Phosphorus forms, lateritic nodules and soil properties along a hillslope in northern Ghana

Catena 33 Ž1998. 1–15

Phosphorus forms, lateritic nodules and soil properties along a hillslope in northern Ghana M.K. Abekoe a , H. Tiessen

b,)

a

b

Dept. of Soil Science, UniÕersity of Ghana, Legon, Accra, Ghana Dept. of Soil Science, UniÕersity of Saskatchewan, 51 Campus DriÕe, Saskatoon, SK, Canada S7N 5A8 Received 8 April 1997; revised 24 April 1998; accepted 24 April 1998

Abstract Inceptisols and Alfisols formed from Volta shales in northern Ghana are moderately to strongly weathered, and contain varying amounts of lateritic nodules and sesquioxides. The plant-available P and fertilization potential of these soils are affected by the presence of the lateritic nodules, which act as P sinks. We examined the amounts and distribution of P and related them to the nodule content, to chemical properties of soil fines Ž- 2 mm. and nodules, and to soil development on gentle hill slopes. Total P of soil fines in the surface horizons ranged from 80 to 280 mg kgy1, and total P of nodules ranged between 430 and 900 mg kgy1. Resin- plus bicarbonate-extractable labile P was between 7 and 18 mg kgy1 and mostly less than crop requirements. On the upper slope, where the topsoil contained large amounts of nodules, small nodules contained more P than large ones. This suggests that the small nodules are an efficient sink for P that cycles actively in the topsoil. Despite this, the labile P of soil fines in the surface horizon was greater on the upper slope Ž18 mg kgy1 . than the lower slope Ž7 mg kgy1 ., suggesting that the sorption is partly reversible and that P-rich nodules can maintain elevated native P levels in surrounding fines. Organic P accounted on average for 35% of total P in the surface horizons, with highest proportions at mid-slope positions. Comparison of the amounts of Ca-bound P Žextractable with dilute HCl. in soil fines and silstone fragments indicated Ži. that there had been external Ca inputs to the upper slope soils Žprobably from fire and dust., Žii. that there was primary mineral P present on the eroded mid slope, and Žiii. that pedogenic Ca–P had been formed on the periodically waterlogged lower slope. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Catena; Alfisol; Phosphorus fractionation; Phosphorus availability; Laterite

)

Corresponding author. E-mail: [email protected]

0341-8162r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 4 1 - 8 1 6 2 Ž 9 8 . 0 0 0 6 3 - 0

2

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

1. Introduction Food production in the semiarid savannas of West Africa is limited by low moisture and nutrient availabilities ŽBationo and Mokwunye, 1991.. Small amounts of total and available P in the soils have been attributed to their advanced weathering, variable P sorption and poor organic matter recycling ŽNye and Bertheux, 1957.. Total P ŽPT . in some soils from Niger ranged from 29 to 349 mg kgy1 ŽManu et al., 1991., and in nine topsoils from the savanna region of Ghana it was between 60 and 173 mg kgy1 ŽOwusu-Bennoah and Acquaye, 1989.. Available P ŽBray 1. values below 9 mg kgy1 were reported by Acquaye and Oteng Ž1972. for 48 topsoils in different ecological zones of Ghana. Similarly, most of the soils studied in Nigeria by Uzu et al. Ž1975. had traces to 12 mg kgy1 available P. Under low-input agriculture, the P supply to plants limits production. An understanding of the nature of soil P and the factors that determine the size of the various P pools within landscapes is necessary to design management practices that raise P fertility. The study of forms of soil P has largely relied on sequential extractions ŽChang and Jackson, 1957; Williams et al., 1967; Hieltjes and Lijklema, 1980.. The fractionation method developed by Hedley et al. Ž1982. has been used successfully in temperate ŽRoberts et al., 1985. and tropical soils ŽBall-Coelho et al., 1993; Agbenin and Tiessen, 1994. to study the distribution and transformations of inorganic and organic P forms. The sequential extraction separates inorganic P ŽPi. from organic P ŽPo. and also distinguishes secondary Fe- and Al-associated Pi from Ca-associated Pi. Relationships between the extracts and plant P availability have been inferred from many experiments: resin and bicarbonate Pi are the most labile Pi pools, and hydroxide extractable Pi is often moderately labile and the residual fraction, that remains insoluble in these extractants, is often the least labile Pi pool. In tropical soils, a further extraction of the residual fraction by hot concentrated HCl and separation into Pi and Po was employed, because nearly half the soil P remained unextracted by the original method ŽTiessen and Moir, 1993.. The characterization of P forms in soils has also been used to study pedogenic processes, since the proportion of total P held in various forms changes as soils develop ŽWalker and Syers, 1976.. Calcium phosphates are the main P forms in young soils, and with progressive weathering, Al–P and Fe–P become more important ŽTiessen et al., 1984; Smeck, 1985; Ibia and Udo, 1993; Cross and Schlesinger, 1995.. In profiles, primary Ca–P contents normally increase with depth, and the surface horizons have greater secondary Pi contents, reflecting increased weathering at the soil surface ŽWalker and Syers, 1976; Harrison et al., 1994.. Soils in semiarid northern Ghana are moderately to strongly weathered, with varying amounts of lateritic nodules, oxyhydroxides of Fe and Al, and clay ŽAbekoe, 1996.. Past work on these soils suggested that the lateritic nodules are a sink for P and that their P sorption capacity depends on the original nodule PT ŽTiessen et al., 1993.. Thus, the presence of lateritic nodules affects the P availability and fertilization potential of the soils. We therefore related the amounts and distribution of P to the nodule content, chemical properties and development stage of soils along a hillslope in northern Ghana

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

3

with the objective of determining the factors that influence P fertility with respect to landscape position in an area typical of the region.

2. Materials and methods 2.1. Site description The study area was at Nyankpala, 20 km west of Tamale in the Northern Region of Ghana, approximately at 9825X N–180X W. The soil parent materials at Nyankpala belong to the coastal beds of the Lower Voltaian Formation and consist of clay-shales, sandstone, siltstone and mudstone ŽAdu, 1952.. Rainfall is monomodal with one rainy season between April and September. The mean annual precipitation is 800 to 1100 mm, which is typical of the semiarid Sudanian ecological zone of West Africa ŽStoop, 1987.. The dry season is severe, with dust-laden air masses ŽHarmattan. often blowing across the region from the Sahara. Average daily temperatures range from a minimum of 268C to a maximum of 318C with a mean of 288C. Relative humidity ranges from 20% in the dry season to 90% in the rainy season. The vegetation in the study area is farmland, grassland savanna and farm regrowth with scattered trees and shrubs. Some common grasses are Andropogon gayanus, Imperata cylindrica and Sporobolus pyramidalis. Trees such as Parkia oliÕeri, Butyrospermum parkii and Parkia biglobosa are commonly scattered over the landscape. Landuse is rotational or shifting cultivation and grazing. Dry season fires occur annually and eliminate most of the aboveground biomass apart from fire-resistant trees. 2.2. Field and laboratory methods A catena with a gradient of about 2% and a length of 390 m was selected from farmers’ fields about 5 km from the Savannah Agricultural Research Institute at Nyankpala. Five profiles were dug, one each on the upper slope, convergent shoulder, and mid-, lower- and toe-slopes. The bottom slope was not sampled because it is seasonally covered by an intermittent stream. The profiles were sampled either by genetic horizons or by layers where distinct horizons were not apparent. In addition to this main study area, topsoils were sampled from the upper-, mid- and lower slopes at two other sites some 2 and 7 km distant. In the absence of detailed grid sampling and geostatistical examination, results from site 1 are compared with those of the two other sites at the end of Section 3 to show common trends among landscape positions. Soil samples were air-dried, crushed and passed through a nest of sieves ranging from 5.6 to 0.5 mm mesh to separate the bulk soil into fines Ž- 0.5 mm. and small Ž0.5–2 mm., medium Ž2–5.6 mm. and coarse Ž) 5.6 mm. nodules. Each nodule size fraction was cleaned of adhering soil particles by gentle but thorough shaking in sieves and then weighed. The total weight of nodules in each horizon was expressed as a percentage of the bulk soil. Microscopy showed that the soil fines were devoid of lateritic nodules, and the coarser fractions contained only nodules or siltstone fragments. Ground subsamples of the different nodule size fractions and the soil fines were analysed separately.

4

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

Siltstone fragments, which were easily distinguished by their angularity and colour, were separated by hand, and results are reported separately. Results for all nodules associated with a soil sample were calculated as weighted averages of the nodule size fractions. Particle size distribution was determined by the pipette method on the soil fines after H 2 O 2 treatment to destroy organic matter and dispersion of soil suspensions by ultrasonic vibrations ŽBraunsonic. for 8 min at 300 W in distilled water. Soil pH was measured in 1:2 soil:water suspensions. Total carbon was measured by Leco CR 12 Carbon analyser at 11008C, inorganic carbon by acid digestion and CO 2 titration, as described by Tiessen et al. Ž1981., and organic C was calculated as the difference. Exchangeable cations ŽCa, Mg, K, Na, Mn, Al. were extracted using unbuffered 1.0 M NH 4 Cl ŽpH 5., and effective cation exchange capacity ŽECEC. was calculated by summation of the basic and acidic cations. Calcium, Mg, Mn and Al in the 1.0 M NH 4 Cl extracts were measured by atomic absorption spectrophotometry and exchangeable Na and K by flame photometry. Dithionite–citrate extractable Fe ŽFed. was determined by the method of Mehra and Jackson Ž1960., and oxalate extractable Fe ŽFeo. was extracted by the method of McKeague and Day Ž1966.. 2.3. Total P and P fractionation procedure Total P ŽPT . was determined by H 2 SO4 –H 2 O 2 digestion ŽHedley et al., 1982. and by summation of P fractions from the sequential extraction. The sequential procedure ŽTiessen and Moir, 1993. extracted P with an anion exchange resin membrane ŽBDH. in distilled water, followed by 0.5 M NaHCO 3 , 0.1 M NaOH, 1 M HCl, hot concentrated Ž11.3 M. HCl, and finally by digestion of the residue with concentrated H 2 SO4 and H 2 O 2 . The bicarbonate, hydroxide and hot concentrated HCl extracts was analysed for total and inorganic P, and organic P was calculated as the difference. The sum of P fractions was within "8% of the independent PT determination for soil fines and most nodules, and the regression equations were highly significant Ž r 2 ) 0.9.. Agreement within "10% was reported recently for a similar comparison on some temperate soils ŽNair et al., 1995.. Some P recoveries from lateritic nodules showed wider errors, up to "15% of the PT values. This was attributed to interference by silicic acid during analysis of the hot concentrated HCl–P fraction. In order to simplify the data presentation, resin and bicarbonate Pi were summed and referred to as labile Pi, the sum of the hot concentrated HCl–Pi and residual P was presented as resistant P, and the three organic P fractions were summed. Cold 1 M acid–Pi is referred to as Ca–Pi, since this fraction is chemically well defined. Hydroxide–Pi is reported as such.

3. Results and discussion Soils of the upper to midslope positions of site 1 were classified as Plinthustalfs, and those of the lower slope as Tropaquepts ŽAdu, 1981.. The textures of the surface horizons ranged from sandy loam on the upper to silt loam on the lower slope ŽTable 1.. The B, BC and C horizons were predominantly clayey. The soil fines were slightly to moderately acid in all profiles, except on the lower slope where pH was slightly alkaline

Table 1 Texture, pH, organic carbon ŽOC., effective cation exchange capacity ŽECEC., and dithionite ŽFed. and oxalate ŽFeo. extractable Ž. soil fines and nodules or siltstone fragments Žsiltstone in italics . at site 1 Horizon

Depth Žcm.

Soil fines Sand Ž%.

Mean nodules and siltstone fragments Clay Ž%.

pH

OC Žg kgy1 .

31 29

15 43

6.2 5.5

12 4.2

ConÕergent shoulder (Profile 2) Ap-E 0–33 31 40 Bt1 33–63 17 25 Bt2-Bt3 63–102 5 37 C 102–112 2 51

29 57 58 47

5.6 5.5 5.6 5.7

Midslope (Profile 3) Ap-AB 0–46 28 Bt1-Bt2 46–70 15 BC 70–93 7 C 93–130 2

39 24 42 51

33 61 51 47

Lower slope (Profile 4) Ap-A1 0–26 32 B-Bt 26–65 20 BC1-BC2 65–126 12 C 126–150 9

45 33 35 38

Lower slope (Profile 5) Ap 0–17 19 Bw-B2 17–105 20 BC-BC2 105–146 23 C1-C4 146–275 19

69 48 28 38

Upper slope (Profile 1) Ap-E 0–46 54 B1-B4 46–210 28

ECEC Žcmol c kgy1 .

Fed Žg kgy1 .

Feo Žg kgy1 .

Nodule Ž%.

pH

OC Žg kgy1 .

3.0 6.2

50 98

1.5 1.5

85 71

6.3 5.6

6.8 2.9

2.5 3.2

286 185

1.8 1.6

15 4.0 1.7 1.1

3.9 7.6 15 15

40 42 30 11

4.9 1.7 0.8 0.7

58 2 53 87

5.6 5.6 5.8 6.0

6.2 1.9 0.6 0.3

2.3 ND a 9.8 10

220 ND 74 10

2.8 ND 0.8 0.6

5.5 5.4 5.4 6.8

13 2.8 0.6 0.3

5.3 13 15 12

40 45 22 21

6.0 2.1 0.7 0.7

25 28 3 2

5.7 5.7 5.7 6.9

6.3 0.5 0.1 ND

2.1 1.9 9.2 ND

276 290 80 ND

2.9 2.8 2.4 ND

23 47 54 53

6.0 5.9 6.8 7.4

6.5 3.6 1.9 1.1

4.0 8.4 15 17

18 26 29 27

2.1 1.5 1.2 0.8

3 10 4 3

6.1 6.1 ND ND

2.0 0.9 0.5 0.5

1.2 1.4 ND ND

244 210 197 179

3.0 2.9 1.9 2.9

12 32 46 43

5.3 5.9 6.0 6.4

6.3 2.8 2.3 0.5

3.4 4.8 9.3 13

4.9 23 34 36

2.0 1.6 1.5 1.0

1 14 17 16

ND 6.3 6.1 6.4

0.4 2.2 1.7 0.4

ND 4.2 2.0 2.3

209 179 164 153

5.4 5.8 8.0 2.2

Fed Žg kgy1 .

Feo Žg kgy1 .

5

Adjacent horizons in each profile with similar properties have been grouped. a NDs not determined because of insufficient sample size.

ECEC Žcmol c kgy1 .

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

Silt Ž%.

6

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

in some subsoil horizons Žnot shown separately in Table 1. because of presence of Ca and Mg carbonates. Organic carbon ranged from 6 to 15 g kgy1 in the surface horizons and decreased with depth. The ECEC was small in the surface horizons and increased in the subsoils, following the increases in clay content with depth. The dithionite extractable Fe contents of the soil fines varied considerably along the hillslope. Smaller Feo than Fed contents in all samples were attributed to high temperatures and prolonged dry seasons, which cause poorly crystalline Fe to dehydrate and develop greater crystallinity ŽJuo et al., 1974.. Amounts of lateritic nodules in the soils decreased downslope ŽTable 1.. Siltstone fragments were present in the B and C horizons of the shoulder and midslope profiles. Nodule pH was slightly to moderately acid, similar to the soil fines. Both organic carbon and ECEC values for the nodules were less than those of the associated soil fines, but the nodules had a significantly greater Fed content Ž p - 0.01. than the soil fines and siltstone fragments. 3.1. Total P The PT contents of the soil fines in the surface horizons of the upper slope and shoulder positions were unusually large ŽTable 2., whereas values in the mid to lower slope surface horizons were within the range reported for semiarid tropical savanna soils ŽOwusu-Bennoah and Acquaye, 1989; Manu et al., 1991.. Concentrations of PT in the C horizons of profiles 2 to 5 were much greater than in the surface horizons. Total P of the soil fines was positively correlated with soil pH Ž p - 0.05.. Total P of the nodules was significantly greater than that of the soil fines Ž p - 0.01.. Ratios of nodule PT to soil fines PT ranged from 1:1 to 8:1. In the Ap horizon of the upper slope profile, nodule PT concentrations increased with decreasing nodule size, but no consistent trend in PT contents of the different nodule sizes was found in the lower horizons ŽTable 2.. Contrary to the trend in the soil fines, nodule PT was greater in the surface horizons than in most of the subsoils. The differences in P content of nodules may result from their activity as P sinks in horizons with different rates of P cycling. In the A horizons with an active P cycle, nodules are constantly exposed to available P from organic matter returns, mineralisation and ash inputs from dry season fires. Small nodules with a larger surface area to weight ratio sorb P more effectively ŽAbekoe and Tiessen, 1998a., and therefore have greater P contents where they are regularly exposed to solution P. However, in lower horizons with less active P cycling, slower reactions such as diffusion into larger nodules can eliminate such differences, and no trend is observed. 3.2. Inorganic P fractions The order of abundance of the Pi forms in the soil fines of the A, E and AB horizons of the upper to midslope soils was resistant P 4 hydroxide–Pi ) Ca–Pi G labile Pi ŽTable 3.. The predominance of moderately labile to resistant P forms and the small available P content of the soil fines conforms to the relatively high degree of weathering in these horizons ŽChang and Jackson, 1957; Ibia and Udo, 1993.. The greater Ca–Pi

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

7

Table 2 Distribution of total phosphorus in soil fines and in different size classes of nodules or siltstone fragments Žvalues for siltstones in italics . Horizon Depth Žcm. Soil fines Žmg kgy1 . Nodules

Nodule PT rSoil fine PT a

Fine

Medium Coarse Average

Upper slope (Profile 1) Ap 0–21 284 E 21–46 233 B1 46–65 234 B2 65–96 235 B3 96–175 246 B4 175–210 128

1030 552 313 260 310 np b

899 970 601 469 316 316

662 863 586 412 410 120

901 917 540 411 368 229

3 4 2 2 1 2

ConÕergent shoulder (Profile 2) Ap 0–12 216 E 12–33 215 Bt1 33–63 261 Bt2 63–90 363 Bt3 90–102 300 C 102–112 316

848 457 np np np np

693 376 380 532 282 295

620 315 np 491 310 483

726 367 380 498 305 433

3 2 1 1 1 1

Mid slope (Profile 3) Ap 0–9 A1 9–17 AB 17–46 Bt1 46–56 Bt2 56–70 BC 70–93 C 93–130

190 225 261 198 172 154 340

np np np np np np np

584 472 450 394 326 181 423

np np 372 282 360 187 np

584 472 418 375 341 184 423

3 2 2 2 2 1 1

Lower slope Profile 4 Ap 0–14 A1 14–26 B 26–36 Bt 36–65 BC1 65–90 BC2 90–126 C 126–150

99 149 116 143 120 215 171

np np np np np np np

639 553 659 695 698 630 524

np np 759 np np np np

639 553 670 695 698 630 524

6 4 6 5 6 3 3

Lower slope Profile 5 Ap 0–17 Bw 17–42 B1 42–70 B2 70–93 B3 93–105 BC1 105–128 BC2 128–146 C1 146–176 C2 176–220 C3 220–250 C4 250–275

84 98 102 141 160 120 91 81 71 121 290

np np np np np np np np np np np

604 np np 331 212 145 np 364 580 590 661

np np np np 169 138 278 173 np np np

604 np np 331 187 141 278 244 580 590 661

7 np np 2 1 1 3 3 8 5 2

a b

Weighted average for all nodulerstone classes. nps Not present.

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

8

Table 3 Phosphorus fractions of soil fines, nodules and siltstone fragments Žin italics . Horizon Depth Žcm. Fines Žmg kgy1 .

Mean nodulesrstones Žmg kgy1 .

a

Labile Hydroxide Ca–P Resistant Upper slope (Profile 1) Ap 0–21 18 E 21–46 6.6 B1 46–65 0.8 B2 65–96 1.7 B3 96–176 2.5 B4 176–210 1.8

b

ÝPo Labile Hydroxide Acid Resistant ÝPo

25 13 21 25 19 5.6

17 5.9 0.3 0.1 0.3 0.1

130 120 99 110 150 98

89 85 113 96 72 21

5.8 5.4 0.9 0.3 0.2 0.7

54 43 29 27 23 7.5

4.8 2.2 0.6 0.2 0.2 0.2

820 850 409 380 320 220

15 14 11 7 17 5

ConÕergent shoulder (Profile 2) Ap 0–12 11 28 E 12–33 3.1 15 Bt1 33–65 1.8 9.0 Bt2 65–90 3.2 8.2 Bt3 90–102 3.4 10 C 102–112 14 23

10 1.1 0.9 1.3 2.7 24

81 110 180 310 210 210

86 83 67 43 70 42

5.9 0.9 ND c 1.1 1.1 11

53 20 ND 7.1 9.1 24

3.8 0.6 ND 4.4 4.3 25

640 320 ND 480 290 370

21 23 ND 3 3 2

Midslope (Profile 3) Ap 0–9 9.7 A1 9–17 5.8 AB 17–46 4.7 Bt1 46–56 1.0 Bt2 56–70 1.4 BC 70–93 3.3 C 93–130 27

26 20 22 13 10 4.7 35

10 1.4 0.7 0.9 0.6 0.3 112

65 73 120 140 120 130 120

79 130 110 48 38 11 49

6.5 2.7 1.1 1.5 0.7 1.4 33

33 30 28 22 21 4.7 16

2.1 1.4 0.6 1.3 0.5 1.9 220

530 430 360 340 310 180 150

10 14 26 9 6 1 0

Lower slope (Profile 4) Ap 0–14 5.9 A1 14–26 5.0 B 26–36 3.1 Bt 36–65 5.1 BC1 65–90 2.7 BC2 90–126 7.5 C 126–150 5.0

8.5 10 7.4 9.5 6.4 5.8 6.1

2.0 0.3 0.1 0.1 0.1 70 43

39 75 64 100 85 120 110

44 59 43 25 26 15 6

5.3 ND 1.4 1.7 1.7 ND 3.1

54 ND 37 34 28 ND 18

2.9 ND 0.7 0.9 0.6 ND 3.6

560 ND 620 640 670 ND 500

20 ND 8 4 3 ND 1

Lower slope (Profile 5) Ap 0–17 7.2 Bw 17–42 8.3 B1 42–70 3.7 B2 70–93 3.6 B3 93–105 2.5 BC1 105–128 1.4 BC2 128–146 1.3 C1 146–178 1.0 C2 178–220 2.5 C3 220–250 4.3 C4 250–275 15

6.6 7.8 7.6 20 16 10 4.8 4.5 3.3 6.2 18

1.4 0.4 1.3 0.7 1.3 0.6 0.1 0.9 0.1 0.6 85

38 48 52 73 97 71 72 68 62 89 110

31 34 38 48 43 38 13 7 3 22 63

7.9 np d ND 0.8 0.3 0.1 0.0 0.6 1.6 1.7 4.3

120 np ND 37 24 22 16 12 13 17 20

10 np ND 0.6 0.5 0.5 0.4 0.5 0.4 0.4 1.0

460 np ND 290 130 87 230 270 540 520 620

6 np ND 6 29 32 31 26 27 22 20

a

Labile P s resin and bicarbonate Pi. determined. d nps not present.

b

Resistant P s Hot concentrated HCl PiqResidual P.

c

NDs not

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

9

fraction of upper and midslope topsoils was accompanied by greater exchangeable Ca Ž2.6 to 3.8 cmol c kgy1 . than was found in the lower slope soils Ž1.3 to 1.5 cmol c kgy1 .. Labile Pi, hydroxide–Pi and Ca–Pi in the A, E and AB horizons were correlated with organic carbon in the soil fines ŽTable 4.. Organic matter may be important for Table 4 Simple correlation coefficients between selected properties and P fractions of soil fines and nodules Soil properties

Labile P

Hydroxide Pi

Ca–Pi

Soil fines: A, E and AB horizons (ns 10) % Nodules pH 0.65) organic C 0.60) 0.76)) dithionite Fe oxalate Fe 0.73)

0.81))) 0.63) 0.73) 0.93)))

Soil fines: B and BC horizons (ns 20) % Nodules 0.82))) pH 0.44) organic C 0.55) dithionite Fe y0.52) 0.80)))

0.77)))

Soil fines: C horizons (ns7) pH Soil fines: All horizons ns 37 % Nodules pH 0.43)) organic C dithionite Fe oxalate Fe

Resistant P

0.68)

0.77))) 0.81)))

0.54))

0.97)))

0.40)

0.54))

0.72))) 0.49))

0.59))) 0.54))) 0.53)))

0.47))

0.64) 0.82) 0.72)

Nodules: B and BC horizons (ns 12) pH 0.60) organic C y0.59) dithionite Fe 0.59) oxalate Fe

Nodules: all horizons (ns 21) organic C 0.52) dithionite Fe Oxalate Fe

0.47)

0.79)))

0.82)

Nodules: A, E and AB horizons (ns 8) pH organic C oxalate Fe 0.75)

Nodules: C horizons (ns 5) % Nodules dithionite Fe 0.97))

ÝPo

0.56))

0.86)))

y0.862) 0.97))

y0.86)

0.52) y0.47)

), )), ))) Significant at 0.05, 0.01 and 0.001 probability levels, respectively. Soil properties with missing correlation coefficients had no significant effect.

0.48)

10

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

maintaining P availability or, alternatively, available P may increase organic matter returns to the soil. In the soil fines, Feo and Fed were positively correlated with hydroxide–Pi and resistant P, showing the importance of Fe in stabilising P. Labile Pi and Ca–Pi contents of the iron-rich nodules were less than those of soil fines in all profiles except the toe slope profile 5 ŽTable 3.. Nodule hydroxide–Pi was greater than that of the soil fines in all profiles. Over 75% of the nodule Pi was in resistant form, as compared to 45–64% of the PT in the soil fines. In addition to the greater stability of P within the nodules, percent nodule content influenced P stability in the surrounding soil fines; it was correlated with resistant Pi of the soil fines in all profiles ŽTable 4.. Percent nodule content was also correlated with the Fed content of the soil fines Ž r 2 s 0.45, p s 0.0001.. It is unlikely that present Fed contents in the fines are related to nodule formation, which took place under a more humid climate in the past. Therefore, the presence of nodules seems to maintain an elevated Fed content in the soil fines, possibly through diffusion exchange, which leads to an increase of P stability in the soil fines. In A, E and AB horizons, percent nodule content was also correlated with PT in fines Ž r 2 s 0.47, p s 0.04., indicating that the large nodule P content may to some extent explain the greater P contents of the soil fines. The labile Pi content of the soil fines was less in the B and BC than in the Ap horizons of all profiles ŽTable 3.. Apart from the BC2 horizon of profile 4, B horizons contained little Ca–Pi and correspondingly greater proportions of hydroxide–Pi and resistant P of the soil fines. The smaller Ca–Pi content in the B than in the Ap horizons is not consistent with reports from temperate soils, in which the content of primary minerals increases with depth ŽWalker and Syers, 1976; Harrison et al., 1994.. In the Ghanaian savannah, deposits of ash and Harmattan dust have increased the Ca supply to the Ap horizons and may maintain greater Ca–P contents in Ap than in the B horizons ŽTiessen et al., 1991; Ball-Coelho et al., 1993.. The pH of the soil fines was positively correlated with Ca–Pi and labile Pi in all horizons, whereas Fed was negatively correlated with labile Pi in B and BC horizons ŽTable 4., indicating that P availability decreases as the soil mineralogy becomes dominated by Fe. As in the A horizons, percent nodule content in the B and BC horizons was correlated with Fed Ž r 2 s 0.64. and hydroxide–Pi ŽTable 4.. The labile Pi contents of nodules and siltstone fragments were generally less than that of the soil fines in the B and BC horizons ŽTable 3.. The nodules contained little Ca–P, but the less weathered siltstone fragments present in the Bt2 and Bt3 horizons of profile 2 were richer in Ca–P, probably representing mineral Ca–P inherited from the parent material. Exchangeable Ca was correlated with Ca–Pi Ž r 2 s 0.41, p s 0.0001.. Dithionite-extractable Fe in the B and BC horizons was correlated with hydroxide–Pi ŽTable 4.. In the C horizons of most profiles, the labile Pi ŽTable 3. and PT ŽTable 2. of the soil fines were greater than in the Ap horizons ŽTable 3.. Except for horizons C1 to C3 in profile 5, the C horizons had relatively large contents of Ca–Pi. The nature of this Ca–Pi in the C horizons was examined by comparing the amounts of Ca–Pi in soil fines, siltstone fragments and lateritic nodules in the C and overlying horizons. In profile 2, Ca–P in the soil fines increased from 1% of PT in the Bt3 to 8% in the C horizon. In profile 3, Ca–P increased from - 1% in the BC to 33% of PT in the C horizon. Both these increases in Ca–P were paralleled by increases in the P content of the coarse

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

11

Ž) 2 mm. fraction: from 1 to 6% of PT in profile 2 and from 1 to 52% of PT in profile 3. The close relationship between siltstone Žparent material. P and soil P in the C horizons of the shoulder and midslope profiles suggests that the Ca–P in the soil fines may be present as primary minerals. In contrast, in the lower slope profiles 4 and 5, increases in Ca–P from - 1% of PT in the BC1 and C3 horizons to nearly 30% of PT in the BC2 and lower C horizons occurred only in the soil fines. In the C horizons of these two profiles, the coarse Ž) 0.5 mm. fraction was lateritic nodules with very small Ž- 1% of PT . Ca–P contents. It was therefore inferred that the Ca–P present in the BC2 and C horizons of the lower and toe slope soils may be pedogenic, precipitated in the soil fines after the lateritic nodules were formed. Some of the C horizons of profiles 4 and 5 were slightly alkaline ŽpH 7.2 to 7.4, not individually shown in Table 1., with greater exchangeable Ca contents than the corresponding horizons of the shoulder and midslope profiles. These horizons therefore show a combination of properties, namely the presence of lateritic nodules and of large Ca and Ca–P contents, which do not reflect weathering, but suggest movement of Ca in the otherwise strongly weathered landscape towards the lower slopes. Impeded drainage on the lower slope then favours the accumulation of pedogenic calcium phosphates and carbonates. Accordingly, the nodules of these horizons, representing an advanced weathering stage, were not related to the P fractions of the soil fines, because the P fractions were probably formed later during pedogenesis. Evidence for large differences in mineralogical properties and the time of formation between laterite and surrounding soil was also presented by Tiessen et al. Ž1996. for a Venezuelan site. 3.3. Organic P In the A, E and AB horizons, organic P ranged from 31 to 130 mg kgy1 ŽTable 3. representing 31 to 55% of PT in these horizons. Significant correlations of Po with Fed ŽTable 4. suggest stabilisation of Po by Fe oxides, which could decrease the mineralisation potential of the Po. Nodules generally had much less Po than the soil fines which, together with their smaller organic C contents ŽTable 1., indicates that their composition does not reflect present soil conditions. 3.4. Comparison of site 1 with sites 2 and 3 Site 1 provided relationships between soil chemistry and phosphorus fertility at different profile depths and slope positions. In order to evaluate whether these relationships are typical of the Northern Ghanaian landscape, we sampled topsoils from upper, mid and lower slope positions at two other sites and analysed them for the same properties as site 1 ŽTables 5–7.. Chemical and physical properties at all sites were similar ŽTable 5.. Textural change downslope was less pronounced at site 3 than at site 1. Lateritic nodule content decreased downslope in a similar fashion at all three sites. The total P content of the soil fines decreased downslope at all sites; amounts were generally less at site 2 ŽTable 6.. The total P content of the nodules was always greater than that of the soil fines, with the smallest nodules containing the largest amounts. On the top slopes of both sites 1 and 3 very large P contents of the small nodules ŽTable 6.

12

Site

1 upper 1 mid 1 lower 2 upper 2 mid 2 lower 3 upper 3 mid 3 lower a b

Depth Žcm.

0–46 0–46 0–17 0–20 0–20 0–20 0–20 0–20 0–20

Soil fines

Nodule Ž%.

Sand Ž%.

Silt Ž%.

Clay Ž%.

pH

OC Žg kgy1 .

ECEC Žcmolc kgy1 .

Fed Žg kgy1 .

Feo Žg kgy1 .

54 28 19 49 45 27 67 52 53

31 39 69 37 40 55 23 37 32

15 33 12 14 15 18 10 13 15

6.2 5.5 5.3 6.1 6.1 5.7 6.8 6.6 5.3

12 13 6.3 6.4 6.7 6.0 9.8 9.7 9.9

3.0 5.3 3.4 3.1 2.9 2.2 3.9 4.0 3.3

50 40 4.9 7.5 11 3.6 16 12 9.7

1.5 6.0 2.0 1.1 1.2 1.3 1.0 1.0 2.7

NDs not determined because of small fraction amount. nps fraction was not present.

85 25 1 68 46 0 66 50 0

Nodules pH

OC Žg kgy1 .

ECEC Žcmolc kgy1 .

Fed Žg kgy1 .

Feo Žg kgy1 .

6.3 5.7 ND a 6.7 6.4 np b 6.9 6.8 np

6.8 6.3 0.4 5.3 3.4 np 3.8 5.4 np

2.5 2.1 ND 2.8 3.1 np 3.0 3.7 np

286 276 209 188 234 np 245 245 np

1.8 2.9 5.4 1.7 1.5 np 1.0 1.3 np

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

Table 5 Physical and chemical properties of the soil fines and nodules at sites 1, 2 and 3

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

13

Table 6 Distribution of total phosphorus in soil fines and in different size classes of nodules at sites 1, 2 and 3 Site

Soil fines

Site 1 upper Site 1 mid Site 1 lower Site 2 upper Site 2 mid Site 2 lower Site 3 upper Site 3 mid Site 3 lower a b

Nodules

284 190 84 126 115 98 234 167 135

Fine

Medium

Coarse

Averagea

1030 np b np 607 616 np 933 641 np

899 584 604 460 432 np 677 817 np

662 np np 381 372 np 725 802 np

901 584 604 396 435 np 700 799 np

Weighted average for all nodulerstone classes. nps not present.

and large amounts of labile and Ca–P in the soil fines ŽTable 7. provide evidence for the link between active P cycling and sorption by the small nodules. The labile P in the fines was similar between sites, but the decline downslope was seen only at sites 1 and 3. Nodule P contents were comparable at all sites ŽTable 6.. In parallel with the nodule content, the resistant P of the topsoil fines declined downslope at all sites. In general, the trends and relationships of soil chemical properties, P fertility and landscape position were similar at the three sites, indicating that P fertility management should be adapted to nodule content and chemical properties, which show predictable patterns according to landscape position.

Table 7 Phosphorus fractions of soil fines and nodules at sites 1, 2 and 3 Site

Fines Žmg kgy1 . Labile

1 upper 1 mid 1 lower 2 upper 2 mid 2 lower 3 upper 3 mid 3 lower a

18 10 7 9 7 8 17 8 10

a

NodulesrStones Žmg kgy1 .

Hydrox.

Ca–P

Resistant

25 26 7 9 7 6 21 8 14

17 10 1 3 3 3 21 9 6

130 65 38 68 59 51 110 82 59

b

Labile P s resin and bicarbonate Pi. Resistant P s Hot concentrated HCl PiqResidual P. c nps not present. b

ÝPo 89 79 31 43 45 32 72 64 49

Labile

Hydrox.

Ca–P

6 7 8 5 5 np c 13 8 np

54 33 120 26 35 np 82 65 np

5 2 10 1 2 np 5 2 np

Resistant 820 530 460 350 370 np 580 710 np

ÝPo 15 10 6 16 26 np 19 18 np

14

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

4. Agricultural implications In the semiarid soils studied, most Po occurred in non-labile forms ŽAbekoe, 1996.. Therefore, the P supply to plants may depend to a large extent on inorganic P fractions. The labile Pi of the soil fines ranged from 9 to 18 mg kgy1 in the Ap horizons of the mid to upper slopes and from 6 to 10 mg kgy1 on the lower slopes. The labile Pi contents of the lateritic nodules were much less than those of the soil fines, but the latter are probably mostly responsible for the supply to crops. This means that the nodules have a substantial effect on P availability. Nodules decrease rooting volume and ‘dilute’ the soil fine earth volume, thus decreasing P Žand other nutrient. availabilities. In addition, they modify the Fe and P contents of associated soil fines. The interactions are complex: strong correlations between percent nodule content, Fed and resistant Pi of the soil fines suggest that P availability on the upper slope should be low. Nevertheless, greater labile P contents were observed in the topsoils of upper slopes than in those of the lower slopes. This labile P was associated with an overall greater total P content, which compensated for the greater stability of the P forms. Since moisture availability in the semi-arid climate favours agricultural use of lower slopes, remedial fertilization of these sites is necessary. The transformations of different types of P fertilizers in these soils are further examined by Abekoe and Tiessen Ž1998b..

Acknowledgements The authors gratefully acknowledge the financial support of the International Development Research Centre ŽIDRC. for this study and the senior author’s scholarship. We thank Dr. H.K. Hauffe, formerly of the Nyankpala Agricultural Experiment Station, for help with the collection of soil samples.

References Abekoe, M.K., 1996. Phosphorus fractions and rock phosphate transformations in soils from different landscape positions from northern Ghana. PhD Thesis, Univ. of Saskatchewan, Saskatoon, Canada. Abekoe, M.K., Tiessen, H., 1998a. Phosphorus sorption by lateritic nodules in soils from hillslopes of northern Ghana, submitted. Abekoe, M.K., Tiessen, H., 1998b. Fertilizer P transformations and P availability in hillslope soils of northern Ghana, Nutrient Cycling in Agroecosystems Žin press.. Acquaye, D.K., Oteng, J.W., 1972. Factors influencing the status of phosphorus in surface soils of Ghana. Ghana J. Agric. Sci. 5, 221–228. Adu, S.V., 1952. Detaileed soil survey of the central agricultural station, Nyankpala, Ghana. Department of Soil and Land-use Survey, 27 pp. Adu, S.V., 1981. The geomorphology, soils, water resources and land use potential of the Nasia River Basin, Ghada. Their relevance to the ‘Middle Belt’ of west Africa. ITC J. 4, 498–525. Agbenin, J.O., Tiessen, H., 1994. Phosphorus fractionations in a toposequence of Lithosols and Cambisols from semi-arid northerneastern Brazil. Geoderma 62, 345–362. Ball-Coelho, B., Salcedo, I.H., Tiessen, H., Stewart, J.W.B., 1993. Short- and long-term phosphorus dynamics in a fertilized Ultisol under sugarcane. Soil Sci. Soc. Am. J. 57, 1027–1034.

M.K. Abekoe, H. Tiessenr Catena 33 (1998) 1–15

15

Bationo, A., Mokwunye, A.U., 1991. Alleviating soil fertility constraints to increased crop production in west Africa: The experience in the Sahel. In: Mokwunye, A.U. ŽEd.., Alleviating Soil Fertility Constraints to Increased Crop Production in West Africa. Kluwer Academic Publishers, Dordrecht, pp. 195–215. Chang, S.C., Jackson, M.L., 1957. Fractionation of soil phosphorus. Soil Sci. 59, 39–45. Cross, A.F., Schlesinger, W.H., 1995. A literature review and evaluation of the Hedley fractionation: applications to biogeochemical cycle of soil phosphorus in natural ecosystems. Geoderma 64, 197–214. Harrison, R., Swift, R.S., Tonkin, P.J., 1994. A study of two soil development sequences located in a montane area of Canterbury, New Zealand: III. Soil phosphorus transformations. Geoderma 61, 151–163. Hedley, M.J., Stewart, J.W.B., Chauhan, B.S., 1982. Changes in inorganic and organic soil phosphorus fractions induced by cultivation and by laboratory incubations. Soil Sci. Soc. Am. J. 46, 970–976. Hieltjes, A.H.M., Lijklema, L., 1980. Fractionation of inorganic phosphates in calcareous sediments. J. Environ. Qual. 9, 405–407. Ibia, T.O., Udo, E.J., 1993. Phosphorus forms and fixation capacity of representative soils in Akwa Ibom State of Nigeria. Geoderma 58, 95–106. Juo, A.S.R., Moormann, F.R., Maduakor, H.O., 1974. Forms and pedogenetic distribution of extractable iron and aluminum in selected soils of Nigeria. Geoderma 11, 167–179. Manu, A., Bationo, A., Geiger, C., 1991. Fertility status of selected millet producing soils of West Africa with emphasis on phosphorus. Soil Sci. 152, 315–320. McKeague, J.A., Day, J.H., 1966. Dithionite- and oxalate-extractable Fe and Al as aids in differentiating various classes of soils. Can. J. Soil Sci. 46, 13–22. Mehra, O.P., Jackson, M.L., 1960. Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. Clays Clay Miner. 7, 317–327. Nair, V.D., Graetz, D.A., Portier, K.M., 1995. Forms of phosphorus in soil profiles from dairies of south Florida. Soil Sci. Soc. Am. J. 59, 1244–1249. Nye, P.H., Bertheux, M.H., 1957. The distribution of phosphorus in forest and savannah soils of the Gold Coast and its significance. J. Agric. Sci. Camb. 49, 141–150. Owusu-Bennoah, E., Acquaye, D.K., 1989. Phosphate sorption characteristics of selected major Ghanaian soils. Soil Sci. 148, 114–123. Roberts, T.L., Stewart, J.W.B., Bettany, J.R., 1985. The influence of topography on the distribution of organic and inorganic soil phosphorus across a narrow environment gradient. Can. J. Soil Sci. 65, 651–665. Smeck, N.E., 1985. Phosphorus dynamics in soils and landscapes. Geoderma 58, 185–189. Stoop, W.A., 1987. Variations in soil properties along three toposequences in Burkina Faso and implications for the development of cropping systems. Agric. Ecosys. Environ. 19, 241–264. Tiessen, H., Moir, J.O., 1993. Characterization of available phosphorus by sequential extraction. In: Carter, M.R. ŽEd.., Soil Sampling and Methods of Analysis. Can. Soc. Soil Sci. Lewis Publishers, Boca Raton, pp. 75–86. Tiessen, H., Bettany, J.R., Stewart, J.W.B., 1981. An improved method for the determination of carbon in soils and soil extracts. Comm. Soil Sci. Plant Anal. 12, 211–218. Tiessen, H., Hauffe, H.K., Mermut, A.R., 1991. Deposition of Harmattan dust and its influence on base saturation of soils in northern Ghana. Geoderma 49, 285–299. Tiessen, H., Stewart, J.W.B., Cole, C.V., 1984. Pathways of phosphorus transformation in soils of differing pedogenesis. Soil Sci. Soc. Am. J. 48, 853–858. Tiessen, H., Abekoe, M.K., Salcedo, I.H., Owusu-Bennoah, E., 1993. Reversibility of phosphorus sorption by ferruginous nodules. Plant and Soil 153, 113–124. Tiessen, H., Lo Monaco, S., Ramirez, A., Santos, M.C.D., Shang, C., 1996. Phosphate minerals in a lateritic crust from Venezuela. Biogeochemistry 34, 1–17. Uzu, F.O., Juo, A.S.R., Feyemi, A.A.A., 1975. Forms of phosphorus in some important agricultural soils of Nigeria. Soil Sci. 120, 212–218. Walker, T.W., Syers, J.K., 1976. The fate of phosphorus during pedogenesis. Geoderma 15, 1–19. Williams, J.D.H., Syers, J.K., Walker, T.W., 1967. Fractionation of soil inorganic phosphate by a modification of Chang and Jackson’s procedure. Soil Sci. Soc. Am. Proc. 31, 736–739.