Why do we till at all? Humans have been tilling since agriculture began to improve seedbed conditions and produce better seed-soil contact, help provide nutrients by encouraging oxidation of soil organic matter to produce nitrogen, and allow roots to reach deeper to extract water and nutrients. It also controls weeds and helps reduce disease pressure by removing some residue and improving soil aeration.
With today's fertilizer and pesticides, many of the reasons to till have diminished. However, we still need good contact between seed and soil, which means either retaining good tilth in the upper few inches of soil or improving tilth mechanically.We also need a good environment for roots to penetrate and proliferate. Medium or lighter soils typically provide a good structure for roots. But in soils that are heavytextured, poorly drained, cold, or compacted by heavy equipment, tillage to depths of 8 to 12 inches performed under good conditions often improves rooting and sometimes increases yields.
Although large tractors and tillage equipment are found on most Illinois farms, it is still common to hear claims that "reduced tillage" or "conservation tillage" is on the increase and "conventional" tillage is on the decline. If "conventional" refers to primary tillage with a moldboard plow in the fall, followed by spring tillage to prepare a seedbed, then it has declined. But deep, primary tillage in the fall has shown little decline; in fact, tractor horsepower continues to increase, as does the depth of primary tillage. So one could say that disk-rippers or other modern configurations of heavy, soil-engaging implements are the "new conventional" tillage.
No-till remains popular, especially for soybeans planted into corn stalks. But the definition has broadened as producers have made this system more reliable. It's rare to find straight "slot planting"-planting into undisturbed soil-especially for corn. Most no-till planters have attachments designed to move residue out of the row and to improve seedbed conditions, allowing seed to be placed at a uniform depth in conditions that allow good contact between seeds and soil particles.While the idea that "no-till is the only way to farm" isn't prevalent today, many producers make no-till work well under their conditions. Improvements in equipment, seed genetics and treatments, and weed control have helped make this work.
Another reason why no-till is said to be increasing is that definitions have changed. No longer is tillage described primarily as a set of practices. Instead, the NRCS considers the results of tillage practices, as measured by residue coverage after planting, as the primary way to define tillage. This works well to describe how practices might affect soil loss, but it does not say much about what was done, how much the soil was stirred and to what depth, or how much horsepower was required to farm a given field.
We have done a number of studies to look at effects of various tillage practices in Illinois.
Deep-ripping: We have found little effect of deep-ripping on corn yield (Figure 1, left), though it's possible that our research fields do not have as much compaction as farm fields. It makes sense that deep-ripping, which requires a lot of power, be done only in fields that have a clear need for such operations. In southern Illinois, deep tillage can let more water infiltrate, which in wet springs can sometimes reduce yields.
Tillage and rotation: Data from 3 years of an ongoing study showed that at Monmouth, no-till plots yielded slightly less than tilled plots in continuous corn. This difference was smaller in corn following soybean. Both tilled and no-till yielded about the same in the somewhat lighter soil at Perry, regardless of rotation (Figure 2).
Strip-tillage: Strip-tillage is usually done in the fall, disturbing only a third or less of the surface soil, with next year's rows planted atop the tilled strip. Strip-till is usually done more easily into soybean stubble than into corn residue, but data from one study in continuous corn has shown little difference between tilled and strip-tilled in most cases (Figure 3).
Vertical tillage: There is considerable interest in "vertical tillage"-fast, shallow tillage designed to improve the seedbed without creating the hard layer in the soil typically made by implements like disk harrows. In a study conducted over 6 years in corn following soybean, Eric Adee found that vertical tillage produced yields similar to no-till in corn following soybean (Figure 4).
It's not uncommon for studies to find that no-till or reduced tillage can provide yields similar to those following thorough tillage. However, the heavier the soil and the colder and wetter the spring, the more likely tillage will provide yield benefits. In warmer, drier soils, no-till usually works well, and it may provide higher yields by helping preserve soil water.
The corn plant primarily needs a good seedbed with good soil structure and good seed-soil contact, in addition to a rooting zone that allows roots to grow well, tapping water and nutrients deeper in the soil. To the extent that tillage helps produce a better seedbed or better rooting zone, it is useful. But if these conditions exist without tillage or without more tillage, then such tillage is unnecessary.
Professor of Crop Sciences