Establishing a soil reference system for fertility assessment and monitoring at plot level in the highlands of Mindanao, Philippines

The study was conducted in the crop production areas of Miarayon Village, along the volcanic footslope of Mt. Kalatungan, Talakag, Bukidnon, Philippines. The elevation range of the longitudinal toposequence is 1,900 to 1,300 m asl. Production areas in the intermediate part of the toposequence (Salsalan) are located at about 1,600 to 1,400 m asl and in the lower part (Mambuaw) at 1,400 to 1,300 m asl. A total of 24 plots (12 in each location) which were planted to potatoes, carrots and corn were investigated. Soils are “Andic” Cambisol in open and convex positions and “Andic” Umbrisol in concave positions and toeslopes. The soil pH values ranged from 5.0-5.9. TOC and TN content were medium to high (4.1-8.9% and 0.30-0.80% respectively) with C/N ratios from 8-15. Range values of available Ca, Mg, K and Na were 1.9-11.24, 0.16-2.14 and 0.20-1.13, 0.04-0.13 cmol + kg -1 , respectively. Top soil horizon exchangeable Al in Mambuaw was higher than in Salsalan. The differences in TN, C/N ratio and available K levels between the two locations were very highly significant, TOC was highly significant and for soil pH and available Ca and sum of bases, their disparities were significant.  Available Mg and Na did not differ between the two sites. Mean potato yield in Salsalan was 8.97 tha -1 more than in Mambuaw. Mean carrot yield in Salsalan was 2.39 tha -1 lesser than in Mambuaw. Mean corn yield in Salsalan was higher than in Mambuaw by 0.29 tha -1 only. Correlations between potato yields with TOC, TN, and available Ca were highly significant while soil pH and K were significant. There was no relationship detected between potato yields and Mg and Na. There was no association detected between carrot yields with topsoil nutrient levels. Correlations were noted in corn yields with available Ca, Mg and K.