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Spatial and time-dependent patterns of selenium (Se) release from selected Se fertiliser granules.

Introduction

Selenium (Se) is an essential trace element for animals. Widespread deficiency of Se has been reported in grazing ruminants in New Zealand (Watkinson 1983; West et al. 2002), Australia (Judson and Reuter 1999), and other countries (Gupta and Gupta 2002). White muscle disease is the most common clinical manifestation of Se deficiency in ruminants (Millar 1983; Judson and Reuter 1999). Se deficiency in ruminants has commonly been controlled by administration of Se via dosing or injection. This method of treatment is cheap and convenient where animals are regularly dosed for internal parasites, but is less satisfactory for large numbers of mature animals, which receive infrequent dosing (Watkinson 1983). An alternative method is to address the primary soil Se deficiency by annually topdressing pasture (at a recommended rate of 10 g Se/ha) with selenised superphosphate or Se prills to increase the uptake of Se by the pasture and in turn raise Se ingestion by the grazing ruminants (Watkinson 1983).

In New Zealand, prior to 1980 pastures were topdressed with sodium selenites to prevent Se deficiency in grazing stock. From early 1980s, sodium selenate at rates up to 10 g Se/ha has been recommended instead of sodium selenite, because at one-fifth of the rate of sodium selenite, sodium selenate will raise pasture Se concentrations to the same level (Grant 1965). Use of sodium selenate is safer since foliar contamination effects are about 5 times less than sodium selenite, and sodium selenate is 4-20 times less toxic to animals and less of an irritant (Watkinson 1983).

Several types of sodium selenate fertiliser have been available for farmers to choose from. However, there is relatively little information available in the literature on the relative effectiveness of the different products in increasing Se concentration in pasture (Whelan and Barrow 1994; McLaren 2005) and forage crops (Gupta and MacLeod 1994). In conventional field trial comparisons of the effectiveness of broadcast Se fertilisers it is difficult to determine the spatial pattern of Se uptake by the sward. Knowledge of the release pattern of Se from a fertiliser granule and its effect on the spatial pattern of Se concentration in the sward is important in understanding the reason for differences in the longevity of Se fertiliser effectiveness. Therefore, the aim of this study was to determine the rate of Se release from 4 types of granular Se fertiliser applied to Se-deficient Pallic Soil/Brown Soil intergrade (Hewitt 1998) by measuring the time-dependent patterns of Se uptake by ryegrass swards at different distances from the point of a single fertiliser granule.

Materials and methods

Selenium fertilisers

Four forms of Se fertilisers were tested in the trial (Table 1). They were obtained from Ravensdown Fertiliser Cooperative Ltd, New Zealand. Unitech Agsel contains Se in the form of sodium selenate in a safe polymer coating (Morton et al. 1999; Morton and Roberts 1999). Selcote Ultra (Nufarm Ltd, Australia) contains 50% of Se in sodium selenate form and 50% of Se in barium selenate form (A. H. C. Roberts, pers. comm.) and therefore has the characteristics of slow and fast release of Se (Morton et al. 1999). The 2 Ravensdown Fertiliser Cooperative products contain Se in sodium selenate form with some additives to provide a degree of slow Se release (A. H. C. Roberts, pers. comm.).

Selenium content variability in fertiliser granules

The study was designed to compare the Se release rates of similar-sized individual granules of the 4 Se fertilisers. Analysis of individual granules of each of the fertilisers, however, showed a high degree of within-fertiliser variability in Se content of granules on a weight basis (Table 1). This variability was highest for Unitech Agsel (cofficient of variation (CV) 70%) and Selcote Ultra (CV 44%) and lowest for the Ravensdown fertilisers (CV 23%). The variability in Se content results from non-uniform aggregation of selenates and other components of the fertilisers. The colour and shape of granules within samples of Unitech Agsel and Selcote Ultra fertilisers also were not uniform and this may reflect the different proportions of components in the fertilisers.

The Se content variability in the individual granules would confound the experimental comparison of Se release rates from individual granules. Therefore, methods of reducing this variability were examined. Granule selection trials based on the differences in colour, as observed by the naked eye, and shape, as determined by rolling the granules on an inclined plastic board and separating the granules into 'rollers' and 'non-rollers', showed that the variability in granule Se content could be reduced by selecting granules of similar shape. Therefore, in the glasshouse trial, granules of similar shape (rollers) were used.

Trial design and treatments

A Pallic Soil/Brown Soil (Hewitt 1998) integrade (0-75 mm depth) from Wanganui in the North Island of New Zealand was used for the study. The soil had a pH([H.sub.2]O) of 5.1, an Olsen P of 26 mg P/kg, and a cation exchange capacity (1 M N[H.sub.4]OAc, pH 7 extraction) of 33 [cmol.sub.c]/kg. The total soil Se concentration was 0.8 mg Se/kg.

In the glasshouse trial, 2.12 g urea, 6.5 g single superphosphate, and 1.36g potassium chloride were mixed with 21 kg of air-dried (equivalent to 19 kg oven-dried soil weight) and sieved soil (<5 mm diameter) and the soil was packed into trays of 0.51 m length, 0.42 m width, and 0.15m depth. The rates of N, P, K, and S added in the fertilisers were equivalent to 100, 60, 70, and 80 kg/ha, respectively. Each tray was brought to 70% field capacity moisture content (26 kg moist soil weight) by adding the required amounts of water and sown with approximately 400-450 ryegrass (Lolium perenne L. cv. Nui) seeds by evenly spreading 0.43 g of seeds on the soil surface and covering the seeds with approximately 100 g dry soil on 23 May 2003.

The trial design accommodated the high variability of Se content in Selcote Ultra and Unitech Agsel granules. The trial consisted of 5 treatments (4 Se fertiliser treatments and a no Se fertiliser control treatment) replicated 3 times and arranged in a randomised complete block design in a glasshouse. To obtain a uniform sward density, the new ryegrass swards were cut to 20 mm height on 21 July, 11 August, 31 August, and 11 September 2003. After the harvest on the 11 September, 4 granules (2-3 mm diameter) from each of the 4 fertiliser types having uniform shape and similar weight were placed on the soil surface in each tray as shown in Fig. 1. The trial design therefore allowed measurements of the Se release rates of 12 granules of each type of fertiliser (3 replicated trays x 4 granules per tray). The weights and expected Se contents of the granules are given in Table 2.

Four granules (2-3 mm diameter) per tray with an average distance between granule centres of 0.265 m in the trial set-up gives a paddock granule density of 142340 granules/ha. This is equivalent to 2.8 kg Selcote Ultra or Ravensdown B/ha and 4.3 kg Unitech Agsel or Ravensdown A/ha. Using the average Se content of the granules, the rates of Se application in these fertilisers were calculated to be 22, 28, 21, and 37 g Se/ha for Selcote Ultra, Ravensdown B, Unitech Agsel, and Ravensdown A, respectively. The actual rates of Se applied in these fertilisers were calculated at the end of the trial by determining the sum of the amounts of fertiliser Se taken up by the plants and that remaining in soil after all plant harvests. Volatilisation losses of Se from the soil by microbial processes and from plants by metabolic processes were not considered in the calculation because such losses are negligible for acid soils (pH <6.0) with no Se contamination and for plants which are not Se hyperaccumulators (Duckart et al. 1992; Brooks 1998).

To prevent earthworms from moving the Se fertiliser granules and thereby influencing the Se release pattern, modified plastic pipette tips were placed over each granule. An arc was cut from 2 sides of each tip to allow free movement of air and water around the granule (Fig. 2).

A complete nutrient solution (Middleton and Toxopeus 1973) was added once a week to all trays. The trays were watered using distilled water to 70% field capacity moisture content at 3-day intervals by weighing the trays.

Sampling of herbage and soil

Because of the variability of the Se content in the fertiliser granules the Se content of herbage in concentric rings around each granule was measured at every harvest. At the end of the trial, Se concentration in soils in concentric rings around each granule was also measured. The amount of Se remaining in the granule and residue in the soil was calculated by multiplying the weight of soil within the rings by the Se concentration in the soil. Added to the Se content in all plant harvests, this gives the total amount of Se applied in each granule. The time-dependent Se release from each granule as measured in herbage was expressed as a percentage of the total Se recovered in soil and herbage analyses.

Ten herbage samplings were conducted 39, 69, 96, 119, 147, 173, 208, 258, 312, and 363 days after Se application. For the first 2 samplings, herbage was collected within 70 mm and 70-100 mm around the granules and from the remaining area (total of 9 samples per harvest) in each of the 12 trays where fertiliser granules were applied (Fig. 1). For the subsequent samplings, an additional herbage sample was also collected at the centre of each tray within a concentric circle of radius 70 mm (total of 10 samples per harvest per Se-treated tray). One sample per tray was also collected from the 3 control treatment trays at each harvest.

Soon after the last herbage sampling (363 days after Se application), soils were collected within 9, 9-29, 29-37, and 37-49 mm around the points of granule application to depths reaching the bottom of the tray. Four 9-mm radius soil cores were also collected from each of the 3 trays with no Se application (control treatment). The 4 soil cores from each of the 3 control trays were combined to make a composite sample for each tray.

Fertiliser analysis

Total and water-soluble Se content of fertiliser granules was determined at Analytical Research Laboratories Ltd, Napier, New Zealand. Total Se content was determined by digesting 0.5 g ground granules in 25 mL of a solution of 20% HCl and 5% HN[O.sub.3] and measuring the Se concentration in the diluted digests by ICP optical emission spectroscopy using standards having matched matrix. Water-soluble Se in the granules was determined by shaking 1 g ground granules with 100 mL water in an end-over-end shaker for 30 min, filtering the suspension, and measuring the Se concentration in the filtrate by ICP optical emission spectroscopy using standards having matched matrix.

Herbage analysis

Herbage samples were dried for 48 h at 65[degrees]C in an oven, weighed, and milled to <1-mm particle size. The Se concentration in the samples was determined at Analytical Research Laboratories Ltd, Napier, New Zealand by digesting I g milled samples in 10 mL mixture of 70% HCl[O.sub.4] and 69% HN[O.sub.3] at a volume ratio of 4 : 10 and measuring the Se concentration in the digest by ICP or hydride generation atomic absorption spectrometry (AAS) (Clinton 1977). Low Se concentrations in the digests were measured by hydride generation AAS and high Se concentrations by ICP optical emission spectroscopy.

Soil analysis

Soil samples were dried for 48 h at 105[degrees]C in an oven, weighed, and ring ground to <100 [micro]m. The Se concentrations in the ground samples were determined at Gribbles Analytical Laboratories, Hamilton, New Zealand by digesting 0.1 g of the ground samples with a 7 mL HN[O.sub.3] (70%) and 2 mL HCl[O.sub.4] (70%) mixture and measuring Se concentrations in the digests by hydride generation AAS (Godden and Thompson 1980). Standard additions of sodium selanate to samples of Se-untreated soils were made and the soils were analysed for Se in parallel with the unknown samples. A recovery of 98-107% of the amounts of Se added to the soils was obtained. This indicates that the method of sample preparation and analysis was acceptably accurate.

Results

Effect of distance from the granules and time after fertiliser application on herbage Se concentration

At all plant harvests, the herbage Se concentration decreased exponentially with increased distance from the fertiliser granule for all fertiliser treatments. The data for the first harvest and for other selected harvests are shown in Figs 3 and 4 respectively.

For all fertilisers the herbage Se concentration decreased with increased time after fertiliser application (Figs 3 and 4). In general, for all harvests herbage Se concentration decreased in the following order: Ravensdown A [greater than or equal] Ravensdown B > Selcote Ultra > Unitech Agsel.

Selenium toxicity and deficiency

Thirty-nine days after fertiliser application, the herbage Se concentration within 40 mm distance from the granules (estimated from the graphs in Fig. 3) reached concentrations considered toxic to grazing animals (5 mg Se/kg DM, Judson and Reuter 1999) for all the fertiliser treatments. This is estimated to be approximately equivalent to 7% of paddock area. The average herbage Se concentration for the entire paddock will therefore be much lower than the toxic level (Table 3).

The herbage Se concentration in soils without any Se treatment (control trays) (0.01 [+ or -] 0.00 to 0.03 [+ or -] 0.01 mg Se/kg DM) (Table 3) was below the Se concentration considered deficient for grazing animals (0.05 mg Se/kg DM, Judson and Reuter 1999) at all harvests. When Se fertiliser was applied the herbage Se concentration reached the Se deficiency level at different times after application and at different distances from the fertiliser granules. For the Unitech Agsel application, the herbage Se concentration reached deficiency level at the centre of the tray (0.12m away from the granules) in 4 months (equivalent to 7% of paddock area). By 9 months all the sward area in this treatment became deficient in Se (Table 4). In contrast, 7% of the sward area in the Ravensdown A fertiliser treatment and 41% of the sward area in the Ravensdown B and Selcote Ultra fertiliser treatments were deficient in Se at this time. Considering the average paddock herbage Se concentration, all fertilisers except Unitech Agsel fertiliser were able to prevent Se deficiency for longer than 7 months after application (Table 3).

Fertiliser Se remaining in soil

For the Unitech Agsel and Ravensdown B fertiliser treatments, the Se concentrations in soils at all distances from the granules at the end of the trial (363 days after Se application) were approximately the same as the background Se concentration of the soil (no-Se fertiliser treatment) (0.8 mg Se/kg) (Fig. 5). For the Selcote Ultra and Ravensdown A fertiliser treatments, however, the Se concentration decreased exponentially with increased distance from the fertiliser granule. The Se concentration beyond 37 mm from the granule was approximately the same as that in the soils with no-Se fertiliser treatment.

Discussion

Selenium release rates

After application of sodium selenate granules in the form of Unitech Agsel to the pure ryegrass sward in the glasshouse trays, the trend observed in decreasing herbage Se concentrations with time (taking 7 months to return to background concentrations, Table 3) is similar to the trends observed in a series of field trials conducted in New Zealand by Watkinson (1983) on pasture topdressed with sodium selenate in superphosphate or in prills. In these trials, Se concentration in pasture treated with 8.5-17 g Se/ha reached the background level after about 6-9 months. Similar results were also reported by Grant (1965) in another field trial on pasture at Wallaceville, New Zealand, where it took 11 months after a much higher rate of sodium selenate application (60 g Se/ha) for the herbage Se concentration to reach the background pasture Se concentration.

The Unitech Agsel was unable to maintain higher Se concentration in the herbage than control treatment beyond 7 months despite approximately 43% of applied Se was still present in soil at 7 months (Fig. 6). This is probably because part of the sodium selanate applied in Unitech Agsel might have been reduced to less plant-available selanite form in the acidic condition of the potted soil (Adriano 2001).

Compared with Unitech Agsel, Ravensdown A and Selcote Ultra maintained adequate pasture Se concentration (>0.05 mg Se/kg DM) until the last harvest (12 months after Se application) (Table 3). Ravensdown B was able to maintain adequate pasture Se concentration for 10-11 months. The reason for Selcote Ultra maintaining the highest Se concentration for a long period is due to the presence of the less-soluble barium selenate in this fertiliser. These results are in agreement with those of a sheep-grazing trial conducted in Western Australia, where Whelan et al. (1994) reported that a single application of sodium selenate fertiliser at 10 g Se/ha maintained adequate Se status (>0.05 mg Se/kg DM) in a subterranean clover based pasture for 15 months, whereas a single application of barium selenate at the same Se rate maintained adequate pasture Se status for most of the 3 years after Se application.

The reason for the longer duration of effectiveness of the Ravensdown products compared with Unitech Agsel, despite sodium selanate being the common form of Se, is due to the lower solubilities of the Ravensdown products compared with the Unitech Agsel (Table 2). These results are in agreement with the results obtained in a very recent field trial conducted by McLaren (2005) on the same 4 fertiliser products in the South Island of New Zealand on a Pallic Soil (Hewitt 1998). McLaren (2005) also reported that the Unitech Agsel fertiliser was effective for only 5 months, whereas Selcote Ultra was effective for the whole year of the trial when the fertilisers were applied at the rate of 10 g Se/ha. The durations of the effectiveness of the 2 Ravensdown fertilisers were between Agsel and Selcote Ultra, as observed in the glasshouse trial reported here.

Selenium application rates

The amounts of Se remaining in the fertiliser granules and residues in the soil in each tray were calculated by multiplying the soil weight in each soil core by the difference in Se concentration between Se-untreated and Se-treated soils in the respective cores and summing these values for all cores in each tray. This amount of Se was added to the total Se taken up by plants per tray at all harvests to give the amount of Se applied to each tray. This method estimated that the amounts of Se applied in granules to the trays were equivalent to 12-16, 18-23, 22-26, and 25-31 g Se/ha for Unitech Agsel, Ravensdown B, Selcote Ultra, and Ravensdown A fertilisers, respectively. These rates are lower than the rates estimated from the average Se concentration and weights of granules in the bulk sample of fertilisers (see Materials and method) but higher than the maximum rate of Se application of 10 g Se/ha (Morton et al. 1999) recommended for field application. The poor performance of Unitech Agsel in elevating herbage Se concentration is partly due to the lower Se content of granules, which lead to the lower rate of Se applied in this fertiliser compared to that in the other fertilisers.

Fertiliser efficiencies

Because different rates of Se were applied in the 4 fertilisers, the efficiencies of the fertilisers in supplying Se to plants were compared in terms of the increase in herbage Se concentration for the Se fertiliser treatments over the no-Se treatment (control treatment) per unit weight of Se applied [[DELTA]Se = mean herbage Se concentration (Se treatment--control treatment) ([micro]g Se/kg DM)/application rate of Se fertiliser (g Se/ha) (application rate calculated by adding soil Se remaining at the end of the trial to Se removed in all herbage cuts--see previous section)]. This method of comparison of fertiliser efficiencies is similar to the method used by Whelan and Barrow (1994) in comparing the efficiencies of sodium selenate and barium selenate in a subterranean clover based pasture field trial in Western Australia (Fig. 7). This method is valid because pasture Se concentration is linearly related to Se application rate up to at least 200 g Se/ha (Watkinson 1983).

[FIGURE 7 OMITTED]

The patterns of the change in A Se values with time show that during the first 3 months after Se application the fertiliser efficiencies decreased in the order: Unitech Agsel [greater than or equal to] Ravensdown B [greater than or equal to] Ravensdown A > Selcote Ultra. During the last 3 months the efficiencies were reversed (Selcote Ultra = Ravensdown A > Ravensdown B > Unitech Agsel). The reason for Unitech Agsel to be more effective in the early stages and Selcote Ultra to be more effective in the later stages is the difference in the solubilities of the 2 fertilisers (Table 2). Unitech Agsel, having Se only in the form of sodium selenate, is 100% soluble in water. Therefore, all Se in this fertiliser is readily available to the plants from the time of application, whereas Se in Selcote Ultra, which consists of 50% sodium selenate and 50% barium selenate and has only 38% of the total Se soluble in water, acted as a slow-release Se fertiliser. Whelan and Barrow (1994) also found that Se in sodium selenate was very available to pasture compared with Se in barium selenate during the first 3-4 months after Se application to grazed pasture in Western Australia (Fig. 7). Selenium availability from barium selenate in their trial, on the other hand, was constant throughout the 7 months of the trial and became similar to that from sodium selenate only after 4 months. The Se release patterns for Ravensdown A and B fertilisers fit in-between those of Selcote Ultra and Unitech Agsel, consistent with their solubilities (Ravensdown A, 71%; Ravensdown B, 76%).

Fertiliser Se recovery in herbage

The percentages of fertiliser-Se recovered in herbage [(mg Se in herbage in Se-treated trays--mg Se in herbage in Se-untreated trays)/mg Se applied in fertiliser per tray] during the 1 year of the trial were 48, 57, 62, and 63 for Selcote Ultra, Unitech Agsel, Ravensdown A, and Ravensdown B, respectively (Fig. 6). The efficiency of plant uptake of Se from Selcote Ultra was lower than that from the other fertilisers because it has only 38% of the total Se in water-soluble form due to the presence of the less-soluble barium selenate as a component in this fertiliser. The other 3 fertilisers have 71-100% of the total Se in water-soluble form (Unitech Agsel, 100%; Ravensdown B, 76%; Ravensdown A, 71%) and therefore had higher Se recoveries.

Even though Selcote Ultra had lower cumulative Se recovery in herbage than the other fertilisers, it had a longer term Se-release characteristic than the other fertilisers, especially Unitech Agsel (Fig. 6). The slope of the curve up to 6 months in Fig. 6 for Selcote Ultra is similar to that for Unitech Agsel, suggesting that the plants were utilising Se mainly from the sodium selenate component in Selcote Ultra. After 6 months the slope for the Selcote Ultra curve remains still positive, whereas that for the Unitech Agsel curve is zero. This suggests that plants in the Selcote Ultra treatment were taking up Se mainly from the barium selenate component at later stages of growth. Though the Se in Ravensdown fertilisers was in the sodium selenate form, the slopes of the curves for these fertilisers do not fit that of Unitech Agsel; instead, they were higher than that of Unitech Agsel and closer to that of Selcote Ultra, indicating a slow-Se-release characteristic probably due to the effect of the additives in these fertilisers or the processes used in the manufacture of these fertilisers.

The cumulative Se recoveries in herbage are much higher than the value of 15% reported by Watkinson and Dixon (1979) in a pot trial with ryegrass fertilised with sodium selenate solution at a rate of 10 g Se/ha, 11 weeks after Se application. The lower value obtained in the trial of Watkinson and Dixon (1979) is probably due to the short duration of their trial and application of sodium selenate in solution form, which would have caused a higher adsorption of Se to soil than when sodium selenate is applied in prills as in our study, and hence lower plant availability of Se. At very high Se application rate (3160 g Se/ha), however, they found that the Se recovery in herbage increased to a maximum of 40%. They explained this high recovery as related to the increased proportion of Se in solution compared with that adsorbed on to the soil.

Relationship between soil and herbage Se concentrations

Figure 5 shows that the Se concentration in soils within 9 mm around the point of fertiliser granule application was highly variable for Selcote Ultra and Ravensdown A fertilisers. This is consistent with the highly variable Se concentration in herbage within 70 mm around these fertiliser granules during the last few harvests in this trial (CV 55-100%). The high variability of the Se concentration in the soils is probably due to the variable contents of the less-soluble Se component in these fertilisers. Selcote Ultra consists of 50% barium selenate and 50% sodium selenate. Barium selenate is highly insoluble in water and the variable contents of this component in Selcote Ultra may have resulted in variable contents of fertiliser-Se residues in the soil. The high positive correlation between the Se concentration in the residues remaining in the soil and Se concentration in herbage in close proximity to these residues at the last 3 plant harvests (Fig. 8) suggests that the Se in these residues is available to the plants at later stages of plant growth.

[FIGURE 8 OMITTED]

Selenium cycling through grazing ruminants

The results showing relatively high plant utilisation rates of soluble fertiliser Se with 48-63% recovery by plant uptake in the first year suggest that soluble fertiliser Se has a fast initial cycle though the grazing animal. The majority of that recovery (35-55%) occurred within 4 months of Se application. Most of the Se ingested by livestock is expected to be excreted in urine and faeces (Judson and Reuter 1999), with the larger proportion being in dung (Millar 1983). In a study of Se metabolism conducted on ewes, Krishnamurti et al. (1997) showed that 82% of Se ingested by ewes was excreted by the animals when the animals were fed with hay having normal concentrations of Se, whereas the excretion was 54% for ewes fed with hay having inadequate concentrations of Se. Future investigations of the fate of Se in dung and urine spots in soils may be worthwhile and may hold the key to extending the persistence of the Se fertiliser effect without having to use slow-release fertiliser materials.

Conclusions

Individual Se fertiliser granules have a high degree of variability in Se content in similar weight granules. In trials on comparison of the Se release characteristics of individual granules, the variability in Se content can be reduced by using granules of the same shape, but it is impossible to completely reduce this variability. However, by analysing the Se content of the fertiliser residues in soil at the end of the trial and adding this to the amounts of Se removed by plants, the Se content of the fertiliser used in the trial can be estimated and trial results can be adjusted for the difference in Se content of the fertiliser granules.

The 1-year-long glasshouse trial conducted on pure ryegrass sward comparing the Se release rates of 2-3-mm granules of similar shape and weights of 4 types of Se fertilisers showed that the increase in herbage Se concentration for the fertiliser treatments over the no-Se treatment per unit weight of Se applied for the first 3 months decreased in the order: Unitech Agsel (containing sodium selenate) [greater than or equal to] Ravensdown B (containing sodium selenate) [greater than or equal to] Ravensdown A (containing sodium selenate) > Selcote Ultra (containing sodium selenate and barium selenate). During the last 3 months, this order is reversed (Selcote Ultra = Ravensdown A > Ravensdown B > Unitech Agsel). These results showed that Selcote Ultra to a larger extent and Ravensdown fertilisers to a lesser extent have slow-Se-release characteristics, whereas Unitech Agsel has a quick-Se-release charcteristic.

The cumulative herbage recovery of Se from Se fertilisers applied to the soil was lower for the less-soluble Selcote Ultra (38% of total Se water soluble; 48% fertiliser-Se recovery in plants) than for the highly soluble Unitech Agsel (100% water soluble Se; 5 7% recovery), Ravensdown B (76% water soluble Se; 63% recovery), and Ravensdown A (71% water soluble Se; 62% recovery) fertilisers. However, Selcote Ultra and Ravensdown fertilisers, especially Ravensdown A, maintained a sustained release of Se over the 1-year period of the trial. These results show that Selcote Ultra to a larger extent and Ravensdown fertilisers to a lesser extent have slow-Se-release characteristics, whereas Unitech Agsel has a quick-Se-release charcteristic.

Acknowledgments

We thank the following members of the Fertilizer and Lime Research Centre, Massey University for their assistance in conducting this project: Mike Bretherton, Ross Wallace, Bob Toes, and Q. Liu for the maintenance of the trial and for soil and herbage sampling, Dr Tin Aye for setting-up the trial, and Lance Currie for organising the soil, fertilizer, and herbage chemical analyses. We also thank ARL Ltd for the herbage and fertiliser Se analyses and Gribbles Analytical Laboratories for soil Se analysis.

Manuscript received 13 September 2004, accepted 14 December 2005

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P. Loganathan (A,B) and M. J. Hedley (A)

(A) Fertilizer and Lime Research Centre, Institute of Natural Resources, Massey University, Palmerston North, New Zealand.

(B) Corresponding author. Email: p.loganathan@massey.ac.nz
Table 1. Granule size distribution (% w/w) and Se content
(within parentheses) of the fertiliser

Fertiliser Chemical form of Se <1 mm

Selcote Ultra Sodium selenate + 0
 barium selenate
Unitech Agsel Sodium selenate 2 (l.55)
Ravensdown A Sodium selenate 1 (1.29)
Ravensdown B Sodium selenate 0

Fertiliser 1-2 mm 2-3 mm

Selcote Ultra 37 (0.73) 48 (0.72)
Unitech Agsel 77 (0.97) 21 (0.46)
Ravensdown A 2 (0.89) 36 (0.87)
Ravensdown B 0 3 (l.04)

Fertiliser 3-4 mm >4 mm

Selcote Ultra 14 (0.97) 1 (1.1)
Unitech Agsel 0 0
Ravensdown A 59 (0.79) 2 (0.88)
Ravensdown B 40 (n.d.) 57 (n.d.)

n.d., Not determined.

Table 2. Weights, water-solubility, and expected Se contents of
fertiliser granules (2-3 mm diameter) applied to the trays in the
trial

Fertiliser Chemical form of Se Water-solubility
 of Se (% w/w)

Selcote Ultra Sodium selenate + 38
 barium selenate
Unitech Agsel Sodium selenate 100
Ravensdown A Sodium selenate 71
Ravensdown B Sodium selenate 76

Fertiliser Granule wt Expected granule
 (g) Se conc. (% w/w)

Selcote Ultra 0.0196-0.0212 0.481-1.156
Unitech Agsel 0.0275-0.0307 0.198-0.763
Ravensdown A 0.0273-0.0291 0.656-1.086
Ravensdown B 0.0191-0.0211 0.767-1.335

Table 3. Average herbage concentration (mg Se/Kg DM) in trays (total
Se uptake per tray, [micro]g/g DM per tray)

Fertiliser Time after fertiliser application (days):
 39 69 96 119 147

Selcote Ultra 1.70 0.91 0.54 0.61 0.32
Unitech Agsel 1.80 0.57 0.56 0.25 0.12
Ravensdown A 2.80 1.20 1.10 1.00 0.57
Ravensdown B 2.30 0.95 1.00 0.89 0.34
Control 0.04 0.01 0.01 0.02 0.02

Fertiliser Time after fertiliser application (days):
 173 208 258 312 363

Selcote Ultra 0.17 0.13 0.14 0.20 0.22
Unitech Agsel 0.06 0.05 0.02 0.01 0.01
Ravensdown A 0.35 0.28 0.27 0.25 0.15
Ravensdown B 0.27 0.21 0.15 0.11 0.05
Control 0.01 0.01 0.01 0.01 0.02

Table 4. Estimated percentage area of paddock deficient in Se

Fertiliser Time after fertiliser
 application (days):

 39 69 96 119

Selcote Ultra 0 0 0 0
Unitech Agsel 0 0 0 7
Ravensdown A 0 0 0 0
Ravensdown B 0 0 0 0

Fertiliser Time after fertiliser
 application (days):

 147 173 208

Selcote Ultra 41 41 41
Unitech Agsel 41 41 71
Ravensdown A 0 0 0
Ravensdown B 0 0 41

Fertiliser Time after fertiliser
 application (days):

 258 312 363

Selcote Ultra 41 41 0
Unitech Agsel 100 100 100
Ravensdown A 7 7 7
Ravensdown B 41 41 71
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Article Details
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Author:Loganathan, P.; Hedley, M.J.
Publication:Australian Journal of Soil Research
Geographic Code:8AUST
Date:Mar 15, 2006
Words:5996
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