Phosphorus stores in dairy farm soils need to be well managed but can be a substantial nutrient resource and a cost saver for farmers.

‘Efficient use of these stores means dairy farmers can make savings on feed and fertiliser inputs,’ says Richmond-based soil scientist Simon Eldridge.

‘Our study of coastal dairy farms in the Sydney area showed that most of the farms studied had a large phosphorus surplus after accounting for phosphorus that left the farm in milk, livestock going to sale and fodder.
 
‘Most of the surplus is likely to be stored in the topsoil and where the cattle spend most of their time grazing close to the dairy but may also accumulate in the effluent pond, or in manure spread around the farm.

‘In our study the highest concentrations of phosphorus were found in topsoil in laneways, holding yards, and paddocks that received effluent, or were used for calf feeding or night paddocks.

‘Fertilisers can also influence phosphorus in topsoil. At a NSW DPI research farm where fertiliser use was tightly controlled and monitored for four years, phosphorus levels increased more than six times at the highest application rate.’

The risk of high phosphorus levels, particularly where the soil has low capacity to absorb the phosphorus, is that the phosphorus will run off the soil surface during rain, or will leach through the watertable to groundwater. 

Simon Eldridge says there are a number of strategies that dairy farmers can adopt to reduce the phosphorus surplus and prevent buildup of phosphorus.

‘Some paddocks have levels well above the point where pastures will respond to additional phosphorus in fertiliser, effluent or introduced feeds. By basing fertiliser applications on regular soil testing some farmers could substantially reduce input costs.

‘They could also benefit by calculating an annual whole farm nutrient budget. This includes all nutrients coming onto the farm as fertiliser and feed, and leaving the farm as milk, fodder and stock.’ 

High soil phosphorus levels in laneways and holding yards are a risk to waterways.

‘Although these unvegetated areas may cover only five per cent of the total farm area, their contribution to phosphorus runoff could potentially be greater than the amount lost from grazed and fertilised paddocks.

‘Collection and sale or redistribution of manure from these areas could help to achieve a more even phosphorus distribution within the farm.

’Rotation of effluent application around several paddocks would also help.

‘Harvesting silage off effluent-irrigated paddocks and feeding to stock in less fertile paddocks is another way of redistributing nutrients more evenly around the farm, and saving on fertiliser bills.

‘Producing more stock feed on the farm would also reduce phosphorus inputs and financial cost.’

Read the findings of the study at www.regional.org.au/au/asssi/supersoil2004/s13/oral/1619_Lawrier.htm

This story appears in Agriculture Today.