2.08 - Soil nutrient critical limits

Acknowledgement: Tables of the minimum and maximum limits for pasture nutrients provided by Cameron Gourley, DPI, Ellinbank, Victoria.

Extractable soil phosphorus

The following tables provide minimum and maximum limits for pastures that have been improved by the introduction of exotic perennial grasses. Raising P levels in native grasslands to above the levels described below may increase the risk of loss of species, such as danthonia.

Recent research has demonstrated that it is increasingly difficult on soils with a moderate phosphorus buffering index to retain native perennial grasses once phosphorus levels exceeded 20 Colwell P. At this point legumes and annual grasses started to replace native grass species. Grazing management, involving pasture rest at critical times, can lessen the rate of loss.

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.

Table 1: Using Olsen and Bray extraction methods

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

Table 2: Using Colwell extraction – Colwell P in mg/kg

* PBI = phosphorus buffering index

Phosphorus Buffering Index (PBI) typically increases with soil texture as it moves from sands through to heavy clays. National Association of Testing Authorities (NATA) accredited laboratories now routinely include a Phosphorus Buffering Index with all analysis reports.

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 ground cover throughout the year.

Extractable sulfur

KCL40 test

• Minimum limit – 6 mg/kg
• Maximum limit – 16 mg/kg

Table 3: Extractable potassium

Organic carbon

Table 4: Organic carbon percentages (%) over a range of conditions

Cation exchange capacity

Table 5: Cation exchange capacity (M NH4Ac, units – cmol(+)/100g or meq/100g)

Soil pH

The following two tables supply limits for soil pH (soil acidity) and aluminium concentrations for commonly introduced pasture species. Each species achieves optimum productivity and stability between the maximum and minimum limits. All will grow outside these limits, but productivity and persistence will be less than its potential.

Native species such as danthonia and microlaena are not included in the tables as they are rarely sown and limits are not as well defined. It is known that microlaena is able to tolerate soils with low pH. Danthonia, on the other hand, has many subspecies that vary in their most preferred range.

Most plants and micro-organisms have defined ranges of pH for optimal growth. The optimal range for plants is generally between 5.5 and 8.0 (pH in CaCl₂) whereas most soil organisms function best between pH 6.0 and 7.0.

Table 6: Soil pH

Exchangeable aluminium

Table 7: Exchangeable aluminium

Native species vary in their sensitivity to soil acidity and exchangeable aluminium.

Table 8: Salinity and sodicity