P. E. H. Gregg

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.

Development and evaluation of an improved soil test for phosphorus. 2. Comparison of the Olsen and mixed cation-anion exchange resin tests for predicting the yield of ryegrass grown in pots

A glasshouse experiment was conducted on four soils contrasting in P sorption capacity and exchangeable Ca content with perennial ryegrass using six phosphate rock (PR) sources and a soluble P source applied at four rates (including a control). After three harvests (11 weeks) replicate pots of each treatment were destructively sampled and Olsen P and mixed cation-anion exchange resin (Resin P) extractions carried out. The remaining replicated treatments were harvested another seven times (during 41 weeks). Yields (for the last seven harvests) were expressed as percentages of the maximum yield attainable with MCP.In general, the Resin P test extracted more than twice as much P as the Olsen test. There was a significant increase in Resin P with an increase in the amount of each P source in all four soils, but Olsen P values were not significantly different for soils treated with different rates of each phosphate rock. The abilities of the Olsen and mixed resin soil P tests to predict the cumulative dry matter yield from 7 harvests and the relative yield of ryegrass were compared. Correlations between measured yield (for the last 7 harvests) and soil test for each soil, and relative yield and soil test for all four soils were assessed by regression analysis using Mitscherlich-type models.When dry matter yields were regressed separately against soil test values for each soil, the Resin P consistently accounted for 18–28% more of the variation in yield than did Olsen P. For Resin P a single function was not significantly different from the separate functions fitted to MCP and PR treatments. However, for Olsen P the separate functions for the MCP and PR treatments varied significantly from the single fitted function. The Resin P test (R2 = 0.84) was a better predictor of relative yields over this range of soils than the Olsen test (R2 = 0.75). Two regression models based on the regression of relative yield for MCP treatments against either Olsen or Resin were developed. These models were then fitted to the relative yield data on soils fertilized with PRs only. The Olsen P model was found to be a poorer predictor (R2 = 0.41) than the Resin P model (R2 = 0.73) because it underestimated the observed yield of the PR treatments.

Plant availability of phosphorus in superphosphate and a phosphate rock as influenced by earthworms

Abstract The effect of earthworms on the plant availability of phosphorus (P) in superphosphate and Chatham Rise phosphorite (CRP) was evaluated in a glasshouse experiment using perennial ryegrass over seven harvests. A mixed earthworm population of Lumbricus rubellus (Hoff.) and Allolobophora caliginosa (Savigny) was used. Increases in the yield of ryegrass in the presence of earthworms varied from 2 to 32%, whereas increases in P uptake by ryegrass ranged from 0 to 40% over seven harvests. With superphosphate, the initial increases in both ryegrass yield and P uptake by ryegrass in the presence of earthworms ranged from 20 to 40% at first harvest to less than 10% by the seventh. In marked contrast, earthworms increased the agronomic performance of pelletized CRP by 15 to 30% throughout the trial period. An increase in plant-available soil N concentrations due to earthworm activity probably explains the initial difference in the performance of superphosphate. The increased agronomic effectiveness of CRP appears to result from the incorporation and intimate mixing of the PR with the soil by earthworms. The implications of the results obtained in the present study to the interpretation of glasshouse and field trials evaluating P fertilizers are also discussed.

Use of potassium bromide solutions to simulate dairy cow urine flow and retention in pasture soils

Abstract A series of field experiments determined the amount of dairy cow urine that moved preferentially via a network of soil macropores beyond the 15 cm depth of soil following a urination event onto pasture. Urination events were simulated using potassium bromide solutions and the amount of bromide that was retained within the top 15 cm of the soil profile was measured. The experiments were carried out on seven different soil types, four of which were sampled in two different seasons. Results showed that between 52 and 106% of the applied bromide was retained in the top 15 cm depth of soil, therefore between 0 and 46% of the “urine” was lost beyond this depth as a result of preferential flow. In general, the amount of urine that was lost was more dependent on the physical conditions of the soil surface (e.g., water-repellent or compacted soil surface, irregular surface microtopography, and macropore distribution) than the soil chemical and mineral characteristics or moisture content. These results ind...

Uptake of potassium and nitrogen by pasture from urine-affected soil

Abstract A field experiment was carried out to determine the depth of soil from which pasture plants growing in urine-affected soil recover urine potassium (K) and nitrogen (N). A comparison of the amounts of K and N taken up by the herbage with the decline in soil K and N levels showed that K and N were absorbed mainly from the 0–12 cm depth of soil. This coincides with the distribution of plant roots within the soil profile, as 86% of the roots in the 0–30 cm depth of soil were found in the top 15 cm. The experiment also showed that immediately after a dairy cow urination event in mid winter (July), up to 59% of the urine K moved preferentially through a network of soil macropores to below a soil depth of 14.5 cm. A simulation of the field trial in the glasshouse confirmed that preferential flow of the urine could result in large losses of urine K. Fortyeight percent of the urine-N was also unaccounted for by soil analysis immediately after the urination event, and is considered to have been lost prefer...

Field evaluation of Chatham Rise phosphorite as a phosphatic fertiliser for pasture

Abstract Results from a 3-year field evaluation of Chatham Rise phosphorite (CRP) at 4 contrasting sites under permanent pasture are presented. On an annual basis, both powdered and pelletised CRP were as effective as superphosphate as a source of phosphorus (P) to pasture from the time of application at all 4 sites. At 2 of the hill country sites (Ballantrae and Wanganui), pelletised CRP showed a marked residual effect in the third year. This was particularly pronounced in the clover component of the sward. At these 2 sites, a single initial autumn application of 70 kg P/ha as CRP was agronomically as effective in the third year as 3 annual applications of superphosphate at 35 kg P/ha. This finding has implications for the strategy of fertiliser use. The residual effect shown by CRP in the third year at Ballantrae and Wanganui appears to result from the effect of CaCO3 on the rate of release of P from CRP. In marked contrast to this, no measurable differences were found in the residual effect of CRP and ...

Ability of chemical extraction procedures to assess the agronomic effectiveness of phosphate rock materials

Abstract The ability of 7 chemical extraction procedures to assess the agronomic effectiveness of phosphate rock (PR) was evaluated using perennial ryegrass and white clover grown in 6 soils in a glasshouse. Of the conventional, single chemicalextraction procedures evaluated, 2% formic acid appeared to be the most useful. Two 2% citric acid procedures (which produced very similar results) and neutral ammonium citrate (NAC) at the first extraction were poor indicators of agronomic effectiveness. Of the sequential extraction procedures investigated, the 2% formic acid (sum of 2 extractions), both of the 2% citric acid (sum of 3 extractions), and the NAC (sum of 4 extractions) gave the best correlation with agronomic performance. The use of sequential extractions also improved the predictive ability of these extractants in the longerterm, a limitation shown by the conventional, single chemical-extraction procedures. Sequential extraction appears to be the only extraction technique suitable for assessing the ...

Development and evaluation of an improved soil test for phosphorus, 3: field comparison of Olsen, Colwell and Resin soil P tests for New Zealand pasture soils

Soil tests suitable for estimating the phosphorus (P) status of soils fertilised with soluble or sparingly soluble P fertilisers (reactive phosphate rock) were evaluated using the New Zealand Ministry of Agriculture Technology (NZMAFTech) ‘National Series forms of phosphate trials’ on permanent pastures located throughout NZ. This included a common core of treatments comparing Sechura phosphate rock (SPR) with triple superphosphate (TSP). At each site, a re-application of twice maintenance TSP was superimposed on one-half plots that previously had received six annual applications of increasing amounts of P (0, 0.5, 0.75, 1.0 and 2.0 times the maintenance rate) in the form of TSP or SPR. Before the re-application of TSP, soil samples (0–30 and 0–75 mm depths) were collected from each plot. All the trials were run for 1 year during which seven to ten harvests were taken. Pasture response was expressed as percent increase in yield obtained with re-application over the previous treatment.The 0.5 NaHCO3 based (Olsen P) extractant with different combinations i.e. soil volume (Olsen (v)), soil weight (Olsen (w)), shaking time variations (Olsen (16 h)) and soil:solution ratio (Colwell), and Resin P soil tests were conducted on soils taken from the plots prior to re-application of TSP. The Olsen (v), Olsen (16 h) and Colwell P values increased with increasing rates of P applied in all soils with values for sparingly soluble P materials being less than where soluble P fertiliser had been previously applied. The Resin P values showed similar increases with P applied regardless of the solubility of previously applied fertiliser. When the yield increases caused by TSP application to all treatments (irrespective of fertiliser source) were regressed against soil test values, Resin P explained 76% of the variation in yield response, compared to 50% by Olsen (v), 42% by Olsen (w), 39% by Olsen (16 h) and 40% by Colwell P. Partitioning the data according to fertiliser source slightly improved the coefficient of determination for Resin P for both the soluble (R2=0.81) and sparingly soluble (R2= 0.80) P fertilisers. With 0.5 M NaHCO3 (Olsen) extractants, R2 values consistently indicated a poorer prediction for the SPR treatments. A Resin P model was able to account for more variance in yield response to re-applied TSP, than an Olsen P model because the Olsen model underestimated the yield response to re-applied TSP on the PR treatments. The Resin test is more suitable than the current Olsen test for assessing the plant available P status of soils previously fertilised with fertilisers of varying solubility.

Fate of potassium in dairy cow urine applied to intact soil cores

Abstract Following an application of dairy cow urine to intact cores of four contrasting soil types, the redistribution of urine potassium (K) as a result of leaching, plant uptake, and soil adsorption was investigated. In climatic conditions that simulated those of August/September in the Manawatu region, 5–39% of the applied urine K immediately moved by macropore flow to beyond the 15cm depth of soil. Leaching losses of K from 90 mm of simulated rain events, which followed the urine application, were smaller than K losses immediately following the urine application and accounted for only 2–6% of the applied urine K. The urine K adsorbed by the soil was retained mainly in the 0–7.5 cm depth of soil in an exchangeable form. During the 30-day period of the experiment less than 10% of the urine K was recovered in the herbage. Overall these results suggest that, irrespective of soil type, a major loss of K occurs as a result of macropore flow of the urine following a urination event. The concentration of K i...

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.