Trade-off between short and long-term effects of mineral, organic or mixed mineral-organic fertilisation on grass yield of tropical permanent grassland

Since the discovery in the early 1950s of fixation of industrial atmospheric nitrogen through physico-chemical processes, mineral fertilisation has become the main means of fertilising cultivated land. However, several studies have shown that continued use of mineral fertilisers can accelerate soil acidification and reduce cation exchange capacity, water-holding capacity, and soil microbial activity, thereby reducing soil fertility. Other studies have underlined the importance of a nutrient balanced fertilisation protocol to sustain high grass yields. On the other hand, by recycling livestock manure, the use of organic fertiliser has been shown to lead to high soil productivity and to provide economic and environmental benefits to farmers. Our study focused on the short-and long-term effects of several types of fertilisers, (mineral, organic, and mixed organic-mineral), and several doses (ranging from 440 to 1,260kgN/ha/year) in a 15-year experiment on highly productive grassland. We showed that, in contrast to organic fertilisation, although the mineral treatments maintained soil nitrogen content, these treatments did not increase phosphorous and potassium concentrations, and in addition, led to soil acidification. Furthermore, although mineral fertilisation led to rapid increases in grass yield, its fertilisation effect weakened over time whereas some of the organic fertilisation treatments resulted in a gradual increase in grass yields. Significant differences in grassland productivity were observed between the effects of liquid manure and compost, liquid manure being more immediately effective while compost had a longer-term positive effect on grass yield. The mixed organic-mineral treatments were shown to be a good compromise, with a significant short-term increase in grass yields plus sustained grass production over time.