Since farmers often wonder how stratification affects no-till, Jeff Schoenau recently tackled a few frequently asked nutrient cycling questions.
A Canadian research scientist located at the University of Saskatchewan in Saskatoon, Saskatchewan, Schoenau offers viewpoints which may fit some U.S. no-till farming situations.
Q: Do the plant nutrients cycle in the same manner and rate when tillage is eliminated?
A: Producers often think they need to change fertilization practices to accommodate nutrient turnover with no-till. Farmers wonder how plant nutrients, especially nitrogen, are supplied in plant-available forms, such as nitrate and ammonium, during the growing season.
Nitrogen supplied by soil depends on the amount of plant-available nitrogen, the rate at which available nitrogen is released from soil organic matter and the amount of crop residues kept on the ground over the growing season plus losses due to leaching, gaseous escape or even temporary nitrogen tie-up as crop residues decompose.
Eliminating erosion by reducing costly tillage can certainly have a positive effect on a soil’s ability to supply long-term plant nutrients.
Concerns have also risen about release of phosphorus and potassium. Both short-term (next growing season) and long-term (after several years) considerations are important to achieve the maximum benefit from no-till.
Q: With no-till, do I need to adjust fertilizer rates to compensate for changes in soil nutrient turnover?
A: Research suggests no-till will reduce levels of plant-available nitrogen in the first few years. This is due to a reduced rate of soil organic-matter decomposition induced by formation and persistence of macro-aggregates (soil crumbs), which protect organic matter from decomposition by soil microbes.
Tillage accelerates the breakdown of macro-aggregates and organic-matter decomposition. Yet the degree to which no-till reduces soil nitrogen turnover depends on the soil, especially the relative importance of soil macro-aggregates.
Since soils intensively tilled in a cereal and mechanical tillage fallow rotation for many years have a relatively low macro-aggregate content, the contribution of macro-aggregate breakdown in nitrogen release is limited.
An intensive two-year study of nitrogen availability in conventional vs. no-tillage fallow was conducted in 1994 and 1995 in southwestern Saskatchewan. No-till had little impact on available nitrogen amounts, rates and wheat nitrogen uptake.
These findings indicate no-tilling soils which had been tilled for long periods do not necessarily result in reduced nitrogen availability or a demand for more nitrogen.
Comparing soils in several years of no-tillage and conventional tillage under similar rotations and fertilization scenarios has shown little difference in nutrient-supplying power when erosion was not a factor.
For example, a southern Manitoba comparison in a field with 16 years of no-till vs. conventional till indicated no significant differences in available rates of nitrate, phosphate, sulfate, potassium, copper or zinc.
There was a slightly higher available ammonium rate in the no-till field, suggesting ammonium nitrogen may be more important as a source of available nitrogen in long-term no-till fields. That may be related to lower nitrification (conversion of ammonium to nitrate) rates.
Q: What happens to those nutrients found in crop residue from the previous year?
A: The residue slows the rate at which decomposition takes place and the release of nitrogen from residue.
This is particularly true with low-nitrogen residues such as cereal and oilseed straw where surface decomposition is considerably slower than when the residue is incorporated.
Fluctuations in surface moisture and temperature often create less favorable conditions for microbial activity. Low nitrogen residues such as cereal and oilseed straws contribute little available nitrogen the following year and often reduce the availability of soil and fertilizer nitrogen due to microbial tie-up.
For this reason, surface accumulation of straw may be beneficial since microbial tie-up of available nitrogen during decomposition will be slower and isolated from the soil compared to incorporated straw. Place fertilizer below the thatch layer to avoid costly tie-up of nitrogen.
With pea or lentil straw residue, surface placement vs. incorporation had little impact on decomposition rate or release of residue nitrogen. This reflects the higher nitrogen content in legume residues and the ability to satisfy microbial needs during decomposition.
While nitrogen exists in crop residues primarily in an organic form and requires microbial decomposition, other nutrients such as potassium exist largely in water-soluble forms. These may make up the majority of the total potassium and over 50 percent of total phosphorus in cereal crop residues.
In no-till, these water-soluble forms are physically leached into the residues and surface thatch by rain and snow melt water.
Because of the immobile nature of phosphorus and potassium, these two elements tend to concentrate near the surface in no-till.
This nutrient stratification may be a disadvantage under dry conditions when the nutrient is stranded in the drier surface layer. However, limitations may be less than anticipated due to greater spring soil moisture under no-till and use of shallower seeding depths.