4Rs: A simple concept and challenging reality

By Matt Reese

With regard to managing phosphorus, the 4Rs are easy to talk about, but it is much harder to actually implement the right source, at the right rate, in the right place at the right time.

“No matter what you do, there are times where there will be run-off and enough water to lose dissolved phosphorus,” said Tom Bruulsema, with the International Plant Nutrition Institute. “Even with great practices like waterways and buffer strips, if water is flowing right through, the dissolved phosphorus is moving right along with the water. The 4Rs are very simple to say and a lot harder to do. What is ‘right?’ The 4Rs take place in the context of the cropping system.”

Right source

“Science has shown that all plants require 17 essential nutrients and we need to apply plant available forms in the amounts needed. We need to credit nutrients from composts and manure for phosphorus and choose a source that you can get placed in the soil rather than on top of the soil,” Bruulsema said.

Right rate

The right rate comes from assessing the nutrient supply in soil with soil tests to make sure the crop demand will be met. It is very important to get a representative soil sample. Some agronomists suggest moving toward yield zone- or soil type-based soil sampling systems rather than grid systems to improve accuracy and reliability of soil tests.

Right time

It is important to apply when the risk of run-off is low. When phosphorus fertilizer is left on the soil surface, any rainfall that results in run-off in the next several weeks can result in losses of the fertilizer.

“You can spilt applications for sandy soils, do more scouting and tissue sampling to improve timing,” Bruulsema said. “Cover crops do capture extra nutrients to help with the timing of fall applications, though there is not much evidence for benefits of cover crops with the loss of dissolved phosphorus.”

Right place

The right place for nutrients is where they are accessible for the crop. For phosphorus, this is not broadcast on the surface.

“Apply nutrients in field management zones based upon soil survey information and band, inject or incorporate,” Bruulsema said. “When I think about phosphorus loss in the Lake Erie Western basin, the greatest volume of run-off is likely coming form the flat, heavy clay soils of northwest Ohio. These are also likely the soils that receive fall broadcast phosphorus.”

The 4Rs are not as simple as they sound, but they will be increasingly important for many reasons.

“As farmers, we need to take stakeholder concerns seriously and apply them at the farm level,” he said. “After all, we impact the food everyone eats, the air everyone breathes and the water everyone drinks.”


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