Agroforestry Chapter 5 (Part 2): Tropical Soils, Nutrient Cycling, Soil Organic Matter and Soil Conservation
Agroforestry Notes · Chapter 5 · Part 2
Chapter 5 (Part 2): Tropical Soils, Nutrient Cycling and Soil Conservation
Only in Nair (1993): soil orders and their FAO names, how trees change soils, nutrient cycling, soil organic matter formulas, litter quality, biomass and roots, and soil conservation with the USLE.
1. Tropical soils and their limits
Cation exchange capacity (Nair)The sum total of exchangeable cations that a soil can absorb. Sometimes called "total-exchange capacity," "base-exchange capacity," or "cation-adsorption capacity." Expressed in centimoles per kilogram (cmol kg−1) of soil (or of other adsorbing material such as clay).Source: Nair (1993), Glossary
Base saturation percentage (Nair)The extent to which the adsorption complex of a soil is saturated with exchangeable cations other than hydrogen and aluminum. It is expressed as a percentage of the total cation exchange capacity.Source: Nair (1993), Glossary
Bulk density, soil (Nair)The mass of dry soil per unit of bulk volume, including the air space. The bulk volume is determined before drying to constant weight at 105°C.Source: Nair (1993), Glossary
Two systems name soils: the quantitative US Soil Taxonomy (Soil Survey Staff, 1975) and the FAO-UNESCO Soil Map of the World (1974). The French ORSTOM system is genetic. ICRAF held its first scientific meeting on soil research in 1979 (Mongi and Huxley).
| US Soil Taxonomy | FAO-UNESCO equivalent | Note |
|---|---|---|
| Oxisols | Ferralsols, Plinthisols | Highly weathered; low K, Ca, Mg; high P fixation |
| Ultisols | Acrisols, Alisols, Dystric Nitosols | Acid; aluminium toxicity |
| Alfisols | Luvisols, Lixisols, Eutric Nitosols, Planosols | Main soils of West African savanna; fairly fertile |
| Andisols | Andosols | Volcanic; allophane fixes phosphorus |
| Vertisols | Vertisols | Swelling clays (black cotton soils) |
| Entisols: Fluvents, Psamments, Lithic | Fluvisols, Arenosols/Regosols, Leptosols | Young soils |
| Inceptisols: Aquepts, Tropepts | Gleysols, Cambisols | Weakly developed |
| Aridisols | Solonchaks, Solonetz | Dry; often saline or sodic |
| Mollisols | Chernozems | Dark, fertile |
| Spodosols | Podzols | Sandy, acid |
| Histosols | Histosols | Organic (over 20% OM), peat |
- All 11 soil orders occur in the tropics. Oxisols (22.7%) and Ultisols (20.4%) together cover over 40%; Entisols 15.7%, Alfisols 15.2%, Inceptisols 14.5%.
- Fairly fertile soils (Alfisols, Vertisols, Mollisols, Andisols) make up only about 23%.
- Tropical America: Oxisols 33.6%. Tropical Asia: Ultisols 28.4% but Oxisols only 1.4%.
- Many tropical soils are low-activity clay (LAC) soils: kaolinite (1:1 clay) and oxides with low CEC.
- Lal and Sanchez (1992) rejected old myths: tropical soils are as diverse as temperate ones, and their organic matter and chemistry follow the same principles.
| Main soil constraint (tropics) | Area (Mha) | Share |
|---|---|---|
| Low nutrient reserves | 1,493 | 32% |
| Aluminium toxicity | 1,160 | 25% |
| Acidity without Al toxicity | 1,018 | 22% |
| High P fixation by iron oxides | 251 | – |
Memory trick: O-F, U-A: Oxisol = Ferralsol, Ultisol = Acrisol. Andisol = Andosol. Spodosol = Podzol.
2. How trees change the soil
Young (1989) grouped the effects of trees on soil into four kinds of process (Nair, Table 15.1).
| Process | What trees do |
|---|---|
| Input (add) | Organic matter from litter, prunings and roots; nitrogen fixation; nutrients caught from rain and dust |
| Output (reduce losses) | Less erosion; less leaching, because deep roots catch nutrients |
| Turnover | Take up nutrients from deep layers and release them in litter |
| Catalytic (indirect) | Better physical properties; lower acidity, salinity or alkalinity; milder microclimate; more soil life |
- Proven: organic matter maintenance (Nye and Greenland, 1960) and N-fixation. Probable: uptake from deep (B and C) horizons. Suggested but not shown: growth-promoting root exudates.
- Forest cover checks erosion mainly through its litter layer, not its canopy. A bare tropical soil surface can heat to over 50 °C.
- Nair says the common term “nutrient pumping” is erroneous; the magnitude of deep uptake is not known.
- Distinguish: N-fixation is a true input; N in litter is mostly an internal transfer of N already taken from the soil.
- Adverse effects: competition for moisture (the most serious problem in semi-arid areas) and nutrients, nutrients removed in harvests (worst with whole-tree harvesting), and toxic litter.
- Chitemene (northern Zambia): branches lopped over a large area are burnt on a small plot, concentrating nutrients there.
| Soil under Prosopis (arid India) | Organic C (%) | Total N (%) |
|---|---|---|
| P. juliflora: under tree | 0.73 | 0.075 |
| P. juliflora: open area | 0.25 | 0.027 |
| P. cineraria (0–30 cm): under tree | 0.37 | 0.045 |
| P. cineraria (0–30 cm): open area | 0.25 | 0.038 |
This micro-site enrichment under single trees is also known under Faidherbia albida in West Africa and Byrsonima in the savanna of Belize (Kellman, 1979).
3. Nutrient cycling
Denitrification (Nair)The biochemical reduction of nitrate or nitrite to gaseous nitrogen, either as molecular nitrogen or as an oxide of nitrogen.Source: Nair (1993), Glossary
Natural forests are closed nutrient cycles; farming systems are open or “leaky”. Agroforestry systems lie in between.
Inputs
- Fertiliser and manure
- Rain and dust
- Biological N-fixation
- Weathering of rock
Outputs (losses)
- Harvest removal
- Erosion and runoff
- Leaching (nitrate is leached most, phosphate least)
- Denitrification and volatilization (N)
- Burning (N and S)
| System | Litter or prunings (t DM/ha/yr) | N (kg/ha/yr) |
|---|---|---|
| Tropical rain forest | 10.5 | 162 |
| Leucaena alley cropping (2 m rows) | about 22 | 200–280 |
| Coffee with pruned Erythrina | 20 | 461 |
- Tree roots are usually 20–30% of total tree biomass (Armson, 1977): as low as 15% in rain forests, 50% or more in dry areas. Most fine roots are in the top 20 cm.
- Dead roots add about 2,600 kg/ha/yr under moist tropical forest (Nye, 1961). Roots hold about 10% of plant N in a rain forest on Oxisol but 40% on a poor Spodosol (Jordan, 1983).
- Alley-cropping hedges supply 100–200 kg N/ha/yr, about what a cereal harvest removes; shade trees in coffee and cacao give 100–300 kg N.
- Legume cover crops (Centrosema, Pueraria) under oil palm add about 150 kg N/ha (Agamuthu and Broughton, 1985).
- Leucaena fallow gave higher CEC, Ca and K than bush fallow (Juo and Lal, 1977), but on Mindoro (Philippines) natural bush fallow gave higher pH than Leucaena (MacDicken, 1991).
- Selective accumulators: palms → K, tree ferns → N, Gmelina → Ca, Cecropia → Ca and P.
Memory trick: Forest closed, farm leaky, agroforestry in between.
4. Soil organic matter and litter decomposition
Soil organic matter = fully decomposed humus + partly decomposed litter. Coarse litter retained on a 2 mm sieve is not counted. The light fraction (density below 2.0 g/cm³) may hold up to 25% of the soil’s nutrient reserves (Ford and Greenland, 1968). Young (1989): soil organic matter is “the prime mover” of soil improvement.
Walkley–Black method: % organic matter = % organic C × 1.724
- Litter to humus (Nye and Greenland, 1960): only 10–20% of the carbon in above-ground litter and 20–50% of that in roots becomes humus; the rest (80–90% and 50–80%) is lost as CO₂.
- Decomposition constant k: 0.03 (3%) under forest fallow and 0.04 (4%) under cultivation (Nye and Greenland); usually 5–10% on cultivated tropical land (Sanchez, 1976).
- ¹⁴C studies: added residues decayed fastest in Nigeria; in England the time scale was about four times longer; in South Australia the rate was half that of Nigeria.
C₁ = C₀(1 − k); Ct = C₀ e−rt; half-life = ln 2 / r = 0.693 / r
Example: with k = 0.04, half-life = 0.693/0.04 ≈ 17 years.
Litter quality
Litter rich in N (low C:N) decays fast and releases N. Litter with high C:N (straw) makes microbes lock up soil N for a while (immobilization), which can starve the crop. Palm and Sanchez (1990) suggested the polyphenol:N ratio as an index of mulch quality: high-polyphenol leaves (Inga edulis, Cajanus cajan) release N slowly.
| Leaves or prunings | N (%) |
|---|---|
| Leucaena leucocephala | 4.33 |
| Gliricidia sepium | 4.21 |
| Cajanus cajan | 3.60 |
| Erythrina poeppigiana | 3.30 |
| Alchornea cordifolia | 3.29 |
| Inga edulis | 3.1 |
| Dactyladenia (Acioa) barteri | 2.57 |
| Cassia siamea (prunings) | 2.52 |
- Budelman (1988), Côte d’Ivoire: K was released fastest from Leucaena. Persistence relative to Leucaena (1.0): Gliricidia 0.7, Flemingia 1.7.
- Erythrina leaves release about 40% of P and Ca and 75% of Mg and K within four weeks.
- Jama-Adan (1993), semi-arid Kenya: decay has two phases, a fast first phase (about 6 weeks for Leucaena, 10 weeks for Cassia siamea) and a slow second phase.
- Slow mulches (Cassia siamea, Flemingia, Dactyladenia) are better for weed control and moisture; fast mulches for a quick N supply.
- Decomposition is studied with the litter-bag technique.
Synchrony: three ways to match nutrient release to crop demand
- Choose species with different decay rates
- Adjust the timing of pruning
- Change the method of application (surface mulch or soil incorporation)
5. Biomass, roots and how much residue is needed
| Natural ecosystem (Young, 1989) | Above-ground NPP (kg DM/ha/yr) |
|---|---|
| Evergreen rain forest | 20,000 (10,000–40,000) |
| Semi-deciduous forest | about 20,000 |
| Moist savanna | 10,000 |
| Dry savanna | 5,000 |
| Desert scrub | 2,500 or less |
IITA example: hedgerows 4 m apart occupy about 25% of the land. Two maize crops (about 10,000 kg) plus a natural rate on 25% of the land (5,000 kg) gives about 15,000 kg/ha/yr. Hedges give 2,000–4,000 kg DM of leaves a year (8–16 t fresh).
| Climatic zone | Topsoil C | Above-ground residues needed to keep soil C (kg DM/ha/yr) |
|---|---|---|
| Humid | 2.0% | about 8,000 |
| Sub-humid | 1.0% | about 4,000 |
| Semi-arid | 0.5% | about 2,000 |
Young’s assumptions: topsoil weight 1.5 million kg/ha, k = 0.04, erosion 10 t/ha/yr, roots 40% of above-ground production, 85% loss from above-ground and 67% from root residues, dry matter 50% carbon. These amounts can be met only if most tree and crop residues go back to the soil.
- Fine roots are those under 2 mm in diameter. Root amount is expressed as root length density (cm of root per cm² of surface or per cm³ of soil).
- Fine roots are constantly shed and renewed, so a one-time sample underestimates root production. Root turnover keeps adding organic matter even when all above-ground biomass is harvested.
- Costa Rica (Ewel et al., 1982): agroforestry systems (cacao + Cordia, coffee + Erythrina) had more root biomass than maize, sweet potato or a Gmelina plantation.
- Mycorrhizae act as an expanded root system; like Rhizobium, they may need inoculation where trees are new to a site.
6. Soil conservation and the USLE (Nair, Chapter 18)
Concern about erosion arose in the 1930s in the USA. FAO’s World Soil Charter came in 1982. Soil conservation now means erosion control plus maintenance of soil fertility (Lundgren and Nair, 1985; Young, 1989). The old barrier approach (bunds, terraces) proved costly, so the stress shifted to the cover approach, which has greater potential, and this brought agroforestry into focus.
Accelerated erosion (Nair)Erosion much more rapid than normal, natural, geological erosion; primarily as a result of the activities of humans or, in some cases, of animals.Source: Nair (1993), Glossary
Sheet erosion (Nair)The removal of a fairly uniform layer of soil from the land surface by runoff water.Source: Nair (1993), Glossary
Rill erosion (Nair)An erosion process in which numerous small channels of only several centimeters in depth are formed; occurs mainly on recently cultivated soils.Source: Nair (1993), Glossary
Gully erosion (Nair)The removal of soil by water concentrated in deep, narrow channels.Source: Nair (1993), Glossary
Splash erosion (Nair)The spattering of small soil particles caused by the impact of raindrops on very wet soils. The loosened and separate particles may or may not be subsequently removed by surface runoff.Source: Nair (1993), Glossary
Cover crop (Nair)A close-growing crop grown primarily for the purpose of protecting and improving soil between periods of regular crop production or between trees and vines in orchards and plantations.Source: Nair (1993), Glossary
USLE (Wischmeier and Smith, 1978): A = R × K × L × S × C × P (A = soil loss, t/ha/yr)
| Factor | Meaning and values (Nair) |
|---|---|
| R, rainfall erosivity | In the tropics about half the annual rainfall in mm: 2,000 mm → R ≈ 1,000; 600 mm → about 300. Usually measured as the EI30 index (storm energy × maximum 30-minute intensity, for storms over 12.5 mm) |
| K, soil erodibility | 0 to 1: about 0.1 for resistant Oxisols with stable aggregates, 0.5 for highly erodible soils. If organic matter falls by 1%, K rises by about 0.04 |
| L and S, slope length and steepness | Ratios to the standard plot; usually combined as the topographic factor LS |
| C, cover | Bare fallow = 1; full year-round cover ≈ 0. Tree crops with a cover crop cut erosion to 0.1–0.01 of bare soil |
| P, support practice | 0 to 1; about 0.3–0.4 is common in ordinary fields |
Exam point: The standard USLE plot is 22 m long with a uniform slope of 9% (5.14°). R and K are largely fixed by the site; the factor agroforestry changes most is C (cover).
- A tall canopy does not always lower erosivity: drops merging on broad leaves of a ~30 m canopy caused severe splash erosion under teak in Indonesia.
- After six years of maize–Leucaena alley cropping at Ibadan, topsoil organic C was 1.1% when prunings were returned and 0.65% when they were removed (Kang et al., 1985).
- US Soil Conservation Service tolerable erosion: 2.2–11.2 t/ha/yr. Young (1989) argues tolerance should be set by whether organic matter and nutrients can be maintained.
- Erosion classes: low under 2, moderate 2–10, high over 10 t/ha/yr.
- Hedgerows are not practical as the main measure on slopes over about 30%; Haitian farmers plant Leucaena hedgerows mainly for erosion control (Bannister and Nair, 1990).
- Land capability classification (Klingebiel and Montgomery, 1961) kept steep land out of cropping, but population pressure made this impossible in many places.
| Land use (Wiersum, 1984) | Erosion, median (t/ha/yr) | Range |
|---|---|---|
| Multistorey tree gardens | 0.06 | 0.01–0.14 |
| Shifting cultivation, fallow period | 0.15 | 0.05–7.40 |
| Natural rain forest | 0.30 | 0.03–6.16 |
| Forest plantation, undisturbed | 0.58 | 0.02–6.20 |
| Tree crops with cover crop or mulch | 0.75 | 0.10–5.60 |
| Shifting cultivation, cropping period | 2.78 | 0.40–70.05 |
| Taungya, cultivation period | 5.23 | 0.63–17.37 |
| Tree crops, clean weeded | 47.60 | 1.20–182.90 |
| Forest plantation, litter removed or burned | 53.40 | 5.92–104.80 |
Exam point: Trees by themselves do not stop erosion; a ground cover of litter does. Only multistorey tree gardens are, by nature, always likely to control erosion. Clean-weeded tree crops and plantations with litter removed lose the most soil.
| Practice (Young, 1989) | Where it applies |
|---|---|
| Plantation crop combinations | Humid to moist sub-humid |
| Multistorey tree gardens and homegardens | Mainly humid; possibly drier areas |
| Hedgerow intercropping and barrier hedges | Humid, sub-humid, possibly semi-arid; gentle to moderate slopes |
| Trees on erosion-control structures | Any |
| Windbreaks and shelterbelts | Semi-arid (wind erosion) |
| Silvopastoral practices | Semi-arid and sub-humid; some humid (South America) |
| Reclamation forestry; integrated watershed management | Any |
7. Quick revision box
- Oxisol = Ferralsol; Ultisol = Acrisol; Alfisol = Luvisol; Andisol = Andosol (P fixation); Spodosol = Podzol.
- Oxisols 22.7% + Ultisols 20.4% > 40% of the tropics; fertile soils only ~23%. Biggest constraint: low nutrient reserves.
- Young’s four processes: input, output, turnover, catalytic. “Nutrient pumping” is an erroneous term (Nair).
- Forest closed, farm leaky, AF in between. Roots 20–30% of biomass (Armson); fine roots < 2 mm, top 20 cm.
- %OM = %OC × 1.724 (Walkley–Black). Litter → humus: 10–20% (above ground), 20–50% (roots).
- k = 0.03 forest fallow, 0.04 cultivation; half-life = 0.693/r (k 0.04 → ~17 years).
- Leaf N: Leucaena 4.33, Gliricidia 4.21; polyphenol:N index (Palm and Sanchez). Two-phase decay (Jama-Adan).
- To keep soil C: humid 8,000, sub-humid 4,000, semi-arid 2,000 kg DM/ha/yr of residues (Young).
- USLE A = R K L S C P; R ≈ half annual rainfall; standard plot 22 m, 9%; C bare = 1.
- Wiersum: tree gardens 0.06; clean-weeded tree crops 47.6; plantation with litter removed 53.4 t/ha/yr. Tolerable 2.2–11.2 t.
8. Interactive MCQ Quiz
15 questions. Tap an option to see the answer and explanation, then go to the next question.
Comments
Post a Comment