1. ISO 5725. 1986(E). “Precision of Test Methods—Determination of Repeatability and Reproducibility for a Standard Test Method by Inter‐Laboratory Tests”.
2. Fertilizer Society of South Africa. 1974. “Soil Analysis—a Manual of Methods”. Pretoria, South Africa
3. Buys, A. J. 1984. Soil Analysis Services in South Africa. Paper presented at FSSA Technical Symposium. September271984, Modderfontein.