Soil Tilth Effects of Surface Residue Management Systems

J.L. Hatfield

J.H. Prueger

Management of the residue layer has an impact on the soil temperature and soil water regimes within the soil. In this overview we will present a comparison of the effect of residue management on the temperature and water variations within the upper soil profile and relate these changes to possible effects on soil tilth and soil quality. Improving our understanding of the processes involved in the modification of the soil microclimate will improve our ability to manage the soil.

Residue layers on the soil surface reduce the soil temperature. This effect has been documented by several studies throughout the United States (Anemiya, 1977; Burrows and Larson, 1962; and Griffith et al, 1977). The impact of residue on soil temperature is quite easy to visualize and understand. The layer of residue acts as insulation and impedes the rate at which thermal energy is exchanged between the soil and the atmosphere. Amemiya (1977) showed that under corn residue, the spring temperatures were reduced by almost 0.5C for each 2.25 mT ha-1 of corn residue. A reduction in the soil temperature below the optimum decreases the rate of emergence of the spring planted crop. If we assume that the temperature response curve for corn seedling emergence is as shown in Figure 1 with an optimum between 25 and 30C, a reduction below that average range will cause a delay in emergence.

Recent studies on the soil temperature regime under no-till, chisel plow, and moldboard plow tillage have revealed that the mean temperature in the spring is not as affected by tillage practice as is the temperature range. In a study conducted at Ankeny, Iowa under long-term tillage plots some interesting seasonal variations were found. Continuous measurements of soil temperature throughout the year were taken at depths of 1, 2.5, 5, 7.5, 10, 20, 50 and 100 cm below the surface. The rate of mean temperature increase at the 10 cm depth in the spring (day-of-year (DOY) 90-150) was not affected by the tillage treatment as shown in Figure 2. Throughout the year there was only minor variation amount the three tillage treatments. There was, however, a significant impact on the diurnal range of temperatures at this depth as shown in Figure 3. The results for the deeper depths showed no differences among the tillage systems. These data would agree with the findings of Burrows and Larson (19620 who also showed an effect of residue on the diurnal temperature range. They, however, did not see any difference in the daily minimum temperatures for a range of tillage systems. In this study we did find an effect of tillage treatment on both the maximum and minimum daily temperature values (Fig. 3) with the greatest effect on the maximum daily temperatures. There was little difference between the chisel plow and the moldboard plow with the largest difference in the no-till treatment.

The largest difference among the three tillage treatments in soil temperature at the 10 cm depth was in the period after harvest as the soil began to cool (Figure 4). The residue layer in the fall was more effective in slowing the rate of cooling compared to the incorporation of the residue in the two tillage systems. Similar results have been reported by Benoit and Van Sickle (1991). They found that the no-till plot with residue had the highest soil temperature at all depths in the winter compared to moldboard and chisel plow. In the study at Ankeny we found that the residue was more effective as a thermal barrier in the fall than in the spring. In the spring the over wintered residue had begun the decomposition process, had decreased in thickness by 70%, and had begun to be covered with soil. The soil covering and the decomposition changed the reflectance to nearly that of the dark color of the tilled soil. The over wintering process alters the effectiveness of residue as a thermal barrier. The implications of the change in the temperature in the fall and winter periods has not been fully evaluated but it changes the rate of microbial processes and could have implications for the rate of mineralization and dentrification of nitrogen. These results warrant further study to completely quantify the changes which occur in residue throughout the year and to determine how residue, either standing or flat, can be managed for the most positive benefit on the soil.

If we couple the results of the diurnal temperature regime in the spring with those for the emergence response of corn in Figure 1, we can then evaluate the impact of residue on the emergence of corn. A reduction in the diurnal range of temperature will affect the rate of emergence given this type of temperature response curve. If we combine the temperature response curve and a reduced diurnal temperature range together, it is apparent that the range is more important that the mean temperature. This biological response curve shows why both the reduction in the mean temperature and the diurnal range in the southern latitudes is beneficial to plant growth while this same change in the northern latitudes is detrimental to seed emergence. The reduction in both mean and range brings the soil temperature within the optimal response range of corn. If we understand the effects of residue management on the biological response, we can more fully develop management systems which utilize this knowledge.

The impact of the residue layer on soil water has not been as extensively studied as the soil temperature regime. Residue reduces the evaporation rate from the soil surface, increases infiltration, and increases the amount of water available for transpiration (Blevins et al., 1971). These are general statements which are found in the literature about the effect of residue on soil water. Phillips (1980) found that no-till corn in Kentucky had a higher transpiration rate and a lower soil water evaporation rate than conventionally tilled corn. The water savings due to residue averaged 150mm over a 4 year experiment and transpiration was increased by 55mm. In each ear of the study the no-till had higher yields than the conventionally tilled plots because the reduction in soil water evaporation increased the available water during the critical period of grain filling when droughts often occur in Kentucky.

In the Great Plains, standing residue had a positive effect on the soil water balance. Willis et al. (1983) showed that standing residue increased the capture of snow and improved the soil water recharge. In the High Plains of Texas, Unger (1972) found that mulches would be ineffective because the amount of residue left by the subsequent crop would be insufficient to reduce the soil water evaporation rate. Hatfield (1990) found that standing wheat stubble reduced the soil water evaporation amount in the early cotton growth stage by 50 mm. The standing stubble improved the water use efficiency of the young cotton crop because of the decrease in the wind speed and atmospheric demand near the surface. In this experiment the cotton plants which were planted into wheat stubble were also provided physical protection from the blowing sand common in this area.

In a study comparing the evapotranspiration rates among chisel plow, ridge tillage, and no-till corn in central Iowa we found that the daily evapotranspiration rates were affected by tillage practice when soil water as not being replenished by rainfall. An example of the changes which were occurring in the evapotranspiration rates is shown in Figure 5. These rates were measured on a 30 minute basis throughout the day with a Bowen ratio system placed in the center of 40 ha corn production fields. The data shown are typical of the two week period prior to silking in 1992. During this portion of the growing season there was no rainfall in 1992. In the no-till and ridge tillage fields, transpiration was maintained throughout the day because of the increased water availability attributed to the decrease in soil water evaporation in the early portion of the growing season.

Throughout the season the total reduction in soil water evaporation ranges from 50 to 150 mm in the no-till system. However, in central Iowa there is not the concurrent increase in the transpiration rate as reported by Phillips (1980) because of limitations on the energy availability (Hatfield and Prueger, unpublished data, 1992). The net radiation values are lower in Iowa and in most farming systems with adequate soil water the evapotranspiration rate is near than of the total energy available.

Placement of residue on the soil surface increases the soil water content through the reduction in soil water evaporation. In Minnesota, Gantzer and Blake (1978) found that no-till had 10% more water available than conventional tillage from May through September. The increase in soil water content can cause problems in poorly drained soils and may impact emergence and crop growth in the spring. Excessive soil water and anaerobic conditions can also lead to denitrification and loss of nitrogen from the soil profile.

The increase in the stored soil water and the decrease in the rate of soil water evaporation decreases the temperature variation which occurs within the surface layer. The temperature data shown in Figure 3 reveals that the variation in the thermal regime is diminished and this reduced variation coupled with an increased soil water content provides a more favorable environment for biological activity. This aspect has not been fully evaluated, although it can be hypothesized based on a temperature response curve typical of that shown in Figure 1. Cook (1992) has found in the Pacific Northwest that when wheat is seeded directly into standing wheat or barley stubble, poor wheat growth is a result of rhizoctonia and pythium root rots which are favored by the cool, moist conditions created by the undisturbed residue. He stated that crop rotation, seed treatment, and an understanding of the soil-disease complex would lead to improved nitrogen-use efficiency and reduce the need for tillage and stubble burning, thereby reducing the environmental concerns associated with tillage. To fully understand the implications of residue management will require an integrated approach across a number of disciplines to fully evaluate the complete interactions among temperature, soil water, residue configuration, and biological activity.

Addition of surface residues invokes a change in the soil system at the surface. The first change induced is the organic matter content in the surface layer because of the decomposing organic matter (Blevins et al., 1977). Dick (1983) measured the organic matter in the top 1.25 cm of the soil profile after 18 years of continues corn without tillage and with plowing. He found that the organic matter content in the neo-till fields as 2.5 times higher than in the plowed fields for a dark silty clay loan and 2.2 times higher on a light colored silt loam. Blow 7.5 cm there as no difference in the organic matter levels. Kinsella (unpublished data, personal communication, 1992) found that the organic matter levels increased in the no-till fields over a 15 year period.

Increased organic matter content in the upper soil profile may be a result of the cooler soil temperatures and a reduced oxidation rate compared to a relatively undisturbed profile of the no-till system. There are several areas of understanding which need to be addressed relative to organic matter changes under no-till conditions. These relate to the rate of organic matter decomposition under no-till systems and the interactions of organic matter changes with nitrogen cycling.

Surface residues alter the conditions of the soil profile starting with the physical environment, for example, temperature and soil water content. Increases in soil organic matter are related to such soil forming processes as aggregation and soil structure which are components of soil tilth. The presence of surface residue is associated with an increase in the biological activity as demonstrated by Cook (1992), and we can assume that if the soil environment is favorable for the pathogens within the soil, it is favorable for the beneficial organisms. Berry and Karlen (1993) have shown that under ridge tillage with a cover crop, there were greater numbers of earthworms than under conventional tillage with a fall chisel-plow. An increase in the earthworm population may provide a signal of the ability of the soil to support biological activity and be near an optimum in terms of tilth. However, there has not been a complete analysis of all of the relationships among the physical and biological factors which could be attributed to soil tilth.

Doran (1987) showed that the microbial biomass was 1.5 times higher in the no-till fields compared to plowed fields. Earlier, Doran (1980) had shown that there was a vertical stratification in the microbial populations. Both aerobes and anaerobes increased in the upper 7.5cm under no-till conditions with the presence of surface residue with little difference or a decrease in the 7.5-15.0 cm layer. These differences in populations are a result of the change in the soil microclimate and the presence of the organic matter as an energy source. Changes in the microbial populations will influence the nutrient cycling within the soil profile and the availability of nutrients to the plant. There will be an effect of residue management on the nitrogen cycling and the nitrate availability and these changes may affect plant growth and development.

Aggregation of soil into more stable structures is influenced by the use of no-till and surface residue management. Mannering et al. (1975) showed that after five years of no-till, aggregation increased in the 0-5 and 5-15 cm layers compared to other tillage practices. However, following tillage in the no-till plots the number of water stable aggregates were similar to those in the moldboard plowed and chisel-plowed fields suggesting that the tillage process can quickly disrupt the stable aggregates formed under no-till conditions. The processes by which these aggregates form need to be quantified in order to improve our understanding of the positive and negative effects of soil management decisions.

Surface residue management under no-till or limited tillage systems alters the microclimate of the soil through the introduction of a physical barrier to water loss from the surface and a thermal barrier to heat exchange. The presence of the residue increases infiltration, decreases soil water evaporation, and increases stored soil water in the profile. Concurrently, residue decreases the diurnal range of temperature because of the addition of a lower thermal conductivity layer on the soil surface. However, there are seasonal differences as to the impact of the residue on the temperature within the soil profile. In the fall there was a decreased rate of cooling because of the residue. In the spring there was a diminished effect of residue on the diurnal range of soil temperature due to decomposition and darkening of the residue and increased soil water content. Seasonal observations of temperature and soil water regimes in northern latitudes need to be more completely understood in order to improve the management process. The capture of snow in the winter by residue has a positive effect on the soil water balance and the reduction in soil water evaporation improves the water supply. In the northern latitudes, management systems need to be developed which can more efficiently utilize this stored soil water.

Surface residue improves tilth because of the change in organic matter, a more stable soil microclimate for biological activity, and a supply of nutrients for microbial populations. These factors contribute to increased tilth; however, tillage can quickly disrupt the positive aspects associated with surface residues. Surface residue and no-tillage systems can create a stable environment within the upper soil profile. To develop more sustainable systems we will have to address many of the processes which are occurring within this regime in a holistic manner. If we build upon what we know, we can improve and enhance the capability of our soil resource and the efficiency of agricultural production.

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