The influence of subsoiling on soil physical properties and pasture production

Abstract

In many areas of New Zealand, soils under pasture have become compacted by a combination of livestock trampling, use of heavy machinery and natural processes, or in the case of sports fields, heavy use at high moisture levels. These processes create soil horizons that have a high resistance to a penetrometer, often a high bulk density and hence a lowered porosity. These horizons reduce hydraulic conductivity, change soil moisture characteristics and restrict root elongation and consequent pasture yield. An arable type subsoiler was re-designed for pasture use and its effects on five soils in the Hamilton Basin were investigated. Soil disturbance was assessed for differing operational depths, tip sizes, operating modes and soil moistures. Soil physical characteristics such as bulk density, soil strength, pore space, water tension and infiltration were measured at intervals after subsoiling. Root growth and yield response of pasture were quantified. Subsoiling in the vibrating mode under relatively dry soil conditions increased soil disturbance by 30% in comparison with non-vibrating; wide tips in comparison with narrow tips increased disturbance by up to 100%. Results indicated that the effectiveness of subsoiling at a standard speed, is a function of operational depth, tip size, pitch angle, soil type and soil moisture. Generally subsoiling below 45 cm depth resulted in a reduction in soil disturbance and was less effective under wet soil conditions at deeper than 35cm Increased water stress in and around the subsoiled slot resulted from summer subsoiling. The addition of press rollers reduced evaporation and reduced ground heave by 25%. Measurements of ground heave following subsoiling using a purpose built heaveometer enabled comparisons between different soils to be made. The more clayey Hamilton and Whatawhata soils showed greatest heave. Subsoiling reduced bulk density by up to 30% within 10 cm distance of the subsoil slot; significantly reduced levels were sustained at this distance in the more clayey soils for up to two years. The effect on bulk density decreased with distance from the subsoil slot. Bulk density estimations in situ showed greater changes in response to subsoiling than laboratory measurements. Penetrometer measurements provided additional crucial information on the extent of soil disturbance and its sustainability with time. Repeated measurements defined “envelopes” of soil disturbance emanating from the subsoiled shank and tip. The lowest soil penetration resistance was found down the slot, where strengths were commonly reduced from 4 MPa to 1 MPa. The depth of influence was shallower further from the slot following a 45 - 50° angle up from the subsoiler tip to the surface. In the more clayey soils a discrete zone of higher strength soil was occasionally identified contiguous to the lateral extremeties of disturbance. Subsoiling shattered large aggregates thus creating a larger volume of soil. Soil mixing was limited to a relatively small area around the subsoiler shank and tip where 17 - 19% of A and B horizon aggregates interchanged. Consequently a redistribution of soil texture and organic matter occurred within soil profiles. Pores >9.76 μm increased from 20 to 35% in A horizons and from 3 to 35% in B horizons. Increased porosity to the full depth of soil disturbance resulted in rapid movement of excess summer rain, and in the case of the poorly drained Whatawhata soil drier soil conditions prevailed in winter. Measurements of root elongation, together with SEM analysis showed that grass roots had difficulty penetrating high bulk density (1.35 - 1.50 Mg m⁻³), high strength (>5.00 MPa) soil layers. In these cases subsoiling facilitated increases of up to 20% in root length and a consequent increase in pasture yield of up to 17.7%. The more clayey soils derived greater benefits from subsoiling than sandy soils, both in the extent of soil disturbance and duration of reduced bulk density and soil strength. In the case of an artificially constructed soil, near surface compaction was ameliorated with a consequent improvement in turf. Provided that subsoiling was carried out under optimum conditions, a positive response in pasture yield could be expected, even where soil physical properties were not limiting. Re-compaction rates following subsoiling are a function of pasture management and soil physical properties, but provided subsoiling is carried out with optimal soil disturbance, then a subsoiling frequency of two years for sandy soils and three years for more silty and clayey soils is recommended.