Edaphological characteristics of selected Philippine acid upland soils as affected by soil amendments and fertilizers

The Philippine uplands offer potential areas for crop production however the soils in these areas are inherently infertile and acidic which may limit crop growth and yield. This study was conducted to characterize the physiochemical and mineralogical properties of selected acid upland soils in the Philippines, identify soil-related production constraints, and assess the effect of various soil amendments and fertilizers in improving their edaphological characteristics. Ten soil series selected for this study were Tigaon, Annam, Antipolo, Bolinao, Cervantes, Mirador, Luisiana, Alaminos, Tacdian and Bantay. These soils are generally clayey, very strongly acidic (pH 4.47-5.40), low in available phosphorus (1.74-5.78 ppm), total nitrogen (0.06-0.14%) and organic matter content (1.07-3.60%). Percentage base saturation (8.80-37.37%) is also generally low however exchangeable acidity (0.13-15.47 cmolc kg-1) and available iron (11.14-125.13 ppm) are high. Cation exchange capacity (CEC) which ranged from 10-36 cmolc kg-1 is reflective of the differences in their mineralogy, and clay and organic matter content. The low available soil P can be attributed to their phosphate retention which ranged from 19.12-76.33%. The clay minerals present based on a single run XRD analysis of the soils were mainly quartz, kaolinite, hermatite matahalloysite and maghemite which are generally associated with later stages of weathering. The use of soil amendments such as: Ca-rich zeolite (22tons/ha), biochar (2% w/w), vermicompost (5t/ha), chicken manure (5t/ha), lime, and phosphate, were assessed as to their effectiveness in improving edaphological characteristics of acid upland soils by conducting incubation and pot experiments. Results of the incubation experiment showed that soil pH, phosphorus content, organic matter and CEC increased while exchangeable acidity, exchangeable aluminum and available iron of the soils decreased upon addition of Ca-rich zeolite and lime. Addition of phosphorus fertilizers increased soil P content only. Addition of biochar, vermicompost and chicken manure increased soil OM with minimal increase in soil pH. In the pot experiment using Luisiana series and corn as test crop, the different cultural interventions resulted in different responses to corn growth and yield. Increasing levels of applied P fertilizer also increased plant height, plant nutrient uptake and corn yield. Addition of Ca-rich zeolite, lime, biochar, and vermicompost without fertilizer application was not enough to produce grain yield. However, combining the soil amendments with fertilizer increased plant nutrient uptake and corn yield. Adequate knowledge of the physical and chemical constraints is vital for further development and sustainable utilization of these areas for future generations. The findings of this study showed that the soils have great variations in physiochemical and mineralogical properties due to differences in parent materials, sampling locations and soil types. Assessment of the behaviour of soils showed that they respond distinctively against the different cultural interventions employed.