Soil treatments for improved timber production in Pinus radiata plantations in South Africa

Forest Ecology and Management 171 (2002) 199±207

Soil treatments for improved timber production in Pinus radiata plantations in South Africa Janusz Zwolinskia,*, Mike Johnstonb, Heyns Kotzec a

Forestry Programme, University of Natal, PO Box 100281, Scottsville 3209, South Africa b School of Applied Environmental Sciences, University of Natal, Private Bag X01, Scottsville 3209, South Africa c SAFCOL Ltd., Sabie, South Africa

Abstract Production of Pinus radiata was greatly increased on poorly drained soils by planting trees on elevated planting beds or mounds and an application of phosphogypsum (PG). At 8.5 years of age, timber yields were improved ®ve times by bedding alone or 10 times by bedding combined with PG application when compared to the yield of trees planted with the traditional method of soil pitting. At 20 years of age, the mean annual increment is expected to improve from 1.4 m3 ha 1 per year in the pitting treatment to 14.9 m3 ha 1 per year where bedding was combined with PG application. The estimated growth improvement is expected to increase pro®ts to over US$ 2000 ha 1 for a 20-year timber production cycle. Such responses were possible due to improved soil physico-chemical properties. It is believed that other tree species that are sensitive to poor drainage (e.g. Pinus patula) could also bene®t from bedding and PG application when planted on poorly drained soils. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Forest regeneration; Bedding; Mounding; Pitting; Phosphogypsum

1. Introduction Pinus radiata was introduced in South Africa more than 150 years ago (Poynton, 1979a). Currently, P. radiata plantations occupy about 75 000 ha of land, which is 5% of all the timber plantations in the country. The species is preferred in the coastal parts of the Western and Eastern Cape Provinces, a region characterised by moderate temperatures with winter or all-year-round rainfall season. P. radiata is not planted in the summer rainfall part of the country due to its susceptibility to a pathogenic fungus diplodia *

Corresponding author. Tel.: ‡27-33-3862314; fax: ‡27-33-3868905. E-mail address: [email protected] (J. Zwolinski).

(Sphaeropsis sapinea). Diplodia infested trees die or they become malformed due to top-dieback of trees. The pathogen is particularly destructive to P. radiata in hail-affected areas. In the Cape Provinces, where P. radiata is planted, the following factors make sites less suitable for its cultivation: (i) nutrient de®ciency or imbalance, (ii) poor plant±soil water relations, (iii) poor physical properties of ``duplex'' soils resulting from a high ®ne sand and silt content (Grey and Taylor, 1983; Grey, 1985; Thwaites, 1986; Grey et al., 1987; SchoÈnau and Grey, 1987). The use of fertilisers has been well studied (Donald and Glen, 1974; Donald, 1986; Donald et al., 1987; De Ronde et al., 1988; Grey and De Ronde, 1988; Payn and Clough, 1988; Payn et al., 1988) and the results are commonly applied in

0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 4 7 3 - 5

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operational forestry. However, aspects of soil amelioration affecting soil physical properties and water availability have not been suf®ciently investigated despite encouraging results reported from overseas experiments (Berg, 1975; Pehl and Bailey, 1983; Froehlich, 1984; Burger and Pritchett, 1988; Wood et al., 1988). An experiment was established on the Harkerville Plateau where afforestation with exotic tree species was implemented in the 1950s. Attempts to improve timber production through planting P. radiata combined with intensive silviculture (soil ripping, weed control and fertilisation) failed. Eventually, the area was left for natural regeneration of P. pinaster, with marginal production rates and mean annual increment (MAI) of about 4 m3 ha 1 per year. The major objective of the current experiment was to investigate whether the application of mechanical and chemical soil treatments could improve soil characteristics for planting P. radiata and result in improved rates of growth. 2. Study area and methods An experimental site was selected on the Harkerville Plateau, in the Kruisfontein State Forest (348050 S, 238150 E, 195 m a.s.l.). Landforms in the region are dominated by the Outeniqua and Tsitsikamma Mountains formed by multiple collisions of the African and South American plates during the Permian (Dingle et al., 1983). Four major physiographic zones are distinguished: (i) sandy costal lowlands and dunes, (ii) a pronounced coastal platform between 180 and 230 m elevation, (iii) a foothill zone from 230 to 550 m a.s.l., and (iv) the mountains rising to 1580 m a.s.l. Tree plantations dominate the land use on the coastal platform which occupies a strip of land between the sea and mountains extending for about 500 km in length and 2±19 km in width. The area at Harkerville has an undulating topography resulting from wind-blown deposits on the coastal platform. The southern slope of the experimental area is approximately 48. The geology consists of a variety of super®cial deposits (aeolian sands, well sorted cobble beds, clay deposits, and local lignite lenses) underlined by steeply seaward dipping Table Mountain Group quartzite. Dune deposits vary in depth between a metre and in excess of 30 m. Sandy clay lenses occur

at depths of 1 m or less. Dominant clay minerals are kaolinite and illite. The sandy clays have weathered deeply to red colours during the Pleistocene and have subsequently been altered to yellow and ``gley'' colours due to reduction. Mobilisation of iron resulted in iron pan formation in the sandy material. A surface mantle of 0.3±1 m of ®ne grey silty loam is often separated from the underlying clay by a gravel layer with iron concretions or a stoneline (Anon., 1984). The soils within the experimental area are relatively uniform, moderately deep, hydromorphic and ``duplex''. They have been classi®ed as ochric Planosol (MacVicar et al., 1977). Water movement in the soil is hampered by physical soil properties. The topsoil is a grey silt loam some 20 cm deep, containing numerous rusty mottles in the lower part. This indicates a high ¯uctuating water table and periods of excessive wetness. A pale yellow-gleyed loam (E horizon) underlies this topsoil to about 60 cm. Rusty mottling occurs in the upper portion of this horizon. A ®rm yellow-gleyed clay abruptly underlies the E horizon. The ¯uctuating water tables and poor aeration for extended periods result from the gentle slope, the impervious clay subsoil, and the ®ne textured topsoil. The other characteristics of the soil include a relatively high pH (5.6±6.1), high levels of magnesium and sodium cations, low available phosphate and low organic matter (Anon., 1984). The climate of the region is in¯uenced by the continental, sub-tropical high pressure in summer and the passage of cold fronts from the south Atlantic in winter. Poynton (1979b) classi®ed the entire area as ``uniform rainfall area, humid, frost-free zone''. The mean annual rainfall is 1005.6 mm and the mean monthly temperature ranges between 12.6 8C in July and 19.7 8C in February (Weather Bureau, 1986). A split-plot, factorial experiment was established by combining three soil cultivation treatments (main plots) and two levels of phosphogypsum (PG) application (sub-plots), replicated four times. Each of the 24 plots occupied 467 m2. Each plot was planted with 64 trees of which 36 measurement trees were located in the plot centre. The measurement trees were surrounded with a guard-row of 28 trees. The soil tillage treatments included: (i) pitting (hoeing in spots of 80 cm diameter and 20 cm depth), (ii) spot mounding (45 cm diameter and 30 cm height), and (iii) bedding (60 cm base width and 30 cm height). All treatments were applied

J. Zwolinski et al. / Forest Ecology and Management 171 (2002) 199±207

by hand. PG (0.4% phosphorus content) was broadcast at the rate of 4.5 t ha 1 before soil cultivation and an additional 7.5 t ha 1 was broadcast 6 months after planting. This resulted in 48 kg phosphorus ha 1 applied to the treated plots. Size-graded bareroot seedlings of P. radiata were planted at 2.7 m spacing in October 1989. At planting, each tree was fertilised with 208 g of superphosphate (10.5% of phosphorus), i.e. 30 kg phosphorus ha 1. Tree mortality in the pitting treatment was very high, necessitating supplemental planting of trees (blanking) to avoid big gaps. The last measurement of height (HT) and diameter at breast height (DBH) of the trees was conducted in May 1998, at the age of 8.5 years. The growth data (DBH, HT and stems per hectare) were extrapolated from the measurement at 8.5 years to the reference age of 20 years. The standard growth model for P. radiata in SAFCOL was used to do the projections. The growth model was derived from the Tokai CCT spacing trial of P. radiata and was implemented in the PCPROJ growth and yield simulator (Pretorius, 1993). The growth model was calibrated for each treatment with quadratic mean DBH, respective mean HT obtained from the relationship between DBH and HT …ln HT ˆ b0 b1 =DBH†, and stems per hectare (including blanked trees) at 8.5 years of age. Growth was then projected to 20 years through a series of thinnings in which numbers of trees were reduced to 650, 400, and 250 stems ha 1 at 11, 15 and 19 years, respectively. The volume of timber to 7.5 cm thin-end stem diameter was calculated by using a locally developed volume equations (Bredenkamp, 1994) and thereafter standing and thinned volumes were calculated on a per hectare basis. In March 2000, samples of untilled soil (pitting plots) were collected to compare the PG treated soil with properties of the undisturbed soil. In each of the pitting plots samples from ®ve randomly selected points were combined into a composite sample. Soil was collected from two depths: 5±10 and 25±30 cm. In addition, at two of the points in each plot undisturbed soil core samples (7.5 cm diameter and 5.0 cm deep) were taken. The analyses performed on the disturbed soil samples were aimed at characterising the fertility status, the soluble salt composition, the texture and aggregate stability. A standard fertility evaluation was conducted at the Laboratory of the KwaZulu,

201

Natal Department of Agriculture at Cedara. Soil pH was measured in a 1:2.5 soil:solution (1 M KCl) suspension; P, K, Zn and Mn were measured on the ``Ambic'' extract (0.25 M NH4 HCO3 ‡ 0:01 M disodium EDTA ‡ 0:01 M NH4F and adjusted to pH 8.0); Ca, Mg and titratable acidity (using phenolphthalein) were measured on a 1 M KCl extract; and Na determined on a 1 M NH4Cl extract. The soluble salt status was measured on the extract of a saturated soil paste in order to establish the salinity and sodicity levels (Richards, 1954). Also measured was the modulus of rupture (Reeve, 1965) in order to evaluate the possible aggregating effect of PG on the soil, as well as the particle size distribution using the pipette method (Gee and Bauder, 1986). The undisturbed soil cores were used to measure bulk density, porosity and water retention characteristics over the range of 0 to 10 kPa matric potential using a tension table. Porosity was calculated from bulk density assuming a particle density of 2.65 g cm 3. Analysis of data was conducted by using analysis of variance with the SAS GLM procedure. A Tukey's honestly signi®cantly different test was performed to compare means of different treatments (SAS Institute, 1990). Signi®cance for speci®c comparisons was claimed only if the F-test had demonstrated signi®cance (Snedecor and Cochran, 1968). Analysis of variance was applied to the survival percentage after the angular transformation of the data with the arcsine had been used to normalise the distribution (Zar, 1984). 3. Results and discussion 3.1. Survival The results indicated that survival of trees planted in October 1989 was signi®cantly affected by the soil tillage and the application of PG (Table 1). Satisfactory survival (not less than 90%) was recorded only in the bedding treatment. In the mounding treatment, tree dieback was excessive (more than 10%) and would require blanking in usual commercial operations. The failure of the pitting treatment was very obvious with only 29% of the trees surviving the ®rst planting. Even with repetitive blanking operations the number of trees planted in pits was insuf®cient to meet the criterion of

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Table 1 Performance of P. radiata trees (including blanked trees) at 8.5 years in response to the experimental treatmentsa Treatment

Survival (%)

Height (m)

DBH (cm)

Stem volume (m3 per tree)

Pitting Mounding Bedding

74 a 84 a 96 b

6.65 a 10.88 b 10.93 b

7.90 a 12.57 b 12.76 b

0.0243 a 0.0615 b 0.0634 b

24.67 a 71.14 b 82.99 b

No PG PG

81 a 88 b

8.59 a 10.65 b

9.83 a 12.60 b

0.0356 a 0.0657 b

39.48 a 79.72 b

Pitting  no PG Pitting  PG Mounding  no PG Mounding  PG Bedding  no PG Bedding  PG

65 83 83 85 94 97

5.27 7.72 9.88 11.87 9.73 12.10

5.71 9.59 11.17 13.94 11.48 14.02

0.0102 0.0352 0.0438 0.0787 0.0458 0.0806

a

Stand volume (m3 ha 1)

9.05 40.29 50.09 92.18 59.29 106.69

Signi®cantly different means are separated with a different letter index.

a successful establishment (Table 1). In the pitting treatment, trees started dying soon after planting without the usual recovery from a transplanting stress visible in other treatments. Zwolinski and Bredenkamp (1999) used the relative growth rate of height to de®ne the acclimatisation period (transplanting stress) to be between 87 days for bedding and 93 days for mounding. It was not possible to calculate the transplanting stress for the pitting treatment, most likely due to the acclimatisation exceeding the study period of 2 years. The use of logistic regression provided information about the environmental factors affecting survival at 1 year after planting (Zwolinski et al., 1994). Higher water holding capacity in the soil improved the chance of survival. Apart from the bene®t of improved water supply during dry periods, bedding and mounding increased the volume of large pores which played an important function in faster soil drainage after rains and improved soil aeration (Zwolinski et al., 1993a, 1995). This was also re¯ected in higher survival being a function of lower bulk density of the topsoil. Higher litter mass also increased the percentage of the surviving trees. Litter increased predominantly due to dieback of weeds. This resulted in reduced competition and increased mulching of the soil surface. Litter also acted as a source of nutrients because of the relatively fast rate of decomposition (Zwolinski et al., 1993b). Most dif®cult to explain was the relation-

ship between rising levels of phosphorus and decreasing survival. A direct harmful impact of superphosphate on trees cannot be excluded (D.G.M. Donald, personal communication, University of Stellenbosch, South Africa, 1991). More likely, however, lower levels of phosphorus in beds were associated with fertiliser leaching due to improved drainage. Pathological analysis indicated that trees stressed by the site conditions were colonised by potentially pathogenic fungi of which Cylindrocarpon sp. and Fusarium oxysporum were most frequently isolated from diseased trees (Zwolinski et al., 1994). At 8.5 years of age, analysis of variance showed signi®cantly improved survival of trees in PG treated plots when blanked trees were included in the analysis (Table 1). This is possibly due to the time required for the incorporation of PG into the soil for its ameliorative effect. 3.2. Growth analysis At 8.5 years of age, a combination of differences in HT, DBH and survival resulted in an impressive improvement of timber volume by over 1000% (Table 1) when the combined bedding and PG application was compared with the routine treatment (pitting and no PG). Pitting resulted in signi®cantly lower timber volumes compared to mounding and bedding, while the application of PG signi®cantly improved timber volumes twofold (Table 1). Trees that survived

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the establishment stage were not showing any symptoms of stress or damage. Despite the site being exposed to strong winds, no leaning (toppling) of trees, uprooting, or stem breakages were observed in the experiment, although wind damage was recorded in the region during the study. 3.3. Changes in soil properties and tree nutrition Growth differences could be associated with alterations of nutrient supply through the ameliorative treatments. At 1 year after planting, bedding resulted in higher concentrations of magnesium and sodium in the topsoil and signi®cantly higher levels of nitrogen, calcium, sodium, and manganese in the mid-soil. The beds were constructed by bringing the soil from nutrient-poor deeper soil horizons, while nutrient rich organic matter and soil from the organic horizons was deposited at the base of the beds (Zwolinski et al., 1993b). In general, the de®cient levels of boron seemed to be improved by the more intensive soil treatment and PG application (Zwolinski et al., 1993b). At 9.75 years of age, sparse crowns of trees were observed in plots which did not receive PG (Schafer, 1999). A foliar analysis was conducted to explain growth differences between the pitting and bedding plots with or without the PG treatments. The lowest levels of nitrogen, potassium, copper, and zinc were found in the best performing trees (bedding and PG treatment). The lower levels of these nutrient elements could be induced by accelerated tree growth on nutrient de®cient soils. Therefore, tree performance could be additionally enhanced by supplemental fertilisation with nitrogen and potassium.

203

The in¯uence of PG on nutrient contents was generally less than might have been expected considering the growth responses. PG had a negligible effect on soil pH, P, K and Zn levels (Table 2). The Mn concentrations appeared to be depressed by PG. The Ca concentration was markedly higher and Mg concentration markedly lower in terms of total extractable ions. The PG caused a slight reduction, particularly near the soil surface, in the acid saturation percentage, i.e. the acidic cations (H and Al) expressed as a proportion of the total extractable cations. The saturation extract analysis showed a marked increase in soluble Ca concentrations, but little change in concentrations of soluble Mg and Na (Table 3). The net effect was that the electrical conductivity (EC) was slightly higher in the PG treatments for both sampling depths, and the sodium adsorption ratio (SAR) is slightly lower. The combination of reduced sodicity, increased electrolyte concentration of the soil solution and an increased Ca:Mg ratio could be expected to have a positive effect on soil aggregation and consequently on soil physical conditions (Shainberg, 1984). Most importantly this would include bene®cial effects on soil aeration and in®ltration rate. Results of the soil physical measurements that were made are consistent with the likely bene®ts of the improved electrolyte status. No measurable in¯uence of PG on bulk density and total porosity was found. However, was a slight increase in the air-®lled porosity at ®eld capacity (air capacity), which is likely to be in the range of 5 to 10 kPa (Table 4). It is normally accepted that an air capacity of at least 10% is required for healthy root growth (Hall et al., 1977). So it is clear that this soil had a marginal to de®cient

Table 2 Fertility status of soils treated and not treated with PG at the Harkerville experimenta Depth (cm)

Treatment

Soil pH (KCl)

Extractable elements (mg kg 1)

Acid saturation (%)

P

K

Ca

Mg

Na

Zn

Mn

5±10

No PG PG

4.2 4.3

1.7 1.8

30 29

169 a 291 b

135 a 70 b

46 50

0.4 0.4

0.4 0.2

20.5 16.8

25±30

No PG PG

4.3 4.3

1.0 0.6

32 26

154 303

157 122

70 57

0.5 0.3

0.4 0.0

14.3 13.3

a Signi®cantly different means are separated with a different letter index. The comparison was carried out only between the same soil depths for the different treatments.

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Table 3 Soluble salt status of the saturation extract of soil treated and not treated with PG at the Harkerville experimenta Depth (cm)

Treatment

SARb

Saturation extract EC (dS m 1)

Cation concentrations (mequiv. l 1) Ca

Mg

Na

K

5±10

No PG PG

0.39 a 0.47 b

0.24 a 0.95 b

0.57 0.51

2.23 2.62

0.07 0.07

3.5 3.1

25±30

No PG PG

0.36 0.52

0.23 a 0.90 b

0.56 0.68

2.53 2.97

0.08 0.07

4.0 3.3

a Signi®cantly different means are separated with a different letter index. The comparison was carried out only between the same soil depths for the different treatments. b Sodium adsorption ratio.

Table 4 Some physical properties of soils treated and not treated with PG at the Harkerville experiment Depth (cm)

Treatment

Texturea Clay (%)

Silt (%)

Sand (%)

Bulk density (kg m 3)

Porosity (%)

AFPb (%) at 5 kPa

10 kPa

Modulus of rupture (kPa)

5±10

No PG PG

10 11

39 40

51 49

1646 1655

37.9 37.5

7.8 8.9

9.4 11.1

128 115

25±30

No PG PG

11 11

41 42

48 49

1755 1763

33.8 33.5

7.0 7.5

7.8 8.8

381 364

a b

Clay: <0.002 mm, silt: 0.002±0.05 mm, and sand: 0.05±2.0 mm. Air-®lled porosity.

aeration status. The improvement in air capacity from 9.4 to 11.1% ( 10 kPa) or 7.8 to 8.9% ( 5 kPa) could conceivably be very bene®cial for the growth of P. radiata. The improved root growth in PG treated soils could result in greater water uptake which would further help to improve the air content of the soil during unfavourably wet periods. Further con®rmation of the improvement in aggregation is provided by

the slight reduction in the modulus of rupture. This re¯ects a reduced tendency for hard-setting in the soil. The soil texture was found to be similar at the two sampling depths, the textural group being a silt loam. It must be recognised that this exercise of sampling and analysis was done some 10 years after the PG had been applied. The impact on soil conditions was probably more dramatic within 2 or 3 years of

Table 5 Growth projections of trees (including blanked trees) to age 20 years at the Harkerville experiment Combination of treatments

Mean height (m)

Mean DBH (cm)

Volume (m3 ha 1)

SI20 (m)

MAI20 (m3 ha

Pitting  no PG Pitting  PG Mounding  no PG Mounding  PG Bedding  no PG Bedding  PG

12.5 17.6 20.2 24.1 20.0 24.9

12.5 20.6 21.8 27.4 22.5 27.4

14.0 54.0 70.0 128.0 74.0 133.0

12.5 17.6 20.2 24.1 20.0 24.9

1.4 5.7 7.3 13.7 8.2 14.9

1

per year)

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205

Fig. 1. Values of site index (SI20) and mean annual increment (MAI20) projected at 20 years for the different treatments (Tmts) at the Harkerville experiment and plotted against the average (standard) relationship between SI20 and MAI20 developed for the regional plantations of P. radiata.

application than it is now, and one could consider the effects measured as being residual. 3.4. Projection of timber yields and bene®ts Results from the growth projections are provided in Table 5. At 20 years of age, the total volume is expected to be improved with bedding and PG application by up to 850%, while the site index (SI20) and the utilisable, thinning-included, MAI20 will likely be higher by up to 100 and 964%, respectively. These two variables are often used to indicate site quality (Von Gadow and Bredenkamp, 1992). Fig. 1 shows how the different treatments are distributed around the standard relationship between SI20 and MAI20 for this thinning regime for the region. This graph clearly illustrates the magnitude of the timber production differences between the treatments. If bedding is done mechanically with such implements as a Savannah plough, then the cost of site preparation with bedding and hand-pitting are the same (about US$ 90 ha 1). This means that the additional volume of timber of 50.24 m3 ha 1 at 8.5 years or an estimated 60 m3 ha 1 at 20 years is produced at no additional cost if pitting is replaced with bedding at establishment (and no PG treatment is applied). With the current and local prices for pine roundwood, it means additional income of about US$ 1200 h 1. On

average, the application of PG doubled the timber volume at the same age in addition to the bene®t achieved with the soil cultivation treatments. The local cost of PG is only US$ 6.30 t 1 but its transport and application can triple this cost up to about US$ 19 t 1. Therefore, the application of 12 t ha 1 may cost up to US$ 228 ha 1. After taking this cost into consideration, the bene®t of additional timber produced by combining bedding and PG application is estimated at US$ 2152 ha 1 for a 20-year pine rotation. 4. Conclusions The growth of P. radiata on sites similar to those found on the Harkerville Plateau can be improved by soil amelioration at planting. Once trees are established, they continue performing well. Therefore, the reason for poor initial growth seems to be poor hydrological conditions causing excessive periods of soil waterlogging. Topsoil drainage is improved by creating elevated planting beds. This site preparation treatment produces ®ve times more volume of timber at 8.5 years of age than the traditional use of pitting. With the current silvicultural costs and timber prices, the estimated growth improvement is expected to increase land owners' pro®ts by over US$ 2000 ha 1 for a 20-year timber production cycle if bedding is combined

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with 12 t ha 1 PG application at planting. More research is required to: (i) de®ne alternatives to intensive soil tillage, and (ii) optimise growth with improved nutrition. Acknowledgements This ®eld experiment was funded by the South African Forestry Company Ltd. The support received from the Regional Manager, Mr. J. LuÈckhoff and the Regional Technical Manager, Mr. C. Carstens is gratefully acknowledged. References Anon., 1984. Proceedings of the IUFRO Symposium on Site and Productivity of Fast Growing Plantations. Tour Guide, Southern Cape. Saasveld FRC, Department of Environment Affairs, Pretoria, 118 pp. Berg, P.J., 1975. Developments in the establishment of second rotation radiata pine at Riverhead Forest. NZ J. For. 20, 272± 282. Bredenkamp, B.V., 1994. The volume of standing trees. In: Van der Sijde, H.A. (Ed.), Forestry Handbook. South African Institute of Forestry, Pretoria, pp. 324±327. Burger, J.A., Pritchett, W.L., 1988. Site preparation effects on soil moisture and available nutrients in a pine plantation in the Florida ¯atwoods. For. Sci. 34, 77±87. De Ronde, C., James, D.B., Baylis, N.T., Lange, P.W., 1988. The response of Pinus radiata to manganese applications at Ruitersbos State Forest. S. Afr. For. J. 146, 26±33. Dingle, R.V., Siesser, W.G., Newton, A.R., 1983. Mesozoic and Tertiary Geology of Southern Africa. A.A. Balkema, Rotterdam. Donald, D.G.M., 1986. The application of fertilizer to pines following second thinning. In: Proceedings of the Seventh Meeting on Forest Site and Nutrition Research Working Group, Piet Retief, South Africa, pp. 64±70. Donald, D.G.M., Glen, L.M., 1974. The response of Pinus radiata and Pinus pinaster to N, P and K fertilizers applied at planting. S. Afr. For. J. 91, 19±28. Donald, D.G.M., Lange, P.L., Schutz, C.J., Morris, A.R., 1987. The application of fertilizers to pines in Southern Africa. S. Afr. For. J. 141, 53±62. Froehlich, H.A., 1984. Mechanical amelioration of adverse physical soil conditions in forestry. In: Grey, D.C., SchoÈnau, A.P.G., Schutz, C.J. (Eds.), Proceedings of the Symposium on Site and Productivity of Fast Growing Plantations. SAFRI, Department of Environment Affairs, Pretoria, pp. 507±521. Gee, G.W., Bauder, J.W., 1986. Particle size analysis. Methods of soil analysis. Part 1. Physical and mineralogical methods. Agronomy 9, 383±411.

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