Summary

Adam Pirie

The physical properties of a soil are extremely important in controlling the health and vitality of the plants growing in them, the health of these plants then determines how productive the system (agricultural or otherwise) will be. During this field trip several key soil physical parameters that contribute to soil quality were investigated. Most parameters were investigated using more than one technique and a number of readings taken for each. Attempts were then made to determine the effect of the differing management techniques (undisturbed pasture, continuous cultivation) on the soil’s physical properties Surface infiltration was investigated using Tension and Ponded Disk Infiltrometers, as well as Single and Double Ring Falling Head Infiltrometers. When analysed there was found to be a significant difference between the Ksat of the pasture and cultivated sites. This was not surprising considering visual inspection of the site showed there to be crusting and ponding occurring at the cultivated site, indicating it’s proneness to slaking. Particles that slake off the peds are able to move into pores and block them, thereby reducing the infiltrative capacity of the soil. No such problems were seen in the pasture site, leading to the conclusion that the slaking present is most likely due to a lack of organic matter to hold the cultivated site’s peds together. Some students also expressed concern as to whether the experiments had been allowed to run long enough to allow steady state conditions to establish. This was of more concern in the pasture site where large cracks present needed time to swell, close up and prevent preferential flow. Subsurface infiltration was evaluated using both Falling Head Lined Borehole and Amoozemeter techniques. In both of these techniques a borehole is dug to the depth of interest before the infiltration rate is measured. This raises the issue of smeared ped faces blocking pores and reducing infiltration, there are techniques to overcome this problem but they weren’t used on this field trip. All lined boreholes were installed on the cultivated site so a comparison between the two management practices is unable to be made. The Amoozemeter showed no significant difference between management regimes. A trend of increased conductivity under pasture appears to exist, but there is insufficient data for this to be established. All sites show low to moderate levels of hydraulic conductivity, which is to be expected of a heavy clay soil such as this Vertosol. These levels of conductivity should not be limiting to plant growth. Bulk density was shown to vary significantly with land use with bulk a mean density of 1.2 gcm-3 in the cultivated and 1.0 gcm-3 in the pasture. These values aren’t very high especially for a clay soil, and it is probable that the anomaly is due to the presence of crack in the soil core. This appears to be very likely in the drier pasture site where large cracks were visible. The higher bulk density measured for the cultivated site may be due to less cracks being present in the soil due to it’s higher water content and swollen nature. The water content of the soil was also found to be significantly different between sites. This is also supported by the EM38 survey and the TDR and Six-Lady Solute experiment (SLSE). The TDR probe of the pasture site shows a major increase in water content down the profile, whereas the cultivated site (SLSE) shows a relatively constant level of water throughout the profile. This difference in water contents through the profile is most likely due to evapotranspiration by the plants in the pasture. This shows clearly the effect of plants on the soil moisture store and should highlight to farm managers the importance of eliminating weeds in order to maximise profile water present for crops.
	The author 'flat out'
Soil strength was estimated using a cone penetrometer and a shear vane. These instruments found the soil subject to continuous cultivation to have less strength than that of the pasture site. However as soil strength increases as a function of decreasing wetness, the effect of increased moisture in the cultivated area makes it difficult to compare values between the two sites. Another problem with the use of a shear vane is that it needs to be rotated at a single constant speed in order two provide comparable data between points (something which is very difficult to do in the field). A plough pan does exist in the profile of the cultivated site (author’s observation); the cone penetrometer however was unable to detect it. Whether this is a function of reduced sensitivity at increased water contents or the plough pan existing at a depth greater than 15 cm remains to be seen. Indicators that a plough pan may be present include the extent of time that this profile has been continually cultivated, as well as the higher bulk density and lower hydraulic conductivities of the cultivated site. It would be interesting to investigate the soils strength at a lower wetness and to greater depth to ascertain whether the cone penetrometer is able to detect it’s presence. None of the soil strength’s measured in the study are limiting to plant growth. Soil temperature was monitored using thermocouples at a variety of depths in the upper profile (1 cm, 3 cm, 10 cm). Once ‘standard’ temperature fluctuations were recorded the effect of covering the soil with a black body i.e. black plastic and mulches were investigated. It was found that black plastic greatly increases the daily surface temperature fluctuation whilst covering the soil with mulch reduces it. This reduction in temperature fluctuation was applied with other data to calculate a Bowen ratio and estimate evaporation for the cultivated area, the graphed results of evaporation showed a large decrease once the soil heat flux was reduced. This highlights the importance of maintaining some surface cover as mulch to minimise water lost as evaporation. To be more conclusive in analysis more samples need to be taken, and experiments need to be run for a longer time to reach a steady state. However it appears that the management regime that this soil has been subject to has had a significant effect on many soil physical properties. This field study does shows that some of the soil physical properties are affected through a regime of continuous cultivation; an increase in bulk density, an increase in the time taken for water to move into and through the profile, as well as a reduction in soil strength. The study also shows the effect of plants in evapotranspiring water out of the soil and the value of mulches in lowering evaporation.