Black Vertosol Research Articles

Changes in phosphorus fractions at various soil depths following long-term P fertiliser application on a Black Vertosol from south-eastern Queensland

Long-term removal of grain P and soil test data suggested that the Colwell phosphorus (P) extraction from the surface 0.10 m of a Black Vertosol from south-eastern Queensland was a poor indicator of run-down of soil P pools. We proposed that plants were also accessing P from layers below 0.10 m or from surface soil P pools not extracted by the Colwell extraction. Both topsoil and subsoil samples in 1994 and 2003 were collected from nil and 20 kg P/ha per crop treatments in a long-term N × P field experiment established in 1985 for detailed P fractionation. An uncropped reference soil was also taken in 2003 from an adjacent area. The long-term effect of the field treatments on soil P fractions was evaluated by comparing the reference site, which was assumed to represent the original soil condition, to the 2003 samples. Without addition of P fertiliser, 55%, 35%, and 10% of total P removal were from 0 to 0.10, 0.10 to 0.30, and 0.30 to 0.60 m, respectively, compared with the uncropped reference soil. Labile fractions comprising resin, bicarbonate, and hydroxide pools in the top 0.10 m decreased by approximately 60% and accounted for 15% of the total P decrease from 0 to 0.60 m depth. Acid and residual-P fractions decreased by 50% and 20%, respectively, and accounted for ~20% and 15% of the total P decrease. In contrast, P addition at 20 kg P/ha per crop over 18 crops doubled the resin and bicarbonate inorganic P (NaHCO3-Pi) pools in the surface 0.10 m. Hydroxide (NaOH-Pi) and acid extracted inorganic P increased by 25% and 10%, respectively, while the residual-P pool decreased by about 15%. Below 0.10 m, very little P was removed by the first 3 extractants. Most of the P was present in the acid and residual fractions irrespective of fertiliser application. The acid and residual-P dropped by 30% and 12%, respectively, at 0.10–0.30 m and 12% and 8% at 0.30–0.60 m. When comparing the experimental soil samples in 2003 with those in 1994, similar trends were observed in the changes of each soil P fraction. In the surface 0.10 m, acid and residual-P pools decreased greatly and explained almost all of the total P decrease in the surface soil without P input. With P addition, labile pools acted as the main sink for P. The acid pool increased by 7%, while the residual-P showed a decrease in the topsoil. Total P level was elevated noticeably in this soil layer. However, at 0.10–0.30 m depth, acid and residual pools were the dominant fractions and decreased significantly irrespective of P fertiliser addition. Below 0.30 m, no significant changes were detected for each fraction and total P. The results suggest that crops had accessed significant amounts of P at 0.10–0.30 m depth irrespective of P fertiliser application, and that subsoil sampling (0.10–0.30 m) should be considered in order to improve the monitoring of soil P status. However, choice of appropriate extractants for monitoring subsoil P reserves is yet to be undertaken.

Improved measurement of conductivity on swelling clay soils using a modified disc permeameter method

Disc permeameters are the preferred method for measuring unsaturated hydraulic conductivity (Kψ) in situ. However, in swelling clays, Kψ measured using 3-D measurement and analysis methods are often several orders of magnitude too high and are of no value for modelling internal drainage. During a series of experiments, the causes of inflated Kψ values were identified and a modified method developed to accurately measure Kψ. On a Black Vertosol, Kψ was strongly correlated with water content and measurement period, with higher flow rates occurring until the soil profile had fully wet up. Near-saturated Kψ measured under permeameters was up to 2 orders of magnitude higher than field-saturated conductivity (KFS) measured in ponded rings. This discrepancy was caused by residual capillarity and unrestricted swelling at the wetting front in unsaturated soils. In addition, lateral flow was common and surface depressions around permeameters filled with water, erroneously contributing to measured infiltration and invalidating assumptions in the 3-D analysis. To prevent this, permeameters were confined within rings to restrict flow to 1D, and measured Kψ (confined) was then compared with Kψ (unconfined) and KFS (ponded rings). Confining flow to 1D reduced Kψ by up to 2 orders of magnitude for a Black Vertosol and by a factor of 3 for a Red Ferrosol. Near-saturated Kψ from confined permeameters agreed well with KFS after similar short measurement durations (e.g. 0.5 h), but Kψ was still overestimated until steady-state flow was established. Therefore, to derive estimates of Kψ that reflect natural flow during internal drainage, we recommend (i) pre-wetting the soil and extending measurement time to attain steady-state flow, and (ii) confining permeameters within rings to restrict flow to 1 dimension. This method was used to compare lucerne ley and annual cropping treatments on 2 Black Vertosols (Bongeen and Waco). Kψ was similar between cropping treatments, suggesting that initial differences in structure and porosity were transient and related to soil moisture content. The Bongeen soil had a significantly lower Kψ (K–1 cm of 0.8 mm/h) than the Waco soil (2.0 mm/h).

Monitoring soil moisture status in a Black Vertosol on the Liverpool Plains, NSW, using a combination of neutron scattering and electrical image methods

Electrical image data were used to estimate soil moisture content beneath smectite-dominated black Vertosol in the Liverpool Plains catchment in northern New South Wales. Measurements carried out over a range of soil moisture conditions included: water use by lucerne sown into a full profile, recharge of profiles after lucerne and long fallow after a cereal crop. Measurements were repeated approximately every 2 months between September 2000 and August 2001. Estimates of bulk electrical conductivity (EC), determined from inversion of apparent conductivity electrical images, were compared to soil moisture measurements made using neutron access tubes. A correlation of 0.77 (n = 228, P < 0.0001) between the 2 datasets indicates that electrical imaging can be used to satisfactorily predict bulk moisture content in soils of this type. Interpretation of the bulk EC images indicates drying of the soil profile to 6 m depth beneath lucerne.

Soil development on dolerite and its implications for landscape history in southeastern Tasmania

Soil genesis has been examined using field description, particle-size distributions, chemical properties, mineralogy and elemental distributions of five soil profiles developed on dolerite on Mt. Nelson and Tolmans Hill near Hobart in Tasmania. The soils form a sequence ranging from a Black Vertosol (P8) to four texture contrast soils, namely, a Eutrophic Brown Chromosol (P5), two Mottled-Subnatric Grey Sodosols (MN8 and P4), and a Mottled-Mesonatric Grey Sodosol (P7). The soil stratigraphic and pedological relationships of these soils have been investigated to help understand their distribution and improve understanding of soil formation history. The knowledge of the soil stratigraphy and weathering features aided in the determination of the broader landscape history. The field observations show the local dolerite has been subjected to both deep weathering and severe erosional periods. Pockets of deeply weathered dolerite occur adjacent to thin A/C soils or hard outcropping rock. Deeper colluvial soil materials occur on lower slopes. The presence of protruding dolerite columns now buried by transported clayey slope-wash materials indicates partial landscape stripping followed by reburial. The presence of buried stone-lines separating the upper profile from the clayey subsoils supports the idea of a second major erosional–depositional cycle. A pronounced variation between the A and B horizons particle-size distribution, mineralogy and elemental distribution supports the conclusion that the modern soils are composed of several sedimentary layers which cap a variable thickness of in situ weathered dolerite (termed “mealy material”) above fresh dolerite. Bedrock jointing, veins and rock fabric extend upward from the bedrock into the mealy material, but are truncated abruptly at the contact with the clayey subsoil. Soil-forming processes have operated to modify soil colours and mottling, soil structure and cation chemistry. These findings have important implications for landscape history, slope processes and the improved understanding of the distribution of dolerite-derived soils in Tasmania.

Management practices for control of runoff losses from cotton furrows under storm rainfall. I. Runoff and sediment on a black Vertosol

Improved practices are needed to minimise soil erosion and related agrochemical transport from cotton fields during rain. We evaluated two options available to cotton growers, namely retention of surface cover and controlling wheel traffic, using simulated rain on a hill–furrow system on a well-aggregated black Vertosol. A companion paper explores effects on pesticide transport. Increasing cover resulted in an increase in the rain ultivar by use the fetus lacks the appropriate enzyme systems, clearance of toxic substances takes place via the placenta and maternal liver. Thus, a suitable in vitro system can effectively differentiate between primary and secondary drug effects. In the present study, 13-day-old fetal testis, at the stage of incipient differentiation, were cultured for 4 days in vitro in the presence of graded doses of caffeine, theophylline or theobromine. It was found that explants exposed to caffeine or theobromine differentiated normally, developing seminiferous cords made up of Sertoli and germ cells, soon followed by the differentiation of functionally active Leydig cells appearing in the newly formed interstitium. However, explants exposed to theophylline failed to develop seminiferous cords and, as a consequence, Leydig cells. In conclusion, insights obtained from different experimental methods, such as organ culture or whole organism studies, are not always identical. It may be prudent, therefore, to take into account that certain experimental techniques, despite providing valuable information, may require confirmation by other

Hydraulic properties of rain impact surface seals on three clay soils—influence of raindrop impact frequency and rainfall intensity during steady state

This paper investigates the influence of rainfall intensity, and its components drop size and impact frequency, on steady state flow through sealed soils. Infiltration rates are often observed to increase with greater rainfall intensity. There appears to be no consensus in the literature on the reasons for this behaviour, making it difficult to incorporate in infiltration models. Seals were formed on 2 swelling Black Vertosols and a contrasting non-swelling Brown Sodosol, under simulated rainfall treatments with a range of rainfall intensities, drop sizes, and drop size distributions. For Vertosols, aggregate size composition of the seal, sub-seal moisture content, and suction were similar across rain treatments and between soils. Steady infiltration rate and conductivity increased 3-fold as rainfall intensity increased from 30 to 120 mm/h for Vertosols. The component of intensity responsible for this trend was isolated, with a strong positive linear relationship found between conductivity and raindrop impact frequency (R2 = 0.87). This trend was attributed to increased water entry by hydraulic penetration of drops through pores in the seal as the frequency of drop impacts increased, on more porous Vertosol seals. In contrast, rain intensity and drop impact frequency had little effect on steady infiltration rates and conductivity for the Sodosol. Abundant fine particles in the seal decreased porosity, apparently leaving few pores of large enough diameter to allow hydraulic penetration. While hydraulic penetration was suppressed, other seal properties for the Sodosol were influenced by rainfall characteristics. Larger, more energetic drops produced seals with lower moisture content and higher subseal suction than seals formed under other treatments. Results of this study indicate that water entry through sealed surfaces may consist of an actively driven component associated with hydraulic penetration of water through larger pores (jetting), as observed elsewhere in early stages of surface sealing. Water entry by this mechanism continues through to steady state on soils where significant numbers of larger pores remain open at the soil surface. This mechanism would act alone, or in addition to other mechanisms such as erosive stripping or microrilling of seals, to explain widely observed increases in infiltration rates with increasing rainfall intensity.

Nitrate and chloride leaching in Vertosols for different tillage and stubble practices in fallow - grain cropping

Research conducted in the mid 1980s on a ‘long-term fallow management trial’, located on a black Vertosol at the Hermitage Research Station, indicated that leaching may have been the cause of low concentrations of nitrate-N within the root-zone of zero-tillage stubble-retained treatments. The ‘fallow management trial’ has 12 management treatments: a factorial combination of zero or conventional tillage×stubble retention or burning×3 nitrogen fertiliser rates (0, 23, and 69 kg N/ha). To test the leaching hypothesis, all trial treatments were analysed for nitrate and chloride concentrations to a depth of 5·4 m in order to assess the relative rates of drainage, solute movement, and nitrate leaching between treatments. Similar analyses were conducted on 2 cultivated sites and 2 permanently grassed sites on-farm, also on black Vertosols, to compare solute movement rates under the continuous winter cereal rotation (trial site) with a winter–summer cropping regime and permanent pasture. Results from the Hermitage trial site showed zero tillage with stubble retention had a chloride concentration peak 2 m deeper down the profile (4·5 m) than all other management treatments, indicating that drainage rates were greatest in zero tillage–stubble retained treatments. Nitrate profiles, however, showed that movement of nitrate-N to below the root-zone was greatest under zero tillage with stubble burning with 69 kg N/ha applied (Z-B 69N), followed by zero tillage with stubble retention and 69 kg N/ha. The large nitrate loss from the root-zone of Z-B 69N (about 30% of applied fertiliser) was considered to be a result of high concentrations of nitrate-N in the top 1·5 m associated with stubble burning and fertilisation. The on-farm cultivated sites had very little nitrate-N throughout the whole profile, suggesting that either the use of summer as well as winter crops reduced residual or ‘spared’ nitrate-N (through control of root-lesion nematodes) and/or mineralisation rates were lower on these sites.

The dielectric behaviour of clay soils and its application to time domain reflectometry

The dielectric behaviour of 3 soils, a sandy loam (Red Chromosol), a highly structured non-swelling clay (Red Ferrosol), and a self-mulching swelling clay (Black Vertosol), was investigated using a waveguide and network analyser technique in the frequency range 3.0 GHz to 4.5 GHz. Curves relating the real part of the relative permittivity to water content are presented and compared with the general Topp curve. The Chromosol generally followed the Topp curve, but the Ferrosol and Vertosol both had curves below the Topp curve. The Ferrosol showed a maximum horizontal offset of 0.05 m3/m3 from the Topp curve in the mid soil-water content range of 0.2–0.3 m3/m3 offset from the Topp curve of 0.10 m3/m3, with a maximum of 0.12 m3/m3 occurring at a soil water content of 0.4 m3/m3. Similar dielectric curves were obtained for the Chromosol and Vertosol using time domain reflectometry (TDR). With this method, the Chromosol showed very close agreement with the Topp curve, but the Vertosol again gave a curve below the Topp curve, similar to the one obtained using the waveguide and network analyser, but with a smaller maximum horizontal offset of 0.08 m3/m3. The difference between the waveguide and TDR Vertosol curves was mainly attributed to low bulk densities in the waveguide where packing was difficult. Some was also attributed to the wider spectrum of frequencies used by TDR. Use of the Topp curve for TDR measurements in the Vertosol would underestimate its water content by at least 0.06 m3/m3. These results are in good agreement with others obtained from similar soils. Deviations from the Topp curve are attributed to bound water associated with the clay particles and this depends on clay mineralogy and clay content. The presented calibration curves improve the accuracy of TDR measurements in these types of clay soils. A field comparison between water contents measured by TDR and gravimetric sampling in a similar Black Vertosol is presented. This calibration showed that soil water contents can be severely overestimated by using TDR with long probes and cables. This unexpected and opposite result is discussed in terms of attenuated high frequencies in the 15-m-long connecting cable used, errors in depth of probe placement, and changes in bulk density and DC conductivity.