Soil spatial variability of drainage properties in relation to phosphate retention and mineralogy on a river terrace of northern Manawatu, New Zealand.
Variability of soil drainage and related properties within soils occurring on terrace lands of the Manawatu district was well known to soil scientists and various explanations have been advanced to explain this phenomenon (Milne 1973; Parfitt et al. 1984). The theories attempt to explain the formation of soils with different drainage properties in relation to the prevailing climatic factors (rainfall) and the past and present vegetation in the area. However, a detailed soil survey carried out at 1 : 25 000 scale in a 60-ha area (2000 m by 300 m) on the Last Glacial terrace near Kiwitea settlement in the Manawatu district found soil map units having drainage ranging from well drained, to moderately well drained, to imperfectly drained within a distance of 2000m (Senarath 2003; Senarath et al. 2004; Senarath and Palmer 2005). Variability of local drainage and related properties occurring within a short distance (2000m) cannot be explained by variation in climate. Previous studies have shown that P-retention is related to soil drainage, and in particular, Parfitt et al. (1984) have shown that soil drainage influences clay mineralogy and hence P-retention. The aim of this study was to investigate the causal factors underlying the short-distance variability of soil drainage and the interrelationships between drainage, P-retention, and clay mineralogy of the soils occurring on the river terraces of northern Manawatu.
Materials and methods
The study area is 3 km north of Cheltenham, near Kiwitea settlement bordering the Oroua River in the northern Manawatu (Fig. 1). Three major ten'aces can be identified within the area: the river flats (180m above sea level (a.s.l.)), covered with recent alluvium (1000-.2000 years BP) and having flat to gently sloping topography; the Last Glacial terrace (200-240 m a.s.1.), covered with a mixture of loamy alluvium derived from a mixture of quarzo-feldspathic sediments, loess, and tephra (<15 000 years BP) overlying river aggradation gravels, and having flat to gently sloping topography; and an upper terrace (240-300m a.s.l.), covered with a mixture of loess and tephra (10000-25 000 years BP) and having flat, undulating, or rolling topography. The alluvium and loess materials have quartzo-feldspathic mineralogy, which suggests that the main source of alluvium was from greywacke and argillite or younger sedimentary rocks with similar source (New Zealand Soil Bureau 1968; Cowie 1978; Senarath 2003).
The loamy alluvial, loess, and tephra deposits on the Last Glacial terrace are generally 0.5-1.5 m thick over gravels and texture varies from silt loam to clay loam. Alluvial layering tends to be gradational with only a slight contrast between adjacent horizons. Alluvium was deposited just prior to the Oroua River beginning to down-cut as climate warmed (Milne 1973). These deposits are of late Ohakean age--15 000-18 000 years old. The underlying terrace gravels were deposited in the early and middle parts of the Ohakean period--18 000-25 000 years ago.
[FIGURE 1 OMITTED]
The area has an annual rainfall that ranges between 900 and 1200 mm and a tendency for dry summers and autumns and wetter winters and springs. The mean annual temperature ranges from 12 to 13.5[degrees]C. The land is used for sheep, beef, and dairy farming.
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An area 2000 m by 300 m (60 ha) was selected on the last glacial terrace (Fig. 2) to investigate relationships between soil drainage, P-retention, and clay mineralogy. A previous study (Senarath 2003) mapped the 60-ha area on the last glacial terrace and identified 3 map units with drainage varying from well drained to imperfectly drained (Fig. 2). On each map unit, a window area 300m by 250m (Blocks A, B, and C) was established and sampling points were selected on a 50-m grid (Fig. 3). Soil sampling methods for drainage class mapping and P-retention are discussed below.
Drainage class mapping
Soil drainage properties were examined by making auger observations to a depth of 1.1 m where possible on a 50-m grid, giving 42 observation points in each window (Fig. 3). The drainage classes were determined on the basis of depth to a redox-mottled horizon and/or reductimorphic horizon (Hewitt 1992; Milne et al. 1995). Soils with gley profile form are considered poorly drained; soils with mottled profile form are considered imperfectly drained; soils containing a redox-mottled horizon below 600 mm are considered moderately well drained; while soils containing no reductimorphic horizon or a redox-mottled horizon within 900mm are considered well drained (Table 1). These criteria follow the drainage class separation criteria used in New Zealand soil surveys (Milne et al. 1995).
The 3D topographic map of the Block B window area (Fig. 4) was generated using the 'Surfer' (version 5) software program. Spot heights on the 50-m grid, established for drainage observations and P-retention sampling, were taken by using a dumpy level and staff.
[FIGURE 4 OMITTED]
Soil samples were collected from surface soils (0-75 mm) on a 50-m grid pattern (Fig. 5). Three soil samples were taken within 300 mm diameter of each observation point using a core sampler and were combined to form a composite sample to determine P-retention (Morton et al. 2000). This is the methodology used in New Zealand to determine fertiliser recommendations for pastures.
[FIGURE 5 OMITTED]
The P-retention of soils was determined according to the method of Saunders (1965) and the results are expressed as percentage values. Eight quality control samples with known P-retention values were incorporated within each 100-sample batch to monitor the possible variations that might arise among different batches. The same P-retention solution and vanadomolybdate solutions were used throughout the analysis of the samples for a fair comparison of results.
Saturated hydraulic conductivity (Ksat)
Saturated hydraulic conductivity measurements were made for undisturbed soil samples taken from each soil horizon, using intact cores (150 mm height and 74 mm diameter) according to the method of Klute (1986). The mean value of 3 measurements was taken as the Ksat value for each soil horizon. Measurements were taken for 4 soil horizons from a selected well-drained and imperfectly drained profile.
Mineralogical properties of soil samples were determined according to methods described by Whitton and Churchman (1987). Clay and sand mineralogy in 4 soil horizons for a selected well-drained and imperfectly drained soil profiles were investigated. Allophane present in soils was determined according to the method of Parfitt and Wilson (1985) and Parfitt (1986). Acid-oxalate-extractable Si was determined according to the method of Blakemore et al. (1987).
Variability in soil drainage
The 1:250 000 scale soil map (New Zealand Soil Bureau 1954) of the study area classifies the entire 60-ha project area (2000m by 300 m) as well-drained Kawhatau silt loam (Table 2). A subsequent land resource map at 1:63 360 scale (New Zealand Land Resource Inventory 1979) reclassifies the area as well-drained Kiwitea loam. When the authors mapped the area at 1 : 25 000 scale (250 m by 250 m grid) it was separated into 3 different soil drainage classes: well drained, moderately well drained, and imperfectly drained (Fig. 2).
Senarath (2003) has explained why neither Kawhatau silt loam nor Kiwitea loam is an appropriate name for the soils on the intermediate terrace, and introduced new series names: Coulter silt loam for well-drained soils, Horoeka silt loam for moderately well-drained soils, and Barrow silt loam for imperfectly drained soils (Table 2). Soil auger observations made on a 50-m grid in selected window areas (Blocks A, B, and C) reveal that the relatively simple soil drainage pattern represented on the 1:25 000 scale map (Fig. 2) is in fact much more complex. Instead of a gradation of drainage status from well-drained to imperfectly drained soils (Fig. 2), there is a mixture of well, moderately well, and imperfectly or poorly drained soils present within close proximity in each block (Fig. 3). The poorly drained soils are identified as Ohakea soils (Table 2), and these were not previously mapped on this surface. At least 3 different soil drainage classes are identified in each of the 300 m by 250 m blocks (7.5 ha). Each of these blocks comprises only one soil drainage class when mapped at 1 : 25 000 scale.
The problem associated with mapping of drainage classes (soil mapping) in this area is that it is difficult to establish an obvious relationship between soil drainage and the topography of the land. The topography of the 3 blocks is relatively flat (0-2[degrees]) and there is no direct relationship between topography and drainage conditions (Fig. 4). A linear regression between land surface elevation and the soil drainage classes calculated for Block B is 0.37 ([R.sup.2] = 0.37). There are some instances where soils in local depressions or areas close to water bodies or streams are imperfectly or poorly drained, but that cannot be accepted as a general rule for the entire area. Figure 4 shows the topography of the land surface of Block B in relation to the topography and the soil (drainage) distribution pattern. Although the poorly drained Ohakea silt loam is located in the lowest position of the landscape, the other soils show no relationship to landscape.
There is a high variability in drainage classes within the previous map units that were identified as containing soils of a single drainage class (Fig. 3, Table 3). Block A consists of 100% imperfectly drained, Barrow silt loam at 1 : 25 000 scale but is in fact a combination of 62% imperfectly drained Barrow silt loam, 21%, moderately well-drained Horoeka silt loam, and 17% poorly drained Ohakea silt loam when grid-surveyed at a resolution equivalent to 1:5000 scale. Block B consists of only 28% moderately well-drained Horoeka silt loam soils instead of the originally mapped 100%. Block C consists of 53% well-drained Coulter silt loam instead of 100%.
The 3 map units have a similar proportion of moderately well-drained soils (21-33%) but there is a distinct trend of increasing percentage of well-drained soils and decreasing percentage of imperfectly drained and poorly drained soils from left to right in Table 3. Notably, Block A contains no well-drained soils and block C contains no poorly drained soils.
Variability in phosphate retention (P-retention)
P-retention values of topsoil samples measured on the 50-m grid vary from 20% to 69% in Block A, from 24% to 82% in Block B, and from 39% to 84% in Block C (Fig. 5). The comparison between soil drainage and P-retention at each observation point indicates that 100% of the poorly drained soils in the study area have low P-retention, whereas 100% of the well-drained soils have high P-retention. P-retention in imperfectly drained soils ranges from low to high: 69% of the observations have medium P-retention, 22% show high values, and only 8% show low values. A majority of the moderately well-drained soils have high P-retention (85%), whereas only 15% of the observations have medium P-retention values (Table 4). The 3 map units contain a wide range of P-retention values but there is a distinct trend of increasing P-retention from Block A to Block C (Fig. 6).
From these observations, there is evidently a close relationship between drainage and P-retention. Poorly drained soils have low P-retention, imperfectly drained soils have medium P-retention, and moderately well-drained and well-drained soils have high P-retention. The relationship has been suspected but not previously demonstrated for New Zealand soils.
Soil drainage, phosphate retention, and clay mineralogy
The sand fractions of both the well-drained and imperfectly drained soils contain volcanic glass (Table 5). The clay fraction of the well-drained soils contains allophone, whereas the clay fraction of the imperfectly drained soils contains no allophane. As expected, P-retention varies in accordance with allophane content (Table 5). The total carbon percentage in the topsoils of Coulter silt loam soils is 6.6% and in Barrow silt loam soils is 4.1%. The organic matter level is medium in both soils (Blakemore et al. 1987). Mineralogical analysis was not carried out for Horoeka silt loam soils; however, the NaF field test on the Horoeka silt loam topsoil samples showed both weak positive reactions and negative reactions for presence of allophone. The topsoil samples of Coulter silt loam soils showed strong positive reaction for presence of allophone. This test confirms that moderately well-drained Horoeka silt loam soils have no allophane or contain a little allophane, less than that of Coulter silt loam soils.
[FIGURE 6 OMITTED]
Parfitt et al. (1984) showed that clay mineralogy is related to soil drainage. It is well known in New Zealand that topsoils dominated by allophane have high P-retention while those dominated by kandite have low P-retention. These results show that the average P-retention of the topsoil is also influenced by the drainage of the whole profile.
According to Parfitt et al. (1984), weathering of rhyolitic tephra is controlled by Si in soil solution. When Si concentration in the soil solution is low (possibly <10 [micro]g/[cm.sup.3]) due to leaching, allophane is formed from volcanic glass; whereas if Si concentration is high (possibly >10 [micro]g/[cm.sup.3]) in the soil solution due to impeded drainage conditions, halloysite is formed. The presence of allophane in the clay fraction of Coulter silt loam can be attributed to weathering of volcanic glass under well-drained conditions. Imperfectly drained Barrow silt loam contains no allophane in the clay fraction due to Si not being leached from the profile, so that kandite minerals (kaolinite + halloysite) form instead.
There has been no satisfactory explanation for short-distance drainage variability of soils occurring on fiver terraces of the Manawatu district. The well-drained Coulter silt loam soils have Ksat values >4 mm/h, whereas the imperfectly drained Barrow silt loam soils have Ksat in 2 horizons slightly below 1 mm/h (Tables 6 and 7). There is a good relationship between Ksat and macro-porosity, except for the 2Ab horizon in the Coulter silt loam soil, which was expected to have lower Ksat related to low macro-porosity (higher Ksat may be related to weaker soil consistence, but this was not measured). The lower Ksat in the imperfectly drained soil is expected and accounts for the imperfect drainage.
As discussed above, the parent material of the soils is 15 000-18 000-year-old alluvial deposits, intermixed with tephric and loess material, having layers of slightly different textural properties. Well-drained Coulter silt loam soils consist of silt loam underlain by clay loam below 950 mm depth subsoil, whereas imperfectly drained Barrow silt loam soils consist of silt loam topsoil underlain by silty clay loam, fine sandy clay loam, and clay loam subsoil (Tables 6 and 7). It is evident from the field data in Tables 6 and 7 that a close relationship exists between soil texture and soil structure. Nutty structure is associated with the silt loam texture class, while blocky structure is associated with the silty clay loam or clay loam texture classes. Soil texture is initially inherited from its parent material, so variations in soil texture are presumed to reflect variations in initial deposition. It is likely that slight textural variations in the initial parent material effected slight variations in soil structural development, silt loam tending to form nutty structure compared with clay loam which formed blocky structure.
The soils on this terrace are >20 m above the Oroua River and are therefore not influenced by a regional water table. It is therefore probable that differences in drainage are related to differences in hydraulic conductivity. Hydraulic conductivity of a soil is directly related to its porosity--more importantly, to its macroporosity. In soils that are aggregated into structural units, porosity is markedly dependent on the size and shape of the structural units. The pore spaces between fine nutty structural units are higher than those between medium or coarse blocky structural units (Griffiths 1985; Griffiths et al. 1999; Webb 2003). Where the proportion of macro-pores is higher in the various horizons of the subsoils, and the pores are continuous, the profile is described as uniform and water moves vertically at the same rate through the profile. Thus, horizons with fine structure or nutty structure have macro-porosity varying from 6 to 12%, whereas the 2 medium to coarse blocky structured horizons have macro-porosity of 3-5% (Tables 6 and 7).
In well-drained Coulter silt loam soils, upper subsoil horizons have nutty structure and the lower subsoil horizons have nutty and blocky structure. The continuity of pore spaces between nutty and blocky structural units may he discontinuous at the horizon boundaries. In imperfectly drained Barrow silt loam soils, subsoil horizons are composed of nutty and blocky structural units. The proportion of pore spaces and their continuity is different between horizons. The hydraulic conductivity also varies accordingly (Table 7). Therefore, water moves very slowly within Barrow silt loam soils, creating imperfect drainage conditions. The whole profile hydraulic conductivity of Coulter silt loam soils is much faster than that of Barrow silt loam soil (Table 6).
It is hypothesised that the differences in soil structure largely caused the differences in drainage status. The shape and the size of the soil structural units have a noticeable affect on the space between them (Griffiths 1985; Griffiths et al. 1999; Webb 2003). Blocky structure is more closely fitting than nutty structure; hence, water can move more readily along structural faces of soils having nutty structure.
Results above show that drainage class varies markedly over short distances (tens of metres) across a large section of a river terrace. We could find no relationship between ridge and hollow topography to account for the drainage differences, and past explanations for variation in drainage class cannot account for short-range drainage variations. We have noted above that there is some correlation of drainage classes with soil texture, P-retention, macro-porosity, and Ksat. We recognise there is a 'chicken and egg' conundrum in determining what has caused the drainage differences because, for example, high P-retention and presence of allophane may be seen as both a result and cause of free-draining conditions. Thus, there is a need to establish some 'trigger condition' that has set the course of soil development. It seems most likely that the drainage variation is related to variations in the permeability profile (probably associated with textural differences) of the initial alluvium. By making this assumption, we propose the following process of soil development.
The initial silt loam layers have higher permeability than clay loam layers and this difference in permeability is enhanced over time. First, permeability is increased in silty layers by the formation of fine nutty structure and is decreased in the clay loam layers by the formation of closely fitting medium to coarse blocky structure (Griffiths et al. 1999; Webb 2003). Second, more permeable conditions promote the formation of allophanic minerals (Parfitt et al. 1984) that confer friable, fine structure (Leamy et al. 1980). Horizons with fine to medium nutty structure have macro-porosity varying from 6 to 12%, whereas the 2 medium to coarse blocky structured horizons have macro-porosity of 3-5% (Tables 6 and 7). These processes have resulted in greater permeability in the well-drained soils (minimum permeability of 4 mm/h) compared with the imperfectly drained soils (minimum permeability of 0.8 mm/h) (Tables 6 and 7).
The variation in soil properties reported here has important land management implications. The variability in P-retention within a paddock should influence the amount of phosphate fertiliser applied by farmers. If fertilised at a rate suited to the low-P-retention soil, then the high-P-retention soils in the paddock will be deficient in P and will have suboptimal productivity. If land is fertilised according to the high-P-retention soil, then surplus P will be applied to the low-P-retention soil, which is uneconomic and may increase the rate of loss of P to waterways. Thus, it may be argued from both an economic and an environmental point of view that soils within different P-retention classes should be treated differently. Therefore, it is important to identify the location of low-, medium-, and high-P-retention areas in the landscape and manage them accordingly. Variable rate of application of phosphate fertiliser through precision agriculture is the solution if the areas of low, intermediate, and high P-retention in a paddock can be identified. This is most cheaply and efficiently achieved by a soil survey delineating soils with differing drainage class.
Soil drainage and topsoil phosphate retention are highly variable, ranging from well drained through moderate to imperfectly or poorly drained and low through moderate to high P-retention at paddock-scale in soils occurring on the Last Glacial fiver terrace of the Manawatu district.
Short-distance drainage variability has a relationship to the textural variations of the original alluvial parent material, which gives rise to the formation of different soil structures and different pathways of weathering. This in turn influences the hydraulic conductivity of the soil and results in variable drainage conditions.
The climatic conditions of the Manawatu district have influenced mixed parent materials consisting of loamy tephra, loess, and quartzo-feldspathic alluvium to form allophone under well-drained conditions and kandite under imperfect and poor drainage conditions.
There is a strong relationship between soil drainage, phosphate retention, and clay mineralogy in soils on the Last Glacial terrace. Well-drained soils have high P-retention and the clay fraction contains allophone, whereas poorly drained soils have low P-retention and the clay fraction contains no allophane but mainly kandite.
The authors are grateful to the technical staff of the Fertilizer and Lime Research Centre, Massey University, Palmerston North, for help in the field and laboratory experiments; farmers of the Kiwitea study area for their co-operation during the soil survey and field experiments, and Trevor Webb, Landcare Research, Lincoln, for his valuable comments and suggestions.
Manuscript received 10 March 2009, accepted 9 September 2009
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A. Senarath (A,C), A. S. Palmer (B), and R.W. Tillman (B)
(A) Landcare Research, PO Box 40, Lincoln 7640, New Zealand.
(B) Soil & Earth Sciences, Institute of Natural Resources, Massey University, Palmerston North, New Zealand.
(C) Corresponding author. Email: firstname.lastname@example.org
Table 1. Criteria for separation of soil drainage classes Drainage class Presence of Presence of redox- reductimorphic mottled horizon horizon Poorly drained Within 150 mm of the A horizon or 300 mm of the soil surface Imperfectly Between 300-600 nun Within 150 mm of the drained of the soil surface A horizon or 300 mm of the soil surface Moderately well Not present within Below 600 nun drained Well 900 mm Not present within drained 900 mm Table 2. Classification of soil types in the Manawatu study area, New Zealand NZ Soil USDA Soil Soil type Classification Taxonomy (Hewitt 1992) (Soil Survey Staff 1999) Kawhatau silt loam Acidic Allophanic Andic Eutrudepts Brown Soil Kiwitea loam Typic Orthic Eutrudepts Melanic Soil Coulter silt loam Typic Orthic Dystrudepts Allophanic Soil Horoeka silt loam Typic Orthic Melanic Andic Eutrudepts Soil Barrow silt loam Mottled Immature Aqualfs Pallic Soil Ohakea Typic Orthic Gley Aqualfs Table 3. Percentage drainage classes in Blocks A, B, and C when mapped at 1:25000 scale (250-m grid) and at 1:5000 scale (50-m grid) Block A Block B 1:25 000 1:5000 1:25 000 1:5000 250-m 50-m 250-m 50-m Drainage class grid grid grid grid Well drained 0 0 0 26 Moderately well drained 0 21 100 28 Imperfectly drained 100 62 0 41 Poorly drained 0 17 0 5 Block C 1:25000 1:5000 250-m 50-m Drainage class grid grid Well drained 100 53 Moderately well drained 0 33 Imperfectly drained 0 14 Poorly drained 0 0 Table 4. Relationship between soil drainage classes and P-retention classes (low, medium, high) on Blocks A+B+C in northern Manawatu Total no. observations (% in parentheses) Low Medium High Drainage class (0-30%) (31-60%) (61-100%) Well drained 0 0 34(100) Moderately well drained 0 5(15) 29(85) Imperfectly drained 4(8) 34(70) 11(22) Poorly drained 9(100) 0 0 Table 5. Relationship between soil drainage, P-retention, and clay mineralogy of the topsoil of 2 Manawatu soil types (Senarath 2003) Drainage: Coulter Barrow silt loam silt loam Well Imperfectly drained drained P-retention 86 50 Sand fraction: Quartz (%) 41 54 Feldspar (%) 32 29 Volcanic glass (%) 13 8 Clay fraction: Kandite (%) 16 31 Allophane (%) 10 0 Table 6. Physical properties of a representative profile of well-drained Coulter silt loam related to water movement in soil (Senarath 2003) Ksat, Saturated hydraulic conductivity [K.sub. Horizon Depth Field Structure Macroporosity sat] (m) texture (%) (mm/h) Ap 0-0.20 Silt Moderate 6 -7 8 loam fine to medium nutty Bwl 0.20-0.65 Silt Moderate 11 8 loam fine to medium nutty Bw2 0.65-0.95 Silt Moderate 8 4.3 loam fine to medium nutty and moderate medium blocky 2Ab 0.95 1.25 Clay Moderate 5 4.6 loam medium to coarse blocky Table 7. Physical properties of a representative profile of imperfectly drained Barrow silt loam related to water movement in soil (Senarath 2003) [K.sub.sat], Saturated hydraulic conductivity Horizon Depth Field Structure Macroporosity Ksat (m) texture (%) (mm/h) Ap 0-0.21 Silt Strong 4-8 6.4--14 loam fine to medium nutty Bgl 0.21-0.34 Silty Strong 12 0.9 clay very fine, loam fine and medium nutty and moderate medium blocky Bg2 0.34-0.65 Fine Strong 9 7.6 sandy medium to clay coarse loam nutty Bg3 0.65-0.82 Clay Moderate 3 0.8 loam medium to coarse blocky
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|Author:||Senarath, A.; Palmer, A.S.; Tillman, R.W.|
|Publication:||Australian Journal of Soil Research|
|Date:||Feb 1, 2010|
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