Soil and Plant Nutrition
Much of WA is dominated by sandy soils characterised by low amounts of organic matter and a poor ability to retain water and nutrients. So it is not surprising that fertilisers are a critical part of agriculture in this State, and that crops and pastures grow poorly without the addition of nutrients.
Plants require 16 nutrients for healthy growth, known as mineral or inorganic nutrients, and are categorised into:
- Major nutrients
- Minor nutrients
- Other nutrients
Plants obtain carbon, hydrogen and oxygen from carbon dioxide and water in the atmosphere and use these to produce carbohydrates in the process of photosynthesis. These three elements are known as non-mineral nutrients.
The major nutrients required for healthy growth are nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca) and magnesium (Mg). Apart from calcium and magnesium, deficiencies of the major nutrients are common in WA unless they are added to the soil in fertilisers.
Among the mineral nutrients, nitrogen (N) is required in the largest amounts. Some plants known as legumes (e.g. peas, beans and clover) obtain most of their N from the atmosphere in a symbiotic association with specific bacteria. This process is called nitrogen fixation.
In general, N is the most common nutrient deficiency in non-legume plants. Increased vegetative growth (and tillering in cereals) is often seen with N application and protein content is also enhanced.
Nitrogen is mobile in both the soil and the plant. Most of the N in soils (98 per cent or more) occurs in organic forms. This must be mineralised to the inorganic ammonium and nitrate forms before plant roots can utilise this N.
Role of Nitrogen
Nitrogen is required for protein formation and determines yield and grain quality.
Deficiency symptoms are often observed as a yellowing of the oldest leaves, reduced plant tillering capacity and in severe cases death of young tillers.
In cereals, the number of tillers is decreased resulting in fewer fertile heads. Poor N supply leads to reduced protein production and low protein content in the grain. The grain also tend to be poorly filled and shrivelled.
Our fertilisers containing N include Urea and Flexi-N, while products such as GranNS, Flexi-NS, Liqui-NS, NS31, NS41, NS51 and NS61 have varying ratios of N and S. Cropping fertilisers including Agras, Agstar and Agflow contain P and S as well as N, while K-Till and MacroPro products contain N, P, K and S.
All soils in WA have the potential to be N deficient as there is not the carry over from legume pastures as seen previously. Only a small proportion of fertiliser N is carried over from one year to the next unless it is converted into organic forms. Hence, N fertilisers need to applied every year for high cereal and canola yields and profits.
Following high rainfall, N is sometimes leached out of the rooting zone and becomes unavailable to plants. In these cases, splitting N applications between seeding and four to six weeks later can increase N recovery and allows you to assess the yield potential of the crop before investing in subsequent fertiliser applications.
Native soils in Western Australia are among the most highly phosphorus (P) deficient in the world. Although N and K are required in larger amounts for plant growth, P deficiency was one of the major limitations to agriculture in this State for many years.
Apart from the very sandy soils with little clay or organic matter, most WA soils retain P tightly. In the year of application, 80-95 per cent of the P applied in fertilisers reacts with soil particles before it can be taken up by the plant roots. This process is known as adsorption.
Over time, as more and more P is added to the soil, the adsorped P builds up and can return to the soil solution to be utilised by plants.
Role of Phosphorus
Phosphorus is an essential component of plant cell membranes, genetic material (DNA), and energy transfer and storage systems. It is critical for early development of plants and is a building block of plant nutrition.
Deficiency during the early stages of plant development stunts growth and reduces yield potential dramatically. Symptoms are first evident on the older leaves which appear stunted and take on a dull dark-green colour, sometimes with a purple or reddish appearance associated with accumulation of sugars and other compounds. Leaves can distort and may turn yellow from the tip before these areas die.
Root growth is also greatly reduced by a lack of P, resulting in poor uptake of other nutrients and water.
Our fertilisers containing P include Super Phos mainly used on legume pastures and Big Phos and Double Phos for legumes crops such as lupins. For cereals and canola, Agras, Agstar, Agflow and K-Till contain varying ratios of P and N. The placement of P fertiliser for crops is important because P is usually highly immobile in the soil. For example, P may only move 1-2 mm away from the fertiliser granule in some highly adsorping soil types. Hence, the plant roots have to grow and intercept the fertiliser granule to obtain the P. Topdressing P on the soil surface in these cases is highly inefficient because the roots need to grow from the seed to the soil surface to obtain the P and this layer of soil is often dry for much of the year. Applying the P close to the seed increases P uptake and early vigour in most cases.
In high rainfall areas of WA, many sandy soils have low native levels of K and K fertilisers are required, especially in highly productive dairy pastures and horticultural crops. Deficiencies of K were first noted in the 1950s in pasture legumes.
Although K levels in other areas were initially adequate for plant growth, continual K export in agricultural products over many decades has resulted in declining K reserves and increased incidence of K deficiency.
Role of Potassium
Potassium (K) is required for photosynthesis, transport of sugars, enzyme activation, maintenance of water status and regulation of stomata (openings that allow carbon dioxide into the leaf and water vapour out). Potassium is critical for controlling water balance within plant cells.
Deficiency symptoms are often described as 'firing' of the oldest leaves. In cereals, this leaves a green spear shape on leaf end. Eventually the whole leaf dies and the crop “hays off” prematurely. The grain produced is often shriveled causing grain quality problems (screenings).
Shallow rooted pasture legumes (eg. Sub-clover) are especially prone to K deficiency. In areas of low K supply, legume seed production is poor and the pastures tend to become dominated by unproductive grasses and other weeds.
Plants deficient in K cannot use other nutrients efficiently, are less tolerant to drought and waterlogging and show greater susceptibility to diseases.
Our K-Till Extra, MacroPro Extra and Lupin Plus products allow farmers to apply N, P and maintenance amounts of K close to the seed. Flexi-NK is also available.
Muriate of Potash, also known as Potash, is the most common fertiliser used for pastures where it is often blended with Super Phos. Potash is also applied to crops.
Potash can be toxic if placed close to the seed in large amounts, hence it tends to be topdressed before or after sowing crops.
While K is more mobile in soils than P, it is retained within the rooting zone more readily than N. Hence, a single large application can rectify K deficiency for several years, without the need for annual applications.
Potassium is not adsorped onto the soil particles as strongly as P, and plants are able to access K fertiliser with greater ease.
Sulphur deficiency was rare in WA, largely because of the widespread use of superphosphate, which contains sulphur in addition to P. However, symptoms of S deficiency were noted in pastures grown on sandy soils in high rain areas.
As with N and P, most of the S in the soil is in organic forms. Hence, soils with low amounts of organic matter are more prone to S deficiency.
Organic S must be mineralised into sulphate before it can be taken up by plant roots. Sulphate is relatively mobile in soils and can be leached out of the rooting zone during winter.
Sulphur deficiencies have become more common over the past few decades with the increased use of fertilisers lacking S, such as All Phos (triple superphosphate or TSP), monoammoium phoshate (MAP) and diammonium phosphate (DAP).
Role of Sulphur
Sulphur is often a forgotten nutrient, but it has an important role in the formation of proteins and is essential for the production of chlorophyll.
Deficiency symptoms are often observed as a yellowing of the youngest leaves, leading to reduced utilisation of applied nitrogen and a reduction in grain yield and quality.
Canola has a high S requirement relative to other plants. Deficient leaves become pale on the top surface and purplish on the under surface, and tend to curl upwards. Flowering and seed set is reduced, and the oil content of the seed is often reduced.
Our fertilisers contain useful amounts of S include Super Phos for pastures and Agras, Agstar, Agflow, K-Till and Big Phos for crops. All our fertilisers contain S in the sulphate form which is immediately available to plants.
Elemental S must be oxidised into sulphate by bacteria in the soil to become available for plant uptake and this may take some time under cool temperatures or dry conditions. The use of fertilisers containing elemental S can result in S deficiency and suboptimal growth, especially in the year of application when the oxidation of the S is slow.
The minor nutrients are boron (B), chloride (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn). These nutrients are also known as trace elements, and are less common than the macro or major nutrients. However, when deficiencies occur, they can cause dramatic reductions in plant growth.
Among the trace elements, deficiencies of copper, zinc, manganese, and molybdenum are relatively common in WA. Deficiencies of boron and iron are also experienced in horticultural crops but these are very rare in broad-acre crops.
Zinc is critical in the production of enzyme systems (chlorophyll, carbohydrate, protein synthesis and growth regulation).
Deficiency symptoms are often observed as lesions in the centre on youngest and middle leaves stunting growth and pale green foliage, reducing yield up to 30%.
Copper is critical in the production of grain quality. It strengthens cell walls, improves resistance to fungal attack, prevents wilting and is required for fertile pollen.
Copper is normally mobile within the plant but deficient plants lose this ability, and symptoms first occur on the youngest growth.
Deficiency symptoms are only visible in severe cases, with rolling of new leaves, leaf tipping and white 'rat tail' heads. The most noticeable symptom occurs when the plant fails to develop grain after flowering due to pollen sterility. These empty heads can turn white, resulting in yield losses of 20 - 90%.
Sodium (Na), cobalt (Co), vanadium (Va), nickel (Ni) and silicon (Si) may also be required when plants are grown in the absence of soil in glasshouses and laboratories. Deficiencies of these nutrients in the field are exceptionally rare.
Some other nutrients such as selenium are not essential for plant growth but they are required by grazing animals and are sometimes added to fertilisers.