Inge Håkansson

A review of the usefulness of relative bulk density values in studies of soil structure and compaction

The state of compactness is an important soil structure and quality attribute, and there is a need to find a parameter for its characterization that gives directly comparable values for all soils. The use of some relative bulk density value for this purpose, particularly the degree of compactness (Hakansson, 1990), is discussed in this review. The degree of compactness has been defined as the dry bulk density of a soil as a percent of a reference bulk density obtained by a standardized uniaxial compression test on large samples at a stress of 200 kPa. The bulk density should be determined at standardized moisture conditions, to prevent problems caused by water content variations in swelling/shrinking soils. The degree of compactness (D) makes results of soil compaction experiments more generally applicable. Whereas the bulk density or porosity optimal for crop growth vary greatly between soils, the optimal D-value is virtually independent of soil composition. Critical limits of penetration resistance (3 MPa) and air-filled porosity (10%, v/v) are similarly related to the D-value and matric water tension in most soils. As the D-value increases above the optimal, the tension range offering non-limiting conditions becomes increasingly limited. The D-value of the plough layer induced by a given number of passes by a certain vehicle is similar in all soils, provided the moisture conditions are comparable. The degree of compactness facilitates modelling of soil and crop responses to machinery traffic. Although this parameter was primarily introduced for use in annually disturbed soil layers, its use may be extended to undisturbed soil layers.

Vehicle and wheel factors influencing soil compaction and crop response in different traffic regimes

Abstract A review is made of effects of machinery traffic on soils and crops, including stress distribution under running gear, soil response to applied stresses, influence of compaction on soil properties and processes, persistence of compaction, and crop response to compaction. The influence of machinery traffic on the crops is divided into several categories: direct damage to growing plants; effects of the state of compactness of the plough layer; residual effects in this layer after re-loosening; effects of subsoil compaction. Traffic intensity and wheel track distribution in different cropping systems are illustrated, and several possibilities for reducing heavy traffic or its negative effects, or to alleviate compaction, are discussed. The present situation in different parts of the world with regard to soil compaction is evaluated, and examples of economic analyses are presented.

Subsoil compaction by vehicles with high axle load—extent, persistence and crop response

Abstract Extent and persistence of soil and crop responses to subsoil compaction caused by vehicles with high axle loads are reviewed and methods to protect the subsoil from permanent deterioration are discussed. Traffic by vehicles with high axle loads on soils with high moisture contents generally causes deep subsoil compaction. At an axle load of 10 Mg, compaction typically penetrates to a depth of 50 cm. With still higher loads, compaction to a depth of 1 m has been reported. Subsoil compaction is very persistent. At depths of more than 40 cm it is virtually permanent even in clay soils in regions with annual freezing. Deep subsoil compaction also causes persistent and possibly permanent reductions of crop yields. Complete amelioration by mechanical loosening is usually impossible and definitely expensive. From a soil productivity point of view, limits for mechanical stresses in the subsoil are needed. These may have the form of axle load limits for the vehicles or a combination of limits for the axle load and for some other important factors, such as the ground contact pressure of the running gear or the per cent water saturation of the soil at the time of trafficking. Guidelines for such limits should preferably be worked out in an international joint effort.

A method for characterizing the state of compactness of the plough layer.

Abstract A method for characterizing the state of compactness of the plough layer is described. The “degree of compactnes” is defined as the ratio (%) of the dry bulk density of the soil and the dry bulk density of the same soil in a compacted reference state. The bulk density in the field is determined by a frame-sampling technique. The reference state is determined by a drained, uniaxial compression test using wet soil samples which are loosely filled in an oedometer and loaded with a pressure of 200 kPa until drainage ceases. In a series of 100 field experiments with spring barley this method was method was used for characterizing the state of compactness of the annually tilled soil layer. On mineral soils, maximum crop yield was obtained at the same degree of compactness irrespective of soil type, which was the main goal of the method. For organic soils a modification of the procedure for determining the reference bulk density is required.

Effect of high axle-load traffic on subsoil compaction and crop yield in humid regions with annual freezing

Abstract The weight of agricultural machines is steadily increasing, and this leads to higher axle load and deeper subsoil compaction. As an international joint effort, the effects of high axle load traffic on subsoil compaction and crop yield are being studied in 26 field experiments in Europe and North America. Results at present available, applicable to humid areas with annual freezing, show that crop yield losses normally become larger and more persistent with increasing soil clay content. The results indicate that restrictions upon axle loads are required.

A model for estimating crop yield losses caused by soil compaction

A computerized empirical model for estimating the crop yield losses caused by machinery-induced soil compaction and the value of various countermeasures is presented, along with some examples of estimations made with it. The model is based mainly on results of Swedish field trials, and predicts the effects of compaction in a tillage system that includes mouldboard ploughing. It is designed for use at farm level and predicts four categories of effects: (1) Effects of recompaction after ploughing. The calculations are based on the wheel track distribution in the field and the relationship between “degree of compactness” of the plough layer and crop yield. (2) Effects of plough layer compaction persisting after ploughing. Crop yield losses are estimated from traffic intensity in Mgkm ha−1 (Mgkm = the product of the weight of a machine and the distance driven), soil moisture content, tyre inflation pressure and clay content. (3) Effects of subsoil compaction. The calculations are similar to those presented under point (2), but only vehicles with high axle load are considered. These effects are the most persistent. (4) Effects of traffic in ley crops. The estimations are based on wheel track distribution, soil moisture content and several other factors.

Influences of degree of compactness and matric water tension on some important plant growth factors

The degree of compactness (D) has been defined earlier as the dry bulk density of a soil in percent of a reference bulk density obtained by a standardized uniaxial compression test on large samples at a stress of 200 kPa. It was primarily aimed for use in annually disturbed soil layers. Field experiments have demonstrated its usefulness for characterizing the state of compactness from a crop production point of view, but knowledge is lacking regarding the relation between D and various plant growth factors. While the bulk density or porosity optimal for crop growth has varied considerably between soils, the optimal D-value has been virtually independent of soil texture. This led to a hypothesis that critical limits of penetration resistance (3 MPa) and air-filled porosity (10%, v/v) are similarly related to the D-value in most soils. With the objective to test this hypothesis, the positions of these limits as functions of the D-value and the matric water tension were studied in four soils with clay content ranging from 70 to 220 g kg−1. In all of them, the positions of the critical limits were similar. The higher the D-value, the more limited was the tension range offering adequate conditions for crop growth, the higher was the water tension (the lower the water content) at which aeration became critical, and the lower was the water tension (the higher the water content) at which penetration resistance became critical. The effects of critical soil conditions were reflected in increased stomatal resistance of plants grown in soil with a high D-value. The results provide basic information for modeling the relationships between soil compactness and plant growth.

Do effects of soil compaction persist after ploughing? Results from 21 long-term field experiments in Sweden

Abstract The extent and persistence of the effect of soil compaction in a system with annual ploughing were investigated in 21 long-term field experiments in Sweden with a total of 259 location-years. Crop yield, soil physical properties and plant establishment were determined. All experiments had two common treatments: control (no extra traffic) and compacted (350 Mg km ha −1 of experimental traffic in the autumn prior to ploughing), using a tractor and trailer with traditional wheel equipment and an axle load restricted to 4 Mg. During the rest of the year, both treatments were conventionally and equally tilled. The compaction was repeated each autumn for at least 7 years, and the yield was determined each year until 5 years after the termination of the compaction treatment. Compaction decreased the porosity and the proportion of large pores and increased the tensile strength of dry aggregates. On clay and loam soils, it decreased the proportion of fine aggregates in the seedbed and the gravimetric soil water content in the seedbed. The yield in the compacted treatment declined compared with the control during the first 4 years, after which it reached steady state. During this steady state, the compaction treatment caused a yield loss of 11.4%, averaged over 107 location-years. Within 4–5 years after the termination of the compaction treatment, the yield returned to the control level. The average yield loss at individual sites increased with increasing clay content. Results from additional treatments indicated that yield loss was linearly correlated with the amount of traffic up to 300–400 Mg km ha −1 . With greater ground contact pressure or a greater soil water content at time of traffic, there was a greater yield loss. Soil compaction effects on yield were similar for all spring-sown crops, and the percentage yield loss seemed to be independent of the yield. In a few location-years with winter wheat there was on average no yield decrease. There were 5.1% less plants in the compacted treatment than in the control. The yield decrease was significantly correlated with the number of plants. Between years results were highly variable, and no consistent correlations between yield loss and soil water content at the time of traffic or the weather conditions during the growing period were found. Soil compaction affected yield during years with good as well as poor conditions for crop growth.

Swedish experiments on the persistence of subsoil compaction caused by vehicles with high axle load

Abstract Final results from nine Swedish field experiments on soil and crop response to traffic by vehicles with an axle load of 10 Mg are reported. The traffic significantly compacted the soils to a depth of 50–60 cm. Eleven years later, virtually no alleviation of the compaction effects in the subsoil was observed, and crop yields were still affected in spite of normal annual freezing to a depth of 40–70 cm.

Soil and crop responses to different tillage systems

An experiment was carried out over eight consecutive years at three sites, on clay or clay loam soils. In a split-plot design, two main treatments (mouldboard ploughing to 25 cm depth and disc or springtine cultivation to 13 cm depth) were combined with two seedbed preparation treatments (three passes with a conventional harrow vs. one pass with a power take off (PTO) driven harrow). Seedbed characteristics and bulk soil properties investigated at one of the sites in 1991 were similar in the different treatments in the 0-13 cm layer. In the 13-25 cm layer shallow cultivation resulted in significantly higher bulk density, degree of compactness and penetration resistance, and lower root density than in mouldboard ploughing. A reduced number of tractor passes achieved by using the PTO driven harrow resulted in significantly lower bulk density and penetration resistance in the unploughed soil, while still providing an adequate seedbed. At 25-30 cm depth, the volume of pores with equivalent diameter > 100 um, saturated hydraulic conductivity and air permeability were higher with ploughless tillage than with conventional tillage. Pore continuity was greater in unploughed soil at all depths investigated. In unploughed plots there was a concentration of organic carbon and potassium in the upper 13 cm. Phosphorus distribution and pH were not altered by the tillage systems. The yield was improved by the PTO driven harrow both in ploughed and unploughed plots.