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Soil Testing: Key Considerations

Tuesday, February 1, 2011
filed under: Fertility

Two key issues to address in soil testing are timing and depth.

• Timing — Debate surrounds the recommendation for timing of soil sampling. Most soil sampling is conducted on fields in the Northern Plains states in the fall. The weather is generally more cooperative, and the information can be learned well ahead of the decision-making period for fertility and crop planning for the following growing season.

But some agronomists say that spring is the best time to sample. That is due to microbial activity working on organic matter in the soil being tied to soil temperature and moisture. The microbes are naturally more active in early April and May. So therefore, a soil sample in the spring when microbial activity has begun might provide more-current soil profiles.

Weather and soil moisture conditions in different geographical areas can dictate the timing recommendation as well. In the northern states, typically a late spring would not allow for accurate testing. By the time the ground thaws and soil testing is done, planting decisions are already complete. Regions further south may lend themselves more to spring testing, as ground thaws earlier and soils dry sooner, allowing access to fields.

• Depth — Since sunflower is a deep-rooted crop, a deeper probe into the soil would give a better idea of N that might be there for sunflower’s taking. Generally, anywhere from 6 to 24” is recommended; but with sunflower you can go deeper to determine what nitrogen is there. Research has shown that sunflower roots will go to a depth of 6’.

Dennis Berglund, of Centrol Ag Consulting in Twin Valley, Minn., says that generally a 2’ test is sufficient, but there certainly is nitrogen below that depth that a deep-rooted crop such as sunflower can use. Seldom does Centrol recommend testing deeper, unless a grower hasn’t tested the soil previously or hasn’t for some time.

“Say you have a grower who hasn’t tested in a number of years. That particular person may not have been applying correct amounts of fertilizer during those years, and therefore could have a buildup. Our growers, since they test every year, know the input on their soil, so we’re confident in that 2’ test,” Berglund explains.

• Other Factors — Even with soil testing information, many farmers won’t be going beyond the 2’ soil level, so growers should also address other key issues before determining a field’s fertilizer needs: (1) what crops have been on that ground, (2) yields of the crops on that ground the past two or three years, and (3) what has the fertility program been.

Nutrients not used by the previous crop are not lost and can be utilized by the following crop. In a drought year, for instance, crops don’t utilize nutrients in a sufficient manner and therefore leave nutrient residue in the soil. The same holds true for a crop failure. Conversely, in wet years crops tend to use more N, but moisture will also push N down faster, leading to a surplus at a deeper level.

Field history is a key component in determining fertilizer tactics. The more nitrogen, phosphorus, potassium and other nutrients routinely put on the ground, the better chance there is subsoil N that sunflower can do go down and use. This is particularly evident with soil that’s been in a no-till system for a good number of years. Nutrients are present in soil from the breakdown of organic matter from previous crops.

Every pound of N in the soil lets the producer reduce by the same amount when calculating how much fertilizer the next crop needs. Bruce Due, agronomist with Mycogen Seeds, calls that “soil bank nitrogen.” This is a big factor with corn and sunflower, given they are late-season crops. They make use of soil nutrients at different stages than early maturing crops like wheat, barley or canola.

Due says there are fields that are four or five years into a rotation without sunflower that could have 20 to 40 lbs of N below the 2’ soil test zone, and more if sunflower hasn’t been on that ground longer than that time period.

Due adds that a soil test in the fall or spring, and traditional nitrogen recommendations, don’t fully account for the biology that goes on during the growing season. Microbial activity is breaking down organic matter, making soil nitrates available to developing crops. A lot of this breakdown in organic matter in May and June becomes available and is used by late-season crops such as corn and sunflower in July and August. The microbial activity is greater in soils with higher organic matter (>3%) and in areas that receive more moisture.

“You fertilize for 2,000 lbs, and get 3,000 lb ’flowers. How does that happen, if you only put down enough N to meet 2,000 lbs? You don’t get good yield unless you have good moisture. If you have good moisture, you’re getting good organic matter breakdown,” says Due. “It’s causing some in the industry to think that maybe the variable-rate application thinking is backward. Maybe you need the extra fertilizer on your poor ground, and not as much on your good ground, because if you’re getting good moisture, there’s enough organic N becoming available in the field to take care of extra yield needs.”

Due says a number of growers with adequate to high fertility levels — or on fields that haven’t seen a deep-rooted crop like sunflower in four or more years — put down a good amount of starter nutrient and allow the sunflower to scavenge for the rest. With the cost of fertilizer where it is, growers are trying to figure out how to get by with less. Since sunflower uses much of its N later in the season, there’s good return there in residual soil N.

Soil-banked N can allow quite a lot of nutrients to become available that were not reported on the soil test. Due illustrates this with a scenario where you take two similarly populated fields, say with 22,000 plants each.

One has adequate N and the other has surplus amounts, keeping in mind that both fields had adequate rainfall. The plants with surplus N will utilize that nutrient to put on more foliage and result in a tall, robust plant. There is much less air movement throughout the field. The field looks great at a distance, causing the grower to anticipate high yields. This extremely healthy-looking canopy can create an ideal environment for fungal diseases.

The field with adequate N, on the other hand, with average plants in terms of size and foliage, will have much better air movement, thus limiting the potential for fungal diseases. The likelihood of fostering disease in the plants that used the excess N is quite a bit higher. That disease can prove to be a detrimental yield-limiting factor.

Due says that soil sampling is merely a tool and not some magic answer to the complicated soil equation. “Whether you take the soil samples in the spring or the fall, it’s going to tell you what’s there right now — and the grower then has to fill in the blanks when it comes time to apply fertilizer.”

Above and beyond a soil test, those aspects growers should consider to fill in the blanks are field history in terms of what crops have been on that ground and the yields achieved, as well as past fertilizer tactics. These factors need to be looked at in order to utilize both residual and applied nutrients, allowing for adjustments to be made for long-term profitability. — Sonia Mullally
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