L. D. Currie

Effect of phosphate fertiliser type on the accumulation and plant availability of cadmium in grassland soils

Cadmium (Cd), a potentially toxic heavy metal for humans and animals, accumulates in the liver and kidneys of older animals grazing New Zealand and Australian pastoral soils. Phosphorus (P) fertiliser is the major input of Cd into these farming systems. A study was conducted to evaluate the effects, over 10 years, of annual application (30 kg P ha−1 yr−1) of four forms of P fertilisers having different solubilities and Cd contents [41, 32, 10 and 5 μg Cd g−1 for North Carolina phosphate rock (NCPR), single superphosphate (SSP), diammonium phosphate (DAP) made from low Cd phosphate rocks and Jordan phosphate rock (JPR) respectively] on soil and herbage Cd concentrations. Ten years of fertiliser application caused a marked increase in surface soil Cd concentrations. Total soil Cd was significantly higher in SSP and NCPR treatments compared to control (no P fertiliser), JPR and DAP treatments in the 0–30 and 30–75 mm soil depths. Plant-available Cd (0.01 M CaCl2 extractable Cd) was higher in SSP treatments than in control and other fertiliser treatments. Chemical analysis of herbage samples showed that there was no significant difference in Cd concentration in pasture grasses between treatments in the second year of the trial but in the eighth and tenth year, plots fertilised with SSP and NCPR had significantly higher Cd in pasture grasses in most of the seasonal cuts compared to control, JPR and DAP. Cadmium recovery by both grasses and clover was less than 5% of Cd applied in fertiliser. Clover Cd concentration and yield were much lower than those for grass and therefore its contribution to pasture Cd uptake was very low (< 7%). A strong seasonal effect on grass Cd concentration, which is inversely related to pasture growth rate, was observed in all three sampling years — Cd concentration was highest during autumn and lowest in spring. Total Cd contents of the fertilisers and their rate of dissolution rather than soil pH [pH (H2O) at 30–75 mm depth of 5.39, 5.20, 5.11 and 5.36 for NCPR, SSP, DAP and JPR treatments respectively]influenced soil and herbage Cd. These results showed that the use of P fertilisers with low Cd contents will reduce herbage Cd levels and has the potential of reducing Cd levels in grazing animals and their products.

Assessment of the influence of phosphate fertilizers on the microbial activity of pasture soils

The objective of the present work was to examine the effects of phosphate fertilizers on the microbial activity of pasture soils. Various microbial characteristics were measured using soils from an existing long-term phosphate fertilizer field trial and a short-term incubation experiment. The measurements included basal respiration, substrate induced respiration, inhibition of substrate-induced respiration by streptomycin sulphate (fungal activity) and actidione (bacterial activity) and microbial biomass C. The long-term field trials was initiated during 1985 to examine the effectiveness of different sources of phosphate fertilizers (single superphosphate, North Carolina phosphate rock, partially acidulated North Carolina phosphate rock, and diammonium phosphate) on pasture yield. The incubation experiment was conducted for 8 weeks using the same soil and the sources of phosphate fertilizers used in the field trial. In the incubation experiment the fertilizer addition caused an initial decrease in basal and substrate-induced respiration but had no effect on total microbial biomass. The initial decline in basal and substrate-induced respiration with the fertilizer addition was restored within 8 weeks after incubation. In the field experiment the fertilizer addtion had no significant effect on basal respiration but increased substrate-induced respiration and microbial biomass C. The short-term and the long-term effects of phosphate fertilizer addition on the microbial characteristies of the soils are discussed in relation to its effects on pH, salt concentration, and the nutrient status of the soils.

Growth and chemical composition of legume‐based pasture irrigated with dairy farm effluent

Abstract Land treatment of farm effluent is becoming popular in many countries because this practice is considered less harmful to surface water and groundwater resources. However, the supply of large quantities of selective nutrients, such as nitrogen (N) and potassium (K), through effluent irrigation could affect the nutrient balance and the botanical composition of crops and pasture. The aim of the project was to investigate the influence of farm effluent irrigation on the growth and chemical composition of legume‐based pasture through a field experiment conducted at Massey University Dairy Unit, New Zealand. Three levels of effluent irrigation with N loading rates of 0,150, and 200 kg ha–1 were applied. Effluent irrigation at these rates also provided 0, 256, and 342 kg K ha–1. The effluent was irrigated either in the presence or absence of calcium (Ca—as gypsum), and magnesium (Mg—as epsom salt) addition. The effluent irrigation was applied once a month and the trial continued for 6 months. The dry matter (DM) yield, botanical composition, and nutrient concentration in the pasture were measured 1 month after each effluent irrigation. The pasture DM yield increased with increasing rate of effluent application. The DM response (kg DM kg–1 N applied) ranged from 4.1 to 7.2, slightly less than the values obtained for fertiliser N. The N and K concentrations in the pasture increased with increasing levels of effluent irrigation. Calcium and Mg fertiliser application had no effect on either pasture yield or the concentration of N or K in pasture. In the absence of Ca and Mg fertiliser application, the concentration of Ca and Mg in pasture decreased with increasing effluent irrigation. Application of Ca and Mg fertiliser increased the concentration of these elements in pasture at all levels of effluent application. Excessive uptake of K by pasture is likely to cause Ca and Mg deficiencies in grazing animals, leading to milk fever and grass staggers.

Application strategies for Sechura phosphate rock use on permanent pasture

At two phosphate (P) responsive sites in hill country the effectiveness of Sechura phosphate rock (SPR) as a direct application P fertilizer for permanent pasture was evaluated. Sechura was applied at two rates, in three different application strategies. The treatments were 16.7 and 50 kgP ha−1 annually, 25 and 75 kgP ha−1 biennially, and 50 and 150 kgP ha−1 triennially giving a total of 50 and 150 kgP ha−1, respectively, over three years. Single superphosphate (SSP) served as the standard P fertilizer. A comparison was also made between SPR and Chatham Rise phosphorite (CRP), another reactive PR. Total pasture and legume production and P uptake by pasture was measured with all fertilizer treatments over a three year period.In the year of application, SPR was as effective as SSP in stimulating total pasture and legume production and P uptake by pasture. This reflects the very reactive nature of this PR. In the second and third years of measurement, SPR did not show superior residual efffects to SSP. The ability of CRP to stimulate legume growth more than SPR in the second year following application demonstrates the danger of generalizing about the residual effects of reactive PR materials. Of the application strategies evaluated, a biennial appplication of 25 kgP ha−1 as SPR maintained legume growth at a higher level than a smaller (16.7 kgP ha−1) annual dressing. The biennial strategy also increased total pasture yield, in addition to legume production to a greater extent in the second and third years than a single (50 kgP ha−1) triennial application.

Potential value of phosphate compounds in enhancing immobilization and reducing bioavailability of mixed heavy metal contaminants in shooting range soil

Shooting range soils contain mixed heavy metal contaminants including lead (Pb), cadmium (Cd), and zinc (Zn). Phosphate (P) compounds have been used to immobilize these metals, particularly Pb, thereby reducing their bioavailability. However, research on immobilization of Pbs co-contaminants showed the relative importance of soluble and insoluble P compounds, which is critical in evaluating the overall success of in situ stabilization practice in the sustainable remediation of mixed heavy metal contaminated soils. Soluble synthetic P fertilizer (diammonium phosphate; DAP) and reactive (Sechura; SPR) and unreactive (Christmas Island; CPR) natural phosphate rocks (PR) were tested for Cd, Pb and Zn immobilization and later their mobility and bioavailability in a shooting range soil. The addition of P compounds resulted in the immobilization of Cd, Pb and Zn by 1.56-76.2%, 3.21-83.56%, and 2.31-74.6%, respectively. The reactive SPR significantly reduced Cd, Pb and Zn leaching while soluble DAP increased their leachate concentrations. The SPR reduced the bioaccumulation of Cd, Pb and Zn in earthworms by 7.13-23.4% and 14.3-54.6% in comparison with earthworms in the DAP and control treatment, respectively. Bioaccessible Cd, Pb and Zn concentrations as determined using a simplified bioaccessibility extraction test showed higher long-term stability of P-immobilized Pb and Zn than Cd. The differential effect of P-induced immobilization between P compounds and metals is due to the variation in the solubility characteristics of P compounds and nature of metal phosphate compounds formed. Therefore, Pb and Zn immobilization by P compounds is an effective long-term remediation strategy for mixed heavy metal contaminated soils.

Effect of serpentine rock and its acidulated products as magnesium fertilisers for pasture, compared with magnesium oxide and Epsom salts, on a Pumice Soil. 2. Dissolution and estimated leaching loss of fertiliser magnesium

Abstract The dissolution rate of magnesium (Mg) fertilisers controls their effectiveness in supplying Mg to plants and the potential for fertiliser‐Mg to be lost via leaching. Results from a field trial, conducted on pasture on an Immature Orthic Pumice Soil (pHwater 6.3) treated with different types of Mg fertilisers (100 kg Mg ha–1), showed that Mg dissolution over a 29‐month period differed, being 15–20% for serpentine rock products, 50–98% for acidulated serpentine products, 95% for E‐mag (magnesium oxide), and 98% for Epsom salts. The percentage dissolution of applied fertiliser‐Mg was related to the water solubilities for all the fertilisers except E‐mag, which had a high dissolution rate in soil but a very low solubility in water. However, E‐mag had high Mg solubility in citric acid, consistent with its dissolution rate in soil. Epsom salts, E‐mag, and acidulated serpentine products significantly increased exchangeable Mg in soil samples collected 9 and 29 months after fertiliser application, whereas the unacidulated serpentine rock increased exchangeable Mg only in soil samples collected after 29 months and only when it was re‐applied annually for 3 years. The recovery of fertiliser Mg in pasture herbage was positively related to the Mg dissolution rate over the duration of the trial, being 4–8% for serpentine rock products, 19–22% for acidulated serpentine products, 17% for E‐mag, and 25% for Epsom salts. For all fertilisers, except E‐mag, total recovery of fertiliser Mg in the soil (0–15 cm depth) and herbage combined was lower for fertilisers with the higher rates of Mg dissolution, being 51% for Epsom salts, 53–90% for acidulated serpentine products, 91–95% for serpentine rock products, and 90% for E‐mag. Fertiliser Mg not recovered was assumed to have been leached below the 0–15 cm soil depth (49% for Epsom salts, 10–47% for acidulated serpentine products, 5–9% for serpentine products, and 10% for E‐mag). The very high fertiliser Mg recoveries in soil (0–15 cm depth) and pasture herbage, and consequently low estimated fertiliser Mg leaching losses from the less water‐soluble fertilisers, suggests that these fertilisers have potential for supplying Mg to pasture over a prolonged period if the rate of fertiliser Mg dissolution does not appreciably slow down with time. However, re‐applications of these less soluble Mg fertilisers may be required on a regular basis to ensure that the supply of Mg is adequate for pasture growth and to meet stock requirements.