Making More from Sheep Australian Wool Innovation Limited Meat & Livestock Australia
MODULE 7: Grow More Pasture
Tool 7.4
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Guidelines for the use of phosphorus

Capital applications

Capital applications of fertiliser are aimed at lifting the soil nutrient levels to target levels based on optimising growth and persistence of a particular pasture species. They involve making one or more applications over and above the level required for replenishing nutrients lost through export off the farm in products. Capital applications often involve phosphorus fertilisers or they can refer to lime. Expenditure on capital applications must be regarded as an investment of financial capital. To get a positive return on capital invested, the increased pasture grown must be converted into increased animal product/ha and profit. The speed at which nutrients are built up will depend on cash flow/access to capital and how quickly stock numbers can be increased (purchase or breed) to utilise additional pasture. The extra capital required to invest in stock is often much larger than that invested in fertiliser. The higher the marginal return per unit increase in turn-off or stocking rate, the higher the return from capital applications. Average returns from capital applications of P fertiliser, for producers involved with the Grasslands Productivity Project, were $1.89 for every $1.00 invested in fertiliser and stock (see table 7.1 in procedure 7.1).

The amount of P needed to increase soil P by one unit varies with soil type. Heavier textured soils (clays) usually require more P than lighter, sandier soils. Depending on the soil’s phosphorus buffering index (PBI), it may take 5–15 kg P/ha to increase Olsen P by 1 unit, or 2–4 kg P to shift Colwell P by 1 unit.

Maintenance applications

Maintenance applications of fertiliser are aimed at replacing the amounts of nutrients exported off the farm in produce (meat, wool, hay, silage) or tied up in the soil. Maintenance applications are designed to maintain the target soil levels for each nutrient and maintain the current level of productivity. Maintenance applications usually involve P and S, but may involve K in hay or silage paddocks. Maintenance rates of P are based on the PBI of the soil. For example:

  • Apply 0.8 kg P/DSE when PBI is low (0–100)
  • Apply 1.0 kg P/DSE when PBI is moderate (100–300) and
  • Apply 1.2 kg P/DSE when PBI is high (>300).

Fertiliser principles

Prioritise applications on the basis of where you will get the best response. Newly sown pastures that have low levels of soil fertility are a high priority. Persistence of the sown species is essential to ensure pasture establishment is a profitable exercise. Introduced species of grasses and legumes have a higher requirement for phosphorus than native grasses so fertilising these paddocks will give a better response. While native grass pastures containing some legume will respond to some fertiliser, many native species will decline under competition from legumes, annual grasses or broadleaf weeds when phosphorus levels are raised.

Correct all nutrient deficiencies where economic responses are indicated. If P, S and K are all identified to be below critical levels, then a fertiliser blend containing all three nutrients should be applied. Correcting only one nutrient will limit the response in pasture growth.

Apply fertiliser when the pasture is most actively growing and you will get best response, eg, autumn for annual or sub clover/perennial grass pastures, or split autumn and spring applications for highly productive perennial pastures (eg, white clover/perennial ryegrass) or spring for summer-active perennial pastures (eg, lucerne).

Do not apply P to waterlogged soils or at the coldest time of the year as most of it will be rapidly converted into a form that is not available to plants.

Avoid applying fertiliser near waterways, waterlogged soils, and on steep slopes to reduce nutrient contamination of streams. Do not apply fertilisers when storm events are likely to increase risk of nutrient run-off. Applying fertiliser in low ground cover situations should also be avoided.

For native grass pastures in high conservation areas, do not apply fertiliser or legume seed.

Guidelines for the use of nitrogen

Nitrogen (N) fertiliser can be used to increase pasture growth rates and fill feed gaps in autumn and winter. It can also be used to boost spring growth for silage or hay production.

Pastures respond best to N:

  • Where there is a high proportion of introduced annual or perennial grasses (60% grass content)
  • Where the other macro and trace elements are adequate and
  • While soil conditions are warm (above 8–10°C) and moist.
  • Do not fertilise nitrophilic weeds.

Pasture growth responses to N can range from 15 kg DM/ha for every 1 kg N applied in autumn, down to 5 kg DM/ha for every 1 kg N in winter. Higher responses are possible in spring. During the growing season, N usually works out to be cheaper than filling feed gaps with grain, hay or silage. For example:

  • Wheat at $150/t equates to 17c/kg DM or 1.4 c/MJ ME*, whereas
  • N at $490/t spread, with a response of 10 kg DM/ha per kg N, equates to 11.0 c/kg DM or 0.9 c/MJ ME.

The economics of N application are dependent on the relative price of grain to N fertiliser as well as the predicted response.

*Megajoules of Metabolisable Energy

Trace elements and plant tissue tests

Molybdenum (Mo) is essential for nodule function in legumes and is commonly deficient in acid soils. Deficient legume plants appear stunted and their roots should be inspected for nodulation and activity of nodules. If there is poor nodulation or nodules are white rather than pink inside, apply Mo.

Alternatively, plant tissue tests can be used to assess the need for trace elements. Samples need to be taken in spring, before the legumes flower. Ensure the lab is ASPAC accredited for the tests you want conducted. Enquire about the correct sampling protocol for the plants you are testing for.

Avoid environmental impacts from nutrient loss

A separate consideration to pasture response is pinpointing areas on your farm where nutrients leak, losing you money and posing a threat to the environment.

Use the Better Fertiliser Decisions CD tool, the ‘Farm Nutrient Loss Index’, to plan your fertiliser needs and applications across the farm with far greater precision.

From this information, you can choose to either reduce nutrient inputs, or stem nutrient losses by applying one or more of the key decisions, critical actions and benchmarks in this module, and in Healthy Soils.

Get your free copy of the Farm Nutrient Loss Index tool by: