What nutrients do plants require
Plants require nutrients for normal growth. These must be in a form useable by the plants and in concentrations that allow optimum plant growth. Furthermore, the concentrations of the various soluble soil nutrients must be properly balanced.
What will you find out in this section?
At the completion of this section, you should:
- Know which nutrients are essential for plant growth.
- Know which nutrients are required in large quantities.
- Understand the principles of the nutrient cycles for the major nutrients.
- Know which nutrients are required in smaller quantities.
6.1 Why do we need fertilisers?
Most Australian soils are old and weathered. In fact, many are considered the oldest soils in the world; and the nutrients have been leached, which has resulted in soils of low fertility. For example, average Australian soil phosphorus levels are 40% lower than English soils and up to 50% lower than North American soils.
Improved pasture species allow a much higher stock-carrying capacity; but to maintain this productivity, they require a higher level of soil fertility than do native pasture species. Fertiliser applications are required to overcome the soil's inherent nutrient deficiencies and to replace the nutrients that are lost or removed from the soil by pasture growth, fodder cropping or conservation, and animal products, such as milk or meat. Nutrient redistribution around the farm and the inherent ability of soils to 'retain' applied nutrients are other reasons for fertiliser applications.
Table 6.1 can be used as a guide to estimate the removal of nutrients from the farm and the addition of nutrients brought onto the farm.
Example 1: Look at the second line of Table 6.1. If you are producing 50,000 L of milk per year, then you are exporting 50 kg of phosphorus (P) and 100 kg of potassium (K) off the farm in the milk. (50,000 L milk x 0.1% P = 50 L of P, which weighs 50 kg; 50,000 L milk x 0.2% K= 100 L of K, which weighs 100 kg.)
Table 6.1 The percentage of nutrients removed or brought in by various pasture and animal components
Meat (kg LWt)
Cereal grain (DM)
0.2 to 0.4%
0.1 to 0.5%
less than 0.03%
Example 2: Now look at the fifth and sixth lines of Table 6.1. If you buy in 100 tonnes of hay (80% DM), you are buying in 200 kg P and 2000 kg (2 tonnes) of K. If you sell 6 tonnes of silage (40% DM), then you are also selling 4.8 kg P and 60 kg K.
Example 3: Now look at the seventh line of Table 6.1. If you have 100 cows who pooh 10 times per day, then the 365,000 poohs, each weighing 1 kg fresh weight (or 0.1 kg dry matter), would involve the cycling of 365 kg P and 292 kg K. If 10% of this is deposited in the laneways and dairy, then 36.5 kg P and 29.2 kg K are lost from the system and 328.5 kg of P and 263 kg K are recycled through the pasture.
Your sale of 50,000 L of milk per year also sends away 300 kg of nitrogen (N) and 30 kg of sulphur (S). The 100 tonnes of bought-in hay (80% DM) adds 1760 kg N and 160 kg S. The 6 tonnes of silage (40% DM) you sold exports 57.6 kg N and 4.8 kg S. The same poohey cows will be cycling 952 kg N and 88 kg S back on to your pasture; and 106 kg N and 10 kg S will be lost from the system on laneways and the dairy.
Note that in this section we are only talking about the plant nutrients that end up in various pasture and animal components. A large quantity of nutrients is also required to 'drive' the system along, in other words, to produce plant growth and to cover leaching, nutrient transfer, soil fixation, and other parts of the nutrient cycles.
Table 6.2 provides some easy-to-use estimates for the amount of nutrients contained in a quantity of hay, grain and animal products.
Table 6.2 The weight of nutrients removed or brought in by particular quantities of hay, grain and animal products
|Product||Amount||Weight of nutrient per quantity of product (kg)*|
|Fresh milk||1000 L||6||1||1.5 to 2||0.6||1.2||1.3|
|Meat (cattle liveweight)||1000 kg||28||8||2||1.5||14.||0.4.|
|Hay (70% grass/30% clover)||1 t DM||20 to 35||2 to 3.5||15 to 25||2 to 3||9||2|
|Lucerne hay or clover hay||1 t DM||30 to 45||2.5 to 3.5||20 to 30||2 to 3.5||8 to 10||5.0|
|Oaten hay||1 t DM||20||2||15 to 20||1 to 2||1.2||3.0.|
|Pasture silage||1 t DM||30||4.3||27||2||2.5||4.0|
|Maize silage||1 t DM||1.3||2.1||1.1||1.3||0.4||n.a.|
|Lupins||1 t DM||50 to 60||3 to 4||8 to 9||2.5 to 3||7||1.7|
|Wheat, oats, barley, triticale||1 t DM||17 to 23||2 to 3||4 to 6||1.5 to 2||3||1.2|
n.a. = not available
* These weights may vary slightly depending on soil fertility and moisture stress.
The nutrient contents of the plant categories in Table 6.2 have large ranges because of such factors as pasture composition, time of season sampled, nutrients applied, and soil type.
Note the high losses of potassium in the fodders. If the same paddocks are regularly cut for fodder and the fodder is not fed back in those same paddocks in the following season, then these paddocks will be quickly depleted, or 'mined', of their fertility, particularly potassium.
Conversely, if the fodder is continually fed back in the same small area (for example, in a sacrifice paddock situation), nutrient build up is greatly enhanced from both the fodder and the return of dung and urine.
Fertiliser applications in the following season on both these types of areas may need to be different from that on the rest of the farm. For example, at least the amount of nutrients removed in the silage or hay should be applied as a maintenance application to the cut paddocks, but a lesser amount of fertiliser may be required for maintenance in the sacrifice paddocks where large amounts of dung and urine were deposited and large amounts of fodder may have been fed.
In addition to the loss of nutrients in fodder, grain, and animal products, a significant amount of nutrients can be lost off the farm in runoff from irrigation and rainfall.
6.2 The essential plant nutrients
Sixteen nutrients are known to be essential for plant growth. They can be divided into two categories:
- Major nutrients (macronutrients).
- Minor nutrients (micronutrients), often referred to as trace elements
These are listed in Table 6.3.
Table 6.3 Essential nutrients required by plants.
|Major Nutrients||Minor Nutrients|
|Carbon (C)||Molybdenum (Mo)|
|Hydrogen (H)||Copper (Cu)|
|Oxygen (O)||Boron (B)|
|Nitrogen (N)||Manganese (Mn)|
|Phosphorus (P)||Iron (Fe)|
|Potassium (K)||Chlorine (Cl)|
|Sulphur (S)||Zinc (Zn)|
Other minor elements, such as sodium, silicon, cobalt, strontium and barium, do not seem to be universally essential, as are the sixteen nutrients listed in Table 6.3, although the soluble compounds of some may increase plant growth. Other elements required for animal health, such as selenium, fluorine and iodine, have no known value to plants.
A deficiency in any one of the 16 essential nutrients will reduce growth and production, even though the others may be abundantly available. Optimum pasture production can only be obtained if all the requirements for plant growth are met.
The first three major nutrients, carbon, hydrogen and oxygen, are generally considered to come from carbon dioxide in the atmosphere and from water. Combined, they make up 90% to 95% of the dry matter of all plants.
The remaining nutrients are found in the soil and are taken up through the root system of the plant. However, legumes (such as clovers, lucerne and medics) also have the ability to convert atmospheric nitrogen into a plant-available form.
6.3 The major nutrients or macronutrients
The macronutrients (nitrogen, phosphorus, potassium, sulphur, calcium and magnesium) are required in relatively large quantities by plants. Plant growth may be retarded because:
- These nutrients are lacking in the soil.
- They become available too slowly.
- They are not adequately balanced.
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.
Nitrogen is mobile in plants, so deficiencies show up in the oldest plant tissues first. Nitrogen deficiency in clovers has similar symptoms to a sulphur deficiency. However, sulphur is immobile in plants, so sulphur deficiencies show up in the youngest plant tissues first.
The nitrogen cycle
Nitrogen is present in the soil in many different forms (Figure 6.1), 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 6.1 Nitrogen cycle of a dairy pasture
Plants can only use two of the many forms of nitrogen, namely, nitrate and ammonium. 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.
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
Nitrogen is returned to the soil via animal manure and urine, bought-in feeds, nitrogenous fertilisers, and legumes.
Where do clovers fit in?
The atmosphere is about 80% N, but only legumes (such as clovers) are able to utilise 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 6.1). The nitrogen becomes available to grasses when the nodules or clover plants (roots, stems and leaves) die. 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 (complete working cycle) or brown (decomposing) if growing in suboptimal conditions.
In a typical 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, the contribution by clover is equivalent to about $55 to $270/ha/year worth of nitrogen fertiliser. Hence, clover is a valuable component in the pasture sward for its nitrogen-fixing ability, as well as for its nutritional value.
The rhizobia bacteria supply nitrogen compounds to the clover, and the clover 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 especially essential to inoculate legume seeds when sowing into virgin, or newly cleared land because the soil will not have enough of the required rhizobia present. Although it may not always be necessary to inoculate when resowing an old pasture, it is advisable and is cheap insurance to give the young clover plants a better chance of survival.
Lime coating of the clover seed ensures that the soil environment surrounding the seed is more favourable (in other words, less acidic) for the rhizobia bacteria and young clover roots.
In addition, several proprietary forms of coating (for example, Prillcote and Agricote) contain ingredients to ensure longer survival of the inoculant if sowing is likely to be delayed; in other words, these coatings extend the life of or protect the rhyzobia bacteria until they are placed in the soil. Also, some insecticides can also be included in the coating to provide a degree of protection against some insect pests (for example, lucerne flea or red-legged earth mite) after germination.
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. 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 fertiliser containing phosphorus (for example, superphosphate) is applied to a pasture, the phosphorus enters a phosphorus cycle. As can be seen from Figure 6.2, 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. 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 6.2, 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 available phosphorus readily available to the plants. The remainder is 'bonded' or 'fixed' in an unavailable form to the surface of the soil 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.
Figure 6.2 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:
- Product removal.
- Redistribution of urine and dung.
- Soil losses:
- Surface runoff.
- Soil fixation.
These forms of phosphorus loss are discussed below.
Removal of phosphorus in plant and animal products
Phosphorus is lost from the pasture in plant and animal products (see Tables 6.1 and 6.2 above). 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 losses of phosphorus than wool production. Redistribution of faeces and urine
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 and urine listed in Table 6.4.
Table 6.4 Fate of nutrients consumed by lactating dairy cows
|Nutrient||% in faeces||% in urine||% in milk||% retained|
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.
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. In some soil types with a high fixing capacity, this may occur within hours; in other soils, the P may remain in the soluble form for several weeks. 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). This 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 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 as yet for this loss in most Victorian soils under a pasture situation, but leaching of phosphorus is known to be relatively low in most soils types. Potassium, sulphur and, under some situations, 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.
Surface runoff on dryland farms: A small amount of applied phosphorus may be lost via surface water carrying away minute amounts of dissolved phosphorus. This amount is much higher if a heavy rainfall occurs within days of applying the fertiliser. Recent 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 fall immediately after application.
Provided pasture cover is good and soil erosion is minimal, the majority of the phosphorus lost via this means is in the soluble phosphorus form. This soluble form is so small (virus size) that pastured or treed riparian (along the banks) buffer strips have very little effect in preventing this particular nutrient loss.
The major effect of this loss is that this 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 of research currently receiving much attention.
Fixation: 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 (in other words, the less available it becomes to plants).
When phosphorus is first applied to soil, a rapid reaction (within a few hours) 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 'fixing' the P to larger and larger 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 adsorbing) 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. Ongoing research is trying to determine the amount of P fixed in a range of soil types. The phosphorus buffering index has been developed to help with differentiating soil P-fixing ability.
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) caused by 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
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).
- Whitish spots along the outer margin of clover (and lucerne) leaves, which subsequently develop a necrosis, or deadening, of the outer leaf margins.
- 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 to identify a K deficiency in grass.
- Excess salinity may also cause brown, necrotic leaf margins, but this occurs mostly in the younger leaves.
Potassium is very mobile in the plant (in other words, rapidly transferred around the plant), and deficiency symptoms initially occur in the older leaves. 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 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 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 6.3) is cycled in a similar way to phosphorous. 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 Tables 6.1 and 6.2, above, which show the rates and quantities of nutrient losses.)
Figure 6.3 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. 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 6.4). 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 6.4 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) and 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 (adsorbed) 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 potassium when it is measured in a soil test. 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.
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.
In trials on irrigated paddocks in northern Victoria and southern New South Wales, potassium has not been found to limit the growth of pastures. Even in soils that have been tested and found to contain relatively low levels of potassium, little response (other than a colour change) has been observed by the application of a potassium fertiliser. However, 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 milk fever and grass tetany respectively
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 clovers
- Small, pale, yellow-green leaves with lighter coloured veins
- Poor development and low numbers of nodules on clovers
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. 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 6.5. Significant amounts of sulphur are removed through meat and plants harvested for fodder, but only small amounts are removed through milk see Table 6.2. 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%). However, if high-analysis fertilisers, such as triple superphosphate and 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 6.5 Sulphur cycle of a dairy pasture
Most sulphur in soils is held by the organic matter and must be mineralised (converted to the inorganic sulphate form, SO42-), before it can be used 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.
Forms of sulphur
Two forms of sulphur are used in fertilisers. They are sulphate sulphur ( SO 4 2- ) , such as in superphosphate, and elemental sulphur (Se), such as used in the Hi-Fert Super M range of products. Sulphate sulphur ( SO 4 2-) is readily available for plant uptake and more effective on very low sulphur soils. The elemental form (Se) must be converted (oxidised) to the sulphate form before it is readily available to the plant. Therefore, this more slowly available form of sulphur (in other words, Se) 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 ( SO 4 2-) will provide a quicker response.
Elemental sulphur (Se) applied at a rate of up to about 30 kg/ha has negligible effect on soil properties but, if applied in large quantities (over 300 kg/ha), can lower the pH of soils. The extent of pH reduction and the reaction rate are 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.
Source: Bolan (1998).
Figure 6.6 Effect of DAP plus MOP on calcium concentration in pasture
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.
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. 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) and nitrogenous fertilisers that produce ammonium ions (for example, DAP) are used together, the potassium or ammonium ions compete at the plant root with the uptake of calcium, thereby raising the risk of inducing milk fever see Figure 6.6. However, nitrogenous fertilisers applied on their own do not cause this problem.
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 Victoria, 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. 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 clovers 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.
Source: Bolan (1998).
Figure 6.7 Effect of DAP plus MOP on magnesium concentration in pasture
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 6.7. The ammonium and potassium ions both compete with the uptake of magnesium ions at the plant root, thus resulting in a lower magnesium concentration the plants. The use of just nitrogenous fertilisers does not cause this poblem.
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.
Grass tetany can be largely prevented by feeding animals a magnesium supplement at a rate of 60 g/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)
6.4 Minor nutrients or trace elements
Although only required in small amounts, minor nutrients (micronutrients or trace elements) are essential for plant growth. These nutrients often act as catalysts in chemical reactions. It is possible to have toxicities of trace elements, as well as deficiencies.
The micronutrients essential for plant growth are listed in Table 6.3 and in more detail in Table D.1. Particular trace element deficiencies are generally restricted to specific soil types or localities.
Many products in the market place extol the virtues of trace elements that are 'absolutely needed' by plants. Some companies use soil tests to determine whether trace elements are deficient in the soil. On the basis of many field and laboratory experiments and much experience, the Department of Environment and Primary Industries has found that soil tests are not a reliable method of detecting a trace element deficiency. Plant tissue tests are far more reliable, but even these are not always correct and must be taken at the appropriate times of the year to increase their accuracy and reliability. In addition, recommendations should be based on research conducted in Australian soils or on Australian plants, not on overseas data.
Some of the micronutrient deficiencies in plants can cause nutrient deficiencies in the animals that graze those plants. In some cases (for example, copper and manganese), these micronutrients are also essential for plant growth. In other cases (for example, selenium), they are not required by the plant. Thus, in many cases of animal nutrient deficiency, it may be better to treat the animal rather than to apply fertilisers to overcome the problem. It is therefore important to discuss trace element issues with your local veterinarian.
Plant tissue testing is the recommended method for testing for trace element disorders in plants, but it can be unreliable for testing for some trace elements required for animal nutrition. Testing of body fluids (blood, urine, saliva) and tissues (liver, bone) is often required to determine whether animals have a trace element disorder. Seeking veterinary advice in addition to, or instead of, plant tissue testing is recommended.
This manual only touches on the complex issue of trace elements and their deficiency and toxicity implications. Several high-quality publications containing colour photographs of deficiency and toxicity symptoms and descriptions are recommended for additional reading. See the References and Further Reading section at the end of this manual.
Some of the more common trace elements likely to be deficient in Victorian soils are discussed below.
Molybdenum (Mo) is essential for the health of the rhizobia bacteria associated with the legume root nodules that are responsible for atmospheric nitrogen fixation. It is also directly involved in nitrogen metabolism and specifically implicated in the electron-transfer system (for example, nitrate reductase and enzyme nitrogenous reactions). Molybdenum is the least abundant of the trace elements in the soil.
Molybdenum deficiency symptoms
Molybdenum deficiency symptoms may look similar to a nitrogen deficiency, and legumes will have green or grey to white nodules rather than the pinkish-coloured nodules of healthy plants.
Consequently, a lack of molybdenum will reduce the nitrogen-fixing ability and growth of clovers. In effect, molybdenum-deficient plants cannot properly metabolise nitrogen, even though their tissues may contain considerable amounts of nitrates.
Molybdenum deficiencies are more likely in acid soils, and the application of lime may increase its availability. However, peat soils, which are usually acidic, should not require additional molybdenum, as these soils usually have high levels of molybdenum held within the organic matter. The application of lime to the peaty soils in the Koo-wee-rup area has been sufficient to rapidly increase the availability of molybdenum and sometimes induce a copper deficiency in livestock.
Applications of 50 to 60 g Mo/ha every 5 to 7 years are required on responsive soils. However, a plant tissue test may show that Mo is not required at the 'due time'.
Molybdenum toxicity is not thought to be significant in plants, but excessive molybdenum levels in plants or high rates of molybdenum applications in fertilisers can sometimes induce copper deficiency in livestock. Therefore, it is generally recommended that copper be included in the fertiliser whenever you apply molybdenum.
Animal health implications
A complex relationship exists between copper, molybdenum, and the sulphates in animal nutrition. Copper and molybdenum are mutually antagonist in plant uptake: if one is applied, uptake of the other may be reduced. Conversely, an oversupply of copper can induce a molybdenum deficiency in animals, particularly on lighter-textured soils or when animals are stressed.
Sulphate can also sometimes restrict molybdenum uptake by plant roots when the two nutrients are applied together. High molybdenum content in plants can be dramatically lowered by addition of sulphates. To avoid copper deficiency in grazing animals, it is generally recommended that copper should be applied when molybdenum is being applied in a fertiliser. However, if plant levels of copper, as determined by a plant tissue analysis are at satisfactory levels or higher, then molybdenum can be applied without copper.
In addition, molybdenum should not be applied to pastures limed within the past 12 months, as the combination of applied molybdenum plus the molybdenum released from the soil by the lime may raise the molybdenum levels enough to cause copper deficiencies in livestock.
Copper(Cu) is required for the formation of enzymes for chlorophyll production, nutrient processing and the plant's exchange of water and oxygen for carbon dioxide. It is also required for seed setting of legumes. Plant responses (in other words, additional growth) due to copper are rare.
Copper deficiency symptoms
Copper deficiency symptoms are not very specific in plants, although 'dieback' is common, showing up first in the young growth.
Copper deficiencies commonly occur in highly leached acid sands (such as coastal sandy and sandy loam soils), in loams from sandstone, in peat soils, and in highly calcareous alkaline soils.
Rates of 1.5 to 2 kg Cu/ha applied as fertiliser every 3 to 6 years are required for deficient soils.
Animal health implications
Problems with copper are more commonly associated with animal deficiencies than with plant deficiencies. Because animals have a higher copper requirement than plants do, animals may become deficient at copper levels that are sufficient for normal plant growth.
Copper deficiency symptoms are also more obvious in livestock than in plants. The symptoms appear as hair or coat abnormalities (red-coated animals tend to be pale-red or orange), retarded growth and skeletal defects, infertility, and diarrhoea.
It may be necessary to treat the animals directly, particularly if copper deficiency symptoms are evident in livestock.
Zinc (Zn) is associated with the formation of chlorophyll and of several enzyme systems required for protein synthesis. It also has a regulatory role in the intake and efficient use of water by plants
Zinc deficiency symptoms
Zinc deficiency symptoms include:
- Small bronze spots on older leaves of clovers; as spots enlarge, leaves develop a mottled appearance.
- Branching of small, dark green, distorted leaves in the centre of clover plants (called the 'little leaf syndrome' and noted at Yanakie, South Gippsland).
Typically, zinc deficiency is associated with leached acidic sandy soils, alkaline soils with considerable calcium carbonate content, and soils with high organic matter. Deficiencies may be temporarily induced by cold, wet weather and have been noted to disappear with the onset of warmer weather. Deficiencies are uncommon in pastures in southern Victoria except on the alkaline coastal soils
Zinc availability is related to pH; and in the north-west and Goulburn Valley areas, zinc availability is often low on heavily cut (lasered) paddocks after landforming, particularly if they are planted to maize and other fodder crops. In this situation, alkaline subsoils become exposed.
Manganese (Mn) has several plant-growth functions. It is closely associated with iron, copper and zinc as a catalyst in plant-growth processes; is essential for rapid germination; and plays a role in enzyme systems in seed and new tissues.
The main factor controlling manganese availability in the soil is pH. The more alkaline the soil, the more likely deficiencies will occur. Conversely, very strongly acidic soils can accumulate toxic levels of manganese, and lime should be considered. Occasionally, a manganese deficiency can be induced by excessive liming on these acid soils.
Manganese deficiency symptoms
Manganese deficiency symptoms include:
- Yellowing between the veins of young leaves.
- Eventually spots of dead tissue may drop out, leaving a 'ragged' leaf.
- Stunting of growth.
- Reduced flower formation.
There is no evidence in Victoria of manganese deficiency affecting pasture growth. However, manganese deficiency in pastures can be treated by applying manganese sulphate.
Manganese toxicity symptoms
Manganese toxicity symptoms include:
- In subterranean clover, symptoms of toxicity appear in late autumn, initially as a light brown discolouration of the leaf margins, which later become reddish. Waterlogging or root rot can produce similar symptoms, so a plant tissue analysis may be necessary to determine the true problem.
- The plant may die in cases of severe toxicity.
Although rare, manganese toxicity can occur in acid soils inherently high in manganese, such as in north-eastern Victoria. Soil compaction and waterlogging (both of which result in inadequate soil aeration) can produce manganese toxicity in plants.
Manganese toxicity can be reduced by working lime into the soil to a depth of 100 to 150 mm and by correcting waterlogging and soil compaction.
Animal health implications
Livestock are susceptible to both manganese toxicities and manganese deficiencies.
A lack of manganese is commonly associated with infertility in cows and impaired growth and bone development. There have been no confirmed cases of manganese deficiency in grazing animals in Victoria.
Deficiencies in livestock can be corrected with manganese supplements. Check manganese levels via a plant tissue analysis of mixed herbage if concerned about manganese deficiency in livestock.
Iron (Fe) is associated with the production of chlorophyll and helps to carry oxygen around the plant cells. Iron is also involved in reactions that convert nitrates to ammonia in the plant.
Iron deficiency symptoms
Iron deficiency symptoms include:
- Chlorosis (yellowing) between the leaf veins of the youngest leaves.
- Tips and margins of leaves remain green for the longest time.
- Affected leaves curve upwards.
- Stunting and abnormal growth.
Iron is very immobile in the plant. Thus, deficiency symptoms affect the youngest leaves first.
Deficiencies usually occur on high-pH calcareous soils or in soils that have been heavily limed.
Boron (B) is mainly involved in the movement of sugars throughout the plant and in seed production in legumes. It is also an important nutrient in the metabolism of nitrogen, carbohydrates, and hormones and is involved in the uptake and efficient use of calcium in the plant.
Boron may induce both toxicities and deficiencies in Australia.
Boron deficiency symptoms
Boron Deficiency symptoms include:
- Distorted and chlorotic leaves with darker pigmentation along the leaf margins.
- Red and yellow discolouration, particularly in sub clover.
- Poor growth.
- Low seedset.
Deficiencies often tend to disappear after rainfall since plant roots may be unable to access soil boron in dry soils. Lucern is the main crop in which boron deficiency has been identified in Victoria.
Boron deficiencies may occur in humid regions, in highly leached acid sands, in organic (peaty) soils, and in calcareous (alkaline) soils and becomes less available in poorly drained soils. Occasionally, liming may heighten a boron deficiency. Boron deficiency can be induced in turnip fodder crops by lime application, usually at 3.5 t/ha or higher during seedbed preparation.
If plant tissue analysis indicates a deficiency, then apply born with a fertiliser application and retest in 2 to 3 years. Seek expert advice to determine the appropriate boron types and application rates.
Chlorine (Cl) is thought to stimulate carbohydrate metabolism, some plant enzymes, chlorophyll production, and the water-holding capacity of plant tissues. Chlorine seems to be more important for animals than for plants. Chlorine deficiencies seldom occur as the chloride ion is continually replenished via rain water, the amount increasing with rainfall quantity and closeness to the sea.
- Nutrients are necessary for plant growth.
- There are two categories of plant nutrients: macronutrients and micronutrients.
- The major nutrients, or macronutrients, supplied by the soil are nitrogen, phosphorus, potassium, sulphur, magnesium and calcium.
- The minor nutrients, also referred to as micronutrients or trace elements, supplied by the soil are molybdenum, copper, zinc, manganese, iron, boron and chlorine.
- Fertilisers are required to overcome nutrient deficiencies and to replace the nutrients that are lost or removed from the soil and pasture.
- Nutrient cycling (soil-plant-animal) involves nutrients:
- Being brought onto the farm in various forms.
- Undergoing ongoing reactions in the soil.
- Being consumed by animals via the plants.
- Being lost to the farm system by various means.
- Nutrients are required for a number of tasks associated with plant growth.
- A deficiency in any one of the 16 essential nutrients will reduce pasture growth and animal production.
- Various trace elements are deficient in some dairying areas.
- Although grazing animals receive their essential nutrients from pasture, plants and animals have different essential nutrient needs. In some cases, it is better to treat animal nutrient deficiencies directly rather than to try to supply the nutrient indirectly by applying it to the pasture.