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A survey of the cation content of piggery effluents and some consequences of their use to irrigate soils.

Introduction

High levels of potassium ([K.sup.+]) in soils are believed to degrade soil structure, and plant production is reduced because the physical fertility of the soil is diminished. Piggery effluent has relatively high concentrations of [K.sup.+] (Kruger et al. 1995; Biswas et al. 1999), and we were commissioned to determine levels of [K.sup.+] in piggery waste and to see if general patterns of chemical and physical consequences of its application to soil might be discerned across a range of soils and climates from south-eastern Queensland to south-eastern South Australia. Redding et al. (2002) surveyed land management of agricultural, and specifically piggery-derived, effluents, and while they focussed on phosphorus, the general issues they identify in relation to effluent management apply to this paper as well. This paper focuses on the fate and effects of [K.sup.+] on effluent properties and on possible soil physico-chemical trends associated with effluent irrigation. It describes some outcomes of a preliminary survey of 6 substantial piggeries where effluent has been applied systematically to the soil.

Literature on the nutritional significance of [K.sup.+] is extensive (Munson 1985), as is that on the reaction chemistry of the element (e.g. Beckett 1964a, 1964b; Sparks and Huang 1985). There is, however, little information on the effects of [K.sup.+] on soil structure beyond some early observations of its general physico-chemical similarity to sodium ([Na.sup.+]) and the implication that their effects on soil structure would be similar (Richards 1954). Nevertheless, Biswas et al. (1999) state that 'if K on the exchange sites exceeds 30% then it behaves like Na and disperses clays'. This statement might be based on observations (Chen et al. 1983) on some effects of [K.sup.+] on 2 Israeli soils.

We focused on 2 principal issues: (a) the distribution of [K.sup.+] relative to other cations in effluents and its accumulation in soils to which effluents had been applied; and (b) possible soil physical effects of enhanced [K.sup.+] levels in soils and effluents. Both of these issues are affected by soil and effluent cation ratios and concentrations, so it is useful to comment on the use of these ratios.

Theory

Exchangeable cation ratios

Exchangeable cations contribute to the material balance required to estimate the fate of cations added to the soil. In non-saline clay soils, for example, the sum of exchangeable cations may greatly exceed that of water-soluble cations and may dominate the exchange consequences of concentration, dilution, or leaching of the soil solution. In saline soils of low exchange capacity, their influence may be trivial. Models such as LEACHM (Hutson and Wagenet 1992) and UNSATCHEM (Suarez and Simunek 1997), which must predict reaction among competing cations, rely on selectivity coefficients or a Gapon exchange constant, [beta], for the cation pairs in the system (Reiniger and Bolt 1972; Bolt et al. 1976; Robbins et al. 1980) with [[beta].sub.Na/Ca] defined for [Na.sup.+] and [Ca.sup.2+], for example, by:

(1) exNa/exCa = [[beta].sub.Na/Ca] [Na]/[square of ([Ca]/2]

where braces, [], represent the solution concentrations of the cations in [mmol.sub.c]/L. This equation is but one of several conventions (Bond 1995) used to describe exchange equilibria. Ideas of this sort also led Richards (1954) to use the sodium adsorption ratio, SAR, defined by:

(2) SAR = [Na]/[square root of ([Ca] + [Mg])/2] [([mmol.sub.c]/L).sup.1/2]

and the potassium adsorption ratio, KAR, defined by:

(3) KAR = [K]/[square root of ([Ca] + [Mg])/2] [([mmol.sub.c]/L).sup.1/2]

to illustrate, in their fig. 9, a relationship between the exchangeable sodium ratio, ESR (= exNa/(CEC - exNa)), and the SAR, and in their fig. 10 the corresponding exchangeable potassium ratio, EKR (= exK/(CEC - exK)), v. KAR relation. These figures also show that [K.sup.+] has a greater affinity for clay than does [Na.sup.+] and that the EKR is almost an order of magnitude greater than the corresponding ESR for the same values of KAR and SAR. These solution ratios provide a useful first estimate of the consequences of the effluent and soil solution cation compositions in our study. However, exchangeable cation ratios are 'peripheral' to arguments relating to structural stability, because structural stability, insofar as it is affected by the dispersion or coagulation of the soil colloid, depends on the properties of the electrical double layer and thus on the equilibrium solution ratios and concentrations (cf. Verwey and Overbeek 1948; Hunter 1993).

Water-soluble cation ratios and structural stability

At a given soil solution salt concentration, soil structural stability decreases as levels of the monovalent cations sodium ([Na.sup.+]) and potassium ([K.sup.+]) increase relative to the divalent calcium ([Ca.sup.2+]) and magnesium ([Mg.sup.2+]). The effects reside in electrical double layer interactions and are parameterised in terms of the ratios and total concentrations of the cations in the ambient equilibrium solution. Quirk (2001) provides a recent discussion. Studies focussed on [Na.sup.+] were initiated by Quirk and Schofield (1955), who set out the consequences of these interactions in terms of soil permeability and parameterised the chemistry in terms of (a)the exchangeable cation ratios and (b) the total equilibrium solution cation concentration. This convention derived from fig. 9 of Richards (1954). Subsequently, Collis-George and Smiles (1963) showed how the solution cation ratios and concentrations controlled the degree of dispersion or coagulation of illitic clay as double-layer theory anticipates. The coagulation or dispersion of the clay in turn affected the structural stability of a remoulded illitic soil, measured using the moisture characteristic method of Childs (1940). The data of Collis-George and Smiles correspond exactly with those of Quirk and Schofield (1955) through fig. 9 of Richards (1954). A review by Rengasamy and Sumner (1998) is based on the Quirk and Schofield approach, but Rengasamy et al. (1984), Oster et al. (1984), and Quirk (2001) adopt the approach of Collis-George and Smiles (1963), and Rengasamy et al. (1984) suggest a similar, but more conservative, separation between stable and precipitating suspensions of Red Brown Earths.

As we note above, there is little information about the effects of [K.sup.+] in soil solutions, but table 1 of Verwey and Overbeek (1948) (reprinted in part as table 2.1 by Hunter 1993) shows that [Na.sup.+] and [K.sup.+] precipitate negatively charged A[s.sub.2][S.sub.3]-sols and Au-sols at almost the same concentrations, although the sol type is very important. We therefore infer that structurally critical values of SAR and KAR in equilibrium with similar total solution cation concentrations should themselves be similar for soils of similar mineralogy. Chen et al. (1983) indirectly supported this contention when they observed that 'the degree of structural damage in soils containing 60-70% exchangeable [K.sup.+] is comparable with that occurring with 10-16% exchangeable [Na.sup.+]. This comparison is insufficient to fully define double layer conditions for clay dispersion/coagulation because the soil solution concentrations are undefined, but we note, from Richards' figures, that an EKR of 60 70% corresponds to a KAR of about 5.5-6.5, while an ESR of 10 16% corresponds to SAR of about 8-12.

We extend this argument to anticipate our measurements of unexpected high concentrations of N[H.sub.4.sup.+] in the effluents. There seem to be no data to guide us on the effects of monovalent relative to divalent cations on the stability of clay suspensions when N[H.sub.4.sup.+] and [K.sup.+] dominate the cation suite; nevertheless, a monovalent cation adsorption ratio (MCAR), defined by:

(4) MCAR = ([Na] + [K] + [NH4 ])/ [square root of ([Ca] +[Mg])/2] [([mmol.sub.c]/L).sup.1/2]

provides a rational basis for assessing the initial effects of these effluents on soil structure. Its use is justified qualitatively (a) by evidence in table 1 of Verwey and Overbeek (1948) that the monovalent cations [Na.sup.+] and [K.sup.+] flocculate a negatively charged colloid at similar concentration, (b) because N[H.sub.4.sup.+] and [K.sup.+] are of similar size and thus of similar effect (Bolt et al. 1976), and (c) by Richards' (1954) use of the sum of divalent cations in the denominator of SAR and KAR as a way to account for them. Preliminary experiments described in Appendix 1 support these ideas. N[H.sub.4.sup.+] appears to be ephemeral in the soil and [K.sup.+] is fixed, so the MCAR must be used carefully.

Finally, we reiterate that Eqns 2 and 3, together with an analogous relation for N[H.sub.4.sup.+] (Eqn 5 defined below), provide rational bases to estimate exchange among competing cation sets and, with the total solution cation concentration, also provide guidance on chemically moderated structural stability. They also permit comparison with the USDA database of Richards (1954).

Experimental

Site selection, descriptions, and sampling

Six piggeries were selected to represent a range of soils and climates. Each piggery has carried >1500 sows for a decade or more. Physical boundaries between irrigated and non-irrigated areas were distinguished confidently, but in contrast with experiences of Redding et al. (2002), operators' records of effluent application and field histories were uncertain.

Farm information

Farm 1

This farm is located in the south-east uplands of climatic Zone 5 (Thackway and Cresswell 1992) with an average annual rainfall of 648 mm. It has been lightly managed for > 10 years. Effluent application rates have been light and various ways to manage effluent have been explored. Analysis and nutrient values of liquids and solids are available. Liquid and solid are separated in contour sedimentation channels and liquid is ponded there until it is irrigated. The solid is dried in the sun and some sold as compost. A cropping cycle seeks to remove excess nutrients associated with effluent irrigation. Existing farm records do not permit nutrient balance calculations.

Farm 2

This farm is in the Coastal Warm Temperate area of climatic Zone 6 (Thackway and Cresswell 1992) with an average annual rainfall of 719 mm. On this farm, effluent has been applied to approximately 100 ha of kikuyu pasture for almost 30 years. Effluent application rates as great as 500 [m.sup.3]/day were once applied but present rates lie in the range 150-200 [m.sup.3]/day. Most areas are grazed but some silage/hay has been taken from about 50% of paddocks. The soils and effluents are routinely monitored but the data do not permit estimates of material balance.

Farm 3

This farm is in the Coastal Warm Temperate area of climatic Zone 6 (Thackway and Cresswell 1992) with an average annual rainfall of 647 mm. Effluent has been applied here to approximately 95 ha of pasture, including some lucerne, for approximately 30 years. Both solid and liquid have been applied but local rates are uncertain. Solid is now composted with the intention of sale off-site. Liquid effluent is sprayed and treated areas are cropped and grazed. Managers assert that good production follows effluent application and they now seek to achieve nutrient balance, although anecdotally, this has not always been so. Soils are routinely monitored but we cannot compare effluent treated and untreated areas from available data.

Farm 4

This farm is in an area transitional from Wet/Dry Monsoon to Central Arid of climatic Zone 8 (Thackway and Cresswell 1992) with an average annual rainfall of 673 mm. Effluent has been applied for 6 years to approximately 80 ha of cropping land, which is double-cropped if the weather permits. Neighbours also use substantial amounts of effluent. Previously, coarse-textured sloping land adjacent to the piggery was used. This land is now rested and all material is applied to heavy clay (Vertosol) alluvial flats. Both the coarser texture and the heavy clays soils were sampled. Good current records of effluent application are maintained on this property; however, in the absence of export data, they do not permit us to assess effects of effluent application.

Farm 5

This farm is in an area transitional from Mediterranean to Central Arid of climatic Zone 3 (Thackway and Cresswell 1992) with an average annual rainfall of 540 mm. Sampling sites are in, or adjacent to, approximately 120 ha of cropping land where effluent has been applied heavily under a centre pivot system for 7 years. Effluent has been used to satisfy the water demand of the crop, and maize/sorghum has been used to export nutrients. Excellent current records are maintained on this property, but they do not permit us to assess the long-term effects of effluent application.

Farm 6

This farm is in the Mediterranean area of climatic Zone 2 (Thackway and Cresswell 1992) with an average annual rainfall of 702 mm. Effluent has been applied to approximately 80 ha of cropping land under a centre pivot system for 6-8 years. Soils are deep, well-draining sands and maize is used to export nutrients from the site. Much thought has been devoted to sustainable waste management in this enterprise. Solids are separated from the liquids using a screw press. The liquid effluent is diluted by up to 1:20 before irrigation and the solid is composted with vegetable matter obtained off-site and sold to help establish perennial horticultural crops. Routine soil analysis is carried out, but again, data do not permit us to assess the material balance.

Sampling procedure

Within each farm, 2 or 3 contrasting pairs of profiles were sampled that had, and had not, received effluent. This discrimination was based on fence lines separating spray irrigation 'treatments' or, in 2 cases, on soils beneath and beyond centre pivot irrigators. Four or five 50-mm-diameter cores were taken within each treatment and bulked according to depth. Sampling depths tended to be in 50-mm increments to 250 mm and thereafter at 100-mm increments to about 0.7 m. We ensured that complementary pairs of soil profiles that had and had not been irrigated were sampled at similar depth increments to assist comparison. Representative soil profiles at all sites were described and classified (McDonald et al. 1990).

Supernatant samples were also taken from effluent ponds being used for irrigation. These were stored frozen until analysed. We also collected previous effluent and soil analyses to help interpret changes resulting from effluent irrigation.

Laboratory methods

Effluent analysis

Water-soluble cations [Na.sup.+], [K.sup.+], [Ca.sup.2+], and [Mg.sup.2+] were measured using inductively coupled plasma (ICP) spectroscopy. N[H.sub.4.sup.+] was measured by reaction with hypochlorite liberated from dichlor-isocyanurate under alkaline conditions. The absorbance was measured at 660 nm using a dual-channel segmented flow analyser. The pH and electrical conductivity (EC) were also measured and C[1.sup.-], N[O.sup.3.sub.-], and S[O.sub.4.sup.2-] were measured using ion-chromatography. Alkalinity (as HC[O.sub.3.sub.-]) of the effluents was calculated from the pH assuming the effluent was in equilibrium with the C[O.sub.2] in the atmosphere (Lindsay 1979).

Soil analysis

The saturation extract (Method 2D1 of Rayment and Higginson 1992) was analysed for [Na.sup.+], [K.sup.+], [Ca.sup.2+], and [Mg.sup.2+]. The pH and EC were also measured on the saturation extracts from all samples, together with [Cl.sub.-] and S[O.sub.4.sup.2-]. Exchangeable [Na.sup.+], [K.sup.+], [Ca.sup.2+], and [Mg.sup.2+] were measured using atomic absorption spectroscopy, and the cation exchange capacity (CEC), based on N[H.sub.4.sup.+] saturation of the exchange, was determined by measuring the displaced N[H.sub.4.sup.+] using the segmented flow analyser.

Particle size distributions were measured using the pipette method for clay and silt fractions (Gee and Bauder 1986), and total carbon (TC) and nitrogen using a Leco CNS-2000 furnace.

Statistical analysis

Changes in the soil properties were analysed using GENSTAT 5 (Payne 1993). Main effects included in the block and treatment model were farm, soil classification, irrigation, depth, and replicates within the farm (Table 3). It was convenient to cluster depths into the increments 0-100, 100-200, and 200-450 mm. Data graphed for illustrative purposes are averages of the cluster for each farm, treatment, and depth.

Results and discussion

The effluents

Table 1 shows chemical analyses of effluents from the farms surveyed. The table also presents historical data ranges from previous analyses. The methods used to derive these historical data are uncertain and their precision is unknown but their values support our general conclusions that effluents tended to be similar across all farms. This is not surprising in view of the similar basic diets for pigs in intensive systems, together with generally similar flushing procedures used to clear dung and urine from the pens. At the same time, concentration variations were evident that reflect climatic conditions prior to sampling in the spring of 2000 and also variation in reuse of effluent as wash-down water.

We discuss the effluent data first with reference to arbitrary values that approximate those used by environmental protection authorities to classify effluents intended for irrigation. Table 2 sets out indicative critical values of key characteristics.

Cation concentrations

Cation concentrations were high, with N[H.sub.4.sup.+] the most abundant, followed by [K.sup.+] then [Na.sup.+], while [Ca.sup.2+] and [Mg.sup.2+] concentrations were relatively modest. The high values of N[H.sub.4.sup.+] suggest that treatment systems were generally overloaded and anoxic. This seems to be a common feature of piggery effluents worldwide (Bernal et al. 1993; Sloan et al. 1999; Pote et al. 2001). The effects of these high concentrations of N[H.sub.4.sup.+] are important and are discussed later.

Cation ratios

The effluent SAR shown in Table 1 was in the range 3.7 < SAR [([mmol.sub.c]/L).sup.1/2] < 11.4. According to conventions summarised by Kruger et al. (1995), values of SAR in the range 6-8 present problems that can be ameliorated with gypsum, but irrigation with waters with SAR >8 is not generally recommended. Richards' (1954) fig. 9 implies that, at an SAR of 8, in the absence of cations other than [Ca.sup.2+], [Na.sup.+] will occupy about 10% of the exchange sites.

The effluent values of KAR were somewhat greater (8.7-14), with an average of about 10 [([mmol.sub.c]/L).sup.1/2]. Richards' (1954) fig. 10 suggests that a KAR value of 10 will be associated with an exchangeable [K.sup.+] percentage of >50% and that exchangeable [Ca.sup.2+] and [Mg.sup.2+] will be substantially reduced by sustained effluent irrigation.

We also defined and tabulated an ammonium adsorption ratio, Am-AR, analogous to SAR and KAR, viz.:

(5) AmAR = [N[H.sub.4]/[square root of ([Ca] + [Mg])/2] [([mmol.sub.c]/L).sup.1/2]

The smallest tabulated value of AmAR was 15.8 and the average of our measurements was 36. Because of the similarity between [K.sup.+] and N[H.sub.4.sup.+], we infer that such high numbers must result in relatively large amounts of exchangeable N[H.sub.4.sup.+] following irrigation.

Our focus on the solution cation ratios and concentrations reflects their central role as determinants of structural stability.

Colloid dispersion and structural stability

Figure 1 shows values of SAR, KAR, AmAR, and MCAR [([mmol.sub.c]/L).sup.1/2], graphed against the total cation concentration. Also shown are the Collis-George and Smiles (1963) relation (which matches data of Quirk and Schofield 1955) and the relationship that Rengasamy et al. (1984) derived for Red Brown Earths in Australia. These define dispersion or coagulation in kaolinite/illites and thence influence the structural stability of the soil. Smectites tend to be more stable. In terms of the SAR, all effluents would be regarded as stable using the Rengasamy criteria. Similarly, KAR and AmAR values by themselves would appear to imply stability if the Rengasamy criteria are any guide. The MCAR data appear problematic, however, and while all effluent data lie below the 'coagulation' line of Collis-George and Smiles (1963), the situation using the Rengasamy et al. (1984) criteria is less clear-cut. Nevertheless, dispersion tests with soils confirm that the clay is flocculated in solutions of similar cation constitution. We reiterate that none of these effluents would be expected to cause clay to disperse if we use criteria, restricted to sodium, of Rengasamy et al. (1984) or the more relaxed criteria of Collis-George and Smiles (1963). These issues are discussed below in the context of the soils data as well as in the Appendix, but require further study.

[FIGURE 1 OMITTED]

Subsidiary issues affecting effluent irrigation Electrical conductivity

Australian environmental protection agencies (see e.g. Anon. 1995) set limits to irrigation waters summarised in fig. 4 on p. 158 of Kruger et al. (1995). In terms of these criteria, all effluents were saline (4 < EC (dS/m) < 10) and are unsuitable for irrigation unless soils are very permeable and crops are highly salt-tolerant.

pH values

The pH values tended to lie between 7.6 and 8, but with one value >9. These values also exceed levels generally acceptable in irrigation waters.

Anions

The total cation charge substantially exceeded that of the measured anions ([Cl.sup.-] , SO[4.sup.2-] , and N[O.sup.3-]). N[O.sub.3.sup.-] estimated from the pH and assuming the system to be in equilibrium with C[O.sub.2] in the atmosphere (Lindsay 1979) was insufficient to account for the difference between cations and anions, which we must attribute to phosphate (Redding et al. 2002). There was no measurable N[O.sub.3.sup.-] (Table 1).

The role of N[H.sub.4.sup.+]

The complicating element in these discussions is the role and fate of N[H.sub.4.sup.+] in the effluent. This cation is not seen as a significant component of 'normal' irrigation water; it rarely features in criteria used to assess water suitability for irrigation, and most authors tend to ignore its influence to focus on the 4 alkaline cations (but see de Haan and Zwerman 1976). High values of AmAR (Eqn 5) associated with high concentrations of N[H.sub.4.sup.+] in the effluent, however, led us to expect, initially, a very high exchangeable N[H.sub.4.sup.+] ratio. This effect will be moderated by volatilisation, which Kruger et al. (1995) estimated to represent about 20% of that in the pond liquor. Nevertheless, a decrease of even 50% in AmAR from values as great as 36 still represents a large load of a competitive cation. Subsequently, oxidation to N[O.sub.3.sup.-] or uptake of the N[H.sub.4.sup.+]-N will occur. This will affect our interpretation of the significance of both the EC and the pH in relation to the suitability of these effluents for irrigation, so critical values of EC that reflect the long-term consequences of irrigation with effluents are required. Explicitly, EC values that are so dominated by high concentrations of N[H.sub.4.sup.+] should be considered more flexibly than those where NaCl is dominant and it may be more appropriate to calculate a corrected EC based only on the 4 alkaline cations and use this value to assess the suitability of the effluent for irrigation.

Soil characteristics

All farms initially asserted that they held good records of paddock treatment but it emerged that only one retained quantitative records, and even there, it was impossible to attempt an accurate material balance for [K.sup.+] or other cations because of local climatic variability, uncertain effluent concentration histories and application rates, and uncertain commodity removal. Farms 2, 3, 4, and 5, for example, had irrigated relatively heavily for more than 10 years. Farms 2, 3, and 4 were grazed with some cropping and commodity removal. Farm 5 maintained an intensive cropping program. Despite these uncertainties, data comparing irrigated and non-irrigated profiles remain useful, but because of the ranges of soils, climate, and treatments selected in this survey, we sought general trends here and defer detailed analyses of well-defined cases to later papers.

We look first at 'general' properties of the soils, then at the specific issues of [Na.sup.+], [K.sup.+], and N[H.sub.4.sup.+] in the saturation extract, and then the exchangeable cations.

Soil classification

Classifications of the soils encountered in this survey are shown in Table 3, which also identifies the surface soil clay content and mineralogy determined by X-ray diffraction. Detailed profile descriptions are available in the CSIRO National Soils Data Base. In terms of Northcote's Principal Profile Forms, soils varied from uniform, heavy clay, self-mulching soils (Vertosols) through texture contrast soils to uniform sandy soils (Arenic Rudosols).

Saturation extract cation ratios

It is useful to illustrate this dataset graphically as well as in tabular form. We ensured that complementary pairs of soil profiles that had and had not been irrigated were sampled at the same depth increments to provide a basis for comparison. For any attribute of concern, we plotted the value of the non-irrigated variant at any depth as the ordinate, and the value for the irrigated partner as the abscissa, in rectangular coordinates. If no change were associated with irrigation, the point would plot on the 1:1 diagonal; displacement from this line is interpreted as a 'consequence' of irrigation. Figures 2 and 3 show the effects of irrigation on KAR and SAR.

[FIGURES 2-3 OMITTED]

There was evidently a general increase in these ratios associated with irrigation, with the greatest effects at the heavily irrigated Farms 2 and 5. Farms 3 and 4, associated with heavy clays, showed little increase, and Farms 1 and 6, where application has been modest or the effluent diluted before irrigation, showed little effect.

Table 4 deals with these data statistically and relates the KAR, SAR, and MCAR of the saturation extract of the irrigated soils to those of their non-irrigated partners. We reiterate that by the time these soils were sampled, all trace of N[H.sub.4]-N had disappeared, so MCAR was the sum of KAR and SAR. The farms differed significantly (P < 0.001) and there were also differences associated with irrigation (P < 0.01). At Farms 2 and 5, the increase in cation ratios in association with effluent application was significant (P < 0.01) and extended down the profiles. In particular, SAR tended to increase with depth in irrigated soils and may have reflected the displacement of [Na.sup.+] by N[H.sub.4.sup.+] and [K.sup.+] immediately following irrigation. The chromatographic chemistry of this process will be published separately. At Farm 1 there was an apparent, though modest, increase in the cation ratios, whereas at the other farms there was no significant effect. In no soil did the KAR and SAR of the saturation extract approach effluent values.

Saturation extract E[C.sub.e]

Table 5 shows that there was an effect of irrigation on E[C.sub.e] between farms (P < 0.001) and clear effects associated with effluent irrigation (P < 0.00l). Explicitly, Farms 2, 5, and 6 revealed significant increases in E[C.sub.e] between the irrigated and non-irrigated profiles (P < 0.001), and in Farms 2 and 5, E[C.sub.e] values of the irrigated soils increased with depth. There was no significant effect among the other farms, although E[C.sub.e] values generally tended to decrease with increasing depth. Generally, no soil presented serious salinity problems. In all cases E[C.sub.e] was substantially less than that of the effluent, E[C.sub.w], shown in Table 1, and no soil had measurable levels of water-soluble N[H.sub.4.sup.+] when sampled. These observations are consistent with our suggestion that the E[C.sub.w] of the effluents might be operationally 'discounted' for the N[H.sub.4.sup.+] that it contains.

E[C.sub.e] v. soluble cations in the saturation extract

Figure 4 shows the relationship between water-soluble cation concentration and E[C.sub.e], using data measured on the saturation extracts for all samples across soils ranging from Arenic Rudosols to self-mulching Vertosols. We expected such a relationship because the solution conductivity is a consequence of the activity of a population of ions over practical ranges of concentration (McNeal et al. 1970; Marion and Babcock 1976; Lindsay 1979; Sposito 1989). Richards (1954) published a similar dataset for Californian soils, and Rengasamy and Sumner (1998) (for example) cite it. The total dataset in Fig. 4 has the equation:

[FIGURE 4 OMITTED]

(6) [log.sub.10](C/([mmol.sub.c]/L)) = (1.007 [+ or -] 0.004) +

(1.012 [+ or -] 0.007) [log.sub.10] (E[C.sub.e]/(dS/m))

with [R.sup.2] = 0.99. It varies slightly from the 'approximate average line' of Richards, which has the same intercept but a slope of 1.093, but corresponds very closely with, and is marginally better than, eqn 9 of Rengasamy et al. (1984), which represents data from their study of Red Brown Earths in Australia. The excellence of this relationship implies that the comments above on the E[C.sub.e] data apply also to the total soluble cation relations within and between soils and 'treatments'. This graph also presents the effluent data of Table 1 and includes, in the soluble cation concentration, the contribution of the N[H.sub.4.sup.+].

Structural stability criteria

Figure 5 relates MCAR to the total cation concentration for the saturation extract data. As with previous general relationships, different soils and treatments behaved differently. Relative to the Collis-George and Smiles (1963) line, all soils appeared to be structurally stable at the saturation paste water content but it is evident that the status of soils in the different farms was different. The Rengasamy et al. (1984) 'stability line' suggests that irrigated soils at Farms 3 and 5 are potentially unstable, although high organic matter at Farm 3 masks this. These observations are consistent with field observation and with a dataset presented in Appendix 1. We offer no further generalisations in this regard but note that the concentration of this extract implies that, in their natural state, these soils must have a greater fraction of exchangeable monovalent cations than the saturation extract concentrations imply because an increase in concentration increases the influence of the monovalent cations according to their concentration, while the influence the divalent cations increases according to the square root of their concentration. The effect is moderated by the relative amounts of exchangeable and water soluble cations (Collis-George and Smiles 1963; Bolt et al. 1976).

[FIGURE 5 OMITTED]

The exchangeable cation ratios

The EKR and ESR complemented KAR and SAR data, and again, data are graphed as well as tabulated. Figures 6 and 7 use the same convention as Figs 2 and 3 to compare irrigated and non-irrigated soils, and EKR data for Farms 2 and 5, in particular, reveal substantial effects of irrigation. Comparison of these figures proved EKR to be a very sensitive indicator of irrigation with effluent containing [K.sup.+], while ESR was relatively insensitive. The difference arises because slopes of the 'isotherms' shown, for example, as figs 9 and 10 of Richards (1954), are quite different. Note too, in the case of Farm 5, that EKR values consistent with the effluent KAR and the Richards (1954) isotherm were approached.

[FIGURES 6-7 OMITTED]

Table 6 statistically relates the EKR and ESR of comparable irrigated and non-irrigated horizons to discriminate between the irrigated soils and their non-irrigated partners. Differences between soils were significant, and Farms 1,2, 4, and 5 showed significant increases in KAR in the surface 100 mm. In Farms 1 and 2 these effects are seen to depth, and in Farm 5 KAR increases significantly with depth. ESR changes were marginal in all soils.

Regrettably, Tables 4 and 6 are formally unrelated because the exchangeable cations were measured at air-dry water content and dilution of the soil water to produce the saturation extract perturbs the distribution of the water-soluble and exchangeable cations according to their exchange isotherms and the amount of charge in solution relative to the cation exchange capacity.

Subsidiary observations

Subsidiary data comparing the effects of irrigation are summarised in Table 7.

The soil pH

Table 7 compares the pH of irrigated soil 1:5 extract in 0.01 M Ca[Cl.sub.2] with that of its non-irrigated partner. Although all effluents were alkaline, there was no particular consequence of irrigation on the surface pH or on the pH trend in the soil profiles that might be associated with effluent irrigation. Redding et al. (2002) made similar observations. At the same time there were systematic trends in some soils. Farm 6 soils were almost pure sand with no buffering capacity and their pH increased throughout the profile (P < 0.01). The pH of soils at Farms 1 and 2 (Chromosols and Kurasols) seemed to decrease (the latter significantly) as did the subsoil for Farm 5 (Chromosols), but patterns were irregular. These effects will be associated with oxidation of N[H.sub.4.sup.+] to N[O.sub.3.sup.-] but will be moderated by the high pH of the effluent (and its origins) and by the potential buffering capacity of the soil.

Total carbon and nitrogen

There was a general increase in TC and TN associated with effluent irrigation, but the effect was greatest at Farm 2. This site had a TC content of 22.9% and TN content of 2.44% compared with the non-irrigated sample of 4.75 and 0.39%, respectively, in the surface 100 mm. We attribute the very large TC and TN values to the irrigated soil being maintained as a grazed Kikuyu pasture for >20 years. Increase in TC will tend to confer structural stability on soils that purely physico-chemical considerations suggest should be unstable (Collis-George and Smiles 1963; Rengasamy et al. 1984). This is consistent with the apparently anomalous stability of the irrigated surface soils from Farm 2.

Concluding remarks

Conclusions from this survey must be qualitative because farm histories were too inexact to estimate material balances that might identify effects of soil type or climate on profile response to effluent irrigation. Nevertheless, systematic patterns emerge in relation to the effluent and to the consequences of its application to soils.

Effluents from all piggeries are chemically similar, with high pH and high EC, and all effluents have very high values of water-soluble N[H.sub.4.sup.+] and relatively high concentrations of [K.sup.+] compared with [Na.sup.+], Ca2-, and [Mg.sup.2+]. Ratios of N[H.sub.4.sup.+] and [K.sup.+] to divalent cations are high. Both of these cations have similar strong affinity for the soil colloid relative to [Na.sup.+], so we expect initially that N[H.sub.4.sup.+] and [K.sup.+] will dominate the exchange complex but N[H.sub.4.sup.+] will then decrease as it is oxidised to N[O.sub.3.sup.-], or taken up by plants. An increase in [K.sup.+] was observed in both the soil saturation extract and the exchangeable cations of irrigated soils when compared with their non-irrigated counterparts. The KAR and EKR seem to be particularly sensitive indicators of irrigation using piggery effluent because of the strong affinity of [K.sup.+] for the exchange complex. Neither the soil nor the extract contained N[H.sub.4.sup.+] at sampling time. Ratios and total concentrations of monovalent and divalent cations in the effluents and the saturation extract of irrigated soils also suggest that these soils will be structurally stable using criteria first defined by Collis-George and Smiles (1963) and data of Quirk and Schofield (1955). However, this conclusion becomes less certain if criteria of Rengasamy et al. (1984) for Red Brown Earth are used, and data presented in Appendix 1 suggest that the effects of all monovalent cations in the soil solution must be considered if a reliable indicator of structural stability based on solution cation ratios and concentrations is to be developed. Basic principles of this approach are discussed.

Because of interactions between the nitrogen oxidation pathways, the high pH of the effluent, and the buffering capacity of the soil, the pH of the soils generally does not appear to respond systematically to irrigation, although the E[C.sub.e] of irrigated soils tends to increase, as does the total carbon content.

All effluents are too salty and should be diluted if they are to meet most irrigation guidelines (Anon. 1995; Kruger et al. 1995; Richards 1954). One might wonder whether the measured EC of an effluent with a high concentration of N[H.sub.4.sup.+]-N and a high pH is the best basis for assessing its suitability for irrigation or whether a reduced value of EC, based, for example, on the sum of the cations other than N[H.sub.4.sup.+], might be more appropriate. Either way, these E[C.sub.w] values remain unacceptable.

Finally it is useful to comment on the experimental procedures we used in this survey. It is easy to identify gaps in data collection in retrospect and costs of analysis demand a parsimonious approach to measurement. At the same time, complementary measurements that test internal consistency of datasets and also permit sound basic comparisons across treatments are most important. The methods described here contain 2 major deficiencies. The first was the failure to ensure that exchangeable cations and those in the saturation extract formed an equilibrium set. The data would then have permitted estimation of selectivity coefficients and thence confirmed basic rules of chemical reaction. They would also, then, have permitted calculation of the consequences of soil water content change and modelling of chemical movement. The other major deficiency was our failure to anticipate the importance of the N[H.sub.4]-N in the effluent and the reaction chemistry cascade that follows irrigation. Experiments dealing with some consequences of high concentrations of both N[H.sub.4.sup.+] and [K.sup.+] applied in irrigated effluents will be reported shortly.

Appendix 1

These experiments relate the potassium and sodium adsorption ratios, KAR and SAR, and their sum, MCAR, v. the total soluble cations, TCC (using the terminology of Rengasamy et al. 1984) to structure stability through the effect of solution ratio and TCC on dispersion (D) or coagulation (C) of the clay in a 1:5 soil:water suspension.

Topsoils from Farm 5, that had (I), and had not (NI) been irrigated, were used. Five soil:water ratios from 1:5 to 5:5 were shaken and allowed to stand for 24 h. These suspensions were then centrifuged and the supernatant solutions analysed. These data are tabulated (Table A1) and the state of the clay mapped on the stability diagram shown in Fig. A1.

These data form part of a more extensive experimental set so our discussion here is cursory but uses the most stringent of the Rengasamy et al. (1984) stability criteria for topsoils (their eqn 4):

(A1) TCC = 1.46SAR + 1.44

Mechanically disturbed topsoils where TCC is less than this equation indicates are said to be stable. Non-irrigated topsoil data (NI data in Table A1) are consistent with this relation insofar as the 1:5 and 2:5 suspensions dispersed and the 4:5 and 5:5 suspensions coagulated (C) and hence bracket this line. Irrigated topsoils (I in Table A1) remained dispersed (D) after 24 h. The use of SAR n TCC (or KAR v. TCC) in relation to Eqn A1 would have incorrectly implied that these materials would be coagulated. The MCAR v. TCC relation meets the Rengasamy criteria for the NI soils, although the irrigated (1) soils bracket the line but were clearly and stably dispersed.

Subsequently, clay from the irrigated soil was equilibrated with SAR (in the absence of [K.sup.+]), KAR (in the absence of [Na.sup.+]), or MCAR (= KAR + SAR) values of 4 [([mmol.sub.c]/L).sup.1/2] and a TCC of 20 [mmol.sub.c]/L (i.e. their SAR or KAR was comparable with the most concentrated. although still dispersed suspensions of field soils). The clay coagulated in each of these systems. The same clay, with TCC values of 8 [mmol.sub.c]/L, tended to coagulate, but when the SAR, KAR, or MCAR values were increased to 8 [([mmol.sub.c]/L).sup.1/2] with TCC remaining at 4 [([mmol.sub.c]/L, they dispersed. That is, the effects of [K.sup.+] and [Na.sup.+] in the adsorption ratios on coagulation and dispersion of this clay appear to be additive.

These relations require more examination but theoretical reasons are sound for proceeding the way we do (but see the qualifications on the use of criteria such as Eqn A1 discussed by Rengasamy et al. 1984). Note too that Rengasamy et al. (1984) measured, but did not consider, [K.sup.+] and give no data, but with samples containing significant amounts of [K.sup.+], their structural stability criteria may be more stringent than is appropriate for general application.
Table A1. Measured equilibrium solution cation concentrations and
adsorption ratios presented as averages of the duplicates shown on the
graph

Solution Na K Ca Mg TCC SAR
 ([mmol.sub.c]/L)

NI[1:5] 0.082 0.235 0.255 0.157 0.729 0.181
NI[2:5] 0.172 0.322 0.535 0.258 1.287 0.273
NI[3:5] 0.257 0.412 0.849 0.397 1.915 0.326
NI[4:5] 0.345 0.503 1.189 0.551 2.588 0.370
NI[5:5] 0.405 0.562 1.438 0.664 3.069 0.395
I[1:5] 1.238 2.097 0.574 0.290 4.199 1.884
I[2:5] 2.393 3.726 1.375 0.662 8.156 2.371
I[3:5] 3.413 4.981 2.221 0.959 11.574 2.707
I[4:5] 4.453 6.194 3.195 1.311 15.153 2.967
I[5:5] 5.550 7.458 4.308 1.758 19.074 3.187

Solution KAR MCAR D/C
 [([mmol.sub.c]/L).sup.1/2]

NI[1:5] 0.518 0.699 D
NI[2:5] 0.512 0.785 D
NI[3:5] 0.522 0.847 D/C
NI[4:5] 0.539 0.909 C
NI[5:5] 0.548 0.944 C
I[1:5] 3.192 5.076 D
I[2:5] 3.692 6.063 D
I[3:5] 3.95 6.657 D
I[4:5] 4.127 7.094 D
I[5:5] 4.283 7.469 D

Table 1. Selected piggery effluent characteristics

The bold values are measurements from this study. Other values bracket
historical data from various sources

 EC pH HC[O.sub.3.sup.-A] [Cl.sup.-] N[O.sub.3.sup.-A]
(dS/m)

 Farm 1

 6.1# 7.6# 1.8# 10.4# <0.1#
 9.5

 Farm 2

 5.1# 7.8# 2.9# 6.8# <0.1#

 Farm 3

 4.8# 7.9# 3.6# 12.1# <0.1#

 Farm 4

 8.4# 7.6# 1.8# 13.2# <0.1#
 6-8 7.7-9 7-12

 Farm 5

 4.3# 8.2# 7.2# 10.4# <0.1#
 5-9 8-9 17

 EC S N[H.sub.4.sup.+] [Ca.sub.2+] [K.sup.+]
(dS/m) ([mmol.sub.c]/L)

 Farm 1

 6.1# 0.7# 48.6# 2.2# 9.9#
 69.3 17.5

 Farm 2

 5.1# 0.6# 50.0# 0.9# 9.4#
 40-46 3-4 10-11

 Farm 3

 4.8# 1.7# 23.6# 2.6# 15.2#

 Farm 4

 8.4# 1.9# 65.0# 1.7# 16.2#
 6-8 37-40 3-4 8-13

 Farm 5

 4.3# 1.7# 25.0 1.3# 13.2#
 5-9 21-62 1-5 10-19

 EC [Mg.sup.2+] [Na.sup.+] Am-AR KAR
(dS/m) [([mmol.sub.c]/L).sup.1/2]

 Farm 1

 6.1# 0.5# 7.0# 42.4# 8.6#

 Farm 2

 5.1# 0.3# 6.5# 65.9# 12.3#
 0.7-0.8 7-8 29-30 7-8

 Farm 3

 4.8# 1.8# 10.4# 15.8# 10.2#

 Farm 4

 8.4# 1.9# 11.3# 48.7# 12.1#
 6-8 1-4 7.4-8 19-25 4-8

 Farm 5

 4.3# 0.4# 7.8# 26.9# 14.2#
 5-9 0.5-2 6-13 15-40 9-14

 EC SAR
(dS/m)

 Farm 1

 6.1# 6.1#

 Farm 2

 5.1# 8.6#
 4-6

 Farm 3

 4.8# 7.0#

 Farm 4

 8.4# 8.5#
 6-8 4-5

 Farm 5

 4.3# 8.4#
 5-9 4-11

(A) Calculated from the pH and assuming the effluent is in equilibrium
with C[O.sub.2] in the atmosphere (Lindsay 1979).

Note: Values are measurements from this study incated with #.

Table 2. Exemplary soil and effluent quality criteria

 Property Units

E[C.sub.e] (saturation extract) dS/m
 E[C.sub.w] (irrigation water) dS/m
 SAR [([mmol.sub.c]/L).sup.1/2]
 ESP %
 KAR [([mmol.sub.c]/L).sup.1/2]
 EKP %
 pH
 Structure stability Depends on relations between SAR,
 KAR, and total soluble cations.
 See text

 Property 'Acceptable' limits

E[C.sub.e] (saturation extract) 4
 E[C.sub.w] (irrigation water) 1-2
 SAR 6-8
 ESP 7-9
 KAR 6-8
 EKP 39-44
 pH 5-7.5
 Structure stability Depends on relations between SAR,
 KAR, and total soluble cations.
 See text

 Property Comments

E[C.sub.e] (saturation extract) Some effect observed if E[C.sub.e]
 >1: cf. Ayers and Westcot
 (1985), Richards (1954)
 E[C.sub.w] (irrigation water) cf. Ayers and Westcot (1985),
 Kruger et al. (1995),
 Richards (1954)
 SAR cf. Kruger et al.(1995) but see
 text
 ESP cf. SAR 'limits' and fig. 9 of
 Richards (1954)
 KAR Inferred from sparse detail
 EKP and fig. 10 of Richards
 (1954)
 pH
 Structure stability Depends on relations between SAR,
 KAR, and total soluble cations.
 See text

Table 3. Soil classification and clay mineralogy, climatic zone, and
qualitative effluent loading

Further detail on these soils is found in the CSIRO National Soils
Database [contact: david.jacquier@csiro.au]. Site 5 is identified as
CP 338 and 339. The others are found under Project Code PRDC1

Farm (site) Australian Soil Classification (A) Topsoil
 clay

1(1,2,3) Red Chromosol [deep] 10-20%
2(1) Red Kurosol [deep] 20-25%
2(2) Yellow Chromosol [moderately 25-30%
 deep]
3(1) Brown Chromosol [very deep] 20-30%
3(2) Brown Dermosol [deep] 14-20%
4(1) Self-mulching Vertosol 50-60%
4(2) Yellow Chromosol 10%
5(1,2) Red Chromosol [very deep] 15-20%
6 Arenic Rudosol 1-2%

Farm (site) Clay mineral Climatic Zone (B)

1(1,2,3) 75% kaolinite; 20% illite Zone 5; SE Uplands
2(1) 55% kaolinite; 20% illite Zone 6; Coastal, Warm,
 Temperate
2(2) NA Zone 6; Coastal, Warm,
 Temperate
3(1) 80% smectite Zone 6; Coastal, Warm,
 Temperate
3(2) NA Zone 6; Coastal, Warm,
 Temperate
4(1) 95% smectite Zone 8; Wet/Dry Monsoon
 transition to Central
 Arid
4(2) NA Zone 8; Wet/Dry Monsoon
 transition to Central
 Arid
5(1,2) 50% kaolinite; 20% illite Zone 3: Mediterranean
 transition to Central
 Arid
6 60% amorphous Zone 2; Mediterranean

Farm (site) Soil
 grouping

1(1,2,3) 1
2(1) 2
2(2) 2
3(1) 3
3(2) 3
4(1) 4
4(2) 2
5(1,2) 1
6 5

(A) Isbell (1996).

(B) The 'Super groups' of Thackway and Cresswell (1992).

Table 4. Effect of effluent irrigation on KAR, SAR, and MCAR
([([mmol.sub.c]/L).sup.1/2]) of the saturation paste extracts of the
soils with depth in the profile

 KAR SAR
Depth Farm
(mm) (soil Non- Non-
 group) Irrigated Irrigated Irrigated Irrigated

0-100 1 (1) 2.08 1.23 0.47 0.29
 2 (2) 8.29 1.85 3.00 0.38
 3 (3) 2.76 3.29 1.90 2.56
 4 (4) 0.23 0.33 2.25 3.08
 4 (2) 4.80 2.89 3.25 0.18
 5 (1) 7.98 0.57 5.01 1.17
 6 (5) 0.57 0.81 3.46 0.86
 s.e.d 1.89 0.92
100-200 1 (1) 1.38 0.90 0.79 0.48
 2 (2) 3.77 1.10 4.03 0.67
 3 (3) 1.76 1.47 2.45 2.67
 4 (4) 0.09 0.11 4.36 6.94
 4 (2) 3.90 2.14 3.86 0.33
 5 (1) 7.37 0.20 5.63 1.84
 6 (5) 0.24 0.44 3.49 0.99
 s.e.d 0.68 1.42
200-450 1 (1) 1.38 0.96 0.79 0.59
 2 (2) 3.01 0.21 6.97 2.15
 3 (3) 1.35 0.68 6.07 4.56
 4 (4) 0.05 0.10 7.09 10.80
 4 (2) 2.45 2.54 1.80 0.41
 5 (1) 6.56 0.20 6.97 2.32
 6 (5) 0.21 0.33 2.71 0.85
 s.e.d 0.93 3.35

 MCAR
Depth Farm
(mm) (soil Non-
 group) Irrigated Irrigated

0-100 1 (1) 2.55 1.52
 2 (2) 11.00 2.23
 3 (3) 4.67 5.86
 4 (4) 2.49 3.42
 4 (2) 8.05 3.07
 5 (1) 13.00 1.73
 6 (5) 4.03 1.67
 s.e.d 2.04
100-200 1 (1) 2.18 1.37
 2 (2) 7.81 1.77
 3 (3) 4.22 4.15
 4 (4) 4.46 7.06
 4 (2) 7.76 2.46
 5 (1) 13.10 2.03
 6 (5) 3.73 1.43
 s.e.d 1.44
200-450 1 (1) 2.18 1.55
 2 (2) 9.98 2.36
 3 (3) 7.42 5.25
 4 (4) 7.14 10.93
 4 (2) 4.25 2.95
 5 (1) 13.54 2.53
 6 (5) 2.92 1.18
 s.e.d 3.17

Table 5. Effect of effluent irrigation on E[C.sub.e], (dS/m) of the
saturation paste extracts of the soils with depth in the profile

 0-100 mm 100-200 mm
 Farm
(soil group) Irrigated Non-irrigated Irrigated Non-irrigated

 1 (1) 0.52 0.46 0.31 0.20
 2 (2) 4.49 1.66 4.65 0.79
 3 (3) 1.27 2.08 0.84 1.25
 4 (4) 1.80 3.42 2.29 1.67
 4 (2) 2.53 1.17 1.53 1.01
 5 (1) 1.44 0.39 1.91 0.24
 6 (5) 1.90 0.65 1.70 0.25
 s.e.d 0.56 0.71

 200-460 mm
 Farm
(soil group) Irrigated Non-irrigated

 1 (1) 0.26 0.19
 2 (2) 6.20 0.55
 3 (3) 0.89 0.96
 4 (4) 0.95 1.30
 4 (2) 0.71 0.42
 5 (1) 3.55 0.22
 6 (5) 1.18 0.10
 s.e.d 0.95

Table 6. Effect of effluent irrigation on EKR and ESR of the soils with
depth in the profile

 EKR ESR
 Farm
(soil group) Irrigated Non-irrigated Irrigated Non-irrigated

 0-100 mm

 1 (1) 0.129 0.083 0.022 0.019
 2 (2) 0.249 0.131 0.025 0.006
 3 (3) 0.189 0.190 0.013 0.013
 4 (4) 0.028 0.040 0.017 0.020
 4 (2) 0.150 0.100 0.015 0.001
 5 (1) 0.36 0.09 0.025 0.013
 6 (5) 0.022 0.047 0.036 0.038
 s.e.d 0.05 0.09

 100-200 mm

 1 (1) 0.151 0.094 0.031 0.025
 2 (2) 0.336 0.102 0.040 0.006
 3 (3) 0.152 0.124 0.021 0.019
 4 (4) 0.012 0.010 0.040 0.065
 4 (2) 0.195 0.110 0.025 0.001
 5 (1) 0.442 0.081 0.042 0.028
 6 (5) 0.013 0.031 0.049 0.071
 s.e.d 0.06 0.018

 200-450 mm

 1 (1) 0.201 0.121 0.033 0.033
 2 (2) 0.330 0.041 0.067 0.031
 3 (3) 0.117 0.064 0.073 0.052
 4 (4) 0.010 0.010 0.075 0.100
 4 (2) 0.175 0.165 0.010 0.005
 5 (1) 0.590 0.080 0.044 0.034
 6 (5) 0.021 0.032 0.079 0.125
 s.e.d 0.07 0.06

Table 7. Effect of effluent irrigation on pH (1:5 0.01 M Ca[Cl.sub.2]),
total C (%), and total N (%) for different soil layers

 pH (1:5 soil:0.01 M Ca[Cl.sub.2])
Depth (mm) Farm
 (soil group) Irrigated Non-irrigated

 0-100 1 (1) 4.85 5.01
 2 (2) 4.88 6.10
 3 (3) 5.52 5.13
 4 (4) 7.35 7.80
 4 (2) 5.70 5.10
 5 (1) 4.92 4.77
 6 (5) 6.98 5.28
 s.e.d 0.34
 100-200 1 (1) 4.40 4.80
 2 (2) 4.93 6.35
 3 (3) 5.77 5.07
 4 (4) 7.73 8.00
 4 (2) 5.45 4.85
 5 (1) 5.33 5.63
 6 (5) 6.89 4.08
 s.e.d 0.30
 200-450 1 (1) 4.63 5.08
 2 (2) 5.53 5.74
 3 (3) 7.11 6.48
 4 (4) 7.95 8.10
 4 (2) 5.40 5.00
 5 (1) 5.78 5.90
 6 (5) 5.60 4.27
 s.e.d 0.76

 Total C
Depth (mm) Farm
 (soil group) Irrigated Non-irrigated

 0-100 1 (1) 2.40 2.39
 2 (2) 22.90 4.70
 3 (3) 3.03 2.19
 4 (4) 2.57 2.79
 4 (2) 4.50 3.75
 5 (1) 1.62 1.25
 6 (5) 1.32 1.28
 s.e.d 4.67
 100-200 1 (1) 0.69 1.04
 2 (2) 3.37 2.20
 3 (3) 1.44 1.17
 4 (4) 2.11 2.19
 4 (2) 0.70 0.58
 5 (1) 0.58 0.36
 6 (5) 1.12 0.98
 s.e.d 0.31
 200-450 1 (1) 0.40 0.47
 2 (2) 1.60 1.00
 3 (3) 0.76 0.57
 4 (4) 1.98 1.76
 4 (2) 0.30 0.17
 5 (1) 0.32 0.30
 6 (5) 0.86 0.39
 s.e.d 0.38

 Total N
Depth (mm) Farm
 (soil group) Irrigated Non-irrigated

 0-100 1 (1) 0.21 0.20
 2 (2) 2.44 0.39
 3 (3) 0.27 0.19
 4 (4) 0.20 0.24
 4 (2) 0.45 0.36
 5 (1) 0.14 0.09
 6 (5) 0.08 0.08
 s.e.d 0.48
 100-200 1 (1) 0.05 0.07
 2 (2) 0.32 0.15
 3 (3) 0.12 0.08
 4 (4) 0.16 0.17
 4 (2) 0.06 0.05
 5 (1) 0.05 0.04
 6 (5) 0.06 0.05
 s.e.d 0.03
 200-450 1 (1) 0.03 0.03
 2 (2) 0.16 0.08
 3 (3) 0.05 0.04
 4 (4) 0.12 0.11
 4 (2) 0.02 0.01
 5 (1) 0.04 0.03
 6 (5) 0.03 0.02
 s.e.d 0.03


Acknowledgments

We thank the pig producers who participated in this project, together with Seija Tuomi and Adrian Beech who managed the chemical analyses. Peter Fogerty described the soils. CSIRO and Australian Pork Limited jointly funded the project.

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Manuscript received 8 May 2003, accepted 25 November 2003

D. E. Smiles (A,B) and C. J. Smith (A)

(A) CSIRO Land and Water, PO Box 1666, Canberra, ACT 2601, Australia.

(B) Corresponding author; email: david.smiles@csiro.au
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Author:Smiles, D.E.; Smith, C.J.
Publication:Australian Journal of Soil Research
Date:Mar 1, 2004
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