Phosphorus Best Practice

How can the research be used? Ka pēhea e whai take ai te rangahau?

• Our research has found that when fertiliser is managed badly, it’s responsible for 30 to 80% of the phosphorus that drains away from a farm. When it’s managed well, that can decrease to less than 10%.
• This research has provided a scientific basis for New Zealand and Australian farmers to follow the fertiliser industry-developed ‘4R guidelines’:  Apply fertiliser in the Right place, at the Right time, at the Right rate, and in the Right form.
  • Right place: Avoid critical source areas (CSAs) that have runoff more frequently than other parts of the farm or soil with poor P sorption capacity (eg anion storage capacity <10%). This includes near stream areas. While low water-soluble products may help avoid an initial burst of P loss, frequent runoff events or an inability of the soil to sorb P will mean that even low water-soluble P will be lost in time. It is best to avoid or minimise P applications to CSAs.
  • Right time: Avoid applying a highly water-soluble P fertiliser if rainfall is scheduled within 7 days that will likely to induce runoff. This might mean applying at a different time of year to avoid excessively wet (eg northerly aspects) or dry (spring instead of summer to avoid soil hydrophobicity) conditions.
  • Right rate: The quantity of P loss increases with the rate of P applied so avoid applying more P than is necessary – that is, no more than agronomically required. Where the soil is unable to hold onto much P (eg many Podzol, Organic and Semi-Arid soils) consider reducing the rate and altering the crops grown. For instance, many ryegrass monocultures do not require the high Olsen P required for mixed clover-ryegrass swards.
  • Right form: Water solubility influences the availability of fertiliser-P for loss in surface runoff or leaching. Superphosphate is highly water soluble (90%), while other forms such as serpentine super or reactive phosphate rock are less water soluble (~15 and <5% respectively) – resulting in a slower availability over time.
• This research indicates that despite adhering to regional regulatory rules, significant phosphorus losses still occur when farm dairy effluent (FDE) is applied at low depths (<2mm) to stony free-draining soils, such as those common in the Canterbury region, the second-largest dairy-producing region in New Zealand.
  • Industry bodies and regional and central government could use these results to strengthen guidelines and regulations regarding the use of FDE on soils of low sorption capacity.
  • Farmers should avoid applying FDE to freely draining soil under irrigation, or use solids removal technology that reduces phosphorus to very low concentrations.