Soil tests give farmers a lot of information, but what does this information mean?
Brian Hefty, CEO of Hefty Seed Co., said in a recent Ph.D in Ag. The first field day growers should look for in a soil test is pH, with a number between 6.3 and 6.8.
And more often than not, soil pH doesn’t drop to that level, according to senior agronomist Rob Fritz at Hefty Seed.
“You find that pH is just a symptom of everything else that’s going on in your soil, but it’s still important and it’s No. 1 on our list for a reason,” Fritz said. “Because if you notice it, it will tell you if you’re too low, you have a lime problem. And if you’re too high, something else is out of balance.”
Growers with high soil pH levels often look for a quick fix, but Hefty said it’s not that easy. “It probably won’t make an immediate difference if your pH is high, but are there things we can do? Absolutely. We just need to figure out what’s wrong; what has caused our pH to go up,” he said. “It could be excess magnesium, it could be excess sodium. Maybe it’s too much. And also in some cases it may happen that you are deficient in something.”
Hefty showed a situation in one of their fields of operation where the pH was in the 8s. “We put in drainage slabs and put in a whole bunch of potash to raise the base K saturation from very low to about 4%,” he said. “And you know what magically happened? pH has dropped. A major cause of high pH levels [was] we had things out of balance, out of proportion in that land.”
Hefty and Fritz gave field day attendees a quick rundown of what they see as important nutrients and numbers from soil tests, and here are some highlights.
Cation exchange capacity
Fritz spoke about the importance of cation exchange capacity in determining soil type, such as sandy soil or heavy clay soil. He explained that CEC indicates the type of clay and the amount of clay in the soil organic matter. “It’s an indicator of the carrying capacity your soil may have to hold nutrients and hold water,” he said.
The higher the number, the higher the carrying capacity. Soils at the Hefty operation near Baltic, SD, have a CEC value of 20, which Hefty said is considered a “heavy” soil.
Hefty emphasized the importance of finding the CEC value of the soil; otherwise, farmers may talk about heavy soil or light soil, which can be subjective and relative.
Hefty recalled a conversation he had with a Canadian farmer. The farmer asked Heft to evaluate his “light, sandy soil.” However, that “light” soil had a CEC of 33, prompting Hefty to reply, “That’s not light, sandy soil.” The farmer replied, “No, this is my light sandy soil. Here is my heavy soil”, which had a CEC of 41.
Hefty described these soils and their CEC values:
- light colored sand, 3 to 5 CEC
- dark colored clay and silt, 15 to 25 CEC
- dark colored shale and painted clay, 30 to 40 CEC
That’s why Hefty said a CEC value is needed to remove subjectivity, as well as give producers an idea of how much nitrogen their soil can hold. Multiplying a soil’s CEC by 10 equals the maximum nitrogen a soil can hold.
Organic matter matters
Fritz said organic matter levels in soil tests in recent years have been higher, “because of the way we’re working. Organic matter has an effect on CEC, but it also has a really high effect on the amount of water your soil can hold.”
Hefty said that every 1% increase in soil organic matter allows it to hold 4% more water.
Fritz says that while organic matter is an important component of healthy soil, it’s also “the smallest part of soil that you can actually have. It’s not the residue you’re seeing on the field. It is not the residue that lies on the surface. It’s not even from the stems. Most of it is actually from the root material as it breaks down.”
Building organic matter is beneficial in more than just improving soil health. “The fact of the matter is that a lot of people want to pay us as farmers to sequester carbon, and the way we do that is by building soil organic matter,” Hefty said.
He suggested that the steps to building organic matter are first to reduce the soil. No. 2 is the planting of crops with many roots. “Corn, for example, has five times more root mass than soybeans. You can build soil organic matter much faster with corn than beans, he said.
Applying compost or manure will help build soil organic matter, as will cover crops and some biological products, Hefty said.
While organic matter is good, Hefty reminded farmers that soil organic matter is constantly mineralized. “So we run into people saying, ‘Well, we’ve got all this nitrate ending up in the water. It must be from the farmers.’ No, no,” he said. “A lot of it just happens because the soil is constantly releasing nitrogen throughout the year. As that soil organic matter breaks down, nitrogen is released.”
He estimated that for every 1% of organic matter in the soils at the Hefty operation, 20 to 30 pounds of nitrogen are released annually. “We have some soils here that are 5% organic matter, so that’s 100 to 150 pounds of nitrogen being released every year.”