2.2.5. Nutrient inputs and balance
We used a simple method of N budgeting to estimate the N surplus from the farm system. Possible inputs of N to the system were: fertiliser, chicken manure, atmospheric deposition and biological fixation by legumes. Outputs of N from the system comprised: commercialised crop products and animals. Soil organic matter could be either a source of N when SOM decreased or a sink when SOM increased. The amount of N mineralised or immobilised was calculated from the change in soil organic carbon, assuming 0.58 organic carbon per unit organic matter and a C/N ratio of 10. N surplus is the difference between N inputs and N output. We determined the input of N fertiliser to each crop following expert recommendations (derived from fertilisation experiments in the region) and taking into account the inter-crop management type. N was applied in such amounts that it never became a yield-limiting factor.
The soils in Canelón Grande have high natural levels of K (0.6–0.8 meq. of exchangeable K per 100 g soil), with no yield response to the application of this nutrient. However we included the application of K fertiliser to crops with a high extraction rate of this nutrient such as potato and sweet pepper following expert recommendations. In most horticultural fields in the region, there is a high accumulation of P as a result of high fertilisation rates for many years. We calculated the application rate of fertiliser P equal to the output of P in commercialised crop products and animals.
2.2.6. Environmental exposure to pesticides
The potential presence of pesticides in the environment as a result of each production activity was estimated following the approach of Wijnands (1997). Environmental exposure to pesticides (EEP) for soil, air and groundwater were calculated based on the amount of the active ingredients (AI), their vapour pressure at 20–25 °C (VP), 50% degradation time (DT50) and mobility (Kom) (PPO, 2001). The EEP per kg of commercial product consisting of various active ingredients was calculated as follows