The effect of zinc fertilizer on take-all and the grain yield of wheat grown on zinc-deficient soils of the Esperance region,Western Australia

Wheat plants were grown in field experiments with five levels of zinc (Zn) fertilizer applied to plots in 1983. The plots were continuously cropped with wheat to allow the build up ofGaeumannomyces graminis var.tritici (Ggt). For experiments 1 and 2, there were high levels of Ggt in the second and third years while for experiment 3 there were high levels of Ggt incidence in the third and fourth year of continuous cropping. The Zn status of the wheat plants, grain yield, and the incidence and severity of take-all were measured for every experiment each year.The Zn-deficient wheat plants were more severely infected by Ggt. However, increasing the Zn supply beyond that required for maximum grain yield had no further effects on decreasing the severity of take-all. The Zn concentration in the youngest emerged blade (YEB) suggested that the Zn status of the wheat plant ranged from severely Zn-deficient through marginal deficiency to sufficiency.The Zn-deficient wheat plant was more susceptible to Ggt infection than Zn-adequate plants. The severity of take-all in the final year was still high in Zn-adequate plants, suggesting high levels of applied Zn (11.2 kg Zn/ha in 1983) had no fungistatic effect on Ggt.     

Mobilization of residual phosphate of different phosphate fertilizers in relation to pH in the rhizosphere of ryegrass

Only 10 to 20% of the P in fertilizers are utilized by crops in the year of application. The value of the remaining 90% to 80% for succeeding crops is uncertain. This paper is aimed at assessing the residual value of several P-fertilizers such as superphosphate (Super-P), Thomasphosphate (Thomas-P), Rhenaniaphosphate (Rhenania-P) and Hyperphos (Hyper-P) a ground rock phosphate. These fertilizers had been applied annually for ten years to supply a total of 520 kg P ha^–1 to a silt loam soil derived from loess. Fertilizer P accumulation compared to the unfertilized plots was 520 kg ha^–1 for Hyper-P and 410 kg ha^–1 for Super-P (4.2 and 3.3 mmol P kg^–1 soil). The residual value of this P was assessed by both conventional soil test procedures and P-depletion at the soil-root interface by ryegrass (Lolium perenne L.) supplied with either NO₃-N, NH₄-N or no N. The different N sources changed soil pH in the rhizosphere and thereby the solubility of P present in this region. To measure P depletion at the soil-root interface, ryegrass seedlings were grown on a soil block covered with a nylon screen, mesh 30µm. A dense root mat developed simulating a plane root surface. After 10 days of growth the soil block was sliced into 0.2 mm layers parallel to the root mat. These soil samples were analysed for P (4N HCl) giving P concentration as a function of distance from the root surface. Phosphorus depletion at the root surface, in mmol kg^–1, was 1.7 for the No-P and 3.2 for the Super-P treatment. Thomas-P and Rhenania-P were in between while for Hyper-P the depletion was only 1.0. This gave a residual effect of 47% for Super-P and a negative 15% for Hyper-P. Acidification of the rhizosphere due to NH₄-N supply had no effect on the residual effect of Super-P but increased that of Hyper-P to +18%. NH₄-N increased the residual effect of Thomas-P from 16 to 28% and of Rhenania-P from 9 to 37%. The supply of NO₃-N increased the rhizosphere pH and decreased the residual effect of Super-P but increased slightly the residual effect of the other P-fertilizers as compared to No-N application.The pattern of these changes of P depletion in the rhizosphere was similar for the No-P and the Super-P treatments suggesting the presence of similar P compounds in both cases. In contrast, Hyper-P apparently remained unchanged in soil, as Ca-P, if not mobilized by acidification. These results are supported by the soil tests. Reasons for the relatively low residual effect even of Super-P (only 47%) are discussed. It is concluded that, because of the low rate of P release from soil, the 10 days growth period of the plants was not long enough to include all P that could potentially be mobilized.     

Accounting for particle movement when assessing the dissolution of slow release fertilizers in field soils

When fine particulate, slow-release fertilizers are applied to pastures, earthworm activity can physically move the particles both horizontally and vertically in the soil. This physical movement needs to be accounted for when researchers attempt to determine the rate of fertilizer dissolution by measuring the quantity of undissolved fertilizer residue remaining in the soil. A hard igneous phosphate rock (Phalaborwa PR) and an inert chromite ore were used as tracers to follow the physical movement of the reactive PR (RPR), NCPR applied to grazed pasture on a Pallic Soil (Aeric Fragiaqualf) isolated in galvanised steel cylindrical cores (150 mm diameter and 100 mm height) with or without nylon mesh sleeve (63 µm) at the bottom. The soil cores were sampled after 113, 270 and 559 days and analysed for PR and chromite residues. Results from both tracers showed that there was a significant movement of particles laterally out of the cores and vertically below 40 mm soil depth. Phalaborwa PR particle movement to 40–60 mm soil depth was 8% in 112 days and 24% in 270 days. Between 270 and 559 days no significant movement of particles was observed. Particle movement appeared to occur in discrete non-continuous events that were associated with the major end-of-autumn peaks in earthworm activity. When corrected for particle movement, the measured rate of NCPR dissolution satisfactorily fitted the rate of dissolution predicted by two models of PR dissolution.     

A discussion of the methods for comparing the relative effectiveness of phosphate fertilizers varying in solubility

Because various phosphate (P) fertilizers differ widely in their solubility, it is commonly observed that crop response to P fertilizers varies under the same soil and crop conditions. Furthermore, a major problem encountered in the methods for determining the relative effectiveness (RE) of water-insoluble P fertilizer (e.g., phosphate rock) with respect to water-soluble P fertilizers, e.g., single superphosphate (SSP) and triple superphosphate (TSP), is that their growth response curves are usually nonlinear and often do not share a common maximum yield. In this paper, we review and discuss the advantages and disadvantages of the three most commonly used methods for calculating the RE of phosphate rock with respect to TSP (or SSP). The three methods are vertical comparison, horizontal (substitution rate) comparison, and linear-response comparison.     

Dissolution of phosphate rocks in soils. 2. Effect of pH on the dissolution and plant availability of phosphate rock in soil with pH dependent charge

The effect of soil pH on the dissolution of phosphate rocks (PRs) and the subsequent availability of the dissolved inorganic phosphorus (Pi) to plants was examined in a volcanic soil adjusted to different pH values. Potassium dihydrogen orthophosphate (KH₂PO₄) and three PRs, Nauru (NPR), Jordan (JPR) and North Carolina (NCPR) were incubated with the pH-amended soils at a rate of 800µg P g^–1 soil for 84 days. The extent of PR dissolution was determined by measuring the increases in the amount of 0.5 M NaOH extractable Pi (NaOH-P) in the PR treated soil over the control soil. The amount of plant available P was measured either by extracting with 0.5 M NaHCO₃ or by growing ryegrass in soil samples incubated with the phosphate sources.At each pH the order of the extent of PR dissolution followed NCPR > JPR > NPR, which was consistent with the decreasing order of their chemical reactivities. As the pH decreased from 6.5 to 3.9 the dissolution of PRs increased from 29.3% to 83.5%, from 18.2% to 78.9%, and from 12.5% to 60.3% for NCPR, JPR and NPR, respectively. In contrast, as the soil pH decreased from 6.5 to 3.9, the proportion of the dissolved P extracted by 0.5 M NAHCO₃ decreased from 38% to 5% and the proportion taken up by ryegrass plants decreased from 46% to 7%. This decrease in plant available P corresponded to an increase in the adsorption of inorganic P with a decrease in pH. However, the uptake of P from PR relative to that from KH₂PO₄ was higher at low pH than at high pH. Further, the amount of P taken up by plants was more closely related to the amount of NaHCO₃ extractable P than to the amount of dissolved P present in the soil.     

Selenium concentration in spring wheat as influenced by basal application and top dressing of selenium-enriched fertilizers

A multisite field experiment was conducted to study the effect of topdressed Se-enriched Ca(NO₃)₂ (CN) and basal applied NPK on the selenium (Se) concentration in spring wheat (Triticum aestivum L.). Selenium was applied either through CN (at the rates of 0, 6.45, and 12.91 g Se ha^–1) or NPK (5.83 g Se ha^–1). Selenium concentration in wheat grains increased consistently with increasing rate of Se-enriched CN or NPK. However, the superiority of Se-enriched CN over NPK in raising the Se concentration in wheat grain depended on location and growth conditions. At the same rate both methods of Se-application were found to be equally effective in raising the Se concentration of wheat grains. The Se concentration of grain was generally higher in the light textured soils than in the medium to heavy textured soils. Without Se application, the Se-concentration in wheat grain was about 16µg kg^–1 which is regarded insufficient to meet the Se requirement for Se in animal and human. Calcium nitrate enriched with 25 mg Se kg^–1 (6.45 g Se ha^–1) increased the Se concentration in wheat grain to a desired level.     

Recovery of 15N-ammonium-15N-nitrate in spring wheat as affected by placement geometry of the fertilizer band

The time course of crop ¹⁵N recovery as affected by placement geometry of nitrogen fertilizer was studied in a field experiment. In frames of30 × 40 cm¹⁵N-ammonium-¹⁵N-nitrate was applied in bands parallel to a single row of growing spring wheat. The fertilizer was banded in nine treatments to a depth of 1.5, 5 or 10 cm combined with a distance from the crop row of 1, 5, 10 or 15 cm, or broad spread on the soil surface. The crop recovery of applied ¹⁵N was calculated on each of 9 sampling dates during the elongation phase. A sigmoid growth function was fitted, and the estimated parameters were analysed statistically. The maximum uptake rate was5.5–6kg N ha⁻¹ day⁻¹, and during an almost linear uptake phase of 7 days the crop recovered 68%of the maximum crop ¹⁵N recovery. Neither the maximum uptake rate nor the maximum crop ¹⁵N recovery was significantly affected by the treatments, whereas the start of the linear uptake phase was affected. By fertilizer placement at 5 cm depth the course of ¹⁵N uptake was delayed 0.5 day cm⁻¹ increase in distance from the crop row. Uptake of ammonium nitrate placed on the soil surface or at a depth of 1.5 cm was delayed approximately 3 days compared to banding at 5 cm depth. This delay corresponded to the time until the first precipitation event. Maximum crop ¹⁵N recovery was obtained before anthesis and 20% of the recovered ¹⁵N was lost during the grain-filling period. In conclusion, the uptake rate of applied nitrogen was unaffected by placement geometry. However, the uptake course of applied nitrogen was delayed both by shallow injection and by increased distance between the crop row and the fertilizer band. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of time of application on the effectiveness of zinc sulphate and zinc oxide as sources of zinc for wheat

Field experiments with wheat were conducted for two years on flood plain alluvial soils to study the effectiveness of soil application of zinc sulphate and zinc oxide at 0, 15, 45, 60, 75 and 90 days after sowing. Yield and zinc uptake of wheat increased significantly with the application of zinc. Delaying the application of both zinc sulphate and zinc oxide up to 45 days of sowing did not adversly affect the zinc nutrition of wheat. However, delaying the application for 75 or 90 days after sowing eliminated the response. Zinc sulphate, when applied within 60 days of sowing performed better than zinc oxide. In a laboratory study, zinc sulphate maintained a higher level of zinc in the soil solution than zinc oxide at least over a 3-week period.     

Evaluation of levels and methods of zinc application to rice in sodic soils

Field experiments were conducted in zinc-deficient sodic soil to study the effect of levels and methods of zinc fertilization on yield, concentration and uptake of zinc by rice. Zinc was incorporated in the soil at the rate of 0, 5.6, 11.2 and 22.4 kg Zn per ha as zinc sulfate; sprayed on the plants at 1% and 2% zinc sulfate solution; and roots of rice seedlings were dipped in 2% and 4% ZnO suspensions in water. Grain yield, zinc content and its uptake increased in all the experiments up to 22.4 kg Zn per ha. Soil applied zinc was significantly correlated with yield of rice (r = 0.80^**) and zinc uptake (r = 0.89^**). Zinc content in 45-day old plants gave a significantly higher correlation with grain yield (r = 0.84^**) than the zinc content of rice straw and grain at maturity. Roots of rice seedlings dipped in 2% or 4% zinc oxide suspension in water were not only comparable with soil application of Zn at 5.6 and 11.2 kg Zn per ha, but also proved to be more economical for sodic soils showing moderate zinc deficiency.     

Cadmium concentration in vegetable crops grown in a sandy soil as affected by Cd levels in fertilizer and soil pH

Field trials were conducted over a three-year period with chinese cabbage (Brassica pekinensis Rupr.) and carrots (Daucus carota L.) grown in a sandy soil with pH adjusted to 5.5 and 6.5. The NPK fertilizers containing 1, 30, 90, and 400 mg Cd kg^–1 P were applied at the rate of 0.07, 2.1, 6.3 and 28 g Cd ha^–1 yr^–1. The amounts of Cd added through phosphate rock also ranged between 0.1 and 28 g ha^–1 yr^–1. The increased Cd application rates through NPK fertilizers increased the Cd concentration in both vegetables but the differences among treatments were not found to be significant. The Cd uptake by both crops was significantly (p<0.01) higher at pH 5.5 than at pH 6.5. Chinese cabbage exhibited lower Cd concentration than carrots. Carrot leaves contained higher Cd than its roots. Cadmium removals by chinese cabbage and carrot were about 0.7 and 1.3 g ha^–1 yr^–1, respectively. At pH 5.5, Cd concentrations in the two crops, based on a three-year average, were 23 and 46% higher than at pH 6.5. Cadmium uptake by chinese cabbage from different sources of phosphate rock was affected to a very limited extent. Cadmium concentration generally increased over the years. Cadmium extracted by ammonium nitrate after harvest of the crops was closely related with soil pH and Cd concentration in the plants.     

Effects of different phosphate fertilizers on yield of barley and rape seed on reddish brown soils of the Ethiopian highlands

The effectiveness of six phosphorus sources at 4 rates were tested for two seasons on reddish brown soil at Holetta, Ethiopia, using barley (Hordeum vulgare L.) and rape (Brassica napus L.) as test crops. The fertilizer sources include: basic slag (BS), bone meal (BM), Ethiopian rock phosphate (ERP), Gafsa rock phosphate (GRP), triple superphosphate (TSP) and mixture of TSP and GRP in the ratio 1:4 (MIX). Yield, P uptake by both crops as well as available soil P showed a marked response to the application of the various P sources. On continuously cropped field, grain yield increase over the unfertilized plot ranged from 2.5 to 16.4 dt ha^–1 for barley and rape respectively. On newly cleared field no significant effects of the different P sources on barley were observed. On the other hand for rape, a grain yield increase over the unfertilized plot ranging from 10.6 to 17.8 dt ha^–1 was recorded. The highest agronomic effectiveness relative to TSP (RAE) for both crops was obtained with BS. Rape was found to utilize P not only from the reactive rock phosphate (RP) but also from the unreactive one, which had a total P content of only 3% and 0.4% ammonium citrate soluble P. Barley, on the contrary, could not utilize P from this magmatic rock phosphate and failed to grow.     

Effect of potassic and phosphatic fertilizer type, fertilizer Cd concentration and zinc rate on cadmium uptake by potatoes

In some areas of southern Australia, cadmium (Cd) concentrations in excess of the Australian maximum permitted concentration (0.05 mg kg^–1 fresh weight) have been found in tubers of commercially grown potato (Solanum tuberosum L.) crops. Field experiments were therefore conducted in various regions of Australia to determine if Cd uptake by potatoes could be minimised by changes in either phosphorus (P), potassium (K) or zinc (Zn) fertilizer management.Changing the chemical form in which either P fertilizer (monoammonium phosphate, diammonium phosphate, single superphosphate and reactive rock phosphate) or K fertilizer (potassium chloride and potassium sulfate) were added to crops had little influence on tuber Cd concentrations. Fertilizer Cd concentrations also had little influence on tuber Cd concentrations, suggesting that residual Cd in the soil was a major contributor to Cd uptake by the crops on these soils.Addition of Zn at planting (up to 100 kg Zn ha^–1) significantly reduced tuber Cd concentrations at four of the five sites studied. However, the largest variation was between sites rather than between treatments, with site mean tuber Cd concentrations varying tenfold (from 0.018 to 0.177 mg Cd kg^–1 fresh weight). Factors associated with irrigation water quality at the sites, in particular the chloride concentration, appeared to dominate any effects of changing fertilizer type or Cd concentration.     

Effect of organic and inorganic phosphate fertilizers and their combination on maize yield and phosphorus availability in a Yellow Earth in Myanmar

Phosphorus (P) deficiency is a major constraint for crop production in many parts of the world including Myanmar and field research into management of P fertilizers and P responsiveness of crops on infertile soils has been limited. The purpose of this study is to determine maize yield response to different forms of P fertilizers on an acidic (pH 4.9) P deficient (Olsen-P 8 mg kg⁻¹) Yellow Earth (Acrisol) in Southern Shan State, Myanmar and to establish relationships between soil Olsen-P test values (0.5 M sodium bicarbonate extracted P) and maize yield. Field experiments were conducted during two cropping seasons. There were 15 treatments in total: P was applied at seven rates of a soluble P fertilizer as Triple superphosphate (TSP) (0–120 kg P ha⁻¹) to establish a P response curve; one rate of a partially soluble P fertilizer (Chinese partially acidulated phosphate rock, CPAPR) and two organic P fertilizers (farmyard manure (FYM) and Tithonia diversifolia) at 20 kg P ha⁻¹; combination of TSP and CPAPR at 20 kg P ha⁻¹ with FYM and Tithonia at 20 kg P ha⁻¹; an additional treatment (TSP 20 kg P ha⁻¹ plus 2.5 t ha⁻¹ dolomite) for assessing the liming effect of a local dolomite. In Year 1, applications of TSP at 40–60 kg P ha⁻¹ produced near maximum grain yields, whereas in Year 2 this could be achieved with a reapplication of 20–30 kg P ha⁻¹ on top of the residual value of the Year 1 application. In both years, CPAPR, TSP and Tithonia at 20 kg P ha⁻¹ significantly increased maize grain yield, but FYM failed to increase grain yield. In Year 1, CPAPR and TSP effects on grain yield were higher than that of Tithonia but in Year 2 the effects were same for all these three treatments. In both years the combination of FYM (20 kg P ha⁻¹) with TSP (20 kg P ha⁻¹) produced significantly higher grain yield than TSP at 20 kg P ha⁻¹ whereas 40 kg P ha⁻¹ of TSP application did not significantly increase grain yield over the TSP application at 20 kg P ha⁻¹. Similar results were obtained when half the P applied as CPAPR was substituted with P from Tithonia and FMP during the first year. The combined data from the two years experiment suggests that 90% of maximum maize grain yields can be obtained by raising the Olsen-P to 30–35 mg P ha⁻¹ soil at the silking stage of growth. Olsen-P for the treatments at silking in Year 1 was: Control < FYM, Tithonia < TSP, CPAPR and in Year 2 was: Control < FYM < Tithonia < TSP, CPAPR. The results showed that for a long-term approach, repeated annual applications of Tithonia can be considered as a potential P source for improving soil P status in P deficient Yellow Earths.     

Nitrogen losses from fertilizers applied to maize, wheat and rice in the North China Plain

Ammonia volatilization, denitrification loss and total nitrogen (N) loss (unaccounted-for N) have been investigated from N fertilizer applied to a calcareous sandy loam fluvo-aquic soil at Fengqiu in the North China Plain. Ammonia volatilization was measured by the micrometeorological mass balance method, denitrification by the acetylene inhibition – soil core incubation technique, and total N loss by ¹⁵N-balance technique. Ammonia loss was an important pathway of N loss from N fertilizer applied to rice (30–39% of the applied N) and maize (11–48%), but less so for wheat (1–20%). The amounts of unaccounted-for fertilizer N were in the order of rice > maize > wheat. Deep placement greatly reduced ammonia volatilization and total N loss. Temperature, wind speed, and solar radiation (particular for rice), and source of N fertilizer also affect extent and pattern of ammonia loss. Denitrification (its major gas products are N₂ and N₂O) usually was not a significant pathway of N loss from N fertilizer applied to maize and wheat. The amount of N₂O emission (N₂O is an intermediate product from both nitrification and denitrification) was comparable to denitrification loss for maize and wheat, and it was not significant in the economy of fertilizer N in agronomical terms, but it is of great concern for the environment.     

N2O and NO emissions from a field of Chinese cabbage as influenced by band application of urea or controlled-release urea fertilizers

A field experiment was conducted in an Andosol in Tsukuba, Japan to study the effect of banded fertilizer applications or reduced rate of fertilizer N (20% less) on emissions of nitrous oxide (N₂O) and nitric oxide (NO), and also crop yields of Chinese cabbage during the growing season in 2000. Six treatments were applied by randomized design with three replications, which were; no N fertilizer (CK); broadcast application of urea (BC); band application of urea (B); band application of urea at a rate 20% lower than B (BL); band application of controlled-release urea (CB) and band application of controlled-release urea at a rate 20% lower than CB (CBL). The results showed that reduced application rates, applied in bands, of both urea (BL) and controlled-release urea fertilizer (CBL) produced yields that were not significantly lower than yields from the full rate of broadcast urea (BC). The emissions of N₂O and NO from the reduced fertilizer treatments (BL, CBL) were lower than that of normal fertilizer rates (B, CB). N₂O and NO emissions from controlled-release urea applied in band mode (CB, CBL) were less than those from urea applied in band mode (B, BL). The total emissions of N₂O and NO indicated that applying fertilizers in band mode mitigated NO emission from soils, but N₂O emissions from banded urea (B) were no lower than from broadcast urea (BC).     

Wheat growth from phosphorus fertilizers as affected by time and method of application in soil with an acidic subsurface layer

Using soils with an acidic subsurface layer, three glasshouse experiments were carried out to evaluate the effect of placement method and application rate of triple superphosphate (TSP) and North Carolina phosphate rock (NCPR) on dry matter (DM) yields. Time of application of NCPR on DM yield response of wheat was also studied.For Experiment 1, soil was collected in depth intervals of 0–2; 4–6; 6–8; and 8–10 cm from a red earth (chromic luvisol). The treatments included two P sources (TSP and NCPR), three placement methods (broadcast, banded or mixed into the subsurface layer, 6–8 cm), and six application rates. In this P deficient soil with an acidic subsurface layer, there was relatively little effect of application method of TSP on wheat yield responses. The maximum dry matter yield responses for broadcast, band and mix application methods was 30, 42 and 50 %, respectively. Responses to NCPR broadcast, band and mix methods were 20, 9 and 44 %, respectively. Mixing NCPR into to acidic subsurface layer produced yields similar to those from TSP although a higher application rate of P as NCPR was needed to achieve this outcome.Treatments for Experiments 2 and 3 were time of application of NCPR (0 and 30 days before sowing) and rate of application of NCPR (0 and 40 mg P/pot). In Experiment 2 (same soil as Experiment 1) application of NCPR prior to sowing, resulted in higher Colwell P concentration than when applied at sowing, but time of application had no effect on final DM yields. Experiment 3 used a red podzolic (chromic luvisol) soil which had a lower P-status, was more acid and had a lower exchangeable Ca²⁺ concentration than the red earth. Application of NCPR prior to sowing resulted in lower DM yield than when it was applied prior to sowing.     

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 g 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.     

Influence of rates of reactive phosphate rock and sulphur on potentially available phosphorous in organically managed soils in the south-eastern near-Mediterranean cropping region of Australia

Reactive phosphate rock (RPR) is the only phosphorous (P) fertiliser allowed for organically managed, broad-acre crop-pasture systems in southern Australia. However, soils are usually deficient in P, and the soils, climate, and plant species grown, do not promote extensive dissolution of RPR so the fertiliser is poorly effective for crop and pasture production. Biological oxidation of elemental sulphur (S) mixed and applied with RPR may sufficiently increase dissolution of P from RPR to improve its effectiveness as a P fertiliser. However, this needs to be confirmed in field studies in the region. Rates of RPR and S required to optimise dissolution of RPR are not known for the soils, environments, and agricultural systems used. Both pot and field studies showed that mixing RPR and S, and incorporating the mix into soil (top 10 cm for field studies), significantly increased Olsen P and soil solution P, even in strongly acidic soils (pH_Ca < 4.6). In general, Olsen P increased linearly with the applied rate of P up to 42–70 kg P ha⁻¹ and the rate of change in Olsen P per unit of applied P increased with the applied rate of S up to 400 kg S ha⁻¹. This interaction suggested that the effectiveness of RPR + S may be compromised by segregation of RPR and S. In addition, there was evidence that S application may not necessarily create a more acidic soil environment necessary for enhanced dissolution of RPR.