We talk to farmers every day. And one of the most common questions we’re asked about our Variable Rate Nitrogen Prescriptions is: how can FarmLogs create a variable rate nitrogen prescription without taking any soil samples from my field?
In this blog post, we’ll identify four truths about the way many farmers’ current N recommendations are being created. We’ll also explain how FarmLogs is able to create accurate variable rate N recommendations without soil samples.
Truth 1: Most university N recommendations are not based on soil samples.
We’ve found that one common misconception about traditional N recommendations is that they’re based on soil samples.
But soil testing is not typically done for N recommendations in most of the Corn Belt, specifically east of the Missouri River where there is more rainfall.
Instead, most soil tests are only taken about every 4 years and focus on measuring P and K levels in addition to soil acidity for liming. They’re not typically used for measuring N levels.
And for P and K, once your soil is built up to the targeted level, most of it is applied at a maintenance or crop removal rate (often specified in land rental agreements).
Truth 2: Most university N recommendations don’t account for the varying amounts of N you’ll lose to differences in weather throughout the fall, spring, and growing season.
One reason that N recommendations from universities aren’t very accurate in a given season is because they don’t consider the difference in the amount of N you’ll lose to unpredictable weather.
In fact, that’s one of nitrogen’s defining features: unlike P and K, it’s easily lost in wet weather conditions.
Jeff Ball from the Noble Foundation describes the relationship between water and N, P, and K in this way:
“Both phosphorus and potassium are immobile in the soil, meaning they don't move readily with water. Let's compare the mobility of nitrogen, phosphorus, and potassium on a scale of 1 to 10, 1 being immobile and 10 being readily mobile. Nitrate nitrogen (NO3-) is a 10. It is extremely mobile and can be lost to leaching, which is downward movement of water through the soil profile. Potassium is a 3…. Phosphorus has a rating of 1. It is extremely immobile in the soil and is likely to stay wherever it is placed unless moved by erosion or crop export.”
Because N is so mobile, most soil samples collected in the fall will not reflect the actual amount of N that will be available to your plants when they start to take it up in June and July. Note also that N-loss doesn’t just happen during fall application, but in-season too. Nitrogen can be lost in a matter of days or weeks depending on the weather.
N-loss from weather is so influential to baseline levels that old university yield goal recommendations that ignored N loss from weather (and the release of N from organic matter) were proven inaccurate and dropped from most of the wetter areas of the Corn Belt.
Truth 3: Most university recommendations don’t account for the differences in the amount of N released by the organic matter that already exists in your soil.
Accounting for organic matter (OM) in N recommendations is important, because for every 1% of OM that’s in your soil, 10 - 20lbs of nitrogen are released. Soils with more organic matter tend to release more N.
Universities pull OM samples from across the state, which means those samples represent a wide range of soil types and OM conditions. But to create recommendations, they tend to average their sample results together, which means you may be using a recommendation that doesn't account for the specific soil types or OM levels in your fields.
Our nitrogen tools do account for differences in the amount of nitrogen released from OM.
Our tools also take into account crop credits. (However, they do not currently consider a field’s manure history.)
Truth 4: Soil tests that look at nitrate near sidedressing times aren’t a practical solution for developing accurate variable rate prescriptions.
Though N-specific soil tests that look at nitrates near sidedressing times will give you a partial snapshot of the N available in the field you’re testing, these tests are meant to work as a barometer for general N availability, and not as an indicator of accurate prescriptions. Why?
- They are only from a one-foot depth and corn roots can take up N from as far as six feet down.
- They only look at the nitrate content at that point in time. In most areas, the critical concentration is 21-25 ppm. This would be roughly 40-50 lbs N/a, which is far less than what the total crop needs.
- The spatial variability of sampling makes it impractical to sample on a variable scale.
There are some soil N tests that can provide a general estimate of the amount of N released from the OM that exists in the field you’re testing. Some correlate heavily with soil OM, and some are helpful in identifying additional N released from fields with a significant manure history. However, these tests vary in their effectiveness depending upon the management history of the field and how the tests are being used. Similarly, there isn’t extensive data to show that spatial recommendations across a broad scope of samples provide a cost effective benefit to the grower, which is why FarmLogs is not utilizing such tests at this time.
What We Use Instead of Soil Tests
As Farmlogs, we emphasize a recommendation strategy that reflects a balanced calculation of 3 factors:
- What the crop needs spatially. As grain yields vary, so does the amount of N taken up by the plant for both the plant growth and ear development. For each bushel produced, another pound of N is needed. A 50-bushel difference in yield across the field means there is over 50 lbs N/a difference in N needs.
- The impact of weather on N losses spatially. More water usually results in more N loss. The lower parts of the field catch more water and usually have heavier soils, with more silt and clay, which means the soil stays wetter longer. The differences in water infiltration, drainage, and wetter conditions in the field all result in losses that vary across the field. We account for all of these things.
- The potential release of N from the soil OM spatially. As the soil types vary, so does the OM. This affects the amount of N released for the crop uptake.