Easy steps to manage phosphorus in grazing
From the August 2011 edition of Agriculture Today.
Gut feel, fertiliser cost, and/or the rhetorical “that is how much I have always used” are some of the common denominators when graziers decide how much phosphorus to apply.
Fertiliser prices heading north in combination with drought meant the levels in grazing systems have varied in recent years from zero to capital applications.
“Grazing systems need to adopt some of the practices of cropping systems and become more precise in our management,” said Tumut district agronomist, Nathan Ferguson.
“There are five easy steps to ensure phosphorus (P) applications are profitable.
“The first step is to use soil testing to assess soil P-fertility and to set management targets.
“Colwell P or Olsen P is typically used as the measure of available P in soil test reports.
“Soil tests should be taken from a known location and sampled at the same time of year under similar conditions every year to track changes.
“This enables adaptation to changing conditions over time.”
Mr Ferguson says next step is to determine the stocking rate appropriate for the current or projected soil fertility level.
Stocking rate is reported as dry sheep equivalents per hectare (DSE/ha) per year.
Using growing season length as a surrogate allows a theoretical maximum DSE/ha to be set.
Knowing the current and potential DSE/ha enables a Colwell P target to be set, taking landscape functions, such as aspect, soil type, soil depth, pasture type into consideration.
Once the targets for Colwell P and DSE/ha have been calculated, the third step is to determine how much P needs to be applied.
Determine the price per kilogram of available P from different sources or fertilisers, for example, single superphosphate (SSP), poultry manure, compost.
“The key is to determine the cost of the amount of available P in each product and divide it by the percentage available,” Mr Ferguson said.
For example, if SSP costs $400 per tonne, and contains 88kg/t of available P, the cost would be approximately $4.50/kg of available P (400/88).
Fourthly, check the investment in fertiliser and livestock will generate an acceptable return, remembering the application of P will increase pasture growth.
Fertilised areas need to run more livestock to capture the extra feed to pay for the investment.
To capture the extra feed it may be necessary to increase the stocking rate with livestock on hand and rest paddocks with lower inputs, or purchase extra livestock.
Finally, what other factors might modify the decision?
Is applying P the best investment that can be made?
Mr Ferguson said SSP represents a major cost to grazing enterprises and paying closer attention to investment decisions will have benefits.
For further information on Five Easy Steps visit the MLA website, www.mla.com.au/Publications-tools-and-events/Tools-and-calculators/Phosphorus-tool) or contact your local NSW DPI office.