Maximizing nutrient utilisation and soil fertility in smallholder coffee and food garden systems in Papua New Guinea by managing nutrient stocks and movement

Abstract

Smallholder farming systems in Papua New Guinea (PNG) are intensifying and becoming increasingly reliant on cash cropping. In the Highlands of PNG, coffee is the main cash crop and sweet potato the predominant subsistence crop. Due to the seasonality of coffee and low prices, farmers are increasingly growing vegetables and fruits for sale. This diversification in the cropping system has implications for nutrient dynamics and soil fertility. The aim of this study was to identify and quantify the movement of nutrient in food garden systems and interpret the effects of these nutrient movements on soil fertility. The study was conducted on six farms in Bena in the Eastern Highlands of PNG. At each farm, soil samples were collected from three food gardens. The aim of the soil sampling was to collect samples from areas with and without application of coffee pulp, fire ash, mulch or fertilizer. Harvested crop samples were also collected, washed and separated into consumable and non-consumable parts (e.g. skin), weighed, oven dried, ground and analysed for nutrient content. The two main pathways of nutrient flow quantified in this study were the output in harvested crop and input in inorganic fertilizers. Soil fertility was generally adequate, except for extractable P and exchangeable K. Many individual gardens also had low soil N concentrations. The application of nutrient sources such as coffee pulp, kitchen peelings and ash was limited, but the areas that had physical evidence of such applications generally had higher soil K concentrations. Crops grown for market had the highest nutrient concentrations because of the addition of fertilizers. Crops like broccoli and sweet potato had high nutrient concentrations but the amount exported per square meter was lower than cauliflower and cassava due to lower planting density and plant biomass. Market demand also affects the net export of nutrients, as greater market demand for certain vegetables like broccoli and sweet potato will result in greater nutrient export. The amounts of N and K exported in harvested crops exceeded the amounts imported in inorganic fertilizers, resulting in a negative balance of those nutrients. The P balance was positive, which may result in its accumulation. However, the extractable P concentration in soil was low so the accumulated P may still not be fully available to crops. The low input farming system currently practiced by smallholder farmers will continue to deplete the soil nutrients and the soil may become deficient in N, P and K. The process of crop harvesting and preparation results in the production of residues or wastes that might be better managed to retain nutrients. However, this option may be perceived as inconvenient and not practiced because the value of the nutrients in the waste is not appreciated. Therefore, adoption of these nutrient retention methods will require education about the value of nutrients in waste products versus the value of convenience.