Recent work has shown that the organic carbon contents and soil nitrogen levels of several important vertisols of the Darling Downs, Queensland, have decreased due to cultivation. An increase in soil bulk density was associated with the decrease in organic C. However, bulk density provides only a limited assessment of the physical state of the soils, and a range of other measurements is required to adequately describe the changes in structural conditions associated with continuous cultivation. As well, some evidence suggests that organic C does not have a major effect on the structural stability of vertisols. Hence there was a need for further investigation of the changes in the structural conditions of these soils.
The objectives of this project were to
1) investigate the changes in soil structure associated with continuous cultivation of two important arable vertisols of the Darling Downs, and
2) relate the structural changes to other physical, chemical, and biological properties that may affect crop growth.
Sites representing cultivation histories up to 64y as well as virgin sites were chosen on the Waco and Langlands-Logie soil types of the order of vertisols. Soil structure was characterized by the size and tensile strength of dry aggregates (dry sieving), the stability of wet aggregates (wet sieving and dispersibility), saturated hydraulic conductivity of repacked cores, and soil and aggregate bulk densities. Clay content and various chemical properties were also determined
The assessment of changes in soil properties using sites with varying cultivation histories assumes that the soils were uniform in their virgin states. The uniformity of the two soils was examined using clay content, clay mineralogy, CEC, and CEC:clay ratio. On this basis the Langlands-Logie soil was split into two smaller groups; the Langlands and the Logie. The Langlands sites covered an adequate range of cultivation histories while the Logie sites did not.
On both the Waco and the Langlands soils, the size and tensile strength of the dry aggregates decreased due to cultivation. However, they remained within a desirable size range, and the pattern of change indicated that further degradation was unlikely. It was therefore concluded that dry aggregation was unlikely to restrict productivity of these soils.
On the Waco soil, results from wet sieving and dispersion measurements indicated that the size of water stable structural units had decreased due to cultivation. They reached relatively stable levels after approximately 15 to 20 years. Dispersibility of the Langlands soil showed a similar pattern. However, results from wet sieving suggested that stability of the Langlands soil was greatest in the young cultivated soils and lower in the other sites.
Hydraulic conductivity of the Waco soil decreased due to cultivation, consistent with the decreased aggregate stability. On the Langlands soil, the hydraulic conductivity was greatest for the young cultivated soils similar to the pattern for the wet sieving results, but in contrast to the changes in dispersibility.
Bulk and aggregate densities increased due to cultivation, but there were few significant differences among the cultivated sites. This indicated that after the initial cultivation there was no further increase in density, and that the swelling and shrinking characteristics did not alter the bulk density (standardised to a constant gravimetric water content) of the cultivated soils.
Changes to other soil chemical properties were also investigated. These properties were then related to the structural parameters. Variation among the sites in dry aggregation was satisfactorily accounted for by an existing equation incorporating ESP and the CEC: clay ratio. The range of ESP values of the soils was insufficient to have a large effect on dry aggregation. No single soil property accounted for more than 15% of the variation in the wet sieving results on either the Waco or the Langlands soils. However, across the soils, the wet sieving results were satisfactorily related to the clay content (R2 = 0.52).
Dispersion was expressed either as a proportion of the fine fraction of the soil (Dispersion Ratios) or as a proportion of the bulk soil (<2 pm: D2, < 20pm: D20). JD2 and D20 were wore strongly correlated with other soil properties than were the dispersion ratios. As well, they are directly measured variables whereas the dispersion ratios are derived variables. For these reasons they were considered to be more useful indices of dispersion.
D2 and D20 of the Waco and Langlands soils were most strongly related to the exchangeable Na content of the bulk soil. Across the soils, clay content was also a major factor affecting dispersion. Equations were developed which related D2 and D20 to the exchangeable Na content and clay content across the soils. These equations explained 88% and 82% of the variation in D2 and D20 respectively, and were shown to apply to some independent data of other vertisols. Hence the dispersion indices can be derived from information available from routine soil tests. Clay content covered the range found in vertisols (35 to 80%), and also a wide range of CEC values were represented. Therefore it is possible that these equations have a wider application to other soils of the vertisol order.
The equations indicate that reduction of dispersibility on these soils is best achieved by decreasing the levels of exchangeable Na and not by increasing the organic C contents. Gypsum application is a possible way to achieve this.
Hydraulic conductivity was highly correlated to D2 and D20, but less well correlated to the dispersion ratios and the results from wet sieving analysis. Small column experiments indicated that infiltration rates were affected by dispersibility in a similar manner to hydraulic conductivity, and the rate of water redistribution in the Waco columns was reduced by increasing dispersibilities. It is known that water relations can limit productivity of these vertisols, and. dispersibility appears to be an important factor governing the entry of water into the soil profile and the subsequent recharge of the subsoil moisture.
It was concluded that the structural status of these soils was best described by the parameters D2 and D20. These parameters are simple to measure, are sensitive to changes with period of cultivation, have agronomic significance through their effect on water relations, and have unifying relationships across soil types. The results indicate that structural stability has decreased due to cultivation of these soils, and it is likely to affect soil productivity through its influence on the soil water relations. Increasing exchangeable Na levels account for much of the change in structural conditions, and therefore gypsum application is appropriate for the maintenance of good soil structure on these soils.