Potential of variable rate fertiliser application for cashew production in Phu Cat district, Binh Dinh province, Vietnam—a case study

ABSTRACT

Sandy soils in the south-central coastal areas of Vietnam have a number of limitations including low nutrient status and retention. Inorganic fertilisers are required for these soils to produce commercial yields. However, these fertilisers are expensive as they account for more than 35% of total production costs. A preliminary study was initiated to investigate ways of increasing fertiliser use efficiency (FUE). The ‘desktop’ investiga­tion included: (1) identifying potential mismatches between nutrient demand and supply using a nutrient replacement approach; and (2) identifying the potential for variable rate (VR) fertiliser applications. The nutrient budget approach indicated that phosphorus and sulfur were applied at rates four to nine times higher than that removed. Conversely, nitrogen and potassium were applied at levels half to three-quarters of that removed during harvest in the cashew nut and apple. Crop yields were interpolated into a VR prescription map with of areas producing high, medium and low yields. While the VR approach had the potential to increase FUE, the lack of yield data over time, lack of tissue testing to diagnose specific nutrient deficiencies and other competing soil limitations limited the validity of VR fertiliser applications in this study