Nutrient Stratification

Farmer led research is a valuable tool for improving crop yields. The hard part is making sense of the data. Marion Calmer, Calmer Farms in Fairbury Illinois has been doing farm research for many years. A major concern has been the stratification of soil nutrients in the upper soil layers. Calmer has been no-tilling (NT) for many years, but he does not use cover crops. Over 14 years, he applied $1,000 worth of surface applied nutrients/acre (average $71.42/acre). Calmer worried he was not getting the best use of that fertilizer. He tried a farm experiment.

First, he soil tested his field taking soil test in 1-inch increments down to 8 inches. Results showed extremely high soil test levels for phosphorus (P) and potassium (K) in the top 1 inch, and about 50% lower in next 1 inch. Approximately 46% of his fertility was within 2 inches of the soil surface and only 16% were in the bottom 2-inches. He then plowed and soil tested a portion of his NT field. Results showed a re-distribution of nutrients with 18% in top two-inches, 39% from 3-4 inches, 27% at 5-6 inches, and still around 17% in the bottom 7-8 inches. On the no-till, the top 3 inches were in the green (high or very high nutrients) but the bottom 5 inches were below average. On the plowed ground, the top 2 inches were low, middle 4 inches were high, and the bottom 2 inches low. He got 9-bushel soybeans higher on the plowed strip worth about $135/Acre. He concluded he needed to do strip tillage to fix his stratification problem.

However, that is not the end of the story. He also did some soil testing on a fence row, virgin soil. Interesting, the soil tests taken at 1-inch increments showed very little stratification with slightly more nutrients near the surface, but the fertility was good all the way down to 8 inches.

His base saturation on calcium was in the mid to high 60’s in the top 5 inches. Calcium is a major nutrient activating 146 key enzymes. Calcium often leaches down in the soil profile to it gets stuck in a hard pan. A common saying, is the “poor man’s way of liming is to plow and inch deeper.” While Calmer’s pH was adequate, his base saturation was in the low to mid 50’s on his no-till fields. Adequate calcium makes all the other nutrients more plant available.

Is nutrient stratification the problem, a lack of calcium, or combination of factors? What about soil compaction and poor soil structure? Every time a tillage tool is used, it creates a zone of compaction. Even planters cause side-wall compaction. Tillage tools smears the soil and prevents water and soluble nutrients from moving down through the soil. On virgin soil with good soil structure and no compaction, the nutrients were fairly evenly distributed. On the plowed ground, the nutrients were above the plow layer. On the no-till, they stayed in the top 2 inches. Why?

The NT field had been no-tilled for many years. However, it was in a corn-soybean rotation. Soybeans do not have many roots but corn has a lot of fine roots which is known to raise your soil organic matter levels (SOM). However, live roots only exist in a corn-soybean rotation, about 1/3 of the time. Those live roots (especially grasses) have fine roots that exude root exudates that promote good soil structure. That allows both water, nutrients and gases to move deeper in the soil profile. The live roots also absorb soluble nutrients (N-P-K, secondary, and micro-nutrients) keeping them recycling in the entire soil profile. The virgin soil (fence row) did not have soil compaction and had plenty of live roots, good soil structure and good drainage to allow water, nutrients, and gases to move deeper. The higher base saturation on calcium allowed the nutrients to be fully used.

Long-term NT plus cover crops mimic virgin soils. It may take 3-7 years to fix the poor soil structure and compaction problems from previous tillage passes, but over time it is a more permanent solution. Mike Starsky, farming 2500 acres near Indianapolis, Indian; did the same 1-inch incremental study on two of his long-term NT and cover crop fields. His soils are acting like a sponge and he did not find any stratification. In a 7-year study on water quality, his farm is hardly losing any nutrients compared to conventional fields nearby which are losing both soil nutrients and soil sediment. In the Chesapeake bay region (Pennsylvania and Maryland) and Lake Erie, where NT and cover crops are being used, water quality is improving.