Using Soil P Tests and P-Sorption Index to Predict P Requirement for Cassava Grown in Tropical Upland Sandy Soils

ABSTRACT

The phosphorus (P) sorption index and efficiency of P extracting methods were investigated to quantify the available P content for accurate recommendations regarding cassava P fertilizer application in the sandy soils of Thailand. Phosphorus sorption isotherms were examined to predict the response of soils to the P fertilizers applied. Six rates of P fertilizers (0, 12.5, 25, 37.5, 50 and 62.5 kg P₂O₅ ha⁻¹) were applied in a randomized complete block design at five field sites in major cassava-growing areas. The efficiency among the Bray II, CaCl₂ and Mehlich III extracts was also tested by comparing their capacity to predict crop response. The P adsorption capacities of all soils were in the range 39.4–106 mg kg⁻¹; therefore, there were classified as low P-sorbed soils. Phosphorus fertilizer produced significantly higher aboveground biomass than did zero-P fertilization. The critical P level adequate for 95% of the maximum yield was in the range 0.14–0.26 mg P L⁻¹. The equation for calculating P fertilizer using soil P extracted based on the Bray II, CaCl₂ and Mehlich III methods had accuracy levels of 73, 65 and 72%, respectively. Bray II was slightly superior to Mehlich III for use in soil-P availability indices, both of which were recommended for P fertilizer assessment.

KEYWORDS:

• Loamy sand soils
• P availability indices
• P fertilizer rate estimation
• upland soils

Acknowledgement

This work was mainly supported by the Kasetsart University Research and Development Institute, Bangkok, Thailand (FF(KU)5.65) and partly funded by a Graduate Scholarship for International Publication, Graduate School, Kasetsart University.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the the Kasetsart University Research and Development Institute, Bangkok, Thailand (FF(KU)5.65).