Introduction to the Long-Term No-Till Winter Wheat Plots
The joint research station of the USDA Agricultural Research Service and Oregon State University has maintained several important long-term experiments that provide an invaluable resource for studying the effects of agricultural practices on soil. One no-till wheat experiment has been in place since 1982. Since 1989 the rotation has been winter wheat/fallow. In 1997 the plots were extended to start another no-till winter wheat/fallow experiment that matches the original. A set of conventionally-tilled and rod-weeded winter wheat/fallow plots was also added. This allowed, for the first time, a comparison of plots having a 16-year history of no-till winter wheat with plots having one year of no-till and plots under conventional tillage. Some of the first year comparative measurements were surprising. Some of the changes were less and others greater than expected. This data is not a complete picture, but it can help us refine our thinking and test our assumptions about the effects of long-term no-till on soil.
Seed-Zone Soil Moisture in the Fall
Chemical fallow produces seed-zone moisture levels unlike traditional summer fallow. Chemical fallow results in less moisture near the surface, and can decrease early germination and fall growth of wheat if conditions are dry (Fig. 1). With chemical fallow it is more often necessary to plant deeper or wait for sufficient rainfall to allow seed germination. Even after 16 years of no-till, the surface soil layer has not mellowed enough to function like the blanket of dust mulch created by intensive tillage.
Tillage breaks up the soil and reduces its strength. This effect is temporary and must be repeated each year to produce a friable soil for planting. A cone penetrometer, which measures cone index (lbs/in2), was used to indicate soil strength (Fig. 2). The first year no-till plots have the greatest soil strength; conventionally-tilled plots, plowed to a depth of eight inches, are weakest. After 16 years of no-till, soil strength more closely matches the tilled than the first year no-till. Given the other features of no-till, as discussed below, it is possible that in many ways the 16-year no-till is a superior environment for plant growth, even though it measures greater resistance to cone penetration than conventional tillage. The fact that soil softens after years in no-till is proof that no-till affects many aspects of soil structure. No-till soil is not just tilled soil minus the tillage, but it evolves to something quite different. Our thinking should also change about how best to use the soil. It suggests we should concentrate on designing seeding equipment for a more mellow soil rather that for the extremely hard soil surface typical of the first or second year in no-till.
Soil Organic Matter
Soil organic matter averages 2.07% in the top eight inches of our 16-year no-till field. Soil organic matter in the conventionally-tilled plots averages 2.02% (Fig. 3). While this difference is smaller than we might expect, the 0.05% difference translates to 1200 pounds per acre of organic matter, which has a dramatic effect on soil properties.
Carbon Loss as Carbon Dioxide
It appears as though some of the increase in organic matter in no-till soils can be explained by carbon dioxide loss. Carbon dioxide loss is accelerated by tillage (Fig. 4). When tillage stops, changes in residue decomposition rates, oxygen exchange, root growth and decay, and soil biology occur. The first year no-till plots are in transition to a new equilibrium of biological activity.
Earthworms are found in very low numbers in almost all of our local agricultural fields, even though they may be numerous in nearby lawns and pastures. It is likely that tillage operations are very detrimental to earthworm populations. The increase in earthworms in the long-term no-till plots indicates a dramatic improvement in their environment (Fig. 5). Whether earthworms actually improve conditions for plant growth in our cropping systems has not been proven yet. Research from around the world reveals that earthworms do not harm plants and have an amazing ability to open the ground to water and air. We might consider an abundance of earthworms a positive indicator of soil quality.
Perhaps the most striking indication that the soil is changing physically is the dramatic increase in water infiltration rate after 16 years of no-till (Fig. 6). A double-ring infiltrometer was used to measure infiltration into 16-year no-till, 1-year no-till, and plowed soil. Infiltration rates are for saturated flow (in/hr) after the soil has been wetted. Even in the first year of no-till, where the soil strength was extremely high, there is an increase in pathways for ponded water to flow deep into the soil. How much of this can be credited to an increase in earthworm and root channels remains to be determined.