If you knew when plants captured the phosphorus in manure, would it change the way you apply manure to your fields?

It should.

According to research conducted at the National Soil Tilth Laboratory in Ames, Iowa, less than 50 percent of the phosphorus in manure applied after harvest is available to crops the following year – a finding that can help you apply manure more accurately.

Soil scientists Tom Sauer and John Kovar arrived at this finding following a year-long experiment at the National Soil Tilth Laboratory, a branch of the USDA’s Agricultural Research Service. Here is a closer look at their research and what it could mean to you:

Phosphorus availability under scrutiny
At the onset, soil scientists selected 15 different soil types from seven states. They added manure from beef, dairy, pork or turkey operations to the soils in order to create 21 different manure/soil mixtures. Then, the researchers incubated the manure/soil combinations under aerobic conditions for one full year.

What’s unique about the study is that the researchers simulated actual field conditions and changes in soil temperature. “We mixed the manure and soil together, then we varied the incubation temperature so that it was cool in the morning and warmer in the afternoon,” Sauer says. “We also changed the temperature and moisture periodically to simulate seasonal changes.”

During the experiment, researchers extracted phosphorus from the samples in order to measure changes in phosphorus amounts in the soil that remained available to plants.

On average, only 43 percent of the phosphorus in manure applied after harvest was available for plant uptake the following year.

Replace soil phosphorus levels accurately
Just because the researchers found that less than half of the phosphorus was available one year after manure application doesn’t mean you should double the manure-application rate on your fields. Instead, use this knowledge to help you replenish soil phosphorus levels more accurately.

But before you do so, consider the soil’s actual phosphorus level. That way, you won’t end up over-compensating or under-compensating your fields with phosphorus, Sauer explains.

For example, if your soil has low to moderate levels of available phosphorus, you may need to apply more phosphorus than is removed at harvest in order to meet crop needs. On the other hand, if the soil in your fields is already saturated with phosphorus, applying more through manure would be a waste because the excess could end up running off or migrating into ground water.

The researchers plan to publish their work in an upcoming issue of the Soil Science Society of America Journal