• Why is it important to me as a farmer?
• Why do we need phosphorus?
• How to recognise the need for phosphorus in the paddock

• Managing the application of superphosphate

• Superphosphate is a fertiliser produced by the action of concentrated sulphuric acid on powdered phosphate rock
• Phosphorus (P) is frequently a limiting nutrient to the growth of pasture. Generally, soils in Australia are depleted in phosphorus due to weathering and erosion. Agriculture removes phosphorus from the soil in farm products
• The minerals in soils have the ability to bind to phosphorus. Some soils (basalt derived clay) bind strongly to phosphorus, others (sands) less so
• The ability to bind phosphorus is known as the phosphorus sorption capacity of a soil. Soils with a high phosphorus sorption require more fertiliser because more of the phosphorus applied is bound in a form not available for plant growth

• Annual soil testing is an effective guide to monitor changes in available phosphorus when in the early stages of a fertiliser programme
• Because phosphorus is so easily fixed in the soil, plants can take up only a small amount of the phosphorus you apply. For this reason it is important that you apply phosphorus every year and test your soil regularly to monitor phosphorus levels
• The growth of a pasture is limited by the nutrient in lowest supply. There is no point in putting on more phosphorus if a lack of potassium or sulphur is restricting pasture growth

• Phosphorus (P) is the most commonly applied nutrient in south west Victoria and so determining the optimum amount to apply has significant impact on farming systems and profitability
• It 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

• 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
• Excessive amounts of applied P can lead to losses in runoff and contribute to the eutrophication of waterways

• Most Australian soils are naturally low in phosphorus due to extensive weathering. While native plants are adapted to these low levels, introduced crops and pasture grasses are not, which means you need to apply phosphorus fertilisers to soil to achieve productive yields. Australian farmers use much more phosphorus than nitrogen and potassium compared with farmers in Europe and USA
• When first settled, all soils in south west Victoria had very low phosphorus levels, the only exceptions being those around the old volcanoes. Regular applications of phosphorus particularly in high rainfall (> 800 mm) areas, along with improved species such as perennial ryegrass, phalaris and subterranean and white clovers have resulted in massive increases in pasture productivity
• Phosphorus fertilisers are available in several forms, all based on rock phosphate
• Superphosphate (8.8% P) - This is produced by treating rock phosphate with sulphuric acid, and is the most commonly used phosphorus fertiliser. Also used are double superphosphate (17.1% P), triple superphosphate (20.7% P), mono-ammonium phosphate (MAP,21.9% P), or di-ammonium phosphate (DAP, 20%P)
• Phosphorus often reacts rapidly with other elements such as calcium and iron in soil, and can result in less than 5% of the total P applied being taken up by plants in the year of application. This adsorption effect increases with declining soil pH
• Nutrient availability (influenced by soil pH), adsorption (nutrient complex with soil components - insoluble), transport and function of nutrients are all critical factors in relation to plant growth. The interception of nutrients by the plant is also crucial; as is the relative uptake and translocation of nutrients from root to shoot, leaves, flower and grain
• 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

• Examples:
  1. If you are producing 50,000 L of milk per year, then you are exporting 50 kg of phosphorus (P) off the farm in the milk. (50,000 L milk x 0.1% P = 50 L of P, which weighs 50 kg)
  2. If you buy in 100 tonnes of hay (80% DM), you are buying in 200 kg P. If you sell 6 tonnes of silage (40% DM), then you are also selling 4.8 kg P
  3. If you have 100 cows who excrete 10 times per day, then the 365,000 excretions, each weighing 1 kg fresh weight (or 0.1 kg dry matter), would involve the cycling of 365 kg P. If 10% of this is deposited in the laneways and dairy, then 36.5 kg P are lost from the system and 328.5 kg of P are recycled through the pasture

• 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

• Without phosphorus, plant growth is retarded. Phosphorus deficiency is difficult to diagnose, and by the time it is recognised it may be too late to do anything. If plants are starved of phosphorus as seedlings they may not recover when phosphorus is applied later
• 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

• 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

Figure 1 - Phosphorus deficient sub-clover (on right) is stunted and darker in colour compared to healthy sub-clover (on left). � Source: DAFWA

Figure 2 - Phosphorus deficient oats with reddish colour on older leaves and stunted growth. � Source: DAFWA

• Phosphorus has traditionally been measured as Olsen P. This measures the amount of nutrient that is immediately available to the plant
• For pastures and cropping in south west Victoria, generalised ranges for Olsen P in 0-10cm surface soils are shown below. Soils in the �adequate� range will generally achieve 95% of potential growth. Levels in the �high� range are more susceptible to environmental losses in runoff and should be avoided

Table 3 - Availability of P at various Olsen P values. � Source: Target 10, 2005

• Decisions about fertiliser use have a large impact on farm profitability, so it is appropriate to put some time and money into making sure the best decisions are made
• Nutrient requirements for phosphorus can be estimated from a relationship between fertiliser rate and the gross margin for the enterprise (this is done instead of a soil test for P.)
• The rule to remember with soluble fertilisers is to apply small amounts regularly, rather than a large amount occasionally. Plants can use only a fraction of any large amount applied, so most will be leached away or react with the soil (P, Cu, Zn, etc). This is not only a waste of fertiliser and money, but it may also pollute ground water and surface waters and may contribute to the development of algal blooms
• Non-mobile nutrients such as P stay close to where they are applied, and are moved only by cultivation and seeding. Fertiliser placement should be used to ensure optimum plant uptake during growth and development, and P should be positioned where it is likely to be in moist soil for longer
• Phosphorus fertilisers can be applied to dry soil and should not be applied if there is a reasonable risk of storms causing runoff. Split applications are preferable when using heavy rates
• The choice of fertiliser depends on price and the need for additional nutrients. Phosphorus fertiliser applications need to be guided by soil test results
• Phosphorus costs are high (around $3.50/kg) and a decision on the rate required to reach the desired test result needs careful consideration
• On farms with high production, e.g. dairy, hay production, six monthly or annual applications of fertiliser may be necessary
• On paddocks where soil test phosphorus levels are adequate, maintenance applications of fertiliser can be made less frequently. Application of phosphorus fertilisers is not necessary in areas where Colwell soil test results exceed 85 g/kg

• The rate of phosphorus that can be applied at sowing may be limited by the sowing equipment used
• If more phosphorus is required it can be applied prior to sowing, immediately after sowing or, in higher rainfall areas, in the spring
• Suggested application rates for grazing pastures are 10kg P/ha (applied as 100 kg/ha of single superphosphate) for stocking rates of 10-15 DSE/ha and 20kg P/ha (as 120kg/ha of double superphosphate) for stocking rates of 15-25 DSE/ha
• In south west Victoria, farmers can refer to the results of the Hamilton Long-Term Phosphate Experiment to calibrate phosphorus application rates. Using the results of this experiment, a �phosphorus calculator� has been developed. The actual amount of phosphorus to apply needs to consider the stocking rate run on the particular farm or paddock. The calculator gives a recommended rate of phosphorus fertiliser to apply per DSE/ha. The calculator also considers a wide range of variables that will affect the required phosphorus rate for a paddock including:
  • Average rainfall
  • Grazing system used (rotational or set stocked)
  • Steepness of the paddock
  • Pasture species present

• See Link to Phosphorus for Pastures � detailing the correct use of phosphorus fertilisers

• The later a crop is sown, the more reliant plants are on fertiliser P, as the roots have less time to explore relatively warm moist soil before the cold sets in
Conversely, with an early sowing, plant roots can exploit soil P very successfully and are less reliant on fertiliser P
• The higher the yield potential, the greater the demand for P. Therefore, the need for fertiliser P has to be considered in terms of the grain produced. The rule of thumb for maintaining P fertility under cereal cropping is:
  • Maintenance rate (kg P/ha) = 10 + (grain yield in tonne/ha � 3) x 3.5 for grain yields greater than 3 t/ha
  • For example, to maintain P supply in a soil producing about 4 t/ha of grain requires:10 + (4�3) x 3.5 = 10 + 3.5 = 13.5 kg P/ha
  • For grain yields of less than 3 t/ha, a maintenance rate of 10 kg P/ha is required

• Such fertiliser rates will ensure that the fertiliser + soil P pool will meet the average long-term needs of the cereal crops
• Although the P content of canola and lupin grain is different to that of cereals, a similar P requirement can be used as an estimate
• Based on Department of Environment and Primary Industries trials on crop responses to P, the optimum rate is about 20 kg P/ha (250 kg single superphosphate/ha) and no less than about 15 kg P/ha
• Fertiliser placed in a band near the roots is a more efficient method of application than broadcasting

• What are the optimum nutrient targets for pastures?
• How do I improve soil without using too much fertiliser?

• Phosphorus for Pastures - Department of Environment and Primary Industries, Victoria


• Phosphorus for Dairy Farms - Gourley (2002) DNRE


• Fertilising Pastures :(Chapter 5)- Greener Pastures for south west Victoria (2006) � Department of Environment and Primary Industries, Victoria


• Why phosphorus is important - Department of Primary Industries, NSW


• Cycling of phosphorus in grazing systems - Department of Primary Industries, NSW


• Fertilisers for pastures - (See page 11) � Department of Primary Industries, NSW


• The Fertiliser budgeting tool (Dairy) - The Ellinbank Phosphorus Budgeting Estimator - Accounting for Nutrients on Australian Dairy Farms

• Improving soil chemical fertility - (Section D8 � Phosphorus) for Grazing and Cropping - SOILpak - southern dryland farmers - Department of Primary Industries, NSW


• Soil Assessment Component - HDLN Soil & Water Dairy Action Program June 2008 - Victorian Resources Online - Department of Environment and Primary Industries, Victoria


• What Nutrients do Plants Require? - Department of Environment and Primary Industries, Victoria


• Phosphorus factsheet - Soilquality.org.au


• Phosphorus - Soil Health Knowledge Bank � Department of Agriculture, Fisheries and Forestry


• How do I manage soil fertility? - - Soil Health Knowledge Bank � Department of Agriculture, Fisheries and Forestry

• What Nutrients do Plants Require? Department of Primary Industries, Victoria.
• Improving soil chemical fertility. (D8 � Phosphorus) for Grazing and Cropping - SOILpak - southern dryland farmers - Department of Primary Industries, NSW.
• Fertilisers for pastures. Department of Primary Industries, NSW.
• Phosphorus for Pastures. Department of Primary Industries, Victoria.

• Phosphorus. Soil Health Knowledge Bank � Department of Agriculture, Fisheries and Forestry.
• Cycling of phosphorus in grazing systems. Department of Primary Industries, NSW.
• How do I manage soil fertility? Soil Health Knowledge Bank � Department of Agriculture, Fisheries and Forestry.
• Fertilising Pastures (Chapter 5) Greener Pastures for south west Victoria.- (2006). Department of Primary Industries, Victoria.

Page Updated: September 2013
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