TAA Scottish and Border Region
6th September 2001
SOIL AND WATER CONSERVATION IN THE TROPICS
RETHINKING EFFECTS AND CAUSES OF LANE) DEGRADATION
T.F.Shaxson
Land degradation:
The loss of ongoing usefulness of land for the chosen
purpose. Regarding biomass production and streamflow, shows as the decline
and loss of productivity, commonly assumed to be caused by erosion.
Need to consider all aspects of 'soil fertility':
physical /hydric /chemical /biological and their interrelations, not just
nutrients alone.
Linkage between blomass production and streamflow:
Plant-available soil moisture essential for biomass
production. Groundwater and derived streamflow is fed by that rainwater
which was not taken up by root systems nor stored below root-depth. Regularity
of streamflow over the year depends on what proportions of rainfall are
partitioned at soil surface into infiltration vs. surface runoff. Infiltration:
slow passage to groundwaterlstreamflow through soil. Surface runoff: rapid
overland flow and concentration at strearnbank.
Linkacie between biomass production and erosion
effects:
Not as assumed: yield difference before-after erosion
due to difference in physical, hydric, chemical, biological characteristics
of what remains, not directly to quantities of what is lost, Excess runoff
and erosional transport are foreseeable ecological consequences of mismanagement
of land - notably by loss of porosity and water-holding capacity in upper
soil layer - not prime causes of the problems. Result of RAINFALL IMPACT,
tillage, trampling, interstitial sealing. Assumed culprits: Deforestation
/ Overgrazing / Over-cultivation. Actual culprits: Loss of cover / Loss
of organic materials and processes / Loss of pare-space.
Restoration and sustained bite'.
Only by biological activity on organic substrates,
in 3 dimensions of space, plus 1 of time.:
(a) Provide protective cover against rain impact
(b) Substrate for organisms - humic materials as
by-products
(c) Re-aggregation can re-form soil pores of various
sizes
(d) Some nutrient re-cycling.
Approach: minimise damage each year and encourage
ongoing, augmented and continuing biological activity year-to-year.
Roots = integral and contributing components of
soil ecosystem
Example: residue-based zero tillage in Brazil,
Paraguay
Good land husbandry - with significant micro-scale
effects - produces conservation-effective farming, with interlinking ongoing
benefits: technical / environmental / economic / social.
Positive 'spin' on raising and sustaining productivity,
(vs. old negative 'defensive' approach which did not, per se, raise
yields).
Land degradation avoided vs. 'controlled'.
Keys: Attention to least disturbance to good root-zone
conditions below-ground, via cover + legumes + rotations (= 'roots of sustainability')
--> have raised yields above-ground with lower costs, made possibilities
for diversification, made streamflow more regular through year, improved
livelihoods, and increased technical and economic resilience to shocks.
Large and small farms, emphasising crops andlor
livestock.
Exponential growth: e.g. Brazil from < 1 000
ha in 1972 to > 13million ha. today, and rising.
Successes with even partial application of the principles
in many other countries: Burkina Faso, Kenya, Philippines, etc.
Think like a root, think like a river .