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Chapter 8
Edaphic adaptability and suitability

As a medium in which roots grow and as a reservoir for water and nutrients upon which plants continuously draw, soil represents an important natural resource and valuable economic asset requiring protection and improvement through good land-use husbandry.

The adequate silvicultural exploitation of the climatic potential or sustained maintenance of productivity largely depends on soil fertility and using soil on an ecologically sound basis. Soil fertility is concerned with the ability of soil to supply nutrients and water as well as the ability of soil to respond to application of production treatments. The fertility of a soil is determined by both its physical and chemical properties whose understanding is essential to the effective utilization of the climatic resources.

In order to assess soil suitability for fuelwood production, the soil requirements of tree species must be determined. Further, these requirements must be understood within the context of limitation imposed by landform and other features (e.g. soil phases) which do not form part of soil composition but have a significant influence on the use that can be of the soil.

8.1 BASIC SOIL REQUIREMENTS

Basic soil requirements for fuelwood tree species relate to the following internal soil properties:

  1. the soil temperature regime as a function of heat balance of soils which, in turn, is related to annual and seasonal and/or daily temperature fluctuations;

  2. the soil moisture regime, as a function of the water balance of soils as related to the soils capacity to store, retain, transport and release moisture for tree growth, and/or to the soil's permeability and drainage characteristics;

  3. the soil aeration regime, as a function of the soil air balance as related to its capacity to supply and transport oxygen to the root zone and to remove carbon dioxide;

  4. the natural soil fertility regime, as related to the soil's capacity to store, retain and release plant nutrients in such kinds and proportions as are required by trees during growth;

  5. the effective soil depth available for root development and physical support of tree stand;

  6. soil texture and stoniness, at the surface and within the whole depth of the soil, required for normal stand development;

  7. the absence of soil salinity and specific toxic substance or ions deleterious to stand growth;

  8. soil accessibility and trafficability under certain management systems;

  9. other specific properties: e.g. soil tilth as required for the germination and early growth.

From the basic soil requirements for forestry land use, a number of responses related to soil characteristics have been derived for fuelwood species.

A correlation between the basic soil requirements for fuelwood species listed above and soil characteristics that can be used as soil factors to rate tree crop performance is given in Table 8.1 and soil requirements for fuelwood species are summarized in Table 8.2.

TABLE 8.1
Relation between basic soil requirements for fuelwood species and soil characteristics

Basic soil requirementsSoil Characteristics (soil factors)
Moisture availability1- Effective soil depth
 - Available soil moisture holding capacity
 - Drainage
Nutrient availability- Nutrient availability
 - Soil reaction
Oxygen availability2- Soil permeability
 - Drainage
Foothold for roots- Effective soil depth
Salinity- Soil salinity
Toxicity- Soil reaction3
Accessibility and Trafficability (workability)- Topsoil consistency and bearing capacity
Soil tilth for plant establishment- Topsoil consistency and bearing capacity

1 Moisture availability is influenced by climatic factors.

2 Oxygen availability is influenced by inundation and flooding characteristics.

3 Chemical properties of soil parent material may also be involved in some cases.

The soil units have been defined in terms of measurable and observable properties of the soil itself; and specific clusters of such properties are combined into “diagnostic horizons” and “diagnostic properties”, which are used in the definition of the soil units.

The diagnostic horizons have been used as defined in the Legend of the FAO-Unesco Soil Map of the World. Diagnostic properties however have been narrowed down in case of ferric properties and widened in case of vertic properties. In Table 8.3 for each soil unit diagnostic horizons and properties of the soil units are summarized. For the ease of interpretation of Table 8.3, these characteristics are summarized below.

TABLE 8.2
Soil requirements of fuelwood species

SpeciesTextureDrainageSoil depth (cm)Reaction (pH)Salinity (mmhos)Flooding
optimumrangeoptimumrangeoptimummarginaloptimumrangeoptimummarginaloptimummarginal
Acacia albidaSL-SCLS-KSWMW-SE>12075–1205.5–7.05.0–7.5<44–6FOF1
Acacia gerradiiL-CSL-KCMW-WI-SE>12075–1205.5–7.05.0–7.5<44–6F1F2
Acacia niloticaL-CSL-KCWMW-SE>12075–1205.5–7.55.0–8.0<44–8F1F2
Acacia senegalSL-SCLS-KCWMW-SE>12075–1205.5–7.05.0–7.5<44–6FOF1
Acacia tortilisSL-SCLS-KCWMW-SE>7550–756.5–8.06.0–8.5<44–8FOF1
Bridelia micranthaSL-SCLS-KCWMW-SE>12075–1205.5–7.05.0–7.5<44–6FOF1
Calliandra calothyrsusSL-SCLS-KCWMW-SE>12075–1206.0–7.05.5–7.5<44–6FOF1
Calodendrum capenseSL-SCLS-KCWMW-SE>12075–1205.5–7.05.0–7.5<44–6FOF1
Cassia siameaSL-CLLS-SiCWMW-SE>150100–1506.0–7.05.5–7.5<44–6FOF1
Casuarina equisetifoliaSL-SiLLS-CLWMW-SE>12075–1206.5–8.06.0–8.5<1212–16F1F2
Casuarina cunninghamianaSL-SCLS-KCWMW-SE>12075–1205.5–7.05.0–7.5<44–6F1F2
Conocarpus lancifoliusSL-CLLS-SCMW-WI-SE>12075–1206.5–8.06.0–8.5<88–12F1F2
Croton megalocarpusSL-SCLS-KCWMW-SE>12075–1206.5–8.06.0–8.5<44–8FOF1
Cupressus lucitanicaL-CLSL-SCWMW-SE>150100–1505.5–7.05.0–7.5<44–6FOF1
Eucalyptus camaldulensisSL-SCLS-KCMW-WI-SE>12075–1205.0–6.54.5–7.5<22–4F1F2
Eucalyptus citriodoraSL-CLLS-SiCWMW-SE>160100–1505.5–7.05.0–7.5<44–6FOF1
Eucalyptus globulusL-CSL-KCWMW-SE>150100–1505.5–7.05.0–7.5<22–4FOF1
Eucalyptus grandisSL-CLLS-SiCWMW-SE>12075–1205.5–7.05.0–7.5<44–6FOF1
Eucalyptus microcorysL-CLSL-SCWMW-SE>12075–1205.5–7.55.0–8.0<44–8F1F2
Eucalyptus microthecaL-CSL-KCMW-WI-SE>12075–1206.5–8.06.0–8.5<44–8F1F2
Eucalyptus salignaSL-CLLS-SiCWMW-SE>12075–1205.5–7.05.0–7.5<44–6FOF1
Eucalyptus tereticornisSL-SCLS-KCWMW-SE>12075–1205.5–7.05.0–7.5<44–6FOF1
Faurea salignaSL-SCLS-KCWMW-SE>12075–1205.5–7.05.0–7.5<44–6FOF1
Gliricidia sepiumSL-SCLS-KCWMW-SE>12075–1205.0–6.54.5–7.0<22–4FOF1
Grevillea robustaSL-CLLS-SiCWMW-SE>12075–1205.5–7.05.0–7.5<44–6FOF1
Leucaena leucocephalaSL-SCLS-KCWMW-SE>7550–755.5–7.05.5–8.5<88–12FOFO
Oleo africanaSL-SCLS-KCWMW-SE>12075–1206.5–8.06.0–8.5<44–8FOF1
Parkinsonia aculeataSL-CLLS-SiCWMW-SE>12075–1206.5–8.06.0–8.5<88–12FOF1
Prunus africanumSL-SCLS-KCWMW-SE>12075–1205.5–7.05.0–7.5<44–6FOF1
Sesbania sesbanSL-SCLS-KCMW-WI-SE>12075–1206.0–7.05.5–7.5<1212–16F1F2
Tamarindus indicaSL-SCLS-KCWMW-SE>12075–1205.5–7.55.0–7.5<44–6FOF1

Texture classes: LS = loamy sand;
SL = sandy loam;
SiL = silty loam;
L = loam;
CL = clay loam;
SC = sandy clay;
SiC = silty clay;
C = clay;
KC = kaolinitic clay.

Drainage classes: I = imperfectly;
MW = moderately well;
W = well;
SE = somewhat excessive.

Flooding classes: FO = no flooding;
F1 = occasional floods;
F2 = frequent floods.

Histic H horizon:Surface layer of organic material more than 20 cm thick.
Mollic A horizon:Surface horizon with dark colour, medium to high humus content, high base saturation.
Umbric A horizon:Surface horizon with dark colour, medium to high humus content, low base saturation.
Ochric A horizon:Surface horizon with light colour, low humus content.
Argillic B horizon:Subsoil horizon with accumulation of illuvial clay.
Natric B horizon:Subsoil horizon with accumulation of illuvial clay and high exchangeable sodium.
Cambic B horizon:Subsoil horizon with a structure and/or colour different from overlying and underlying horizons.
Spodic B horizon:Subsoil horizon with accumulation of iron and/or humus.
Oxic B horizon:Subsoil with residual accumulation of sesquioxides and low CEC.
Calcic horizon:Horizon of accumulation of calcium carbonate.
Gypsic horizon:Horizon of accumulation of calcium sulphate.
Sulphuric horizon:Horizon with strong acidity and prominent mottling.
Albic E horizon:Eluvial horizon from which clay and free iron oxide have been removed, light colour.
Calcareous material:Calcium carbonate present at least between 20 and SO cm from the surface.
CEC high or very high:Exchange complex dominated by allophane or montmorillonite.
CEC low:Exchange complex dominated by kaolinite (CEC less than 24 meq/100 g clay).
CEC very low:Less than 1.5 meq/100 g clay.
Cracking clays:Formation of deep and wide cracks upon drying.
Plinthite:Mottled subsoil layer which irreversibly hardens upon exposure to repeated wetting and drying.
High salinity:Electrical conductivity (EC) higher than 15 mmhos/cm.
Moderate salinity:Electrical conductivity (EC) between 4 and 15 mmhos/cm.
High alkalinity:Saturation with exchangeable sodium of more than 15 percent.
Moderate alkalinity:Saturation with exchangeable sodium of 6 to 15 percent.
Indurated subsoil:Subsoil layer with firm or hard consistence, but can still be penetrated by spade or auger.
Cemented hardpan:Extremely hard continuous subsoil layer which cannot be penetrated by spade or auger.
Coarse texture:Less than 18 percent clay and more than 65 percent sand.
Heavy texture:More than 35 percent clay.
Abrupt textural change:Considerable increase in clay content within a very short vertical distance.
Tonguing:Deep and irregular penetration of an albic E horizon into an argillic B horizon.

8.2 EDAPHIC SUITABILITY

The edaphic suitability classification is input-specific and based on:

  1. matching the soil requirements of fuelwood species with the soil conditions of the soil units described in the soil inventory (soil unit evaluation); and

  2. modification of the soil unit evaluation by limitation imposed by texture, phase and slope conditions.

The soil unit evaluation for fuelwood species production is expressed in terms of ratings based on how far the soil conditions of a soil unit meet the growth and production requirements under a specified level of inputs. The appraisal is effected in five basic classes for each species and level of inputs, i.e. very suitable (S1), suitable (S2), moderately suitable (S3), marginally suitable (S4), and not suitable (N).

A rating of S1 indicates that the soil conditions are optimal, and that suppression of potential yields (if any) are assumed to be nil or slight. A rating of S2 indicates that there are slight to moderate soil constraints and there would be a suppression of potential yields of the order of 25%. A rating of S3 indicates that there are moderate to severe soil constraints and there would a suppression of potential yields of the order of 50%. A rating of S4 indicates that there are severe soil constraints and there would be a suppression of potential yields of the order of 75%. A rating of N indicates that soil conditions are not suitable for fuelwood production.

The soil unit ratings for fuelwood are given in the Appendix in Table A8.1 and apply as indicated provided there or no additional limitations imposed by soil texture, phase or stoniness. Modifications are required where such limitations are present.

In the case of soil texture, soil unit ratings remain unchanged if the soil is an albic, cambic, ferralic, calcaro-cambic or luvic Arenosol (Q, Qa, Qc, Qf, Qkc, Ql) or a vitric Andosol (Tv), or where textures are medium (fine sandy loam, sandy loam, loam, sandy clay loam, clay loam, silty clay loam, silt), or fine (sandy clay, silty clay, peaty clay, clay). In all other cases (i.e. with coarse textures: sand, loamy coarse sand, fine sand, loamy fine sand, loamy sand) the soil unit rating is one class (25%) lower.

Limitation imposed by phase and stoniness are rated using the five basic classes already described The ratings are presented in Appendix in Table A8.2 for phase modification and in Table A8.3 for stoniness modifications.

TABLE 8.3
Diagnostic horizons and properties of soil units

 AAcAgAhAicAlfAioAoApAthBBcBdBeBfBgBhBkBncBtcBteBvCChCk
Histic H horizon                         
Hollic A horizon                      xxx
Umbric A horizon  (xx     x     (xx        
Ochric A horizonxx(x xxxxx xxxxx(x xxxxx   
Argillic & horizonxxxxxxxxxx               
Natric B horizon                         
Cambic B horizon          xxxxxx xxxxxxx 
Spodic B horizon                         
Oxic B horizon                         
Calcic horizon                 x      x
Gypsic horizon                         
Sulferic horizon                         
Albic E horizon (mat.)                         
Calcareous Material                 x    xx 
CaCo3 > 5% (topsoil)                         
CEC high/very high                     x   
CEC low (<24 meg)    x x       x          
CEC very low (<1.5 meg)     x                   
Base sat. >50%, (pH>5.5)          xx x   xxxx xxx
Base sat. <50%, (PH<5.5)xxxxxxxxxx  x   x        
Cracking clay                     x   
Depth <25 CM                         
Depth 25–50 cm                         
Drainage very poor/poor  x                      
Drainage imp./moderate        x      x         
Drainage excessive                         
Plinthite        x                
Salinity high                         
Salinity moderate                         
Alkalinity high                         
Alkalinity Moderate                         
Indurated subsoil                         
Cemented hardpan                         
Iron concretions     x        x          
Texture coarse                         
Texture heavy           x      xx x   
Abrubt textural change                         
Tonguing                         
Reddish colour x  x      x      xx     
Yellowish colour                         
Sat. fertility high          x  x  xx  xxxxx
Hat. fertility moderatexx xx xx x xx  x  xx     
Hat. fertility lot  x  x  x     x          
 AAcAgAhAicAlfAioAoApAthBBcBdBeBfBgBhBkBncBtcBteBvCChCk

 EEcEoFFaFhFnhFnrFoFrFxGGcGdGeGhGmGvHHgHhHnlHolHrlHth
Histic H horizon               (x(x        
Hollic A horizonxxx             (x xxxxxxx
Umbric A horizon     xx        (x         
Ochric A horizon   xx  xxxxxxxx  x       
Argillic B horizon                   x     
Natric B horizon                         
Cambic B horizon           xxxxxxxx x   x
Spodic B horizon                         
Oxic S horizon   xxxxxxxx              
Calcic horizon            x            
Gypsic horizon                         
Sulferic horizon                         
Albic E horizon (mat.)                         
Calcareous materialxxx         x            
CaCO3 > 5% (topsoil) x                       
CEC high/very high                         
CEC low (<24 meg)   x xxxxxx              
CEC very low (<1.5 meg)    x                    
Base sat. >50%, (pH>5.5)           xx x  x       
Base sat. <50%, (pH<5.5)   x    x x  x xx        
Cracking clay                 x       
Depth <25 cm                         
Depth 25–50 cmxxx                      
Drainage very poor/poor           xxxxxxx x     
Drainage imp./moderate            xxxxxx x     
Drainage excessive                         
Plinthite                         
Salinity high                         
Salinity moderate                         
Alkalinity high                         
Alkalinity Moderate                         
Indurated subsoil                         
Cemented hardpan                         
Iron concretions                         
Texture coarse          x              
Texture heavy       x x               
Abrubt textural change                         
Tonguing                         
Reddish colour         x             x 
Yellowish colour          x              
Hat. fertility highxxx        xx xxxxxxxxxxx
Hat. fertility Moderate     xxx x   x           
Hat. fertility lot   xx   x x              
 EEcEoFFaFhFnhFnrFoFrFxGGcGdGeGhGmGvHHgHhHnlHolHrlHth

 HtlHvlIIrJJcJeJtKKhLLaLcLfLgLicLifLioLkLncLnfLoLvMMo
Histic H horizon                         
Hollic A horizonxx      xx             xx
Umbric A horizon       (x                 
Ochric A horizon    xxx(x  xxxxxxxxxxxxx  
Argillic B horizonxx        xxxxxxxxxxxxxxx
Natric B horizon                         
Cumbic B horizon        xx               
Spodic B horizon                         
Oxic & horizon                         
Calcic horizon     x            x      
Gypsic horizon                         
Sulferic horizon       x                 
Albic E horizon (mat.)           x             
Calcareous material        xx               
CaCO3 > 5% (topsoil)                         
CEC high/very high                         
CEC low (<24 meg)               x x       
CEC very low (<1.5 meg)             x  x   x    
Base sat. >50%, (pH>5.5)    xxx xxxxxxxxxxxxxxx  
Base sat. <50%, (pH<5.5)                         
Cracking clay x                    x  
Depth <25 cm  x                      
Depth 25–50 cm   x                     
Drainage very poor/poor              x          
Drainage imp./moderate                         
Drainage excessive                         
Plinthite                         
Salinity high                         
Salinity moderate                         
Alkalinity high                         
Alkalinity Moderate                         
Indurated subsoil                         
Cemented hardpan                         
Iron concretions   x         x  x   x    
Texture coarse                         
Texture heavy                      x  
Abrubt textural change                         
Tonguing                         
Reddish colour            x  x   x     
Yellowish colour                         
Hat. fertility highxx  xxx xxx       x  xx  
Hat. fertility Moderate            x xx x x   xx
Hat. fertility lot           x x  x   x    
 HtlHvlIIrJJcJeJtKKhLLaLcLfLgLicLifLioLkLncLnfLoLvMMo

 MvoNNdNeNhNmNthNveNvmOOdQQaQcQfQkQlRRcRdReRtcSSgSlo
Histic H horizon         xx              
Hollic A horizonx    x  x                
Umbric A horizon    x x                  
Ochric A horizon xxx   x   xxxxxxxxxxxx x
Argillic B horizonxxxxxxxxx       x        
Natric & horizon                      xxx
Cambic B horizon           x x           
Spodic B horizon                         
Oxic B horizon                         
Calcic horizon               x         
Gypsic horizon                         
Sulferic horizon                         
Albic E horizon mat.)            x            
Calcareous material               x  x  x   
CaCO3 > 5% (topsoil)                         
CEC high/yen high                         
CEC low (<24 meg)              x          
CEC very low (<1.5 meg)                         
Base sat. >50%, (pH>5.5) x x x xxx       xx xx   
Base sat. <50%, (pH<5.5)  x x x   x        x     
Cracking clayx      xx                
Depth <25 cm                         
Depth 25–50 cm                         
Drainage very poor/foor                       x 
Drainage imp./moderate                         
Drainage excessive           xxxxxx        
Plinthite                         
Salinity high                         
Salinity moderate                         
Alkalinity high                      x x
Alkalinity tolerate                         
Indurated subsoil                         
Cemented hardpan                         
Iron concretions              x          
Torture coarse           xxxxxx        
Texture heavyx      xx                
Abrubt textural change                         
Tonguing                         
Reddish colour                         
Yellowish colour                         
Hat. fertility high x x x xxx       xx xx   
Hat. fertility moderatex x x x            x     
Hat. fertility lot          xxxxxxx        
 MvoNNdNeNhNmNthNveNvmOOdQQaQcQfQkQlRRcRdReRtcSSgSlo

 SmSoTThTmTvUVVcVpWWdWeWhWsWveXXhXkXyZZgZoZt
Histic H horizon                        
Hollic A horizon    x        (x          
Umbric A horizon   x  x                 
Ochric A horizon xx  x    xxx(xxxxxxxxxxx
Argillic B horizon          xxxxxx        
Natric B horizonxx                      
Cambic B horizon  xxxx          xx  xxxx
Spodic B horizon                        
Oxic B horizon                        
Calcic horizon                  x     
Gypsic horizon                        
Sulferic horizon                        
Albic E horizon (mat.)          xxxxxx        
Calcareous Material                        
CaCO3 > 5% (topsoil)                        
CEC high/very high  xxx  xxx              
CEC low (<24 meg)                        
CEC very low (<1.5 meg)                        
Base sat. >50% (pH>5.5)x   x     x x xx  xx    
Base sat. <50%. (pH<5.5)   x       x x          
Cracking clay       xxx     x       x
Depth <25 cm                        
Depth 25–50 cm      x                 
Drainage very poor/poor                     x  
Drainage imp./moderate          xxxxxx        
Drainage excessive                        
Plinthite                        
Salinity high                    xxxx
Salinity Moderate              x         
Alkalinity highxx                      
Alkalinity Moderate                        
Indurated subsoil                        
Cemented hardpan                        
Inn concretions                        
Texture coarse     x                  
Texture heavy       xxx             x
Abrubt textural change          xxxxxx        
Tonguing                        
Reddish colour                        
Yellowish colour                        
Hat. fertility high    x  xxx      xxx     
Hat. fertility Moderate  xx      x xx x        
Hat. fertility lot     xx    x  x         
 SmSoTThTmTvUVVcVpWWdWeWhWsWveXXhXkXyZZgZoZt

(indicates Mutually exclusive horizons


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