Nutrient flows in smallholder production systems in the humid forest zone of southern Cameroon

The flows and balances of N, P and K were studied in 20 farms in the Campo Ma'an area in Cameroon between March and August 2002 to assess the nutrient dynamics in smallholder farms. Data were collected through farmer interviews, field measurements and estimates from transfer functions. Nutrient input from mineral (IN1), animal feed (IN2a) and inorganic amendments (IN2b) was absent. Major outputs were through crop (OUT1a) and animal (OUT1b) products sold. Partial budgets for farmer managed flows were negative: -65 kg N, -5.5 kg P and -30.8 kg K ha^-1 year^-1. For inflows not managed by farmers, deep capture (IN6) was the major source: 16.6, 1.4 and 6.6 kg ha ^-1 year^-1 of N, P and K, respectively. Atmospheric deposition (IN3) was estimated at 4.3 kg N, 1.0 kg P and 3.9 kg K ha ^-1 year^-1, and biological nitrogen fixation (IN4) at 6.9 kg N ha^-1 year^-1. Major losses were leaching (OUT 3a): 26.4 kg N, and 0.88 kg K ha^-1 year^-1. Gaseous losses from the soil (OUT 4a) were estimated at 6.34 kg N, and human faeces (OUT 6) were estimated at 4 kg N, 0.64 kg P and 4.8 kg K ha^-1 year^-1. The highest losses were from burning (OUT 4c), i.e. 47.8 kg N, 1.8 kg P and 14.3 kg K ha^-1 year^-1. Partial budgets of environmentally controlled flows were negative only for N -4.8 kg N, +2.4 kg P and +9.6 kg K ha^-1 year^-1. The overall farm budgets were negative, with annual losses of 69 kg N, 3 kg P and 21 kg K ha^-1. Only cocoa had a positive nutrient balance: +9.3 kg N, +1.4 kg P and +7.6 kg K ha^-1 year^-1. Nutrients reaching the household waste (1.9 kg N, 2.8 kg P and 18.8 kg K ha^-1 year^-1), animal manure (4.9 kg N, 0.4 kg P and 1.6 kg K), and human faeces (4 kg N, 0.64 kg P and 4.8 kg K ha ^-1 year^-1) were not recycled. Five alternative management scenarios were envisaged to improve the nutrient balances. Recycling animal manure, household waste and human faeces will bring the balance at -62.6 kg N, 0 kg P and +1 kg K ha^-1 year^-1. If, additionally, burning could be avoided, positive nutrient balances could be expected.