Soil phosphorus, potassium and sulphur excesses, regularities and heterogeneity in grazing-based dairy farms

• Nutrient surpluses on dairy farms have led to excessive soil accumulation.
• We studied soil nutrient levels on 43 contrasting grazing dairy farms across Australia.
• More than three quarters of paddocks had excessive soil P, K and S levels.
• Patterns of nutrient distribution were associated with within-farm nutrient flows.

Efficient nutrient management is a critical component of profitable and sustainable milk production on modern dairy farms. While the impact of intensification on nutrient surpluses at the farm scale is well recognised, there are few studies that have quantified the corresponding soil nutrient concentrations within dairy farms at a national scale. In this study, we examine soil phosphorus (P), potassium (K), and sulphur (S) concentrations on 2092 grazed dairy paddocks and animal containment areas within 43 contrasting dairy farms from all dairy producing regions of Australia. More than three quarters of the routinely grazed paddocks sampled had available P, K and S concentrations above agronomic requirements, while 20% of paddocks were at least 3 times above agronomic requirements. An exception to these generally high levels were paddocks sampled on organic or biodynamic dairy farms, which had lower soil P and S concentrations. Within farm nutrient heterogeneity was substantial, irrespective of the characteristics of the dairy farm or the regional location. Animal holding areas often had excessively high soil nutrient concentrations. Overall, there were significant relationships (P < 0.01) between soil P, K and S concentrations and farm management and paddock characteristics including distance from milking parlour, application frequency of mechanically applied effluent, frequency of grazing herd visits, frequency of feeding of conserved forage and frequency of mechanical pasture harvesting. These results highlight generic management practises that exacerbate elevated soil nutrient concentration within grazed dairy farms and have direct implications to farmers and advisors. There are clear opportunities to strategically apply, and more often reduce, P, K and S fertiliser inputs on dairy farms without a loss of production. A greater understanding of farm nutrient fluxes and expected patterns of within-farm nutrient distribution, complemented by comprehensive soil testing, will help guide profitable and environmentally beneficial nutrient management decisions.