Phosphorus - how low can you go?

1 August 2016

Although fertiliser is an important production driver, it is also a significant cost, making up almost 10 percent of operating expenditure (DairyNZ Economic Survey 2014-15).

Determining what is required to support a farm system so it can operate at its best is an essential part of creating a fertiliser programme, and phosphorus is usually a key focus.

So how do you decide how much phosphorus your system requires? Josh Verhoek, Science Extension Officer, Ballance Agri-Nutrients says: “It’s about operating in the optimal zone for your farm. What is going to give you the greatest benefit? Is it saving money by cutting fertiliser costs, or are you actually going to lose more by not producing as much? The answer to that depends on where you sit on the soil fertility scale. For ash and sedimentary soils, the aim is to have an Olsen P (soil fertility and soil quality monitoring indicator) of around 20-30, while the goal for pumice and peat soils is an Olsen P of 35-45. If your farm’s milk solids production is in the top 25 percent for your area, then your target range will be a bit higher.”

If your farm’s Olsen P is above the optimum level, fertiliser can be withheld for a year with very little, if any, production loss. Farmers who have an Olsen P at the upper end of the optimum range can afford to halve their fertiliser application for a year, while those at the lower end of the optimum range will notice a drop in production if they withhold fertiliser. Farmers who are operating below the optimum Olsen P range will have more to gain by applying fertiliser than by reducing it to save money, because there is still room to increase production. At the very least they will need to apply maintenance rates of fertiliser to sustain current production levels.

“Interestingly, studies have shown if you withhold fertiliser, pasture and animal production will decline before you see a drop in the Olsen P,” notes Josh. “Also, the different properties of each soil type will influence how quickly the Olsen P rises and falls. If you have a soil with a high anion storage capacity (ASC), such as a volcanically derived ash soil with an ASC of 70, you’ll have more room to move, as the Olsen P will take a while to fall. On the other hand, a soil with a low ASC, like a sedimentary soil with an ASC below 30, will react faster.”

The ASC is a measure of the soil’s ability to bind negatively charged ions, like phosphate. Soils with a high ASC have a greater ability to bind phosphate, as they have more sites for it to attach to than those with a low ASC. This means more phosphorus is required to raise the Olsen P of these soils, but once they have bound the phosphorus they will have greater reserves and their Olsen P levels will be slower to fall.

Ongoing monitoring is essential for successfully adjusting phosphorus fertiliser inputs. “If you decide you want to halve your fertiliser additions for a period of time, or even cut them out altogether, then soil testing and monitoring are essential,” says Josh. “The cost is relatively small for the benefit gained from finding out what is actually happening on your farm.”

Your local Fonterra Farm Source team can help you work out the best phosphorus fertiliser policy for your particular farm system and the level of inputs that will give you the greatest return.

Article supplied by Ballance Agri-Nutrients