Changes in Soil Properties: Conclusions

This study indicates that soil pH, organic C and total N tended to decrease with time in the oil palm agro-ecosystem. However, the decline in soil pH was slight whereas those related to organic C and total N corresponded to the period where the oil palm biomass was burnt or partially burnt at replanting. The large increases in soil organic C with the current norm of zero burnt replanting technique were favourable in regards to sustainability and land degradation.

There were large positive changes in soil P and exchangeable K, which might be attributed to the applications of higher rates of phosphate rocks and K fertilizer especially from the 1990s following the results of fertilizer response trials. However, excessive build-up of soil nutrients on the highly weathered tropical soils of Ultisols should be avoided due to their generally low nutrient holding capacity, which may increase the risk of pollution. On the other hand, nutrient depletion should also be prevented as they commonly lead to lower production in the long-term. Thus, close monitoring of the changes in soil nutrients in the oil palm estates is essential.

This study shows explicitly that the soil fertility status of the Ultisols under oil palm in Johor was enhanced through sound fertilizer management practices and zero burnt replanting technique.