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Effect of ammonium, organic amendments, and plant growth on soil pH stratification.

Introduction

Soil acidification is a major concern in Australia (Donald and Williams 1954; Williams 1980; Coventry and Slattery 1991), especially in the wheatbelt area of eastern Australia (Greenland 1971; Cregan et al. 1979; Helyar et al. 1990). Acidification may not be confined to the surface soil but may extend to depths of 30 cm (Williams 1980) to 60 cm (Bromfield et al. 1983) into the subsoil in eastern Australia and to 80 cm in Western Australia (Dolling and Porter 1994). In some studies the investigation of profile acidification has been determined for soil samples taken in depth intervals of 5 or 10 cm (e.g. Williams 1980; Bromfield et al. 1983; Haynes 1986b; Jacobsen and Westerman 1991; Haynes and Williams 1992). In other studies, Pinkerton and Simpson (1986b), Conyers and Scott (1989), McLaughlin et al. (1990), Black (1992), and Young et al. (1995) measured the pH of soil samples collected in 2-cm depth intervals. These latter studies identified changes in soil pH with depth within the surface 10 cm, with the pH being highest at the surface and decreasing with depth. The magnitude of this pH decrease was in the order of 0.4-2.0 [pH.sub.Ca] units. This pattern of change in pH with depth is referred to as stratification of soil pH.

Few studies have investigated the rate at which this [pH.sub.Ca] stratification develops within the surface 10 cm of soil. Conyers and Scott (1989) reported the development of pH stratification at 3 different field sites, 5 years after the surface soil had been thoroughly mixed. The glasshouse data of Black (1992) indicated a rapid stratification of soil pH within 2 months after the addition of nitrogen sources (urea solutions and sheep urine) in his pot experiment. It would appear that nitrogen transformations may influence the development of soil pH strata.

The objective of this study was to investigate the effect of a range of added substrates and of plant growth on the rate of development and the magnitude of stratification of soil pH.

Materials and methods

This investigation included a glasshouse and a field experiment.

Glasshouse experiment

The soil was collected from an established lime trial (Scott et al. 1992) at Borambola, 20 km east of Wagga Wagga, in south-eastern New South Wales. The soil of the area is an albic luvisol (FA0 1974), Dy 3-42 (Northcote 1971), and an intergrade between a Yellow Sodosol and a Yellow Chromosol (Isbell 1996). The soil characteristics were: [pH.sub.Ca] 4.11; ECEC, 2.51 cmol(+)/kg, Al/ECEC, 24%; and organic carbon, 0.7%. The clay mineralogy for the 0-10 cm soil layer was illite 55%, kaolinte 24%, and quartz 17%. Soil from the surface 0-10 cm layer was collected from plots which had [pH.sub.Ca] approximately equal to 4, 5, and 6.

Air-dried soil (750 g) was placed into plastic bags. A basal nutrient solution containing the equivalent of 12 kg P/ha (as [KH.sub.2][PO.sub.4]) and 100 kg [MgSO.sub.4]-[7H.sub.2]O/ha was added to the soil 3 days prior to the start of the experiment. On the day prior to the start of the experiment, the following amendments were added to each bag of soil: no amendment (control), ammonium sulfate added at a rate of 200 [micro]/g N/g soil (+N), sucrose added at the rate of 2000 [micro]g C/g soil (+C), and finely ground lucerne hay (+HAY) with a nitrogen content of 2.78%, at the rate equivalent to 200 [micro]g N/g soil. The soil and amendment were shaken thoroughly and then placed into a cylindrical pot (8 cm in diameter by 15 cm high) lined with a plastic bag.

These amendments were chosen as they directly affect nitrogen transformations. The added ammonium would stimulate nitrification. The added sucrose would promote immobilisation of available nitrogen. The addition of hay is equivalent to an addition of organic nitrogen to the soil which should enhance mineralisation and nitrification. Each treatment was carried out in triplicate.

Pots from each amendment were left either bare or sown with Italian ryegrass (Lolium multiflorum Lam.). Italian ryegrass var. Concord was chosen for the glasshouse experiment because it is acid-tolerant but slower growing than cereal grasses, therefore reducing the possibility that roots could become pot-bound.

Pots to contain plants were sown with approximately 20 seeds of Italian ryegrass. After germination, plants were thinned to 10 plants per pot. Pots were placed in random order on 2 trolleys in the middle of a glasshouse and remained in this position throughout the experiment. All pots (with and without plants) were watered to field capacity at the start of the experiment. Field capacity was determined on the basis of 24-h drainage in a glass column with dry soil at the base. All pots were kept at approximately field capacity by watering to weight every second day throughout the experiment.

To establish the initial effect of each amendment on [pH.sub.Ca] pots containing soil and amendment were sampled immediately prior to sowing (pre-sowing sampling). The remaining pots were sampled after 5 weeks (final sampling). This final sampling time was determined using a calculation based on the estimated plant dry matter and the total nitrogen concentration, which suggested that plants would have utilised all of the available mineral nitrogen in this time period. Subsequent analysis of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] in the soil confirmed this assumption (Evans 1995).

Soil from each pot was sampled using a tube 6 cm. in diameter and 10 cm long, which took a core of a smaller diameter and length than the pots. This strategy was adopted to negate any edge effects. Each 10-cm core was then cut into 1-cm intervals, except for the 0-2 cm soil layer where 0.5 cm depth intervals were taken. A weighed subsample of soil from each depth interval was placed into a plastic tube and frozen, to halt microbial activity, prior to mineral nitrogen analysis. The remaining soil, to be used for [pH.sub.Ca] analysis, was placed into a labelled paper envelope and dried in a coolroom at 10 [degrees] C to reduce the potential effect of microbial activity on soil [pH.sub.Ca] during drying (Conyers 1992).

Field experiment

A set of unlimed buffer plots, 50 m by 1.6 m, from a lime experiment site was used for this experiment. Four blocks, each 10 m by 1.6 m, were established with each of the 4 amendments present in each block. Each main plot was 2.3 m long by 1.6 m wide. The trial was cultivated to 7-8 cm with a rotary hoe. Treatments were the control, +C (2000 [micro]g C/g soil in the form of sucrose), +N (200 [micro]g N/g soil in the form of ammonium sulfate), and +HAY (equivalent to 200 [micro]g N/g soil in the form of finely ground lucerne hay) as in the glasshouse experiment, based on a 10-cm soil depth and 1-3 g/g bulk density. Amendments were then applied evenly over individual plot surfaces, leaving a 20 cm. border between plots. Each plot was then individually rotary hoed again, with the blades being lifted and cleaned to minimise carryover between adjacent plots. The following day it was noted that soil had not settled to the original ground level after cultivation. This would have led to soil sampling having inaccurate depth intervals. In an attempt to reduce this problem a 200-L drum with an 80 kg weight was rolled over the plots to consolidate soil before sampling occurred. After soil sampling, oats (Avena sativa L.) var. Cooba were sown on one-half of the plots with a tyned implement at a rate equivalent to 83 kg/ha. The other half of the plots was kept devoid of plants by spray applications of Roundup. Therefore each split plot was 2.3 m by 0.8 m.

Soil was sampled at various times throughout the growing season, in 2-cm. intervals to 10 cm, using a 2-cm diameter coring tube. Plants and soil were generally sampled at the same time, but in August the soil was too wet to sample where there were no plants growing and in September the soil was too wet to be sampled. At the final sampling time, plants were cut and grain was removed for analysis.

Laboratory methods

Soil [pH.sub.Ca] was determined using a 1:5 ratio of soil to 0.01 m [CaCl.sub.2] solution and a 1-h shake. Soils were extracted for mineral nitrogen analysis using a 1: 5 ratio of soil to 1 m KCl. Mineral nitrogen concentration was determined using a Technicon Autoanalyser II. Ammonium concentration was measured using a modified Berthelot reaction (Willis et al. 1993). Nitrate concentration was determined using cadmium reduction and colorimetric analysis (Henriksen and Selmer-Olsen 1970). Effective cation exchange capacity (ECEC) of soils was determined as the sum of cations after the soils were extracted with an ammonium chloride plus barium chloride solution (Gillman and Sumpter 1986), and cation concentrations were measured by flame atomic absorption spectrophotometry.

Plants were cut at ground level, washed to remove any soil, and oven-dried at 80 [degrees] C, and dry matter weights were recorded. Dried plant samples were ground (particles passing a 150 [micro]m screen) before total nitrogen analysis. A Kjeldahl digest using a selenium catalyst (Simonne et al. 1993, quoting Lauro 1931) was performed followed by automated titration using a Tecator Kjeltec Auto 1090 Analyser.

Statistical methods

For the glasshouse data, 1-way analysis of variance was used for the statistical analysis of the mean profile [pH.sub.Ca] results. Analysis of variance was run using the GENSTAT program (Alvey et al. 1977). Individual depths within a profile could not be analysed using analysis of variance as strong correlations between depths meant that an independent analysis of variance was inappropriate. Therefore, there are no I.s.d. values on Fig. 1. Instead the analysis of variance was conducted using the mean pH for each pot, and significant difference is indicated. Plant data were analysed by 2-way analysis of variance using the GENSTAT program (Alvey et al. 1977). In some cases, data were analysed using square root transformations to ensure homogeneity of variance.

[Figure 1 ILLUSTRATION OMITTED]

The soil and plant data from the field experiment were analysed by 1-way analysis of variance using the GENSTAT program (Alvey et al. 1977). Data for dry matter at anthesis and grain yield were transformed to log, before analysis due to the large range of values among the treatments. All other plant data were analysed as untransformed data.

Results and discussion

Glasshouse experiment

Plant data from the glasshouse experiment are shown in Table 1. The lowest dry matter yields were obtained on the +C amendment, whereas the greatest dry matter yields tended to be on the +N amendment. Within the amendments, dry matter tended to increase as initial soil pH increased. Plant total nitrogen concentration was greatest on the +N amendment with a tendency for lowest nitrogen concentrations to be in the plants from the control amendment. The +C amendment had greater total nitrogen concentration than the control as a result of its low dry matter yield. Plant total nitrogen concentration tended to increase with an increase in initial soil pH for the control and +C amendments. There was no effect of initial soil pH on plant total nitrogen for the 2 amendments with nitrogen added (+N and +HAY). Nitrogen uptake was calculated as the product of dry matter yield and plant total nitrogen concentration. The greatest nitrogen uptake was in the +N amendment and the lowest in the +C amendment. Nitrogen uptake tended to increase with pH within each of the 4 amendments.

Table 1. Dry matter yield (g/pot), nitrogen concentration in shoots (%), and nitrogen uptake by plants (mg N/pot) in the glasshouse experiment

Within columns, means followed by the same letter are not significantly different at P = 0.05
Amendment Plant ANOVA(A) Shoot total N Nitrogen
 DM yield conc. uptake
 at harvest at harvest of shoot

Control pH 4 0.52 0.72b 2.01a 10.4
Control pH 5 0.68 0.82c 2.27ab 15.3
Control pH 6 0.69 0.83c 2.59be 17.9
+C pH 4 0.04 0.19a 2.70bed 0.90
+C pH 5 0.05 0.23a 3.03cde 1.60
+C pH 6 0.06 0.24a 3.31ef 1.80
+N pH 4 1.30 1.14a 5.46h 71.0
+N pH 5 1.93 1.39f 5.64h 108.5
+N pH 6 2.24 1.50g 5.44h 121.6
+HAY pH 4 1.05 1.03d 3.82g 39.7
+HAY pH 5 1.37 1.17e 3.70fg 50.8
+HAY pH 6 1.35 1.16e 3.90g 52.4


(A) After square root transformation of data.

There was little change in soil [pH.sub.Ca] with depth at the pre-sowing sampling time for any treatment at any initial [pH.sub.Ca] (Fig. 1). The mean [pH.sub.Ca] of the soil at the pre-sowing sampling time was increased by the addition of hay at each initial soil [pH.sub.Ca]. After 5 weeks, there were differences in mean soil [pH.sub.Ca] among pre-sowing -plants and +plants as indicated in Fig. 1. The principal changes in mean soil [pH.sub.Ca] occurred in the +N and +HAY amendments. In the +N amendment the soil profile acidified at [pH.sub.Ca] 4, 5, and 6 over time, due to both nitrification and NH+ uptake by plants (Table 2). At [pH.sub.Ca] 4 in the +N amendment there was no change in soil profile acidification in the absence of plants, and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] remained the dominant form of inorganic nitrogen (Table 2), 4 suggesting that nitrification was inhibited by low soil [pH.sub.Ca]. In the presence of plants, it was assumed that [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] was the dominant form of nitrogen taken up, 4 which would cause some acidification of soil. In the +HAY amendment at the final sampling time, soil profiles were more alkaline than in the control due to the alkaline nature of the added hay. The gross increase in [pH.sub.Ca] which had occurred due to the oxidation of organic anions in the hay, and prior to nitrification (Fig. 1), is not known. In the absence of plants at initial soil [pH.sub.Ca] 5 and 6, there was an acidification of the profile relative to the pre-sowing state due to nitrification (Table 2), but in the presence of plants, nitrate uptake resulted in higher mean pH than the pre-sowing state.
Table 2. Mineral nitrogen content ([micro]g N/g) of
soil in the glasshouse experiment at final harvest

Values in parentheses refer to the percentage of total
mineral nitrogen in the form of ammonium Data are the
depth-weighted average of the 12 sections of the pot

 Control +C
 -Plants +Plants -Plants +Plants

pH 4 24 (8) 0.6 (100) 0.9 (78) 1 (100)
pH 5 26 (5) 0.2 (100) 1.4 (43) 0.6 (100)
pH 6 45 (3) 0.8 (100) 4.6 (17) 0.5 (100)

 +N +HAY
 -Plants +Plants -Plants +Plants
pH 4 262 (87) 96 (100) 91 (30) 28 (30)
pH 5 276 (69) 46 (95) 143 (3) 9.8 (23)
pH 6 261 (41) 32 (38) 140 (1) 28 (5)


The appearance of stratification of the soil profile was minimal over the length of the experiment (Fig, 1). In the absence of plants there was a tendency for the soil [pH.sub.Ca] of the surface 1 cm to be higher than the rest of the profile. For the control soil at initial soil [pH.sub.Ca] 5 and 6, the change in soil [pH.sub.Ca] within the top 2 cm depth was up to 1.0 and 0.8 [pH.sub.Ca] units, respectively. In these surface layers the quantity of nitrogen involved in nitrogen transformations was small (Evans 1995), so the cause of such large [pH.sub.Ca] gradients is uncertain. The +C amendment exhibited no change in soil [pH.sub.Ca] with depth over the experiment period.

[Figure 1 ILLUSTRATION OMITTED]

At the final sampling in the presence of plants, the general trend was for the surface 1-2 cm to be the least acidic layer of the profile. One notable exception to the general trend occurred in the +HAY amendment at initial soil [pH.sub.Ca] 4 where the soil was less acidic in the middle of the profile. This pattern of [pH.sub.Ca] change with depth was unexplained.

From the experimental results, it appeared that under glasshouse conditions and in a closed system where water movement was minimised by frequent applications of small volumes of water, stratification of soil [pH.sub.Ca] was minimal. This differed from the observations of Black (1992) who found stratification of soil [pH.sub.Ca] after 6 weeks in a pot experiment under glasshouse conditions. Black found that soil [pH.sub.Ca] changed little in the 0-2 cm and 8-10 cm layers but between 2 and 8 cm, soil [pH.sub.Ca] decreased by up to 1.2 [pH.sub.Ca] units. We attribute the difference in stratification between the 2 experiments to the amendments used in each experiment. Black (1992) used urine and urea which were applied to the surface of the soil. Subsequent movement and concurrent nitrogen transformations of these water-soluble amendments would have caused differential changes in [pH.sub.Ca] within the pot. In our experiment, amendments were homogenously mixed throughout the soil and not spread upon the surface. The ammonium sulfate and hay amendments used in our experiment were also less mobile than the urea and urine used in the experiment of Black (1992).

Field experiment

Plant yield parameters are shown in Table 3. The dry matter yields (Table 3) in August showed significant differences between all amendments. The +C and +N amendments had lower dry matter yields than the control. The +HAY amendment had a higher dry matter yield than the other amendments. In September, dry matter yield of the +C amendment remained lower than that of the control. The +N amendment was not different from the control, and the +HAY amendment remained higher in dry matter yield than the control. At anthesis (October) the +C amendment had dry matter yields that were not different from the control, which suggested that the effect of sucrose had diminished by this time. The dry matter yield of the +N amendment was also not different from the control. The +HAY amendment produced dry matter yields which remained higher than in the other amendments. In December, grain yields showed the same pattern as the October dry matter yields.

Table 3. Dry matter yield (g/10 plants), total nitrogen concentration (%), and nitrogen uptake (mg/10 plants) in oats during the season in the field trial

Within columns and parameters, means followed by the same letter are not significantly different at P = 0 - 05
 Aug. Sept. Oct. (anthesis)

 Dry matter yield

Control 1.85c 9.9913 36.9a
+C 0.54a 3.19a 25.3a
+N 1.48b 12.6b 46.5a
+HAY 2.28d 30.9c 142.2b

 Total N concentration

Control 2.82a 1.14(n.s.) 0.75(n.s.)
+C 2.36a 1.61(n.s.) 1.17(n.s.)
+N 4.55b 2.07(n.s.) 0.99(n.s.)
+HAY 4.97b 1.35(n.s.) 0.77(n.s.)

 Nitrogen uptake

Control 52.2b 113.9a 276.8a
+C 12.7a 51.4a 296.0a
+N 67.3b 260.8b 418.5a
+HAY 113.3c 417.2c 1094.9b

 Dec. (harvest, grain)

 Dry matter yield

Control 15.7a
+C 16.4a
+N 14.0a
+HAY 58.2b

 Total N concentration

Control 1.81a
+C 1.77a
+N 2.28b
+HAY 1.79a

 Nitrogen uptake

Control 284.2a
+C 290.3a
+N 319.2a
+HAY 1041.8b


(n.s.), not significant.

The total nitrogen concentration in plant tops (Table 3) was significantly different between the amendments only at the August sampling. The concentrations of nitrogen in the control and +C amendments were lower than in the nitrogen amendments (+N and +HAY). At the final sampling time, there was no significant difference in grain total nitrogen concentration between the control, +C, and +HAY amendments, whereas the nitrogen concentration of the +N amendment was higher than the other amendments.

The data for nitrogen uptake by plants (Table 3) showed that in August there was no difference between the control and the +N amendment, the +C amendment was lower than the control, and the +HAY amendment higher than the other amendments. In September, nitrogen uptakes of the +C amendment and the control were not different, the +N amendment was higher than the control, and the +HAY amendment was higher again. At anthesis there was no difference in nitrogen uptake between the control, +C, and +N amendments, whereas the +HAY amendment had higher uptake than the other amendments. The results were similar for the grain nitrogen uptake in December.

The low dry matter yield and nitrogen uptake of plant tops and grain in the control and +C amendments were attributed to the plants only recovering nitrogen following mineralisation of soil organic nitrogen. In the +C amendment, immobilisation of available nitrogen reduced the dry matter yield relative to the control until September.

The significantly higher dry matter yield and nitrogen uptake of plant tops and grain in the +HAY amendment were attributed to (a) an increase in available nitrogen following mineralisation of the added organic nitrogen, and to (b) the increase in initial soil [pH.sub.Ca] with the addition of hay (Fig. 2). The suggested mechanisms for the relatively low dry matter yields and consequent low nitrogen uptake of the +N amendment were reduced uptake of nutrients and/or water as made evident in the following discussion.

[Figure 2 ILLUSTRATION OMITTED]

Grain yield was highly correlated to nitrogen uptake at anthesis (the product of dry matter yield at anthesis and total nitrogen concentration) across all amendments ([r.sup.2] = 0 - 897, with n = 15, P [is less than] 0.001), suggesting that plant uptake of nitrogen was the factor controlling yield. Grain yield was also correlated with the average soil [pH.sub.Ca] 0-10 cm across the season ([r.sup.2] = 0.582, P [is less than] 0.001), which also suggested that the soil [pH.sub.Ca] influenced grain yield. This led to the hypothesis that decreased [pH.sub.Ca] caused a reduction of root development with a corresponding limit to dry matter production. The mean [pH.sub.Ca] of the 0-10 cm layer in the +N amendment was 3.82 (50% exchangeable aluminium) at the end the experiment, which was the lowest soil [pH.sub.Ca] of all amendments, as may be expected from the addition of ammonium-based fertilisers (Haynes 1986a). This low soil [pH.sub.Ca] might have caused a decrease in the development of oat roots, thus limiting water and nutrient (especially nitrogen) uptake in the +N amendment. This would be consistent with the findings of Pinkerton and Simpson (1986a, 1986b) where the roots of subterranean clover, wheat, lucerne, and rapeseed were found to be stunted in acidic soils ([pH.sub.1:5water] 4,7). They suggested that crops with poor. root growth will be susceptible in drought conditions as roots will be unable to reach deep soil water. The present study supports these findings of Pinkerton and Simpson (1986a, 1986b) and even suggests that poor root growth may limit water and nutrient uptake without drought conditions.

Fig. 2 gives the soil [pH.sub.Ca] profiles for each amendment at the initial and final sampling times. The mean initial soil [pH.sub.Ca] (0-10 cm) was 3.99, 4.03, 4.15, and 4.28 for the control, +N, +C, and +HAY amendments, respectively. In general, there was a decrease in soil [pH.sub.Ca] over time regardless of whether or not plants were grown (Fig. 2). The mean final soil [pH.sub.Ca] reported is an average of the final soil [pH.sub.Ca] of the +plant and -plant treatments as there was no significant difference between the plant treatments. The mean final soil [pH.sub.Ca] (0-10 cm) was 3.92, 3.82, 4.02, and 4.04 for the control, +N, +C, and +HAY amendments, respectively.

At the initial sampling time, there was no [pH.sub.Ca] stratification in the control, +N, or +C amendments. In the +HAY amendment there was evidence of stratification of soil [pH.sub.Ca] which might be a result of the poor mixing of hay particles through the soil (Fig. 2). Between sowing (initial sampling) and harvest (final sampling) the soil [pH.sub.Ca] decreased in most layers, except in the surface 2 cm. This led to the appearance of [pH.sub.Ca] stratification. Between 2 and 10 cm, the change in soil [pH.sub.Ca] over time was significant except in the +C amendment in the 6-8 cm layer. There was no significant difference between the +plant and -plant treatments (except for the +N amendment in the 6-8 and 8-10 cm layers), which indicated that the magnitude of the [pH.sub.Ca] changes caused by the nitrogen transformations within the soil was greater than any changes in [pH.sub.Ca] caused by plant uptake of nitrogen.

The addition of hay increased the initial soil [pH.sub.Ca] relative to the control. The +HAY amendment had the greatest decrease in soil [pH.sub.Ca] over time, but at the final sampling time the soil [pH.sub.Ca] between 2 and 10 cm was similar to the initial pH of the control.

Fig. 3 shows the changes in soil [pH.sub.Ca] over the growing season for the +HAY amendment, which had the greatest magnitude of [pH.sub.Ca] change. However, for all amendments the same general trend of [pH.sub.Ca] change existed. By August, the [pH.sub.Ca] had not significantly changed compared with the initial soil [pH.sub.Ca]. The soil acidified from August to October to 10 cm depth. In the surface 0-2 cm layer, soil [pH.sub.Ca] decreased between August and October, then increased between October and December. The [pH.sub.Ca] decrease to 10 cm depth between August and October was attributed to mineralisation and nitrification followed by nitrate leaching. Little nitrate production would have occurred over winter because of the cold conditions. In spring, with the onset of warmer temperatures, any nitrate produced would have leached from the surface soil with the 210 rum of rain which fell over the intervening period. The increase in [pH.sub.Ca] in the 0-2 cm depth between October and December could be due to nitrogen mineralisation continuing but nitrification being suppressed by the decrease in soil water. Conyers (1992) also attributed the [pH.sub.Ca] increase to manganese reduction occurring as the soil dries.

[Figure 3 ILLUSTRATION OMITTED]

The general trend was a [pH.sub.Ca] decrease throughout the season regardless of the presence or absence of plants. The [pH.sub.Ca] decrease was presumed to be due to mineralisation and nitrification followed by nitrate leaching from these layers. Once nitrate has been leached from a layer, that layer remains acidic in that growing season regardless of whether or not the nitrate in deeper layers is taken up by plants.

For each amendment, the magnitude of the [pH.sub.Ca] changes within each layer varied throughout the season. In the control and the +C amendment, [pH.sub.Ca] changes between sampling times were relatively small, less than 0.25 [pH.sub.Ca] units, whereas the changes in the +N amendment were between 0.3 and 0.4 [pH.sub.Ca] units and in the +HAY amendment up to 0.5 [pH.sub.Ca] units.

This observation of [pH.sub.Ca] strata within the surface 10 cm is potentially significant to crop and pasture production. Pinkerton and Simpson (1986b) identified problems with soil water availability to plants in the most acidic layers, with plant roots being stunted by the acidity. Pinkerton and Simpson (1986b) and McLaughlin et al. (1990) showed that as [pH.sub.Ca] decreased through the 0-10 cm depth interval, phosphorus availability also decreased. Young et al. (1995) found that nitrification was negatively correlated with soil depth as [pH.sub.Ca] decreased within the strata of the surface 10 cm. Hence, the availability to plants of water and nutrients in the profile may be restricted if root growth becomes impeded by acidic subsurface layers of soil. Further, the bioavailability of nutrients from within these acidic subsurface layers may also be restricted.

Previous studies have suggested that the development of soil [pH.sub.Ca] stratification was apparent after 2 months in a glasshouse (Black 1992) and 5 years in the field (Conyers and Scott 1989). The rate of stratification in this field trial was shown to occur within a season and, in fact, occurred between the months of October (mid spring) and December (early summer).

Conclusions

Under the closed conditions of the glasshouse, where mass flow of water was minimised by frequent applications of small volumes of water, stratification of soil [pH.sub.Ca] was minimal and occurred only in the surface 1 cm. There was no opportunity for complete removal of nitrate through leaching after nitrification, as any nitrate movement was confined within the pot.

Stratification of soil [pH.sub.Ca] was identified under field conditions and has been found to occur within the latter part of a 6-month growing season. The [pH.sub.Ca] difference between the 0-2 cm and 2-10 cm soil layers ranged from 0.22 [pH.sub.Ca] units for the control to 0.51 [pH.sub.Ca] units for the +HAY amendment. The processes which determined grain yield in the field trial were also the processes which contributed to the formation of stratified [pH.sub.Ca] The presence or absence of plants had little effect on the development of stratification. Therefore, the stratification of [pH.sub.Ca] is believed to be due to nitrification followed by the spatial separation of the [H.sup.+] and the N03 by leaching of N03 with a cation other than [H.sup.+].

Acknowledgments

Our thanks to Steve Morris, the late Cheryl Wilson, Brian Cullis, Bernard Ellem, Fiona Thompson, and Phil Eberbach for their assistance with the statistical analyses, Les Rodham for sowing the field trial, and to the casual staff who helped with the glasshouse and laboratory work. LWRRDC funded the project.

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Manuscript received 17 October 1997, accepted 23 February 1998

C. M. Evans(AB), M. K. Conyers(A), A. S. Black(B), and G. J. Poile(A)

(A) NSW Agriculture, Agricultural Research Institute, PMB, Pine Gully Rd, Wagga Wagga, NSW 2650, Australia. Email: evansc@agric.nsw.gov.au

(B) Charles Sturt University, PO Box 588, Wagga Wagga, NSW 2678, Australia.
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Author:Evans, C.M.; Conyers, M.K.; Black, A.S.; Poile, G.J.
Publication:Australian Journal of Soil Research
Date:Jul 1, 1998
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