pH testing on the go

Technology makes variable-rate liming easier

Maintaining suitable soil pH has long been a concern and headache for many crop producers. Conventional soil sampling is laborious and costly - especially true when farming in a site-specific manner. However, new technology makes variable-rate lime application affordable and practical with on-the-go soil pH testing.

“Variable rate liming is one of the most profitable and popular practices in site-specific crop management,” says Viacheslav Adamchuk, precision agriculture engineer at the University of Nebraska - Lincoln. “In addition to knowing acidic field areas, having knowledge of areas with alkaline soil conditions (high pH) can be useful to avoid lime application in these areas and also aid in the selection of crop varieties tolerant to problems associated with high pH.”

Currently, most variable rate lime prescription maps are gener­ated based on soil samples collected manually and analyzed in laboratory conditions. These samples are usually obtained with a 2.5-acre sampling frequency. With on-the-go testing, samples are taken at a greater density and at a relatively same or lower cost. Adamchuk says that the economic benefits of variable liming through advanced pH maps can range between $5 and $15 per acre, with significant environmental benefits as well.

In 2004 Veris Technologies of Salina, Kan. began marketing the first on-the-go pH mapping device- the Mobile Sensor Platform (MSP) pH Manager.

“People see it work and say, ‘It’s so simple anybody could have thought of that’,” says Eric Lund of Veris. “The fact that the unit is one moving part makes it jaw dropping simple.” Lund explains the main concept of design is a cutting shoe that goes beneath the ground allowing soil to flow through horizontally creating a soil core. The shoe is then picked up hydraulically and the soil is held against two pH electrodes for 10 seconds while a reading is stabilized. During each cycle the shoe is cleaned off by a scraper and the pH electrodes are washed off with water nozzles. The sampling process is controlled with an external electric control module and the pH data is recorded.

Lund says the system is predominately used in places where people apply a lot of lime. “pH is really an issue in the eastern Midwest as well as other pocketed areas. Producers that spend the money to put on lime need to determine the pH better than what they have been.” Lund also says that the majority of pH Manager sales are to Co-op’s, agricultural retailers and large farmers. Approximately one third of Veris sales are over seas.

Steve Kramer, technology manager for Servi-Tech in Stromsburg, Neb. has been using the pH Manager since 2003 and sees the benefits the technology has to offer.

“It takes a lot of variability out of the picture when you’re trying to record pH,” Kramer says. “In fields that have pH issues you only want to address the areas that need it. With the price of lime you want be as efficient as possible - it’s too expensive of an input to go out there and put two tons on the whole field. It makes more sense to put it where you need it or to not put it where you don’t need it.”

The greatest advantage of on-the-go testing is variation in sampling density and reduced labor costs. Lund says that lime requirements simply vary too much within a field to allow the computer to fill in the gaps between a handful of samples.

“Using grid sampling the pH at each point is determined in laboratory testing, which is better than on-the-go readings,” Lund says. “With grid sampling the space between the points is then automatically filled in. However, there’s more area of the field between the points than there is at the points.”

When grid sample sites are overlaid on a pH Manager map, it’s evident that there is more variability than grid samples will be able to capture, Lund says. “Software may contour the grid spots into a pretty map, but it can’t accurately map what wasn’t measured.”

After a pH map is made using on-the-go testing Kramer and other consultants use the results as well as buffer pH and soil texture information to generate lime recommendations. Kramer says this helps the producer put the right amount of lime where it needs to go. “With the Veris map producers typically use about the same amount of lime. The difference is that they’ll know to put less where they don’t need it and more where they do.”

K-State agronomist, Scott Staggenborg, says that on-the-go pH testing can help producers make more informed management decisions. “When researchers began looking at variable rate inputs in the early 2000’s it became obvious that lime would be significant because of the cost of the material and the fact that it is spatially oriented,” Staggenborg says. “Grid sampling was just not dense enough to make the decisions that needed to be made. The real-time sensors make the most sense and the biggest advantage is that you get a very dense data set.”

Certain crops are more susceptible to suffering from lime deficiencies Staggenborg says Soybeans, alfalfa and wheat are crops that can experience harmful lime deficiencies. In legume crops, low pH inhibits the bacteria necessary to starting the nitrogen fixation process which reduces nodulation. Wheat under low pH can suffer from toxicities, most notable being aluminum toxicity. This means that having soil with suitable pH can be very important and fixing pH problem areas more accurately only makes sense.

Is it really possible to test soil with out the laboratory? As with any new technology, some may be skeptical of on-the-go pH testing and the accuracy it provides. “Not only is it being done in a field, its being done on a piece of equipment that is bouncing and moving along at about 4-8 mph.” Staggenborg says. “The trade off is while the samples may not be as accurate as those from a laboratory, we get them at a more dense rate and we get them right now.” Staggenborg also says that it doesn’t necessarily have to be completely accurate because producers are putting on tons of material. Lime applications are not as finely tuned as other elements and on-the-go testing is accurate enough to pick up substantial variations. Results from field, static and lab tests show that the on-the-go method produces pH measurements that are highly correlated with laboratory analysis of soil pH.

While the system may not be flawless, the technology is by far a step up from other forms of pH sampling. “Pinpoint lime application is the main benefit; you know what your pH is,” Lund says. “You know you need some lime here, a little here and a lot over there. Lime is expensive, with this type of mapping it easier to choose which areas to apply lime and help farmers make better management calls.”