Unexpected N and K nutrition diagnosis in oil palm smallholdings using references of high-yielding industrial plantations

The rising demand for vegetable oil is inducing an expansion of oil palm cultivation in the tropics. In southern Cameroon oil palm smallholdings have been growing fast since the mid-1990s. Now, industrial plantations and smallholdings exist side by side. The current technical advice given to smallholders originates from agroindustrial practices. However, industrial plantations were created by planting on previous forest cover with no food intercrops, whereas for smallholdings food crops are a common previous cover and an intercrop during the juvenile phase. Technical advice used for industrial plantations may therefore not apply to smallholdings. Huge yield differences are observed in oil palm smallholdings, ranging from 2 to 14 t·ha-1 of fresh fruit bunches, while in industrial plantations yields average 14-16 t·ha-1. As no agronomic evaluation to date had explained those variations, we carried out a regional agronomic diagnosis of N and K nutrition on smallholder plots planted with selected oil palms. To prepare leaf samples and determine mineral contents, we used the same standardised method and the same laboratory as the regional industrial plantations. We compared smallholder leaf N and K contents with reference models of critical mineral contents, previously built with data from the high-yielding industrial plantations. Statistical links were also established between nutritional status and practices. Our results showed two groups of oil palm plantations: a group with N deficiencies ranging between 80 and 90% of the reference and K deficiencies ranging from 45 to 90% of the reference, and another group with satisfactory N and K status. The N deficiency was statistically linked to food cropping as the previous cover or as an intercrop, whilst K deficiency was qualitatively linked to an absence of K fertilisation. N deficiency is a specificity of oil palm smallholdings that had never been encountered in African industrial plantations. To conclude, the current technical advice given to smallholders is not well adapted.