Ll Burkitt

A simple phosphorus buffering index for Australian soils

Soil phosphorus (P) buffering capacity (PBC) is an important soil property that influences the amount of P fertiliser available for plant uptake. However, current methods of determining PBC are time-consuming and uneconomic in most commercial soil testing programs. The current study examined simpler methods of measuring the PBC of a wide range of Australian soils. Phosphorus sorption and extractable P data from 290 soils (initial data set) were collated to define the range of PBC values of Australian agricultural soils. Independently, detailed chemical and physical analyses were undertaken on a second set of 90 agricultural soils (principal data set), which were selected to represent the range of soil properties measured on the initial data set. Relationships between PBCO&S (Ozanne and Shaw 1968) values (P sorbed between solution P concentrations of 0.25 and 0.35 mg P/L) and 11 different single-point P sorption indices and selected soil properties were examined for the principal data set. Whilst relationships between PBCO&S values and selected soil properties such as oxalate-extractable iron and aluminium, and clay content, were generally poor, strong relationships existed between all of the single-point P sorption indices and PBCO&S. Results suggest that PBCO&S values were most closely related to the P buffering indices (PBI+ColP and PBI+OlsP) when a single addition of 1000 mg P/kg was added to soil and either the Colwell or 4.59 × Olsen extractable P were added to the amount of P sorbed:

Comparing irrigated biodynamic and conventionally managed dairy farms. 1. Soil and pasture properties

Ten paired irrigated dairy farms under biodynamic (BD) and conventional (CV) management were compared over a 4-year period (1991–94). The paired farms were located in the irrigation districts of northern Victoria and southern New South Wales and were matched for soil type, climate, cattle breed and farm area. Farms had been practising BD principles for an average of 16 years before the commencement of the study and had not received phosphorus (P) fertiliser for an average of 17 years. The effects of farm management on soil chemical and biological properties and the nutritive properties and botanical composition of pasture were examined at varying sampling times during the study. Soil Olsen extractable P concentrations were consistently 2–3 times higher under CV management at various sampling depths (mean = 22 mg/kg, 0–10 cm), and were generally marginal under BD management in the surface 10 cm (mean = 8.5 mg/kg). Low soil extractable P concentrations were also reflected in consistently lower mean pasture P concentrations under BD management (0.25 compared with 0.35% on CV farms). Lower soil and pasture P concentrations under BD management were the result of a large negative P balance across BD farms (–17 kg P/ha.year). A mean negative P balance under BD management was a result of low P imports (2 kg P/ha.year) in comparison with large quantities of P (19 kg P/ha.year) effectively lost from the farming system through animal products, estimated losses in water runoff and slowly reversible soil P reactions. These results suggest that greater P imports are required to ensure the future sustainability of BD dairy pasture farming systems. There were few differences in soil biological properties, with earthworm weights significantly higher under CV management, but no difference in soil organic carbon, humus concentration, the weight of the organic mat or microbial biomass, between the two management systems.

Soil phosphorus buffering measures should not be adjusted for current phosphorus fertility

Soil phosphorus (P) sorption is an important and relatively stable soil property which dictates the equilibrium between sorbed and solution P. Soil P sorption measures are commonly adjusted for the effect of current P fertility on the amount of P a soil sorbs. In the case of highly fertilised agricultural soils, however, this adjustment is likely to be inappropriate as it may mask changes in a soils capacity to sorb P, which could affect future P fertiliser applications. A study was undertaken to compare adjusted or unadjusted methods of measuring P sorption using 9 pasture soils sampled from southern Victoria which had previously received P fertiliser and lime. The P sorption assessment methods included: P sorption isotherms, P-buffering capacity (PBC) measures (slope between equilibrium P concentration of 0.25 and 0.35mgP/L), and single-point P-buffering indices (PBI), with methods either adjusted or unadjusted for current P fertility. A single application of 280kgP/ha, 6 months before sampling, resulted in a general negative displacement of unadjusted P sorption isotherm curves, indicating reduced P sorption on 8 of the 9 soils. Adding the Colwell extractable P concentration to the amount of P sorbed before calculating the slope (PBC+ColP), tended to negate this fertiliser effect and, in 2 of the 9 soils, resulted in a significant increase in PBC+ColPvalues. Increasing rates of P fertiliser application (up to 280kgP/ha)resulted ina consistent trend todecreasingPBI values (unadjusted for ColwellP), whichwas significantat 4of the 9 sites after 6 months. However, only minimal changes in PBI values were determined when PBI was adjusted for current P fertility (PBI+ColP). Phosphorus sorption properties appeared reasonably stable over time, although 2 soils, both Ferrosols, indicated significant linear increases in PBI values when these sites remained unfertilised for 30 months. Lime significantly increased both PBI and PBI+ColP values at all sites 6 months after application, but the effect generally diminished after 30 months, suggesting PBI measurements should not be taken immediately after liming. These results demonstrate that unadjusted measures of P sorption are more likely to accurately reflect changes in soil P sorption capacity following P fertiliser applications and suggest that the unadjusted PBI be used in commercial soil testing rather that the currently adjusted PBI+ColP.

Changes in bicarbonate-extractable phosphorus over time when P fertiliser was withheld or reapplied to pasture soils

Field studies were established on 9 different soil types used for pasture production in the high rainfall zones of southern Victoria. Sites were selected to represent a range of phosphorus (P) buffering capacities (PBC) and were analysed for a series of chemical and physical properties before receiving P fertiliser treatments. A single application of P fertiliser in the form of triple superphosphate (TSP), single superphosphate (SSP), or TSP and lime (5 t/ha) was applied at amounts ranging from 0 to 280 kg P/ha at the start of the experiment, whilst treatments of 35 and 70 kg P/ha were reapplied at 6-monthly intervals. Soils were analysed for bicarbonate-extractable P concentration, using both the Olsen P and Colwell P methods, 6, 12, 18, 24, and 30 months after P fertiliser was applied. A strong positive linear relationship existed at all sites and time periods between the amounts of P applied as a single application and both the Olsen P and Colwell P concentrations. The slopes of these relationships measured the change in extractable P concentration per unit of P applied (ΔEP) and the rate of decline in ΔEP values represented the decline in the effectiveness of the P fertiliser with increasing time from application. The decline in these ΔEP values varied with soil type. The ΔEP values of some low to moderate P buffered soils remained 2–3 times higher compared with the most highly buffered soils, after 30 months. Despite this, the decline in ΔEP values between 6 and 30 months was difficult to predict using a single soil property. Multiple linear regressions involving a measure of PBC and either organic carbon or exchangeable hydrogen were useful methods of predicting the decline in ΔEP values across the 9 field sites examined in this study. In general, the reapplication of P fertiliser every 6 months resulted in significantly higher extractable P concentrations compared with the same rates applied in a single application, across all sites and rates of P applied. The application of SSP, TSP, and TSP and lime had little impact on ΔEP values 18–30 months after treatments were applied.

Relationship between phosphorus concentration in surface runoff and a novel soil phosphorus test procedure (DGT) under simulated rainfall

There is a need to be able to identify soils with the potential to generate high concentrations of phosphorus (P) in runoff, and a need to predict these concentrations for modelling and risk-assessment purposes. Attempts to use agronomic soil tests such as Colwell P for such purposes have met with limited success. In this research, we examined the relationships between a novel soil P test (diffuse gradients in thin films, DGT), Colwell P, P buffering index (PBI), and runoff P concentrations. Soils were collected from six sites with a diverse range of soil P buffering properties, incubated for 9 months with a wide range of P additions, and then subjected to rainfall simulation in repacked trays growing pasture. For all soil and P treatment combinations, the relationship between DGT (0–10 mm) and runoff P was highly significant (P < 0.001, r2 = 0.84). Although there were significant curvilinear relationships between Colwell P and runoff P for individual soils, there were large differences in these relationships between soils. However, the inclusion of a P buffering measure (PBI) as an explanatory variable resulted in a highly significant model (P < 0.001, R2 = 0.82) that explained between-soil variability. We conclude that either DGT, or Colwell P and PBI, can be used to provide a relative measure of runoff P concentration.

Factors affecting the change in extractable phosphorus following the application of phosphatic fertiliser on pasture soils in southern Victoria

Nine pasture soils from high rainfall zones of southern Victoria were analysed for a range of chemical and physical properties before receiving a single application of P fertiliser in the form of triple superphosphate (TSP), single superphosphate (SSP), or TSP and lime (5 t/ha) at amounts ranging from 0 to 280 kg P/ha. Soils were analysed for bicarbonate-extractable P concentration, using both the Olsen P and Colwell P methods, 6 and 12 months after fertiliser application. A strong positive linear relationship existed at all sites between the amount of P applied and both the Olsen P and Colwell P concentrations. The slopes of these relationships measured the change in extractable P concentration (∆EP) per unit of P applied, whilst the inverse of the ∆EP value indicated the amount of P fertiliser required above maintenance to increase the extractable P concentration by 1 mg/kg. These values ranged from 5 to 15 kg P/ha, depending on soil type. The ∆EP measured by the Olsen (∆EPOlsen) method was closely related to selected soil properties and P sorption measures, whilst the ∆EPColwell values were also closely related to selected soil properties and P sorption measures, but only when one particular site, an acidic sand, with a high organic carbon content was excluded from the analysis. In general, simple, direct measures of soil P sorption could allow the estimation of ∆EP values on different soil types. The application of P in the form of SSP resulted in a trend for higher ∆EP values than occurred with TSP. This difference was significant on 3 sites (P < 0.05), but depended on the method of extraction and the time after fertiliser application. The application of lime significantly (P < 0.001) increased soil pH (H2O and CaCl2) and decreased the concentration of exchangeable Al, 6 months after treatments were applied, but generally had little impact on ∆EP values.

Low rates of phosphorus fertiliser applied strategically throughout the growing season under rain-fed conditions did not affect dry matter production of perennial ryegrass (Lolium perenne L.)

Pasture is the cheapest source of feed for dairy cows, therefore, dairy pastures in Australia are intensively managed to maximise milk production and profits. Although soil testing commonly suggests that soils used for dairy pasture production have adequate supplies of phosphorus (P), many Australian dairy farmers still apply fertiliser P, often by applying smaller rates more frequently throughout the year. This study was designed to test the hypotheses that more frequent, but lower rates of P fertiliser applied strategically throughout the growing season have no effect on dry matter production and P concentration in perennial ryegrass (Lolium perenne L.), when soil extractable P concentrations are above the critical value reported in the literature. Three field sites were established on rain-fed dairy pasture soils ranging in P sorption capacity and with adequate soil P concentrations for maximising pasture production. Results showed that applied P fertiliser had no effect on pasture production across the 3 sites (P > 0.05), regardless of rate or the season in which the P was applied, confirming that no P fertiliser is required when soil extractable P concentrations are adequate. This finding challenges the viability of the current industry practice. In addition, applying P fertiliser as a single annual application in summer did not compromise pasture production at any of the 3 sites (P > 0.05), which supports the current environmental recommendations of applying P during drier conditions, when the risk of surface P runoff is generally lower. The current results also demonstrate that the short-term cessation of P fertiliser application may be a viable management option, as a minimal reduction in pasture production was measured over the experimental period.

Effect of variable soil phosphorus on phosphorus concentrations in simulated surface runoff under intensive dairy pastures

Intensive dairy operations in Australia regularly apply P fertiliser to maintain productive pasture species. However, extractable soil test P (STP) concentrations in this industry commonly exceed those required to maximise pasture production, a situation which can increase the risk of P loss to surrounding waterways. The current study examined relationships between STP (Olsen P and CaCl2 P) and surface runoff P concentrations from a red silty loam (Ferrosol), commonly used for pasture production in south-eastern Australia. Soil was mixed and re-packed into soil trays and a rainfall simulator was used to generate surface runoff. A wide range of soil Olsen P concentrations (0–20 mm, 15–724 mg/kg; 0–100 mm, 9–166 mg/kg) was created by surface-applying a range of P fertiliser rates 8 months before the rainfall simulations. A comparison of the 2 STP methods suggests that Australian soils have higher labile P concentrations for given Olsen P concentrations compared with those measured internationally, suggesting a greater likelihood of P loss in runoff. Furthermore, significant curvilinear relationships between STP and dissolved reactive P (DRP <0.45 µm) in surface runoff for both Olsen P depths (0–20 mm, r2 = 0.94; 0–100 mm, r2 = 0.91; P < 0.01) were determined, as well as significant linear relationships between DRP and both CaCl2 depths (0–20 mm, r2 = 0.83; 0–100 mm, r2 = 0.92; P < 0.01). This confirmed that the concentrations of P in surface runoff increased with increasing STP, providing further evidence of an urgent need to reduce excessive STP concentrations, to reduce the risk of P loss to the environment.

A review of nitrogen losses due to leaching and surface runoff under intensive pasture management in Australia

This paper reviews the literature on nitrate leaching and nitrogen (N) runoff under intensive dairy pasture systems in Australia and draws comparisons with research undertaken under similar climates and farming systems internationally, with the aim to inform future research in this area. An Australian nitrate-leaching study suggests that annual nitrate-leaching loads are lower (3.7–14.5 kg N ha–1 year–1 for nil N and 6–22 kg N ha–1 year–1 for 200 kg N ha–1 applied) than the range previously measured and modelled on New Zealand dairy farms (~18–110 kg N ha–1 year–1). It is likely that nitrate-leaching rates are higher in New Zealand because of the prevalence of free-draining soils and higher average stocking rates. However, this review highlights that there are insufficient Australian nitrate-leaching data, particularly following urine application, to undertake a rigorous comparison. Median N surpluses on Australian dairy farms are higher (198 kg N ha–1) than values for an average New Zealand farm (135 kg N ha–1). Given the facts that many soils used for intensive pasture production in Australia are lightly textured or free-draining clay loams receiving average rainfall of >800 mm year–1, that herd sizes have risen in the last 10 years and that water quality is a concern in some dairy catchments, nitrate leaching could be an issue for the Australian dairy industry. Australian data on surface runoff of N are more available, despite its overall contribution to N losses being low (generally <5 kg N ha–1 year–1), except under border-check flood irrigation or hump-and-hollow surface drainage (3–23 kg N ha–1 year–1). More research is needed to quantify surface N runoff and leaching following effluent application and to examine dissolved organic forms of N loss, particularly in view of the continued intensification of the Australian dairy industry.

Phosphorus auditing cannot account for all of the phosphorus applied to different pasture soils

Five field sites established in the high rainfall zone of southern Victoria were used to examine the downwards vertical movement of phosphorus (P) fertiliser on soils which ranged in P sorption capacity. Fertiliser was applied either as a single application of 280 kg P/ha at the beginning of the experiment (April 1998), or as 35 kg P/ha reapplied every 6 months (totalling 210 kg P/ha by the end of the third year). Soil cores were sampled in June 2001 to a depth of 40 cm, and soil at depths of 0-5, 5-10, 10-20, 20-30, and 30-40 cm was analysed for a range of soil properties and total P concentration. Total P concentration changed very little down the profile, indicating that there was minimal vertical movement of P fertiliser below the 10 cm layer of 5 pasture soils following the single application of 280 kg P/ha or 35 kg P/ha reapplied every 6 months. Soils with low to moderate surface P sorption capacity showed a trend for higher total P concentrations at depth. However, quantitative relationships between vertical P movement and soil properties at depth were poor. A P audit resulted in variable recovery of the applied P (45-128%) in the surface 40 cm at each of the 5 sites. Consistently low P recoveries were achieved at one site, where the surface soil had a high P sorption capacity. Some applied P may have bypassed the high P sorbing surface layers at this site through macropore flow and moved beyond the 40 cm sampling zone, or have been lost to surface runoff. These results question the usefulness of P audit or mass-balance methods for accounting for P movement in a pasture-based system, as spatial heterogeneity of soil properties, both horizontally and vertically, was high in the current study.