• Electro-ultrafiltration (EUF) was developed and introduced by Karoly Nemeth in 1979 as an innovative soil testing methodology.
• The technique applies an electric field across a soil suspension using deionized water as the sole extractant under ultrafiltration vacuum.
• By varying voltage and temperature over successive extraction fractions, EUF simultaneously extracts nutrient intensity, quantity, and replenishment reserve pools (e.g., K, P, N) in a single run.
• Lindsay and Norvell (1978) developed the DTPA micronutrient soil test, and Cate and Nelson (1971) developed graphical critical level models.

Mehlich No. 1 extractant (double acid: 0.05 N HCl + 0.025 N H2SO4) is specifically suited for determining available phosphorus in sandy acidic soils.

• Olsen: Sodium bicarbonate (0.5 M NaHCO3 at pH 8.5) extractant used for neutral, alkaline, and calcareous soils.
• Mehlich No. 1: Specifically developed by Adolf Mehlich for extracting available P and exchangeable cations in coarse-textured acidic soils of low cation exchange capacity.
• Morgan-Wolf: Sodium acetate-acetic acid universal extracting solution utilized primarily in acidic soils of the northeastern United States.
• AB-DTPA: Ammonium bicarbonate-DTPA method designed by Soltanpour and Schwab for alkaline soils.

The alkaline potassium permanganate distillation method for available soil nitrogen was developed by Subbiah and Asija in 1956.

• Bremner (1996): Pioneer in soil nitrogen methodology and steam distillation analysis.
• Hanway and Heidel (1952): Developed flame photometric procedures for soil potassium.
• Walkley and Black (1938): Formulated the chromic acid wet oxidation method for organic carbon.
• Subbiah and Asija (1956): Standardized distillation with 0.32% KMnO4 and 2.5% NaOH to estimate hydrolyzable and mineralizable available nitrogen in Indian soils.

• < 10 mg/kg (or 5-10 mg/kg) represents the low / deficient category of available sulfur.
• Under the standard 0.15% CaCl2 extraction method (Williams & Steinbergs), available sulfur status in soil is calibrated as: Low (< 10 mg/kg), Medium (10 to 15 mg/kg), and High (> 15 mg/kg).
• 15-20 mg/kg falls in the high / adequate available sulfur category.
• 20-25 mg/kg represents very high sulfur status.