Prolonged drought has resulted in a marked reduction in the amount of Phosphorus (P) applied to pastures. The reduction has been driven by two factors:
- Cost Cutting. During drought years P applications were cut to reduce expenditure.
- Decreased Production. At the onset of drought, pasture fertility is often high and moisture is the most limiting factor to pasture growth. In this instance reducing P input is prudent. However sustained reduction in inputs leads to a decline in soil P level, resulting in a decline in productivity.
The reduction in application of P has seen soil P levels in many pastures decline to the point where pasture growth and carrying capacity are compromised. A perennial pasture that is Phosphorus deficient will:
- Be slow to respond to rainfall
- Produce less feed (dry matter)
- Be less competitive with weeds
- Become invaded by undesirable species
- Likely become Nitrogen deficient as well due to pasture legume species becoming weaker
The current buoyant livestock market provides an opportunity to review the program for application of pasture fertiliser and increase pasture productivity and carrying capacity. In doing so the following should be considered:
- Pasture composition. Priority paddocks for single super application should be those containing productive pasture species.
- Soil pH. The most limiting factor to plant growth should be addressed first. Paddocks with soil acidity may require liming prior to applications of P. Raising the pH increases the availability of soil P, hence reducing the requirement for P fertiliser.
- Current soil P levels. The critical value is the soil P level at which a good legume and grass pasture could be expected to reach 95% of its potential production. In southern NSW the critical soil value is approximately 35 mg/kg Colwell P.
- Target Stocking Rate. The amount of Phosphorus required to maintain the critical soil P value will be determined by the stocking rate. Target stocking rate should be a level that can be conservatively maintained throughout an average year
- Phosphorus removal. Phosphorus removal varies from 0.4 to 1.4 kg P/DSE/ha/year. On an annual basis, assuming a removal figure of 0.9 kg P/DSE/ha and a stocking rate of 8 DSE ha, livestock remove 7.2 kg P/ha year which equates to approximately 80 kg/ha of Single Super. In other words, to maintain the existing soil P level, 80 kg/ha of Single Super is required each year. Where soil P is below the critical value, the rate must be increased in order to build the soil P level over time.
On a whole farm basis, every 1,000 DSE removes 900 kg of P annually. This equates to approximately 10 tonnes of Single Super.
Single Super should be applied to annual pastures during autumn, while application to productive perennial pastures can occur in autumn or spring.