URBANA, Ill. – Understanding soil productivity ratings (SPR) in Illinois and their relation to optimal yields was the topic of a Sept. 5 webinar offered by the University of Illinois farmdoc team. Utilizing data gleaned from studies conducted on farmland under contract with Precision Conservation Management (PCM), U of I farm economist Gary W. Schnitkey and Laura Gentry, Illinois Corn’s director of water quality science, spent an hour guiding webinar attendees through the basics of SPR and examined ways to improve field ratings based on the data.

“Some people call (SPR) a soil productivity index or PI or SPI, but they are the same thing. However, SPRs are unique to the state of Illinois, to my knowledge,” Gentry said. “It is my understanding that (the development of SPR) is the work of one person, Dr. Kenneth Olson, who was a soil scientist who specialized in soil conservation and management at the University of Illinois for over 30 years. At PCM, we used Olson’s Bulletin 811 which focuses on optimum crop productivity ratings for Illinois soil types.”

SPRs are based on the expected yield for five different crops under optimum management practices dated back to 1990, Gentry explained. The SPR assigned to a soil is a benchmark based on a productive Illinois soil known as Muscatune silt loam. Soil productivity indexes for Illinois farmland typically run from around 47 at the low end to 147. A rating of 133-147 represents the healthiest – or Class A – soils.

“At PCM we have very few soils with a SPR of below 90, but we do not cover the entire state, and we have a lack of representation in the southern part of the state,” Gentry said. “At PCM we consider a rating of 135 as the cutoff between low and high SPR scores.”

The number of crop fields in Illinois with a high SPR has increased from 57 in 2016 – when PCM first began – to 5,281 fields in central and northern Illinois in 2023. Through their research, however, PCM learned that many more crop fields in the upper regions of the state still possess low SPRs despite educational efforts such as the state’s voluntary Nutrient Loss Reduction Strategy (NLRS) of 2015 that encouraged farmers to address soil and water health on their operations.

Schnitkey devoted his portion of the webinar to summarizing the various data points gleaned from PCM field studies in relation to improving SPRs. A pie chart illustrated that the most profitable corn fields with low SPRs by tillage practice utilized a one-pass light system (28 percent) followed by no-till (25 percent) and strip till (15 percent). For soybeans, the most profitable fields with a low SPR were not tilled (60 percent), followed by one-pass light (16 percent). 

The relation of cover crops to profitability on low SPR fields was also examined, as was the most profitable N timing in lower SPR fields. Conservation practices and financial results for lower SPR fields were also dissected by Schnitkey.  

In addition to improving soil health, yield and profitability, achieving a high SPR will increase farmland value. Many land companies will list SPRs alongside properties for sale or lease to justify land prices. In addition, many food buyers are paying premiums for food grown on cropland with documented higher SPRs.

Based on nine years of research, PCM has developed the following recommendations for increasing a field’s SPR while keeping an eye of profits:

Tillage: 

- Less is more: consider one less tillage pass to reduce fuel costs, save soil and increase profitability.

- Light Tillage: More than two passes of heavy tillage is never more profitable than other tillage management programs. 

Nitrogen management:

- Applying rates above MRTN (maximum return to nitrogen) is less profitable.

- The most profitable acres use pre-plant or side-dress N applications. The practice also improves water quality.

Cover crops:

- Farmers new to cover crops should start with cereal rye ahead of soybeans.

- Protect your profitability by managing seed, planting and termination costs. Consider cost-share programs like PCM to cut costs.

The farmdocDAILY webinar, “PCM Analysis: Lower Soil Productivity Rating (SPR) Fields,” can be viewed in its entirety at farmdoc.illinois.edu/webinars