By Luke Schulte, CCA, Beck’s Field Agronomist
In this time of unusually high input prices and commodity price uncertainty, crop input selection has become even more crucial.
Often, as we feel pressure to tighten our belts regarding input decisions, we resort to scrutinizing those inputs that are less visual and believed to be less impactful. For example, altering our herbicide program or lowering the amount of applied nitrogen will often visually show the impact of our decision. That said, managing soil fertility is one of those inputs that “pulling back” is not as obvious as the impact.
Over the years, annual rainfall accumulation throughout the eastern U.S. has increased modestly. However, the intensity of our rain has increased dramatically. As difficult as recent springs have been to complete field work, climatologists project spring precipitation to continue to increase as well. Currently, we are faced with rising fertilizer prices, more violent rain events, fewer spring days to complete fieldwork, and sustained periods with limited soil oxygen.
How can we adapt?
As soil fertility inputs are evaluated, it’s important to keep in mind how changing weather patterns impact soil oxygen and, ultimately, nutrient availability and water infiltration. Contrary to what may appear to be the case throughout some agronomic textbooks, soil fertility is not quite as simple as a mathematical equation. The state of crop nutrients, available versus unavailable or “tied up,” are continually evolving.
Take soil test phosphorous (P), for example. The average recovery rate or volume of P that leaves the field the year following a P fertilizer application is approximately 17% to 18%. So, if we apply 200 pounds of 11-52-0 this fall, approximately 35 to 40 pounds is all that will be removed in the harvested grain next year. Where did the remaining P go? The remaining P largely contributes to the organic phosphorus (OP) pool and is temporarily unavailable. OP accounts for approximately 30% to 65% of all soil P. This is like having an operating loan of $100,000 that you are paying interest but only have access to $20,000.
One of the primary mechanisms for maximizing nutrient availability is through close monitoring of soil test calcium (Ca) levels. Years ago, simply maintaining soil pH often meant Ca levels were also adequate or nearly adequate. However, as the Clean Air Act has resulted in less sulfur deposition, thus, less acid rain, soil pH is not declining as rapidly. The need to apply lime or Ca to maintain pH has also declined. In the 1980s, the pH of our rainwater was 4.0-4.5. Today it is much higher. I’ve been testing the rainwater at our home in central Ohio throughout the spring and summer for 2 years and have never found the pH to be less than 6.2.
High calcium lime is a great source of Ca. However, as soils require less frequent lime applications to elevate pH, what are soil test Ca levels doing? No longer can we simply rely upon maintaining soil pH to also provide sufficient Ca to maximize soil functionality. Think of it this way, the higher the base saturation Ca level, the higher the soil oxygen content.
While soil pH was optimal, we still observed a yield advantage of 5.3 bushels per acre with an ROI of $38.50 per acre. Ideally, we need to maintain a Ca base saturation level greater than 70%.
According to 2021 research from the University of Illinois, the addition of pelletized lime (Ca) to fall-applied P on soils with a pH of 7.0 results in an increase of available P the following spring by 14 to 53%.
Why did the addition of Ca lead to a yield benefit or increase in nutrient availability when soil pH was already optimal?
- ↑ Ca = ↑ Soil oxygen = ↑ Soil microbial activity → Converting OP to plant available P
- ↑ Ca = ↑ Microbial activity → Improved aggregate stability/Soil structure → Increased water infiltration
- ↑ Water infiltration → Reduced soil crusting
- ↑ Water infiltration throughout the soil profile = ↓ Seedling diseases
- ↑ Oxygen = ↑ Soil biological activity = ↑ Nutrient release = ↓ Dependency on applied nutrients
Bottom line, as large rains become increasingly common, soil oxygen is inherently the most yield-limiting element year in, year out. Maximizing soil functionality and reducing dependency on applied nutrients comes down to increasing soil oxygen.
Products to influence soil Ca levels include pelletized lime, high calcium ag lime, and gypsum.