An evaluation of the rationale for fertiliser management of tropical fruitcrops

Sustainable fertiliser management involves maximising production, preventin g on-site soil degradation and minimising off-site movement of nutrients. I n the past, fertiliser management has been driven by the need to maximise p roduction. Fertiliser costs are generally less than 10% of the variable production cos ts in horticultural enterprises and this has often led to excessive applica tion of nutrients. Fertiliser experiments have not generally provided calib rated soil or leaf test data because of their shortterm nature, the biennia l or variable production of many tree crops, their narrow focus and the dif ficulty in demonstrating yield responses because tree crops have relatively low rates of nutrient removal over long periods of time. Nutrient balance is a basically sound approach to developing fertiliser recommendations and can be easily estimated from crop nutrient removal data. This approach has been used successfully for crops such as low-chill stonefruit (Huett and St ewart 1999) and a simplified approach is presented for passionfruit, mango and avocado based on nutrient uptake by well-managed, mature, productive or chards. Macronutrient removal by a 20 t/ha passionfruit crop was (kg/ha) 55 nitroge n (N), 78 potassium (K), 6 phosphorus (P), 6 sulfur (S), 5 calcium (Ca) and 4 magnesium (Mg). For a 10 t/ha mango crop it was (kg/ha) 11 N, 15 K, 2 P, 1 S, 2 Ca and 2 Mg. For a 10 t/ha avocado crop it was (kg/ha) 41 N, 61 K, 8 P, 4 S, 7 Ca and 8 Mg. Passionfruit, in contrast to tree crops, is genera lly a 3-year crop and nutrient uptake by developing leaf, vines and roots w ill inflate nutrient uptake by a factor of 2-3. Nutrient losses due to leac hing, runoff and fixation will further inflate fertiliser requirements. How ever, the inclusion of these additional factors still means that nutrient a pplication rates will be substantially less than current fertiliser recomme ndations.