Soil properties of Imperata grasslands and prospects for tree-based farming systems in Northeast Luzon, the Philippines

Ambiguity in the potential of soils under Imperata cylindrica grass contributes to the slow pace of grassland development in Northeast Luzon. The aim of this study is to investigate the soil properties of Imperata grassland and the prospects for tree-based farming systems in northeastern Luzon. Soils are developed over Miocene-Pliocene sedimentary rock and Plio-Pleistocene volcanic and fluvial deposits. There is a clear distinction between relatively well-developed fertile soils (Cambisols, Luvisols) along hill slopes, clayey soils in lowlands (Vertisols) and inferior leached soils (Planosols) restricted to palaeo terraces and elevated surfaces. Low soil organic matter content is a regional constraint to grassland development. Other constraints mainly occur on a local level and are associated with soil physical, rather than soil chemical, properties such as low infiltration, surface crusting, gully formation, limited soil depth, and stony surfaces. It is argued that variable soil conditions and topography are inadequately considered in regional attempts to develop and rehabilitate grasslands. Fertile, low-angle foot slopes are most suitable to permanent cultivation with crop rotation and husbandry practices such as mulching of crop residues, efficient use of animal manure and inclusion of nitrogen-fixing plants. Enrichment of natural forest patches is proposed, together with mixed tree plantations on rocky and steep terrain, field boundaries and stream banks in cultivated areas. Less fertile palaeo terraces, watersheds and upper slopes are most suitable to silvipastoral systems.