Down to Earth: Research Encourages Savvy Interpretation of Fertilizer Recommendations

Alongside many of life’s essentials, the cost of fertilizer has jumped significantly in recent years, with some components doubling or tripling in price due to supply chain disruptions and global competition. Fertilizer is one of the most expensive parts of crop production, and that cost comes out of farmer’s pockets and profits.

A key financial question then is: How much fertilizer is enough?

“You’d think that it would be a pretty straightforward question for common crops,” said Matt Yost from Utah State University’s Department of Plant, Soils & Climate. “But it is surprisingly hard to get a straight answer.”

Yost is part of a team that recently published research on the topic.

Fertilizer levels affect not only a farm’s bottom line, but crop yields, water quality and airsheds far beyond the field where they are applied.

Farmers can get fertilizer recommendations based on soil samples they send to be chemically analyzed, including from USU’s Analytical Laboratories and other public and for-profit labs. But these analyses and the resulting recommendations can vary greatly from lab to lab — commercial labs tend to be more liberal, and university and public labs more conservative with nutrients and amounts, according to Megan Baker, lead author on the research and recent PhD graduate from PSC.

To better understand the variability coming out of soil labs, the research team sent soil from 12 different farms in Utah and Wyoming to five different professional laboratories for analysis and fertilizer recommendations — two university labs and three private commercial ones.

They returned wildly different recommendations, with costs varying up to $820 per acre. In one case, a lab recommended the equivalent of a handful of fertilizer when another suggested enough to fill a 5-gallon bucket for the same area in a small test plot.

To evaluate which advice would perform the best, the research team applied the recommended amounts to experimental fields of alfalfa, corn and small grains. For corn and alfalfa, the expensive commercial recommendations did nothing to increase crop yields. University recommendations grew the same yields at a fraction of the cost. Even smaller amounts of fertilizer, and sometimes none, had similar results, they found.

The difference in the recommendations was likely based in the varying strategies of the lab doing the testing: differences in tolerance for risk and motivation for profit, Baker said.

Commercial labs often suggested extra fertilizer to "build up" the soil and prevent the slightest risk of a poor harvest — which covers all bases as well as helping them sell more product. University labs typically recommended only what a crop needs for a specific year, aiming to save the farmer money and protect the environment.

“Recommendations can be driven by different goals. Maybe to optimize yield for a crop, to replace nutrients taken up by a particular crop, or to boost soil nutrient concentrations so that they won’t limit growth in future years,” Yost said. “Which goal you begin with has a big impact on end recommendations, and each has its place.”

Along with staples like nitrogen, phosphorus and potassium, commercial labs also frequently recommended micronutrients like zinc, boron and copper, the researchers found, even though crop response to these nutrients has rarely been documented in the region. These additions accounted for up to 30% of the total fertilizer cost at some labs, without improving yield.

The takeaway, Yost said, is not that farmers should stop using fertilizer, which could eventually be disastrous, but rather, they should pay close attention to what recommendations they use and year-to-year nutrient levels in their soil, aiming to add enough fertilizer to give themselves flexibility to skip or reduce applications when prices are too high.

“And consider sending soil samples to labs that are near home, within your region,” Baker said, “because soil characteristics, nutrient responses and fertilizer needs vary a lot by geography.”

Finally, farmers do not have to follow the recommendation provided by the lab that analyzes the soil, the researchers emphasize. They have the option to take the raw soil test results and compare them to university-backed sufficiency guidelines to compare costs and recommendations from different labs, which can protect both crop yield, profits and the environment.