Dry soil immobilises potassium, thereby reducing its availability temporarily. Waterlogging also reduces K uptake due to lack of oxygen. Unlike the P in single superphosphate, if K is applied to dry soils, it will not be utilised until rain or irrigation occurs.

Potassium is found in four forms in the soil: mineral non-exchangeable potassium, non-exchangeable potassium, exchangeable potassium, and potassium in soil solution (water-soluble potassium) – see Figure 3.7. The total amount of K present in each form will depend on the potassium content of the parent material, extent of weathering and leaching, redistribution by plants (fodder) and animals, and the amount of applied potassium.

Figure 3.7  Forms of potassium in the soil and their plant availability.
Figure 3.7 Forms of potassium in the soil and their plant availability.

Approximately 90% to 98% of the total soil K is in the non-exchangeable form, although some becomes available very slowly due to weathering. In the non-exchangeable form it is part of the internal structure of clays, mineral particles and parent rock material. This form is not available for plant uptake. Approximately 1% to 2% is in the exchangeable form and is lightly bound or held (retained) on the surface of clay particles and organic matter. This form becomes available rapidly and easily to plants when it exchanges with other cations and moves back into the soil solution. Hence, it is referred to as ‘exchangeable K’ when it is measured in a soil test – see ‘Exchangeable potassium’ in Chapter Approximately 0.1% to 0.2% is in the soil solution and readily available for uptake by plants. Both the soil solution and exchangeable potassium are measured in a soil test as ‘available K’ – see Chapter 9.2.6. Losses of potassium

The potential for fixation or leaching of potassium depends largely on the soil clay content and its mineralogy; the level of soil organic matter; and the climate, particularly rainfall or irrigation levels.

In sandy soils low in clay, potassium largely remains in the soil solution and can be leached below the plant root zone and potentially into the ground water. Such lighter soils, especially in high-rainfall districts or under high irrigation levels, are more prone to potassium deficiencies due to this leaching effect.

Heavy soils (such as clays) or soils high in organic matter are usually high in potassium. However, some can be low in potassium. Responses to potassium fertilisers

In rainfall-dependent pastures, soil testing and test strips provide an excellent prediction of likely potassium responses. If a response is to be seen, it will occur in the spring following an autumn or early winter application because of the rapid demand for potassium in spring. The value of soil testing versus plant tissue testing for potassium needs to be assessed in light of soil texture and extent of the following rainfall. Light textured (sandy) soils can leach the soil potassium below the pasture root zone, so in these situations plant testing will be more reliable.

It must be recognised that dairying is an intensive system and significant rates of potassium are being removed, so responses to potassium may occur sometime in the future. Animal health implications

High rates of potassium fertilisers can cause low plant calcium and magnesium levels, which may induce the metabolic disorders hypocalcaemia (milk fever – see Section and hypomagnesaemia (grass tetany – see Section respectively. It is important to understand that dairy cows can acquire the grass tetany disorder from grazing pastures that contain excessive potassium; whether from fertiliser or inherent levels, in particular from effluent fields that accumulates in some cases very high soil potassium levels. For more information about grass tetany follow the links:

3.4.4 Sulphur

Sulphur (S) is required for the formation of several amino acids, proteins, and vitamins and for chlorophyll production. It also helps the plant to resist stress from weather, insects and diseases. Sulphur deficiency symptoms

Sulphur deficiency symptoms include:

  • Plants appear stunted.
  • Plants tend to become spindly with thin stems and petioles on legumes.
  • Small, pale, yellow-green leaves with lighter coloured veins.
  • Poor development and low numbers of nodules on legumes.

Plants severely deficient in sulphur show similar symptoms to nitrogen deficiency. The major difference between sulphur deficiency and nitrogen deficiency is that sulphur is immobile within the plant, and deficiency symptoms appear first in the younger leaves, whereas nitrogen deficiency affects the older leaves first. This is true for grasses, however S is more mobile in subterranean clover so the whole plant rather than young leaves typically becomes lemon yellow and, if the deficiency is severe, clover leaves cup upwards. When sulphur levels are low, grasses, because of their larger root system, will compete very strongly for the available sulphur, to the detriment of the legumes. This results in a grass-dominated sward and reduced pasture quality. The Sulphur cycle

The sulphur cycle is shown in Figure 3.8. Significant amounts of sulphur are removed through meat and plants harvested for fodder, but only small amounts are removed through milk – see Appendix H.

In the past, sulphur deficiencies have been rare because most farmers used low-analysis fertilisers, such as single superphosphate, which contains high levels of sulphur (11%) – see Chapter 11.3.1. However, if high-analysis fertilisers, such as triple superphosphate and Diammonium Phosphate (DAP) are used, then the potential for sulphur deficiencies may increase because these fertilisers contain much lower levels of sulphur. For example, triple superphosphate contains only 1% S.

Figure 3.8  The sulphur cycle of a dairy pasture
Figure 3.8 The sulphur cycle of a dairy pasture


Most sulphur in soils is derived from soil organic matter and must be mineralised (converted to the inorganic sulphate form, SO42-), before it can be taken up by the plants. In this form, it is very soluble and may be more readily leached, particularly from sandy soils or in high rainfall conditions or under high levels of irrigation. In some soils, the sulphate is adsorbed on (fixed to) soil particles, which reduces leaching. This adsorbed sulphur becomes available as it is released back into the soil solution – see Chapter 9.2.7. Forms of sulphur

Two forms of sulphur are used in fertilisers. They are sulphate sulphur (SO42-), such as in superphosphate, and elemental sulphur (S elemental). Sulphate sulphur (SO42-) is readily available for plant uptake and more effective on very deficient soils. The elemental form (S elemental) must be converted by bacteria in the soil to the sulphate form before it is readily available to the plant. Therefore, this more slowly available form of sulphur (S elemental) may be more suitable on sandy soils that have less organic matter and are susceptible to leaching. Where a soil test reveals a sulphur deficiency, then the sulphate form (SO42-) will provide a quicker response.

Elemental sulphur (S elemental) applied at a rate of up to about 30 kg/ha has negligible effect on soil properties but, if applied in large quantities (over 800 kg/ha), can lower the pH of soils. The extent of pH reduction and reaction rate is influenced by the pH buffering capacity of the soil and the original pH level. The rate at which elemental sulphur converts to sulphate sulphur depends on the type of sulphur applied, particle size of the material, soil temperature, soil moisture content and population levels of the sulphur-oxidising bacteria.

If soil sulphur levels are high, then it is usually a lower-cost option to use a low-sulphur phosphorus fertiliser, such as triple superphosphate.

3.4.5 Calcium

Calcium (Ca) is usually in adequate supply for plant growth. It is involved in the proper functioning of growing points (especially root tips), maintaining strong cell walls, and seed set in clovers. Calcium deficiency symptoms

Deficiency symptoms are rare because calcium is common in the earth’s surface. It is also a component in many fertiliser products and in lime and gypsum. Soils low in calcium usually have associated adverse conditions, such as low pH and high aluminium, iron, and manganese – see ‘Exchangeable calcium’ in Chapter In very rare situations, heavy applications of potassium may induce a calcium deficiency, particularly on very acid soils, possibly resulting in hypocalcaemia, or milk fever.

Deficiency symptoms can also occur in strongly acidic peaty soils, where the calcium content may be less than 0.1%. Animal health implications

Milk fever is caused by low levels of calcium in the blood stream of cattle. This often occurs at or soon after calving when the cow’s requirements for calcium are high. When high rates of potassium (for example, muriate of potash – MOP) and nitrogenous fertilisers that produce ammonium ions (for example, DAP) are used together, the potassium or ammonium ions interfere with plant root uptake of calcium, thereby raising the risk of inducing milk fever – see Figure 3.9. However, nitrogenous fertilisers applied on their own do not cause this problem.

For more information about milk fever follow the links:

Figure 3.9  Effect of DAP plus MOP on calcium concentration in pasture. Source:  Bolan (1998).
Figure 3.9 Effect of DAP plus MOP on calcium concentration in pasture.
Source: Bolan (1998).

3.4.6 Magnesium

Magnesium (Mg), like calcium, is usually present in sufficient quantities in the soil for plant growth; and pasture deficiencies are rare. It is an essential component of chlorophyll and is required for the transport of phosphorus around the plant. Magnesium deficiency symptoms

Magnesium deficiency symptoms, rarely seen in most Australian states include:

  • Yellowing of leaves while the leaf veins remain green.
  • Abnormally thin leaves.
  • Older leaves mainly affected and affected first.

Magnesium is mobile within the plant, and a deficiency presents itself in the older leaves first.

The main source of magnesium for pasture deficiencies is dolomite (a compound mineral of calcium carbonate and magnesium carbonate containing 8% to 13% Mg).

As with calcium, magnesium plays an important role in the cation exchange capacity in the soil – see ‘Exchangeable magnesium’ in Chapter However, magnesium is more exchangeable than calcium, and the magnesium ion is more soluble and susceptible to leaching. Animal health implications

In most pasture situations, magnesium is present in adequate quantities for plant growth. However, the level of magnesium in the grass may be too low to meet the animals’ requirements and may lead to a condition known as grass tetany. Pasture magnesium levels are highest in summer and lowest in late winter and early spring. Grasses, which contain less magnesium than legumes do over most of the year, are usually dominant in late winter and early spring. Thus, grass tetany has typically occurred in late winter and early spring. Also, low temperatures and wet soils can reduce magnesium levels in forage.

However, high application rates of potassium fertilisers or dairy shed effluent can result in a luxury consumption of potassium. In other words, the plant takes up more soluble K than it requires and no yield increase occurs. This high concentration of plant potassium can often result in a lower proportion of other nutrient cations in the plant, such as calcium, sodium and, in particular, magnesium. These low magnesium levels may induce hypomagnesaemia, or grass tetany, in cattle. With more farmers applying more potassium and potassium blends in early spring, there appears to be anecdotal evidence that grass tetany is becoming more prevalent in the following autumn.

Grass tetany may also be caused by applying high rates of nitrogenous and potassium fertilisers, thus releasing ammonium ions and potassium ions together – see Figure 3.10. The ammonium and potassium ions both compete with the uptake of magnesium ions by the plant root, thus resulting in a lower magnesium concentration the plants. The use of nitrogenous fertilisers on their own does not cause this problem.

Also, animals consuming pasture or fodder high in potassium concentration can often upset the magnesium movement through the rumen and intestinal walls, consequently inducing a magnesium deficiency leading to grass tetany.

The cation exchange section of your soil tests can be used to determine the ratio of magnesium to potassium in the tested paddocks – see ‘Exchangeable magnesium’ in Chapter

Figure 3.10  Effect of DAP plus MOP on magnesium concentration in pasture. Source:  Bolan (1998).
Figure 3.10 Effect of DAP plus MOP on magnesium concentration in pasture.
Source: Bolan (1998).


Grass tetany can be largely prevented by feeding animals a magnesium supplement at a rate of 60 grams/head/day mixed with hay or a grain supplement, or dusted on pasture. The main sources of magnesium used in this way are Granomag, Magox or Causmag (magnesium oxide containing 50% to 56% Mg) and Epsom salts (magnesium sulphate containing 9.6% Mg).

For more information on grass tetany, refer to the web links listed in Section