The rate of extraction exceeds the rate of recharge
The 42% reduction in biomass in the pastures of Southern Zambia, recorded between January and March 2026, is not an isolated figure. It is the result of a continuous extraction rate that exceeds the soil’s recharge capacity. Herds, forced to move to common areas due to the El Niño phenomenon, extract 18,000 tons of forage per month. This extraction rate is higher than the natural recharge rate, which stands at 12,000 tons per month. The difference of 6,000 tons per month represents a cumulative deficit that translates into a structural degradation of the pasture system.
The tension is not only environmental. It is economic. Each ton of forage extracted represents a marginal cost of €45/ton for the transport and management of the herds. The total monthly cost of extraction exceeds €810,000. This cost is not included in the production budgets of local farms, but is borne by small farmers, who do not have financial buffers. The biomass deficit is not a management problem, but a thermodynamic problem.
The recharge does not compensate for the extraction
The soil carrying capacity in Southern Zambia has been estimated at 23,000 tons of phosphate needed to restore soil fertility after a year of degradation. However, the natural recharge rate is only 12,000 tons per month. This means that, even under optimal conditions, the soil takes more than two years to restore its capacity to support biomass. The recovery time is greater than the lifespan of the herds, which is around 15 years.
The dynamics of extraction are not linear. Under conditions of water stress, the extraction rate increases by 30% compared to the average. This increase is not compensated by the recharge, which remains constant. The system is in a state of thermodynamic disequilibrium. Biomass is not produced sufficiently to support the extraction, and the soil is not recharged sufficiently to support production. The result is a self-perpetuating cycle of degradation.
The threshold of irreversible degradation
The critical threshold is reached when the available biomass falls below 30% of the reference level. At this point, the soil is no longer able to support forage production. The recharge rate drops to zero, and the extraction continues to increase. The system enters a state of collapse. The soil’s buffer capacity is exhausted. There are no management alternatives that can restore the balance.
The empirical evidence is provided by the SoilFER program, which monitored 18,000 hectares of pastures in Southern Zambia. The data indicate that, when biomass falls below 30%, the recharge rate decreases by 75%. The soil is no longer able to produce enough biomass to support the extraction. The system is in a state of irreversible collapse. The threshold is not a breaking point, but a point of no return.
Implications for the decision-maker
The marginal cost of degradation is €810,000 per month for forage extraction. This cost is not included in the production budgets of local farms, but is borne by small farmers. The total annual cost exceeds €9.7 million. This represents a direct impact on the working capital of the farms.
The solution is not to increase forage production, but to reduce extraction. The soil’s buffer capacity is exhausted. Investment in soil recharge is no longer effective. The strategy must be to contain the extraction. The cost of containment is €1,200/ton for each ton of forage not extracted. This cost is lower than the marginal cost of extraction. The strategic choice is clear: reduce extraction to preserve the soil’s carrying capacity.
Photo by Ed Wingate on Unsplash
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