OPTIMIZING LAND-USE DISTRIBUTION TO MINIMIZE NUTRIENT DEPLETION - A CASE-STUDY FOR THE ATLANTIC ZONE OF COSTA-RICA

Nutrient depletion may be reduced by effective land use planning using a nutrient balance model and linear programming techniques to optimiz e the distribution of land use over different land units. This techniq ue plans the geographical distribution of land utilization types and c reates a more sustainable basis for agriculture in an area. In contras t with traditional land use planning where land utilization types are matched with land units on the basis of maximizing present agricultura l production, this approach focuses on long-term effects. A case study was carried out in an area of 3340 ha in the southern part of a settl ement scheme in the Atlantic Zone of Costa Rica. Land units vary betwe en young fertile volcanic soils and relatively old, leached unfertile soils. Land utilization types vary between extensively grazed pasture and intensively cultivated annual and perennial crops. Information on land units with its land qualities and on land utilization types was s tored in a geographical information system which was integrated with a nutrient balance model (NUTBAL). This model, initially developed for Kenya and calibrated for local conditions in Costa Rica, was based on separate assessments of 5 nutrient input factors: mineral fertilizers, organic manure, wet and dry deposition, nitrogen fixation and sedimen tation, and 5 nutrient output factors: harvested product, crop residue removal, leaching, denitrification and erosion. For every combination of land unit and land utilization type the nutrient balance was model led. With a linear programming model, the land utilization types were distributed over the different land units, such that nutrient depletio n was minimized in the area within a set of boundary conditions. Two d ifferent scenarios were elaborated. One in which all land utilization types were redistributed and one in which the location of forest areas was fixed at their present location. The optimized land use distribut ion based on cuffent land use types was compared with the actual distr ibution which had an average nutrient depletion of 22, 5 and 13 kg ha- 1 yr-1 for respectively N, P and K. There was little difference in nut rient depletion between the two scenarios. The first scenario resulted in a nutrient depletion of 18, 3 and 9 kg ha-1 yr-1 and the second on e in 19, 3 and 10 kg ha-1 yr-1 for N, P and K respectively. However, t here was a large difference with the actual situation on one hand and the two scenarios on the other. In both cases there was a slight incre ase in the estimated agricultural production. The introduction of a nu trient depletion model in land use planning seems to constitute a usef ul complement to the existing procedures.