29.1. Soil physical properties and plant growth

The shear strength, penetration resistance and hydraulic properties of soils change according to the level of compaction. Both these mechanical and hydraulic properties have an effect on the rate at which plant roots can grow to depth in a soil, and the flow and availability of water and nutrients for plant use. This fact has been quite obvious in numerous comparative observations of root and crop growth in different soils.

Taylor et al. (1966), for example, measured the number of tap roots of the cotton plant which penetrated compacted layers of different soils, and characterized the degrees of  compaction by means of measurements with a cone penetrometer. As their results in figure 3.1 indicate, the number of roots penetrating the soil was reduced drastically as the penetration resistance approached 2 MPa pressure. In fact, at soils compacted to more than 2 MPa resistance, virtually no roots at all were able to grow.


Fig 3.1. Percentage of cotton tap roots penetrating 2.5 cm thick layers of soil compacted to different degrees of cone penetration resistance pressures (PR) in four soil types (Taylor et al. 1966)

Similarly, Raghavan et al. (1979) dug trenches in an experimental clay field of silage corn, in which plots had been treated to varying levels of compaction by machinery traffic. Washed root samples at different depths for eaqch level of soil compaction were weighed, and the results are shown in figure 3.2 as cross sectional maps of root distributions at harvest time for the various treatments.


Fig 3.2. Root density distributions in a clay field of silage corn wherein plots were subjected to different levels of compaction by machinery traffic, at an average compact pressure of 61.7 kPa (Raghavan et al. 1979)

The heaviest compaction treatment (15 tractor passes) evidently decreases the maximum depth of rooting by one half, and the depth of dense roots to about one third of that in uncompacted soil. This effect, together with soil water status alterations had the net result of considerably reducing the growth and yield of the silage corn compared to uncompacted plots in the same field, (figure 3.3).


Fig 3.3. Dry matter yield of silage corn as a function of soil dry density between 0 and 20 cm depth in a clay field of silage corn

The results of figure 3.3 were obtained in successive plantings of the same hybrid of silage corn on a field of clay in the years 1976, 1977 and 1980. The various densities of top soil were obtained by applying different levels of machinery traffic in plots following an initial rotary cultivation of the field in the spring of each year to a depth approximately 25 cm. Between the years 1977 and 1980, the yields in plots of different density were close together. In 1976, the yields were higher at lower density, around 16 t ha-1 at 1 t m-3. The curves in figure 3.3 fitted to the data points illustrate two important aspects of the effects of soil density upon crop growth, namely phenomenon of optimum density, and the role of local seasonal precipitation.

The occurance of an optimum density for soil was observed by Vomocil (1955) and reporte by Rosenberg (1964). Vomicil noted the yields of field corn, sweet corn and potatoes on some New Jersey soils, at different degrees of compaction, and concluded that a soil which is either looser or more dense than a particular density value will incur yield losses compared to that optimum structure. He proposed an equation to describe this phenomenon with a parabolic shape, such that the loss in crop yield increases as the square  of the difference in soil dry density from the optimum, as shown in equation 3.1.

      294.png                               ------------------------------------ (3.1)


                        Y* = the maximum obtainable crop yield

                        Y  = the actual crop yield

                        C  = a constant

                        Υdry = the actual soil dry density (averaged from 10-40 cm depth)

                        Υ*dry = the optimum soil dry density for maximum yield

Vomocil (1955) suggested that the constant C, which can be referred to as a factor of sensitivity to soil compaction, depends upon both the type and variety of crop, and the weather. The results of figure 3.3 confirm the concept of a parabolic change in crop yields. However, there appears to be a distinct difference in yield behaviour between the 1976 and the other two years’ seasons. The rainfall pattern was also markedly different in these periods, and may well provide a sufficient explanation for the different behaviour. In 1976, one of the wettest summers on record in the Montreal area, 330 mm of rain fell in the combined months of June, July and August. In the years 1977 and 1980, 215 and 220 mm, respectively, were experienced in the same period. These latter two amounts of precipitation are essentially the same, as are the silage corn yieldsat each soil density in figure 3.3 for those years.  

With about 50% more rainfall in 1976, not only has the sensitivity factor C of equation 8.2 diminished from 282 to 90 t ha-1 per t m-3, but the apparent optimum soil dry density has been reduced as well from about 1.13 to 0.99 t m-3, and the maximum possible crop yield, all other factors being equal, has increased from 12.3 to 16 t ha-1. It would appear then that it is not only the compaction sensitivity constant C, which is dependent on annual weather patterns, but also the optimum soil density, and the maximum obtainable crop yield.

Such behaviour is reasonable in view of the preceding comments on soil hydraulic properties as a function of compaction. A loose soil with large macropores, such as the clay of figure 8.9 at a dry density of 1 t m-3 and porosity 62%, might normally be expected ti drain too quickly, and to retain an insufficient quantity of moisture during dry periods of the growing season for optimum crop growth. However, during a year such as 1976, when the summer rainfall exceeded the normal average by some 50%, there were no extended dry periods at any time in the growing season. Therefore, the loose soil fabric, with its reduced impedence to root growth and increased conductivity for moisture and nutrients, allowed the soil-plant system to take advantage of the plentiful rainfall and to produce higher than normal yields.

The years 1977 and 1980 represented average weather conditions from June to September, and were thus more typical of what is to be expected in most years. In such a case, the soil density of 1 t m-3 was too low for water retention in periods between rainfalls, which extended up to ten days in those years. The available water in the soil fabric was used to an extent that there was considerable water stress in the plants at some times, and over the growing season the total crop yields were reduced by some 30% belowthe optimum quantity which occurred at a somewhat higher soil density (1.13 t m-3).

In all of the years of the study illustrated in figure 8.9, and in all studies reported on the effects of compaction on crop growth, an excessive amount of compaction above the optimum dry density also results in reduced crop growth. In the case of clay soil of figure 3.3, the dropping off of yields was quite rapid, with nearly half of the yield being lost for a dry density increase of only 0.13 t m-3 above the optimum. Other soil types such as the sandy loam of figure 3.4, appear to be less sensitive to the absolute amount of density change. The difference here is logical when one considers that there is a much larger discrepancy between the sizes of macropores and micropores in a clay soil, and only small overall density changes may be needed to close off the macropores among clay structural units. In a sandy soil, however, there is a more gradual distribution of macro and micropore volumes among soil particles, and larger overall density increases are required in order to have an equivalent effect on soil structure as it influences the movement of roots and moisture in the growing season.



Fig 3.4. Relationship between silage corn crop yields and the dry density of the 0-20 cm layer of a sandy loam soil (Negi et al., 1981)

In a detailed study of root growth and water movement in field plots of silage corn on clay soil, Douglas and McKyes (1983) outlined the stresses which can arise in soils compacted or tilled to different structures. Figure 3.5 shows an example of the rate of water extraction from corn roots at varying depth in the soil, along with the dry density patterns in the corresponding plots of different cultural treatments. The roots in the plots tilled by a moldboard plow and a subsoiler were not active as deep as the roots in other treatments, most likely owing to a dense “pan” layer of soil occurring at the 20-25 cm depth..


Fig 3.5. Patterns of corn root extraction rate of water at depths in clay soil plots subjected to different tillage treatments, after 57 DAS, and the dry density profiles of the various plots (Douglas and McKyes (1983)

This phenomenon shows up also in figure 3.6 which contains plots of crop growth stress factors as a function of time during the growing season. The two crop stress factors shown, one related to water deficits and the other to root growth impedance, were calculated as the difference between the initial and the later rates of crop growth divided by the initial and subsequent actual water transpiration rate, and effective rooting depth respectively.


The curves in figure 3.6 indicate that both the loose “control” clay plots and the compacted untilled plots suffered more water stress for crops than the others. However, it was the moldboard plow and subsoiler treatments which resulted in greater stresses for root growth. In this case, the root impedance stresses in the moldboard plow and subsoiler treatments were more severe than water availability stresses, because as the results of figure 3.7 show, the final crop yield was lower in these treatments than in the loose control soil. The compacted untilled treatment caused the highest soil packing of all, and the combination of root and water stresses led to low corn yields. The pattern of crop yield versus soil dry density in figure 3.4 is similar to that described by the Vomocil (1955) equation 3.1 and those of figures 3.3 and 3.4.


Fig 3.7. Dry matter harvest yield of silage corn on plots of clay soil subjected to different treatments of compaction and tillage as a function of soil dry density (Douglas and McKyes (1983)


Last modified: Wednesday, 19 March 2014, 12:18 PM