Fertilizer stratification occurs when a farmer surface applies soil nutrients like phosphorus (P) and potassium (K) without doing any tillage. Deep tillage (plowing 6-8 inches deep) generally moves and mixes surface applied nutrients down about 3-4 inches, or roughly 50%. Some farmers worry that nutrients applied at the surface will not be plant available.
Marion Calmer, an experienced no-till farmer and researcher in Illinois, found that roughly 54% of his P and 43% of his K was found in the top 2 inches of his soil. Since he plants corn 2 inches deep, many nutrients were above his corn roots. In dry weather, he was seeing stunted corn and nutrient deficiencies (P deficient purple corn). For every $1 in fertilizer (P and K) applied every year, he got back about $.40 in additional corn yield. He had been applying commercial fertilizer for 30 years to his no-till fields by surface applying nutrients. When he started checking with soil test every year, he found that he was gaining about 5# P at the surface (0-2 inches) while his P soil tests were decreasing 1-2 pounds per year at 8 inches deep. Should he be concerned?
First, soil stratification is normal. In undisturbed soil, nutrients are naturally deposited at the surface since plant residue accumulates at the surface. In prairie and woodland soil, this is not a major issue because the soil is high in soil organic matter (SOM) and has good soil structure. The biology (worms, centipedes, springtails, mites) all decompose the residue and the nutrients will slowly move down into the soil with rain and melting snow. Even in extensively tilled soil, nutrients are stratified. Often a plow pan forms due to tillage, which limits nutrients from moving deeper into the soil profile.
In conventional soils that are converted to no-till, poor soil structure and lower soil organic matter (SOM) generally leads to soils that are compacted. The soil compaction prevents many soil nutrients from easily moving downward, so they tend to accumulate at the surface. Surface accumulation of nutrients may either runoff or follow a crack, leading to nutrient losses in surface water.
Calmer decided to try plowing on a small strip (60 feet by 1000 feet). Although it did help move the nutrients down about 50%, the surface was bare in the fall and soil erosion from wind and water becomes an issue. He has also experimented with applying fertilizer in a strip with a SoilWarrior to get the nutrients below the soil surface. That helped keep his crops healthier, at least this past year. Calmer will be sharing his results at the 2023 National No-Tillage Conference Jan. 10-13 in St. Louis, Mo.
A second way to fix the stratification problem is to use cover crops. Soils that are naturally healthy have plenty of earthworms and root channels that allow nutrients to move deeper into the soil profile. Tillage destroys these channels so the nutrients stay where they are applied. Also, tillage burns up or oxidizes the SOM where many soil nutrients are stored. Cover crops help restore soil structure and increase SOM so nutrients van be absorbed more efficiently.
Dr. Paul Jasa, a University of Nebraska no-till researcher, found that moisture is the key to optimal nutrient uptake. No-till soils have higher soil moisture due to the undisturbed crop residue at the surface. Jasa has several rules for nutrient placement. First, place the nutrients in the soil where the roots are located. Second, you have to have water where the roots are located because the roots need water to absorb the nutrients. No-till fields conserve soil moisture and allow better nutrient uptake during dry weather.
Fertilizer research by Dr. Ray Weil, University of Maryland, and Dr. John Grove, University of Kentucky, found that no-till crops tend to have greater uptake of surface-applied fertilizer than conventionally tilled crops. No-till corn’s uptake of K was 130% higher than conventional corn that was plowed despite significant no-till fertilizer stratification.
Jasa found at P stratification in no-till corn was not an issue if the P fertilizer was placed in-furrow. On his corn plots, P fertilizer was placed 5 inches deep on one side and on both sides and compared to placing P in-furrow. There was a 10-bushel increase in P applied in-furrow or on both sides of the corn. The in-furrow stratified P yielded same/more, so P was not limited. In dry years, he found that banded or P dribbled on the surface resulted in about 15-bushel increase. Overall, either placing fertilizer close to the roots or using no-till with cover crops makes nutrient stratification a non-issue in no-till.