There are many characteristics and indicators of soil quality, including bulk density, good soil pores and water-holding capacity, good infiltration rates and overall tilth, and high levels of organic matter and beneficial soil organisms. Tillage can negatively impact almost every one of those characteristics.
Tillage in perspective
For newly mechanized farmers, tillage was a way to solve problems. Tillage was used for seedbed preparation, weed suppression, soil aeration, turning over cover crops and forages, burying heavy crop residue, leveling the soil, incorporating manure and fertilizer into the root zone and activating pesticides.
The effect of tillage on soil
However, tillage has all along been contributing negatively to soil quality. Since tillage fractures the soil, it disrupts soil structure, accelerating surface runoff and soil erosion. Tillage also reduces crop residue, which help cushion the force of pounding raindrops.
Without crop residue, soil particles become more easily dislodged, being moved or 'splashed' away. This process is only the beginning of the problem. Splashed particles clog soil pores, effectively sealing off the soil's surface, resulting in poor water infiltration.
The amount of soil lost from Iowa farmland each year is directly related to soil structure, levels of crop residue remaining on the soil's surface, and the intensity of tillage practices.
Choosing frequent tillage within a season
Every growing season is different and the best managers make decisions based on frequent scouting and our knowledge of soil conservation practices.
A couple years ago, drought left production short at the end of the season in some areas. Crop residue levels also fell short, or could at least be classified as insignificant. In the following season, a producer managing soil quality through tillage would have accounted for that in the choices made about tillage throughout the season. Converting to no-till or reducing tillage or cultivation would have been 'in-season' choices made the following year based on the desire to limit the impact of tillage on soil erosion and soil physical, biological, and chemical properties.
Producers who used conventional in-season tillage plans under those circ*mstances may have undermined soil quality on their land. In general, frequent tillage can have the same negative impact on soil quality without the special circ*mstances.
Frequent tillage over a couple seasons -- the impact
When producers use unnecessary tillage, more serious problems begin to occur.Without a break from tillage, a total break down of soil structure is possible. Soil organisms can be affected, bringing microbial activity to a halt. Soil pores are closed, imposing severe limitations on infiltration and increasing runoff.
There may even be some initial loss of productivity with moderate levels of erosion. With frequent tillage sustained over a couple years, topsoil loss begins to exceed replacement. In time, the soil is in danger of yield setbacks due to organic matter and nutrient loss as well as the damage done to the soil's physical properties.
Frequent tillage over many seasons -- the impact
When frequent tillage is sustained over a period of years, the impact grows even more severe. A total break down of soil structure and overall soil quality is almost assured. A hardpan can develop, effectively cutting off root elongation, crop development and yield.
Producers reaching this point may experience high erosion rates and degradation of topsoil, where nearly all organic matter is located. Removal of topsoil by erosion contributes to a loss of inherent soil fertility levels. Approximately half of plant-available phosphorus is concentrated in topsoil as is nearly all of the plant-available potassium.
While producers can supply needed crop nutrients to offset the loss of inherent fertility, the productivity of eroded soils can be restored by adding inputs only when favorable subsoil material is present. Where unfavorable subsoils exist (limited rooting depth, coarse sand and gravel, or high soil densities), there is little or no ability to recover yield losses -- the impact on soil quality and productivity is devastating and final.
Frequent tillage and water quality
Frequent tillage can also contribute to deterioration in overall surface water quality. Sediment from soil erosion is a major water quality pollutant. Sediment also transports nitrogen and phosphorus from fields into lakes and streams, leading to 'eutrophication' -- a serious surface water quality problem.
Frequent tillage -- costly in many ways
Frequent tillage can be costly in terms of soil quality, soil productivity and surface water quality as well as extra wear on machinery and extra labor requirements. Before making choices about tillage, producers should consider the impact of sustained frequent tillage on soil quality.
Table 1. Effectiveness of commonly used practices for controlling soil erosion.*
| Practice | Effectiveness for control of | |||
| Water erosion | Water runoff | Sediment delivery | Wind erosion | |
| Permanent vegetation | 3 | 2 | 2-3 | 3 |
| Crop sequence | 1-3 | 1-2 | 1-3 | 1-3 |
| Strip cropping | 2-3 | 1-2 | 2 | 2-3 |
| Cover crops | 2-3 | 2 | 2-3 | 3 |
| Contouring | 1-2 | 1-2 | 1 | n.a. |
| Conservation tillage | 1-3 | 1-2 | 1-2 | 1-3 |
| Nutrient and pesticide management | 0-1 | 0-1 | 0-1 | 0-1 |
| Terraces, gradient | 2-3 | 1 | 3 | n.a. |
| Terraces, level | 2-3 | 3 | 3 | n.a. |
| Terraces, tile outlet | 2-3 | 1 | 3 | n.a. |
| Grassed waterway | 1-2 | 0-1 | 0-2 | n.a. |
| Grade stabilization structure | 2-3 | 0-2 | 1-3 | n.a. |
| Land use change | 3 | 2 | 3 | 3 |
*Scale of effectiveness: 0 = not effective
1 = slightly effective (less than 10% reduction)
2 = moderately effective (11-50% reduction)
3 = substantially effective (51-100% reduction)
n.a. = not applicable
Source:A Technical Assessment of Nonpoint Pollution in Iowa.College of Agriculture, Iowa State University. March 1978.
Comment: Note that the same practices that are beneficial for controlling soil erosion are the same practices that encourage the development of soil quality.
This article originally appeared on pages 78-79 of the IC-492(13) -- June 28, 2004 issue.
