1. Blakemore L.C., Searle P.L. and Daly B.K. 1987. Methods for Chemical Analysis of Soils. NZ Soil Bureau Scientific report 80, N. Z. Soil BureauDept. of Scientific and Industrial ResearchLower HuttNew Zealand.
2. Nitrogen-cycle processes and acidification of soils in grazed pastures receiving or not receiving excreta for 23 years
3. Chien S.H. 2001. Factors affecting the agronomic effectiveness of phosphate rock: a general review. In: Rajan S.S.S. and Chien S.H. (eds), Direct Application of Phosphate Rock and Related Appropriate Technology – Latest Developments and Practical Experiences. Proceedings of an International Meeting, Kuala LumpurMalaysia. International Fertilizer Development CenterMuscle Shoals, AL, pp. 50–62.
4. LONG-TERM REACTIONS OF PHOSPHATE ROCKS WITH AN OXISOL IN COLOMBIA