Making More from Sheep Australian Wool Innovation Limited Meat & Livestock Australia
MODULE 7: Grow More Pasture
Tool 7.3
Previous Index Next

The levels described in the following tables apply to tests performed by accredited laboratories using procedures defined by the Australian Soil and Plant Analysis Council (ASPAC) proficiency testing program. Sheep producers are advised to use only ASPAC accredited soil testing laboratories. Check the accreditation status of your regular soil testing providers on the website: www.aspac-australasia.com

Tool 6.5 in the Healthy Soils module lists key benchmarks for problem soils, and signposts a number of resources to help you manage acid, saline, sodic and compacted soils, and non-wetting sands.

Extractable phosphorus

Most soils in Australia are responsive to applications of phosphorus (P) fertiliser.

The following tables show the responsive levels for commonly used P tests, for pastures that have been improved by the introduction of exotic legumes and perennial grasses.

High levels of P application can make it difficult to retain some native perennial grasses. As P levels increase, legumes and annual grasses may replace native grass species. Grazing management, involving pasture rest at critical times, can lessen the rate of loss. The potential reduction in native grass content and the likely change in profitability through the application of P fertiliser need to be considered.

Sheep producers wishing to optimise pasture productivity while retaining native grass species should seek advice from agronomists to establish grazing management strategies and minimum and maximum P limits for their region and soils.

Using P Olsen and P Bray extraction methods (based on 0–10 cm sample)

  Olsen P (mg P/kg)
Bray P (mg P/kg)
Responsive <12 <12
Marginal response 12-15 12-15
Generally non responsive >15 >15

Note that soil type (texture) has no impact on levels when using Olsen or Bray test procedures

Using P Colwell extraction method (based on 0–10 cm sample)


  Phosphorus status at 95% of maximum production
    Low  Medium  High 
 Category  Phosphate
Buffering Index
 Critical Colwell soil test P (mg/kg)
Extremely Low > 10-15  <15 15-20 >20
Very,very low  >15-35 <20 20-25 >25
Very low  >35-70 <25 25-29 >29
Low  >70-140 <29 29-34 >34
Moderate  >140-280 <34 34-40 >40
High  >280-840 >40  40-55  >55

Note: the interpretation of Colwell soil test values is dependent on soil type, or more specifically the Phosphate Buffering Index (PBI) of the soil. Seek further advice before applying Phosphorus fertilisers to light sandy soils with a PBI below 10 (such as those in Western Australia).

The Phosphorus Buffering Index (PBI) typically increases with soil texture as it moves from sands through to heavy clays. Some laboratories now routinely supply a PBI with all analysis reports. The PBI affects the rate at which the soil test value changes with the addition of phosphorus fertiliser.

P is a relatively immobile nutrient once it has entered the soil, usually remaining within a few centimetres of where it was applied. Losses of P by fixation and leaching depend on soil type and rainfall and are generally greater as rainfall increases. Well fertilised pastures are generally protected from P loss associated with soil erosion as they retain high levels of groundcover throughout the year as P is attached to the soil particles.

Extractable sulphur (S)

Soil sulphur benchmarks using the KCl40 method for measuring soil sulphur levels are:

Responsive

Less than 6mg/kg

Marginal response

Between 6 and 8 mg/kg

Non responsive

Above 8 mg/kg

Extractable potassium (K)

  Colwell or Skene K (mg/kg) Exchangeable K (meq/100g)
Soil texture
light
medium
heavy
light
medium
heavy
Responsive <100 <140 <180 <0.25 <0.35 <0.45
Marginal response 100–150 140–220 180–300 0.25–0.4 0.35–0.55 0.45–0.75
Non responsive >150 >220 >300 >0.4 >0.55 >0.75

From Brown et al. (1980)

Understanding Soil pH

To better understand the relationships between soil pH, pasture growth and availability of soil nutrients read the Acid Soil Action brochure from NSW Agriculture at: www.dpi.nsw.gov.au/agriculture/resources/soils/acidity/publications/ph

Acknowledgements

  • Some information extracted from More Beef from Pastures manual (published by MLA).
  • Tables of the critical levels compiled by Jim Shovelton (MS&A) using data from:
    • K.I. Peverill, L.A. Sparrow, and D.J. Reuter (1999). Soil Analysis - an interpretation manual (CSIRO), and
    • A.J. Brown, K.K.H. Fung, and K.I. Peverill, (1980). A manual on the soil testing service provided by the Division of Agricultural Chemistry, Department of Agriculture, Victoria, Tech Rep Series No 34.