Improving soil specification for landscape tree planting in the tropics

Tree growth in urban areas is beset by multiple stresses, of which the soil component has imposed severe constraints. In the tropics, the inherently infertile and acidic soils present challenges that demand dedicated solutions. Yet, the key edaphic issues have remained poorly understood and often inadequately evaluated and improved. Where site soil is poor or absent, a soil specification usually prescribes a fabricated soil mix. Unfortunately, these documents frequently include non-essential attributes or exclude essential ones. The specified ranges and thresholds tend to detach from soil science concepts or are irrelevant to local conditions. Some projects would adopt specifications prepared for other local or international sites without modifications to match site requirements. Errors tend to be copied and propagated in different documents. Knowledge transfer from researchers to practitioners can improve the soil specifications to resolve a significant and chronic weakness in urban forestry. Five representative local documents and five from other countries were critically evaluated in detail to distill 21 principal concerns and 20 principles of soil specification design. The knowledge base informed the development of a rationalized and improved soil specification for urban forestry in the tropics. It included the fabricated topsoil mix and fabricated subsoil mix to meet most planting needs, and a dedicated fabricated lightweight mix for planting on rooftops with limited load-bearing capacity. Further explanations justified using local raw materials, soil sampling strategy, soil properties, recommended ranges, and standard laboratory testing methods. Other tropical regions can modify the proposed specification to fit local circumstances and specific landscape needs.