land clearing, (2) the allocation of labor and land to available options of investment,
and (3) the diminishing and increasing marginal returns on soil fertility and land
productivity. The annual simulation loop of FALLOW is built on the ‘Trenbath’
soil fertility approach (Trenbath 1989), in which soil fertility at the plot-level
declines proportionally during cropping periods based on a crop specific depletion
rate, and increases during fallow periods with a characteristic half-recovery time.
Fertilizer application affects soil fertility and yield by reducing the specific depletion rate, with crop yields derived from crop specific conversion factors and current
soil fertility at the plot-level.
The model scenario analysis was built on a set of participatory discussions
conducted with different groups of stakeholders in Ban Put, representing upland
farmers and local government organizations. Results of the participatory discussions
were used to calibrate a baseline scenario within the FALLOW model, which was
successfully validated with the goodness-of-fit algorithm of Costanza (1989) (see
also: Lippe et al. 2011 for further details). Based on the validated baseline set-up, a
series of scenario simulations were conducted for the time period 1975–2018, as the
principal investigation period. The year 2019 coincides with the time at which the
provincial government is expected to reallocate land use rights (the so-called ‘red
book’ certificates) among villagers guaranteed after their introduction. The following
scenarios were chosen: Scenario 1 (IncFert) focused on the stakeholders assumption
that an increase in fertilizer application rates or the application of organic manure
after the year 2000 improved soil fertility levels; Scenario 2 (EarlyFert) assessed the
assumption that a 5 year earlier start of fertilizer application, when compared to the
actual implementation in 2000, would result in a faster recovery of soil fertility, while
Scenario 3 (ReIFallow) tested the assumption that the reintroduction of a 3 year
swiddening system after 2008, together with fertilizer application from 2000
onwards, would greatly enhance soil fertility levels. For each of the chosen scenarios,
two levels of fertilizer application rates were used (K fert : dimensionless, with 0.2 low,
and 0.4 high fertilizer efficiency), aiming to assess soil fertility evolutions and
following stakeholder suggestions on how to improve upland soil fertility.
7.4.2 Scenario Results and Discussion
In the simulation outputs (Fig. 7.7) only a high fertilizer application rates increase
(IncFert), e.g., cover crops and fertilizer use, resulted in a moderate build-up of soil
fertility at the landscape level. Earlier use of fertilizer delayed the decline of soil
fertility and only in association with improved crop management (EarlyFert)
stabilized soil fertility at acceptable levels suitable for maize production. The
simulated scenarios of reintroduction of a 3 year swiddening (ReIFallow) showed
a positive effect on soil fertility development for all tested fertilizer use efficiencies.
When taking a snapshot into 2018, soil fertility further declined with most plots
pertaining to moderate to low fertility conditions (e.g., < red soil conditions).
7 Soil Conservation on Sloping Land: Technical Options and Adoption Constraints
249
and (3) the diminishing and increasing marginal returns on soil fertility and land
productivity. The annual simulation loop of FALLOW is built on the ‘Trenbath’
soil fertility approach (Trenbath 1989), in which soil fertility at the plot-level
declines proportionally during cropping periods based on a crop specific depletion
rate, and increases during fallow periods with a characteristic half-recovery time.
Fertilizer application affects soil fertility and yield by reducing the specific depletion rate, with crop yields derived from crop specific conversion factors and current
soil fertility at the plot-level.
The model scenario analysis was built on a set of participatory discussions
conducted with different groups of stakeholders in Ban Put, representing upland
farmers and local government organizations. Results of the participatory discussions
were used to calibrate a baseline scenario within the FALLOW model, which was
successfully validated with the goodness-of-fit algorithm of Costanza (1989) (see
also: Lippe et al. 2011 for further details). Based on the validated baseline set-up, a
series of scenario simulations were conducted for the time period 1975–2018, as the
principal investigation period. The year 2019 coincides with the time at which the
provincial government is expected to reallocate land use rights (the so-called ‘red
book’ certificates) among villagers guaranteed after their introduction. The following
scenarios were chosen: Scenario 1 (IncFert) focused on the stakeholders assumption
that an increase in fertilizer application rates or the application of organic manure
after the year 2000 improved soil fertility levels; Scenario 2 (EarlyFert) assessed the
assumption that a 5 year earlier start of fertilizer application, when compared to the
actual implementation in 2000, would result in a faster recovery of soil fertility, while
Scenario 3 (ReIFallow) tested the assumption that the reintroduction of a 3 year
swiddening system after 2008, together with fertilizer application from 2000
onwards, would greatly enhance soil fertility levels. For each of the chosen scenarios,
two levels of fertilizer application rates were used (K fert : dimensionless, with 0.2 low,
and 0.4 high fertilizer efficiency), aiming to assess soil fertility evolutions and
following stakeholder suggestions on how to improve upland soil fertility.
7.4.2 Scenario Results and Discussion
In the simulation outputs (Fig. 7.7) only a high fertilizer application rates increase
(IncFert), e.g., cover crops and fertilizer use, resulted in a moderate build-up of soil
fertility at the landscape level. Earlier use of fertilizer delayed the decline of soil
fertility and only in association with improved crop management (EarlyFert)
stabilized soil fertility at acceptable levels suitable for maize production. The
simulated scenarios of reintroduction of a 3 year swiddening (ReIFallow) showed
a positive effect on soil fertility development for all tested fertilizer use efficiencies.
When taking a snapshot into 2018, soil fertility further declined with most plots
pertaining to moderate to low fertility conditions (e.g., < red soil conditions).
7 Soil Conservation on Sloping Land: Technical Options and Adoption Constraints
249