Low-Cost and Rapid NPK Soil Test Kit Using Colorimetric and Turbidimetric Methods for Field-Level Analysis of Tropical Acidic Mineral Soils

This study developed and evaluated a low-cost, rapid soil test kit optimized for tropical acidic mineral soils (Oxisols). The system utilizes a modified 0.02 M strontium chloride (SrCl2) universal extractant, adjusted to pH 1.8 to ensure effective recovery of plant-available phosphorus. Nitrate (NO3−), phosphorus (P), and potassium (K) were determined using adapted Zinc-Griess, molybdenum yellow, and sodium tetraphenylboron reagents, respectively. Validation via 90 inverted agitations against standard laboratory methods yielded high Pearson correlation coefficients (r = 0.87 for NO3−, 0.98 for P, and 0.99 for K). Categorical accuracy, measured via percent agreement, was 61% for NO3−, 83% for P, and 81% for K. Discrepancies primarily occurred between adjacent ranges, confirming that while visual interpretation carries inherent subjectivity, the kit reliably distinguishes between critical deficiency and excessive levels. Providing results within 10 minutes, the kit significantly reduces the logistical turnaround of conventional laboratory services, which often need weeks of delay. This rapid feedback loop enables real-time, on-site decision-making for smallholders during critical crop growth stages. At a cost of USD 2.32 per test, the kit facilitates high-density spatial sampling to map intra-plot variability. To optimize sustainable fertility management, a hybrid approach is recommended, integrating frequent on-site screening with periodic laboratory validation. This instrument-free tool offers a robust, cost-effective solution for precision nutrient management in resource-limited agricultural settings.