Charles T. Bryson, USDA Agricultural Research Service, Bugwood.org
Research has shown that, compared with no-till, deep or surface tillage does not increase yields on well-drained soils in our state. Considering that most cropland soils in Pennsylvania are well-drained, it makes a lot of sense that more than 65% of our cropland is now managed with no-tillage. However, no research had been done to determine if subsoiling would benefit soils with 'hardpans'. We got interested in this because 30% of the soils in Pennsylvania have a so called 'fragipans'. In fact, Pennsylvania has the largest acreage of fragipan soils (7.9 million acres) of any state in the U.S.! Fragipans are layers in the soil that have formed naturally. They usually have a loamy texture, are very dense and, when dry, are very hard and brittle. Because of the high bulk density and limited porosity few roots are found in fragipans and their organic matter content is very low. If close to the surface, fragipans result in a seasonally high watertable due to slow water percolation.
On the other hand, they may be droughty in the summer because rooting depth is limited. Although most fragipan soils are under forest vegetation, they are also used for pasture and crop production. Research in other parts of the United States suggests that deep tillage may result in yield improvement on soils with hardpans. However, it is also important to preserve the benefits of continuous no-tillage. Most soil improvement benefits of no-till can be preserved by using non-inversion tillage. This tillage is done with tools that do not inverse the soil but merely fracture the soil below the surface. This way most crop residue is left on the soil surface to provide its soil improvement benefits. We did a comparison between no-tillage and non-inversion subsoiling on an Andover loam soil at the Agronomy Research Farm in Centre County. The fragipan starts typically at about 20 inches depth, but a grey mottled zone is observed above the fragipan because it is saturated for extended times of the year. The soil had been in continuous no-tillage for more than 10 years. The subsoiler used was a 4-row Unverferth Zone Builder with spring-loaded straight shanks with a narrow tip on 30 inch spacing, rippled coulter in front of each shank, and two 2" fluted coulters behind the shank to bust up clods and close the slot left by the shank. Subsoiling was done at proper moisture content each spring. The corn yields were increased on average 14 bu/A by the subsoiling operation (see table).
Effects of continuous no-till and non-inversion subsoiling on corn grain yields (bu/A) on an Andover loam soil with shallow fragipan:
We measured penetration resistance to 18 inches in the rows with a recording compaction tester. It showed that penetration resistance was significantly reduced immediately after subsoiling but it was still measurable one year later at the 7-13 inch depth. This suggests that subsoiling may not be needed every year, which is beneficial because it may not be possible to do subsoiling every spring if the soil sits too wet, or subsoiling can be done in the fall. The results also suggest that the soil is compacted below the depth of the subsoiler. This research has shown that by using non-inversion subsoiling, corn yields can be improved on soils with shallow fragipans while many benefits of no-tillage can still be preserved by avoiding soil inversion and maintaining crop residue cover.
One disadvantage that may discourage the use of this practice on these soils is the presence of rocks that are brought up to the surface by the tillage tool. Some of these rocks are of sufficient size that they may damage equipment. Another potential disadvantage is that the soil is made more susceptible to subsequent rutting. It may be that trafficability of the soil is decreased by the use of subsoiling. It is also unavoidable that the subsoiling operation increases soil roughness - this should be kept in mind if forages are part of the farming operation. It should also be remembered that the deeper the fragipan is below the soil surface, the smaller the expected yield improvement of subsoiling is. It therefore pays to check the soil types on your farm and the type of crop rotation you have to determine if this practice is the right choice for you.