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Changes in chemical properties of 48 intensively grazed, rain-fed dairy paddocks on sandy soils over 11 years of liming in south-western Australia.

Introduction

Dairy production in south-western Australia occurs on predominantly sandy soils in areas with annual average rainfall >800 mm. Most dairy pastures are rain-fed in the Mediterranean-type climate of the region, with typically cool, wet May-October growing seasons and dry, warm to hot November-April. The pastures comprise the annual species subterranean clover (Trifolium subterraneum L.), hereafter called clover, and annual ryegrass (Lolium rigidum Gaud.) and Italian ryegrass (L. multiflorum Lam.).

Due to diminishing returns from milk production in southwestern Australia, dairy farmers have intensified their production to remain profitable. Paddock-grown pasture is the cheapest source of feed for milk production in the region, so farmers have improved grazing management to use as much paddock-grown pasture as possible for milk production (Bolland and Guthridge 2007a, 2007b, 2009). This was achieved by adopting the 3-leaf grazing strategy for ryegrass-based pastures, described by Fulkerson and Donaghy (2001). Pastures are rotationally grazed by starting grazing when ryegrass plants have 2-3 leaves per tiller, and fertiliser nitrogen (N) is usually applied after each grazing. About 80% of the pasture is grazed, but clover in the pasture can no longer provide sufficient N through symbiotic [N.sub.2] fixation for pasture production, hence the need to apply fertiliser N after each grazing (Bolland and Guthridge 2007b). Both clover and ryegrass respond to the applied N by growing bigger leaves and stems, but between grazings, ryegrass grows over the top of and shades out clover, so clover rapidly disappears and ryegrass comprises >90% of the pasture (Bolland and Guthridge 2007a, 2007b, 2009).

When newly cleared of native vegetation for agriculture, most soils used for dairy production in the region had pH values, as measured in calcium chloride, of 5.5-6.5 (McArthur 2004). Many of these soils have since acidified and pH values of 3.7-4.5 are now common. Aluminium toxicity, induced by soil acidification, is a major problem for crop production (Dolling et al. 1991; Dolling and Porter 1994) and dairy production (Bolland et al. 2002a) in south-western Australia, and is ameliorated by applying sufficient lime to raise the pH of the top 0.10m of soil to [greater than or equal to] 5.5 (Whitten et al. 2000).

When newly cleared, the soils in the region were acutely deficient in phosphorus (P) (McArthur 2004). In addition, the soils are predominantly sandy and the surface soils explored by roots of pasture plants mostly have low capacities to retain potassium (K) and to sorb sulfur (S). Both elements leach below the root-zone in wet years, inducing deficiency of the elements for pasture production, and requiring application of fertiliser to combat the deficiency (K, Bolland et al. 2002b; Bolland and Guthridge 2009; S, Bolland et al. 2003).

To ensure that P, K, and S deficiency does not reduce pasture production, most dairy farmers in the region usually apply fertiliser P, K, and S to pastures every year. The fertilisers are typically applied twice a year: in autumn, usually 3 weeks after pasture has emerged at the start of the growing season; and in late winter-early spring (August-September) (Arkell 1995). Many dairy farmers now apply fertiliser N, P, K, and S after each grazing to their intensively grazed ryegrass pastures.

The fertiliser P, K, and S requirements are known for clover but not for ryegrass (Bolland et al. 2002b, 2003; Bolland and Guthridge 2007a). Annual ryegrass has lower requirements than clover for P (Ozanne et al. 1969; Barrow 1975) and K (Bolland et al. 2002b; Bolland and Guthridge 2009). Consequently, the fertiliser P and K requirements of intensively grazed ryegrass pastures are likely to be less than those of clover in the traditional clover-ryegrass pastures not treated with fertiliser N after each grazing.

Fertiliser P has a good residual value for pasture production on sandy soils of the region, even in sandy soils with low capacities to sorb P (Barrow 1966). As a result of application of fertiliser P each year, the P status of most dairy soils is high and well above critical soil-test P of the various soils used. When fertiliser P is applied to these soils, no pasture dry matter responses to applied P are obtained (Bolland and Guthridge 2007a). On sandy soils in high-rainfall areas, continual application of fertiliser P each year, even when none is required, has increased wasteful loss of P from fertilised pastures to drainage, resulting in eutrophication of the many waterbodies in the region (Hodgkin and Hamilton 1993; McComb and Davis 1993).

The predominantly sandy soils used for dairy production in south-western Australia were initially infertile, and the physical and chemical properties of the soils have been improved by increasing organic matter contents of the topsoils (Barrow 1969). Mineralisation of sulfate-S from soil organic matter is a major source of S for high-rainfall pastures in south-western Australia (Barrow 1966). Cultivating soil increases mineralisation and depletion of the organic matter, so the pastures are usually permanent, and the soils are rarely cultivated or cropped. The amount of organic matter in the soils through time is routinely estimated by measuring organic carbon (C) content of the soils (Rayment and Higginson 1992). In addition, soil salinity occurs in patches in grazed pastures in the region, reducing pasture production (McArthur 2004). Soil salinity levels in soil are monitored by measuring electrical conductivity (EC) of the soil (Shaw 1999).

During 1999-2009, soil and tissue testing was undertaken in 48 paddocks used for intensively grazed, rain-fed dairy production on sandy soils on the Vasse Research Centre, near Busselton, south-western Australia. The aim was to measure changes in chemical properties of the soils, as determined by the soil-test procedures routinely used by commercial soil-testing laboratories in the region for pH, P, K, S, organic carbon, and EC. The results are reported in this paper. We hypothesised that soil testing for pH and P are reliable for indicating paddocks requiring lime and fertiliser P applications, and that no fertiliser P is required if soil-test P is above the critical value for that soil. Although soil testing is used in the region to provide fertiliser K and S advice, we propose that neither of these soil tests is reliable for high-rainfall pastures because both elements can be leached below the root-zone, and reliance should be placed on tissue testing instead. We suggest tissue testing is required in conjunction with soil testing to help monitor the success of decisions based on soil testing, and indicate deficiencies of elements not covered by soil testing and monitor success of fertiliser decisions made for the elements not covered by soil testing. We do not report the results of other soil properties also measured and reported by the laboratories, because the results are either unreliable for predicting deficiency for pasture production in the next growing season (total N, ammonium-N, nitrate-N) or not calibrated in the region because tissue testing is used rather than soil testing to determine fertiliser requirements (copper, zinc, manganese), or because the elements are not deficient for pasture production in the region so it has not been possible to develop local calibrations for the elements (calcium, magnesium, iron and boron).

Materials and methods

Location and climate

The 48 paddocks were at the Department of Agriculture and Food Vasse Research Centre (VRC, 33[degrees]45'S, 115[degrees]21'E, elevation 30m), ~15km south of Busselton, south-western Australia. Average annual rainfall at VRC is 876 mm, with ~85% falling in the May-October growing season and ~70% of the annual evaporation occurring during the hot, dry November-April period.

Soil classification and topography

Soils in the 48 paddocks were 1-2 m sand to sandy loam over massive clay, known locally as Abba sand (Tille and Lantzke 1990) and classified as Chromosolic Redoxic Hydrosol (Isbell 2002) and Glossic Natrudalf (Soil Survey Staff 1987). For many soils in the region, including Abba sands, the topography is flat and the soils are waterlogged from June to early September (Tille and Lantzke 1990; McArthur 2004). The capacity of Abba sands to sorb P and S, and to retain K, increases as the clay content of the soil increases (McArthur 2004).

Grazing studies conducted in the 48 paddocks

The 48 paddocks were used for two intensively grazed dairy studies during 2000-09. The first grazing study, called the Vasse Milk Farmlets (VMF), was conducted from mid 2000 to early 2005 and measured the effects of stocking rate and feeding different amounts of concentrate on pasture and milk production (Staines et al. 2007). The second grazing study, called Greener Pastures 1 (GP1), was conducted from mid 2005 to early 2009 and measured pasture, milk, and profit responses to five fertiliser N levels applied after each grazing (0.0, 0.5, 1.0, 1.5, and 2.0 kg N/ha.day). If it was 30 days since the last grazing, then 0, 15, 30, 45, and 60 kg N/ha was applied after that grazing, and a total of 0-331 kg N/ha was applied for all grazings in each growing season (Staines et al. 2008).

Collecting soil samples

Standard procedures for soil testing of high-rainfall pastures were used (Russell 2010). The top 0.10 m of soil was collected from each paddock in April 1999 and January-February 2000-09, during the dry period before fertiliser was applied in the next May-October growing season. The samples were collected by walking on the same diagonal path across each paddock each year between two permanent markers located on fences. Samples were collected using 25-mm-diameter metal tubes that were pushed into the soil by foot every 2-3 m, with 50-100 samples collected per paddock, depending on the size of the paddock. The samples from each paddock were bulked and air-dried, and subsamples of the <2 mm fraction were sent for analyses to the major commercial soil and tissue testing laboratory in the region, CSBP Laboratories at Bibra Lakes, Western Australia.

Soil properties measured and reported

Soil pH is used to determine the need for lime in south-western Australia and is measured using 1 : 5 soil : 0.01 M Ca[Cl.sub.2] (w/v), as outlined by Rayment and Higginson (1992, p. 19).

The Colwell (1963) sodium bicarbonate procedure, which is a modification of the original sodium bicarbonate procedure of Olsen et al. (1954), is the soil P test used in south-western Australia. The capacity of soil to sorb P has a large effect both on the amount of fertiliser P needed for near-maximum yields (Ozanne and Shaw 1967) and, for the Colwell procedure in particular, on the critical value required for soil-test P (Moody 2007). Consequently, both soil-test P and the capacity of soil to sorb P are routinely measured in south-western Australia to provide P fertiliser advice (Bowden et al. 1993; Bolland and Allen 2003). Three procedures have been used to rank the capacity of soil to sorb P in south-western Australia. Reactive iron (Fe) was the first procedure used from the mid 1970s until 2007 and is the amount of Fe extracted from soil by ammonium oxalate (Schwertmann 1964). Subsequently, two single-point P sorption indices were used to directly measure P sorption by soil: (i) from the mid 1980s to 2007, the P retention index (PRI; Allen and Jeffery 1990); (ii) since 2006, the P buffering index (PBI; Burkitt et al. 2002). All three methods were used and compared in this study.

Other soil-test procedures used in south-western Australia are as follows. The Colwell soil P test procedure is also used for soil K testing (Bolland et al. 2002b). The KCl-40 procedure (Blair et al. 1991) is used for soil S. Organic C content of soil is determined using procedures outlined by Walkley and Black (1934). EC is measured using procedures described by Rayment and Higginson (1992, pp. 15-16).

Liming

No major liming program had been undertaken in the 48 paddocks before April 1999. Soil testing in 1999 indicated that soil acidification was a major problem in all 48 paddocks, so a liming program was undertaken to rectify the problem.

Local ground limestone (from Redgate), with a neutralising value of 80-90%, was used to ameliorate soil acidification in the 48 paddocks. The lime was spread over the soil surface in March-May before the start of the growing season and was not incorporated into soil after application.

Price limited the amount of lime purchased and applied each year. No lime was applied in 2001, 2002, and 2005. The same level of lime was applied to all 48 paddocks in 1999 (3 t/ha), 2000 (4 t/ha), and 2003 (2 t/ha). In 2004, 17 of the 48 paddocks with the lowest soil pH were treated with 1 t/ha of lime. Lime was applied each year during 2006-09, the level applied varying with soil pH. In 2006, levels of lime applied were 5 t/ha if pH was <4.5, 2.5 t/ha if pH was 4.5-4.9, 1 t/ha if pH was 5.0-6.0, and no lime if pH was >6.0. Levels of lime applied in 2007 were 5 t/ha for pH <4.5, 3 t/ha for pH 4.5-4.9, 2 t/ha for pH 5.0-6.0, and no lime for pH >6.0. Levels of lime applied in 2008 and 2009 were 5 t/ha for pH <5.0, 3 t/ha for pH 5.0-5.4, 1 t/ha for pH 5.5-6.0, and no lime for pH >6.0.

Fertiliser

Tissue testing indicated copper (Cu), zinc (Zn), and manganese (Mn) applied in previous years were still adequate for pasture production, so no fertiliser Cu, Zn, or Mn was applied during 1999-2009.

During 2000-03 in the VMF project, fertiliser N was applied after each grazing, when 1 kg N/ha.day was applied, and a total of ~121 kg N/ha was applied in each growing season (Staines et al. 2007), normal practice at the time. In the VMF project, depending on treatment, 3-49 kg P/ha was imported in concentrate, and to ensure P introduced in the concentrate did not affect pasture dry matter production, 51 kg P/ha was applied as fertiliser P each year (Staines et al. 2007). The district average for dairy farms at the time ranged from 20 to 40 kg P/ha, depending on the intensity of grazing. During 2000-03, using district practice, a total of 100 kg K and 75 kg S/ha was applied each year. One-third of the P, K, and S was applied after the start of the growing season in autumn and two-thirds in late winter when paddocks were dry enough to be trafficable (Staines et al. 2007).

During the GP1 project (2005-09), fertiliser N levels applied varied from 0 to 331 kg N/ha.year, depending on the N treatment (Staines et al. 2008). In each year during 2004-09, fertiliser P was only applied when soil-test P was below the critical value for the sand to sandy loam soils, with critical values being 15-40 mg P/kg soil. Therefore, the few paddocks that were treated with P fertiliser in these years received 10-20 kg P/ha applied 3 weeks after pasture emerged at the start of the growing season (Staines et al. 2008). During the GP1 project, fertiliser N, together with ~10 kg K and 5-7 kg S/ha, was applied after each grazing (Staines et al. 2008), the practice now adopted by dairy farmers for intensively grazed ryegrass pastures in the region. A mean total of 63 kg K and 34 kg S/ha was applied each year during 2005-09 (Staines et al. 2008).

In both the VMF and GP1 projects, fertiliser N, P, K, and S was applied as various mixtures of urea, ammonium sulfate, single superphosphate, triple superphosphate, potassium chloride, and ammonium phosphate fertilisers in custom N : P : K : S fertiliser blends.

Tissue testing

Samples of ryegrass herbage were collected from selected paddocks at different times in each growing season during 1999-2009 for tissue testing. The ryegrass samples were collected just before grazing when ryegrass plants had 2 3 leaves per tiller. Using district practice, young tissue was collected at each sampling by collecting samples 5 cm above the soil surface, being the pasture typically grazed by lactating dairy cows at each grazing. The samples collected from each paddock were bulked and dried in a forced-draught oven at 50[degrees]C for 2 days. The dried samples were ground and sent to CSBP laboratories for analyses. Subsamples of the dried, ground herbage were dissolved in a nitric and perchloric acid mixture (Yuen and Pollard 1954) and concentrations of elements in the digest were measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES) (Zarcinas et al. 1987).

Analysis of data

For each soil property in each year, data presented are mean and range (minimum and maximum) values for all 48 paddocks.

Results

Soil pH

Liming increased soil pH from the range 4.0-5.0 in 1999 to 5.2-6.0 in 2009 (Fig. 1). Soil pH of 5.5 was achieved in individual paddocks by 2007-09, i.e. 9-11 years after the liming program started, and required a total of 12-21 t/ha lime (Table 1). For the last soil testing in 2009, 14 paddocks (~29% of paddocks) were yet to achieve pH 5.5, but pH values for these paddocks were 5.2-5.4, so close to the target value.

Critical soil-test P for the 48 paddocks, and ranking the capacity of the soils to sorb P

Based on all available published and unpublished data for various soil types of south-western Australia, most unpublished, critical Colwell soil-test P has recently been summarised for different soils with different capacities to sorb P, as ranked using reactive Fe, PRI, and PBI (Table 2).

[FIGURE 1 OMITTED]

Mean reactive Fe, PRI, and PBI values measured in the different years for each of the 48 paddocks are listed in Table 1. Based on these mean values, data listed in Table 2 were used to provide critical Colwell soil-test P for each of the paddocks, as ranked using reactive Fe, PRI, and PBI (Table 1). The general trend was for reactive Fe to determine lower critical soil-test P values than the two single-point P-sorption indices, with PBI often estimating the largest critical soil-test P values (Table 1). For each paddock, there were variations between years for all three procedures, with variations being larger for reactive Fe than PRI and PBI. Note that comparisons are limited to 3 years for PRI and 4 years for PBI, compared with 9 years for reactive Fe.

Soil-test P

Soil-test P significantly increased from 1999 to 2004 during the VMF project when 51 kg fertiliser P/ha was applied each year (Fig. 2a, b). In the VMF project, P was also imported in concentrate fed to the cows, increasing P surpluses from 9 to 21 kg P/ha.year as the amount of P imported in concentrate increased from 3 to 49 kg P/ha.year (Staines et al. 2007).

During 20044)9 in the GP1 project, fertiliser P was only applied in the next growing season when soil test P was below the critical value for the sand to sandy loam soils. Most soil test P values were above the critical value (Fig. 2c), so few of the 48 paddocks were fertilised with P in these years. Mean soil-test P significantly declined gradually in these years, with soil-test values remaining above critical values in most paddocks up to 2009 (Fig. 2c).

Soil-test K

Soil-test K varied greatly among paddocks (Fig. 3) and, for each paddock, varied greatly between years (Fig. 3b). Soil-test Kwas mostly lower for sand than sandy loam soils (Fig. 3e) and tended to decline after silage and hay crops and increased in paddocks where silage and hay were fed to stock.

Soil-test S

Soil-test S values ranged from just over l0 to just over 300mg/kg, with mean values being ~40-77 mg/kg, and were always above the critical value of ~6.5mg/kg for pasture production (Fig. 4).

Organic C content of soil

Organic C content of soils ranged from ~21 to 95 g/kg, with mean values for all 48 paddocks in each year being ~38-53 g/kg (Fig. 5). There was a significant long-term (11-year) trend for organic C to increase slightly with time (Fig. 5).

Electrical conductivity

Electrical conductivity ranged from 0.08 to 1.57dS/m, with mean values for all 48 paddocks in each year being 0.29-0.47 dS/m (Fig. 6).

Tissue-test results

Tissue testing indicated that P, K, S, Cu, Zn, Mn, Fe, Ca, Mg, and B were adequate to high for ryegrass production (Table 3). Tissue testing indicated that N was adequate during the VMF and GP1 projects, except that it was deficient for the nil-N treatment of the GP1 project (Table 3).

Discussion

Soil pH

If soil testing identifies soil acidification as a problem, liming with high levels of good quality lime should be undertaken as soon as possible and continued until the desired pH is achieved. Where possible, lime should be incorporated into soil after application to increase its effectiveness. Soil acidification continues, both during liming and after liming has been successfully completed, particularly for intensively grazed pastures treated with fertiliser N after each grazing. Therefore, once the target pH has been achieved, soil pH should be measured annually and lime again applied when pH declines below 5.5. This strategy requires smaller amounts of lime (1 t/ha) to return soil pH to [greater than or equal to] 5.5 than allowing soil pH to decline to lower values.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Progress in ameliorating soil acidification was disappointing during 1999-2003 (Fig. 1). There were three possible reasons for this. First, lime was not incorporated into soil after application to the soil surface, so liming was less effective (McLaughlin et al. 1990; Scott et al. 2000). Second, continued soil acidification during 1999 2009 increased the requirement for lime to ameliorate the acidity. Third, amounts of lime applied were limited by cost; no lime was applied in some years and only low levels were applied in other years. As the liming program continued and liming eventually increased soil pH, applications of lime after 2006 were more effective at ameliorating the problem (Fig. 1).

Although lime is more effective at ameliorating soil acidification when it is incorporated into soil after application (McLaughlin et al. 1990; Scott et al. 2000), dairy farmers in south-western Australia are reluctant to cultivate their soils. This is because in most situations the topography is flat and the soils are waterlogged for most of the growing season. After cultivation, managing the wet, unconsolidated soils can be difficult (Bolland et al. 2002a). In addition, cultivating soil increases mineralisation of soil organic matter, resulting in loss of soil fertility built up by growing pasture over many years when the soils were not cultivated.

[FIGURE 4 OMITTED]

Ranking the capacity of soil to sorb P

The capacity of soil to sorb P decreases as more P is applied to soil (Barrow 1974; Bolland and Allen 2003). This makes it difficult to rank the capacity of soils to sorb P using single-point P-sorption indices. However, adding soil-test P to P sorption when calculating single-point P sorption indices estimates P sorption for newly cleared, P-deficient soil, regardless of the P fertiliser history of that soil, making it simpler to rank the capacity of soil to sorb P (Barrow 2000). Therefore, PBI is calculated by adding soil-test P to P sorption (adjusted PBI; Burkitt et al. 2002). However, soil test P is not added to P sorption when calculating PRI; thus, PRI provides an estimate of the current capacity of soil to sorb P (Allen and Jeffery 1990). Phosphorus is likely to leach in sandy soils of south-western Australia when PRI is <2 L/kg (D. G. Allen and R. C. Jeffery, unpubl, data). We suggest that laboratories report both unadjusted and adjusted PRI and PB1 so that both values are available to provide appropriate fertiliser advice.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Spatial variation has been found to be large for reactive Fe and PRI, particularly reactive Fe (Bolland and Allen 1998).

Therefore, for the same paddock, different values for reactive Fe, PRI, and PBI measured in different years in this study are attributed to spatial variation.

In this study, there was a poor relationship between PRI and reactive Fe, a better relationship between PBI and reactive Fe, with the best relationship between PBI and PRI. These findings support results of previous studies showing poor relationships between direct measures of P sorption and reactive Fe (Singh and Gilkes 1991; Brennan et al. 1994; Bolland et al. 1996; Burkitt et al. 2002; Bolland and Windsor 2007), and better relationships between PRI and PBI (Burkitt et al. 2002; Bolland and Windsor 2007).

Soil-test P

Soil-test P increased dramatically from 1999 to 2004 because 51 kg P/ha was applied in each of these years to all 48 paddocks. However, during 2004-09, when no P was applied to most paddocks because soil-test P was above the critical value, soil-test P decreased gradually while remaining above critical values in most paddocks up to 2009 (Fig. 2c). For paddocks not treated with P in this period, tissue testing indicated P was adequate for ryegrass production (Table 3).

The GP1 project was an N study, not a P study, but a similar result to that shown in Fig. 2c for this study occurred in a P x N field experiment in the region (Fig. 7, presenting data of Bolland and Guthridge 2007a). The data in Fig. 7 were not presented as such by Bolland and Guthridge (2007a). The P x N experiment was done on an intensively grazed dairy pasture where 6 levels of fertiliser P (0, 15, 30, 45, 90, and 180kg P/ha.year, as triple superphosphate) were applied with either no or adequate fertiliser N. The pasture was dominated by ryegrass when fertiliser N was applied after each grazing. When no N was applied, the pastures comprised >90% clover and ryegrass, and 30-70% of each species, depending on the N status of soil, grazing and fertiliser management, and seasonal conditions (Rossiter 1966). The soil-test P values were not significantly different for the two N treatments. The soil-test data shown in Fig. 7 are for the nil-P treatment and significantly, gradually decreased from 73 to 48mg/kg during 2000-04 when no fertiliser P was applied, and were always above the critical Colwell soil-test P of ~35 mg/kg for the soil type on which the experiment was located. Consequently, no response of pasture dry matter to applied P was obtained. The amount of pasture dry matter consumed by dairy cows at all grazing events in each year when adequate fertiliser N was applied is shown near the top of Fig. 7 and was the same for the nil-P and the five P levels

applied each year. Corresponding pasture yields when no fertiliser N was applied were ~2 t/ha less than the yield data for the adequate N treatment. The experiment started in mid-June 2000; hence, total pasture dry matter consumed in that year (4.9t/ha v. 9.3-12.3t/ha in the other years) was smaller because pasture yields were not measured early in the 2000 growing season before the experiment started.

[FIGURE 7 OMITTED]

We conclude soil P testing can be used to determine when to apply fertiliser P to pasture in the next growing season. No fertiliser P is required when soil test P is above the critical value for that soil, particularly if it is well above the critical value. If fertiliser P is applied to such soils, no pasture dry matter responses to applied P are obtained. When soil test P is above the critical value and no fertiliser P is applied, soil test P decreases gradually.

Soil-test K

Because of K leaching in the next growing season, soil K testing could not be used to determine fertiliser K requirements of ryegrass in the next growing season. In addition, soil-test K values were very variable. Previous studies in high-rainfall areas of south-western Australia (Bolland et al. 2002b; Bolland and Guthridge 2009) have shown that soil K testing is unreliable for indicating K deficiency for pasture production in the next growing season. In addition, while cows graze pasture they deposit urine, containing plenty of K, in patches on the pasture. The urine patches are deposited in the 5 to 8 grazings undertaken in each paddock in each growing season. Soil test K is greatly affected by how many soil samples are collected for soil testing each year within and between the urine patches deposited during the previous growing season (Bolland and Guthridge 2009).

Therefore, in this study, fertiliser K was applied each year, and tissue testing was used to assess whether the fertiliser K strategies provided adequate K for ryegrass production. Each year during 2000-03 in the VMF project, ~30kg K/ha was applied in autumn and ~70 kg K/ha in late winter-early spring, so a total of ~100kg K/ha was applied each growing season. In the GP1 project, ~10 kg K/ha was applied after each grazing, and a total of ~63 kg K/ha was applied in each growing season. Tissue testing indicated that both methods of applying fertiliser K provided adequate K for pasture production (Table 3).

Soil-test S

A critical KC1-40 soil-test S value of-6.5 mg/kg has been determined for subterranean clover-based pasture (Blair et al. 1991), for pasture (various species) in >560 mm average annual rainfall areas of Australia (Blair et al. 1997), and for canola (Brassica napus L.) grain production on sandy soils in southwestern Australia (Brennan and Bolland 2006). In this study, soil-test S values were always >6.5 mg/kg (Fig. 4).

Because S mineralised from soil organic matter or dissolved from applied fertiliser can be leached, soil S testing cannot be used to determine fertiliser S requirements of high-rainfall pastures in the next growing season. The extent of leaching of S cannot be predicted. Therefore, the recommendation for these pastures is to apply fertiliser S after July every year and use tissue testing to assess and improve S management for the pastures. In the VMF study during 2000-03, fertiliser S was applied twice a year, with ~25 kg S/ha applied in autumn and ~50kgS/ha applied late winter-early spring, so a total of ~75kg S/ha was applied in each growing season. In the GP1 project during 2004-09, 5-7kgS/ha was applied after each grazing, and a total of-34kg S/ha each growing season. Tissue testing indicated that both fertiliser strategies provided enough S for ryegrass production (Table 3).

Organic C content of soil

The data from this project confirm that organic C contents of the top 0.10 m of dairy soils in the region are high. The organic C content of the soils tended to significantly increase in time, supporting results of an earlier study for permanent annual pastures in the region when the soil was not cultivated (Barrow 1969). Fluctuations of soil organic C contents in different years may be due to large spatial and/or temporal variations for this soil property in different years (Brown 1999).

Electrical conductivity of soil

For sandy loam soils, soil salinity is likely to reduce annual ryegrass DM production when electrical conductivity values are >0.33 dS/m (D. L. Bennett, unpubl, data). Depending on year, electrical conductivity was above this critical value in 23-70% of the 48 paddocks (Fig. 6).

In most of the 48 paddocks there were small patches that were more saline than the remainder of the paddock. Electrical conductivity was not measured in these patches, but would have been larger than for the remainder of the paddock. Emergence of pasture at the start of the growing season was delayed in these patches, particularly following drier growing seasons.

Soil properties measured by commercial soil testing laboratories

Commercial soil-testing laboratories always measure and charge for a standard number of soil properties that are potential candidates for soil testing in all agricultural industries in south-western Australia. However, for specific agricultural industries in specific regions, we encourage the laboratories to measure only those soil properties that research has shown to be reliable and for which the results can be interpreted by advisors and consultants. This would reduce costs of analyses, encouraging farmers to soil test. For the dairy areas of southwestern Australia, this would include measuring soil pH and soil-test P every year, together with PBI, organic C content of soils, and EC every 3-5 years. For soils prone to P leaching, PBI/ PRI should be measured every year.

Conclusions

This study confirmed that both soil and tissue testing need to be undertaken when assessing the reliability of measuring different soil properties to help determine lime and fertiliser requirements for different agricultural enterprises in different regions, and to identify nutrient elements deficiencies not covered by soil testing. This applies to all agriculture.

In this study, soil testing for pH and P each year successfully targeted paddocks requiring lime to ameliorate soil acidification and requiring applications of fertiliser P to maintain ryegrass production. Once the acidity has been ameliorated, measurement of soil pH needs to continue regularly to target paddocks requiring lime, to prevent acidification reoccurring. No fertiliser P is required when soil-test P is above the critical value for that soil. Tissue testing indicated that P was not deficient for ryegrass production when no P was applied if it was not required. These findings are relevant to all soils, climates, and environments where soil acidification and P deficiency are problems.

In soils where leaching of K and S below the root-zone occurs, soil K and S testing cannot be used to predict fertiliser K and S requirements in the next growing seasons. Because it is not possible to predict the extent of leaching of these elements below the root-zone, the solution is always to apply K and S fertiliser each year, and use tissue testing to determine the appropriate fertiliser K and S fertiliser strategies required to ensure K and S are sufficient for pasture production. These findings are relevant to all soils prone to K and S deficiency due to leaching of the elements below the root-zone.

For soil testing in each region, it is worthwhile to measure only soil properties that research has shown to be reliable for helping to determine lime and fertiliser requirements for specific agricultural systems, such as dairy production in south-western Australia. Commercial soil and tissue testing laboratories need to be flexible, and rather than routinely measuring, reporting, and charging for a standard number of soil properties, they should only measure and report those soil properties required. This will reduce cost of analyses and encourage soil testing. This applies to all regions where soil testing is used.

10.1071/SR09199

Acknowledgements

Technical assistance was provided by John Baker, Patrick Donnelly, Leonarda Paszkudzka-Baizert, Kathy Lawson, and Andrew Lindsay. Martin Staines, Richard Morris, and Peter Jelinek helped collect soil samples. Peter Jelinek was responsible for data management of the VMF and GPI projects, including the soil test data reported in this paper. Funds were provided by the Government of Western Australia and Dairy Australia (Vasse Milk Farmlets DAW046, and Greener Pastures 1 DAW11011). CSBP did not charge for chemical analyses during 19992004. Positive comments of anonymous referees helped us improve the paper.

Manuscript received 4 November 2009, accepted 29 April 2010

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M. D. A. Bolland (A,B,C) and W. K. Russell (A)

(A) Department of Agriculture and Food Western Australia, PO Box 1231, Bunbury, WA 6231, Australia.

(B) School of Plant Biology, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia.

(C) Corresponding author. Retired. Present address: 35 Casuarina Street, Eaton, WA 6232, Australia.

Email: mbolland46@gmail.com
Table 1. Total lime applied during 1999-2009 for the 48 paddocks,
and mean reactive Fe (mg/kg), P retention index (PRI, L/kg), and
P buffering index (PBI, no units), and corresponding critical
Colwell soil-test P (critical P, mg/kg) determined from the
reactive Fe, PRI, and PBI ranges listed in Table 2

Paddocks are listed in order from Greener Pastures 1 (GPI)
paddocks 12 48 and 50-60. ND, No data

Paddock        Lime        Reactive   Fe (1999 2007)      PRI
no.       applied (t/ha)     Mean       Critical P       Mean

1              18.5           350           25             3.5
2              19.5           187           15             ND
3              16.5           249           20             6.1
4              18.0           230           20             1.8
5              17.5           178           15             0.1
6              14.0           224           20             1.0
7              19.5           234           20             0.9
8              19.5           286           25             2.2
9              19.0           255           20             0.8
10             19.5           295           25             2.6
11             19.5           273           20             1.8
12             15.5           485           25             9.1
13             15.0           343           25             4.9
14             14.0           323           25             6.1
15             14.0           355           25             6.1
16             14.0           419           25             5.1
17             15.0           493           25             6.6
18             14.0           609           25             8.6
19             14.0           544           25             6.9
20             13.0           614           25             8.8
21             15.5           571           25             9.1
22             15.5           719           30            10.3
23             14.0           728           30            12.3
24             14.0          1337           35            32.0
25             14.0          1193           30            19.8
26             14.0           818           30             9.5
27             16.0           846           30             9.8
28             17.5          1199           30            17.1
29             21.0           695           30            13.5
30             19.5           909           30            14.9
31             17.5          1127           30            72.1
32             17.0           822           30            16.2
33             16.0           847           30            20.4
34             15.5          1233           30            37.8
35             14.0          1635           35            68.0
36             19.5          1592           35            73.9
37             17.5          1498           35            95.3
38             19.5          1074           30           370.6
39             19.0          1067           30           259.7
40             16.0           537           25            83.1
41             16.0           449           25            24.5
42             18.5           472           25            15.1
43             19.0           311           25            11.4
44             19.0           228           20             7.4
45             20.0           188           15             3.2
46             21.5           168           15             3.8
47             20.0           195           15             2.9
48             15.0           222           20             3.1

Paddock    (2004-07)      PBI      (2006-09)
no.       Critical P     Mean     Critical P

1             25          32.2        25
2             ND          16.3        25
3             25          38.3        30
4             20          18.8        25
5             20          14.2        20
6             20          17.6        25
7             20          17.6        25
8             25          21.5        25
9             20          18.5        25
10            25          23.6        25
11            20          20.7        25
12            30          50.1        30
13            25          35.6        30
14            25          35.3        30
15            25          40.8        30
16            25          42.1        30
17            25          38.4        30
18            25          48.1        30
19            25          46.5        30
20            25          47.7        30
21            30          45.3        30
22            30          49.3        30
23            30          59.4        30
24            35          99.2        35
25            30          88.1        35
26            30          67.8        30
27            30          68.4        30
28            30          66.4        30
29            30          57.8        30
30            30          71.3        35
31            35         105.2        35
32            30          64.6        30
33            30          71.8        35
34            35          90.7        35
35            35         132.7        35
36            35         143.0        40
37            40         126.7        35
38            40         265.9        40
39            40         215.0        40
40            35         153.6        40
41            30          85.0        35
42            30          64.0        30
43            30          51.7        30
44            25          46.2        30
45            25          34.1        30
46            25          27.4        25
47            25          32.8        30
48            25          35.2        30

Table 2. Critical Colwell soil-test P for different soils in
south-western Australia and corresponding P sorption of soil as
ranked using reactive Fe, PRI, and PBI

From published and unpublished data summarised early 2009 by R. N.
Summers and D. M. Weaver. Critical soil test P is for 95% of the
maximum pasture yield

Critical Colwell      Reactive Fe    PRI       PBI
soil-test P (mg/kg)     (mg/kg)     (L/kg)

15                      100-200       <0      5-10
20                      201-280      0-2      11-15
25                      281-650      3-9      16-30
30                     651-1250     10-28     31-70
35                     1251-2500    29-87    71-140
40                     2501-4950    88-275   141-280

Table 3. Published ranges for critical tissue test values for
young tissue of perennial ryegrass as summarised by Pinkerton et
al. (1997, p. 345), and range of tissue test values obtained in
this study as determined for young tissue of mixed annual and
ltalian ryegrass pasture measured on samples collected in
selected paddocks severai times each growing season during 2000-09

Element      Published critical   Range of tissue test values
             tissue test values   obtained in this study (A)

N (g/kg)           30-35                   37-50 (B)
P (g/kg)          2.0-2.8                   3.1-3.8
K (g/kg)           14-19                     23-36
S (g/kg)          1.8-2.2                   3.6-5.7
Ca (g/kg)         1.5-2.0                  3.6--7.2
Mg (g/kg)         1.5-2.0                   2.5-3.9
Cu (mg/kg)        4.0-6.0                  8.1-15.2
Zn (mg/kg)         10-15                     30-54
Mn (mg/kg)         15-20                     29-89
Fe (mgikg)         40-60                    87-1033
B (mg/kg)         3.0-5.0                  8.3-16.6

(A) For each element, the lowest and highest values obtained for
all samplings in all years.

(B) Did net include values for the nil-N treatment of the GP1
project, which were 22-29 g/kg of N so below the critical range
of 30-35 g/kg.
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Publication:Australian Journal of Soil Research
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Date:Dec 1, 2010
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