The effects of subsoiling on soil physical properties and crop production

In the Canterbury and North Otago regions of New Zealand, many intensively cropped yellow-grey earths contain subsoils which have become compacted by traffic and/or natural processes and which therefore restrict root growth and potential crop yield. The effects of loosening such soils by subsoiling were investigated under dryland and irrigated conditions at four sites (three in North Otago, one in Canterbury (Lincoln)). To widen the range of soil compactness within the study, a roller was used at one site (Lincoln) to create artificial pans immediately beneath the topsoil. A range of depths, degrees, and uniformities of subsoil disturbance were achieved by subsoiling to different depths (35, 50 cm) with two types of implements (a conventional subsoiler, and a 'Paraplow'). Soil physical properties, root growth, crop water use, and yield of spring-sown peas, wheat, and barley, were assessed. By creating large soil pores which roots could easily enter, and by reducing penetration resistance, subsoiling increased maximum rooting depth, subsoil root density, and total root length, of dryland peas. As a result, the crop was able to extract more water from the subsoil during short-term droughts. In the 1987/88 season, which was typical of the Canterbury and North Otago climate, water use of the dryland peas was increased by 19-53 mm. Under dryland conditions, subsoiling increased the seed yield of peas by 12-61% (0.33-1.35 t ha⁻¹), and yields of wheat and barley crops by 5-22% (0.54-1.15 t ha⁻¹). Responses were minimal under frequent irrigation, but under that typical of district practice, subsoiling reduced water stress between irrigations. Pea and cereal yields were then increased by up to 34% (1.2 t ha⁻¹). Results indicated that, in average to moist seasons, subsoiling can be as effective as irrigation in reducing crop water stress during short-term droughts. In drier seasons, it may not be able to fully substitute for irrigation, but it can, however, reduce the total irrigation requirement. By creating soil fissures and packing voids, subsoiling increased the volume of macropores (larger than 30 µm ESD) by up to 30% of the total volume of soil. Because many of those pores were continuous to nearly the full depth of loosening, they rapidly transmitted water into the subsoil. In yellow-grey earths, the penetration of water below the topsoil is often slow, therefore subsoiling reduced the risk of anaerobic conditions occurring in the root zone during rain and irrigation. Consequently, more excess rainwater was accommodated before topsoil waterlogging occurred, and more plant-available water was stored during irrigation. Improvements in root growth and water transmission resulting from subsoiling depended not only on the volume, but also on the continuity of the macropores produced. The most relevant soil physical measurements were therefore hydraulic conductivity and air permeability, because these reflect the functionally effective macroporosity. Air permeability, in particular, proved to be a sensitive indicator of the ability of each of the subsoiling treatments to create continuous macropores and thus improve root growth. At the 17-30 cm soil depth, subsoiling increased both air permeability and hydraulic conductivity by up to 3-4 orders of magnitude. Subsoil disturbance which created a wide range of fragment sizes produced more continuous macropores than did less disruptive disturbance which created mainly large soil fragments. The more effective disturbance therefore resulted in the lowest soil penetration resistance and bulk density, and the greatest increases in macroporosity, hydraulic conductivity, air permeability, root growth, and crop yield. The effects of subsoiling were also greatest when that type of disturbance was achieved to the greatest depth (i.e. to 50 cm rather than 35 cm). Measurements of soil physical properties, together with dye staining of the macropore system, showed that even the most uniform subsoiling treatment substantially increased short-range soil variability. In order for measurements of soil physical properties to accurately reflect macropore continuity, large volumes of soil are therefore required to provide a representative sample of the macropore system. For hydraulic conductivity and air permeability, large (47-108 dm³) undisturbed blocks of soil were sampled and encased in gypsum. This procedure was successful for assessing the overall effects of subsoiling, and is recommended for future subsoiling work. Repeated measurements of some soil physical properties and crop yield suggested that, provided significant soil disruption was achieved, the effects of subsoiling diminished only gradually. When post-loosening traffic was kept to a minimum, a subsoiling frequency no greater than three years was indicated