Standard Overview

NYT 295-1995 is a Chinese agricultural industry standard that specifies the determination methods for cation exchange capacity (CEC) and exchangeable base cations in neutral soil. This standard applies to neutral soils with a pH value between 6.5 and 7.5, providing key data for soil fertility evaluation and improvement. Engineering technicians and procurement personnel need to accurately understand the standard content to ensure the reliability of test results.

Determination Principle

The ammonium acetate exchange method is used: the soil is repeatedly treated with 1 mol/L ammonium acetate solution, so that the exchangeable cations on the soil colloids are replaced by NH4+. Then ethanol is used to wash away excess ammonium acetate, and NH3 is distilled out. The cation exchange capacity is calculated by titration. Exchangeable base cations are obtained by measuring the contents of K+, Na+, Ca2+, and Mg2+ in the exchange solution.

Reagents and Instruments

Main Reagents

  • 1 mol/L ammonium acetate solution (pH 7.0)
  • 95% ethanol
  • Magnesium oxide (analytical grade)
  • 0.1 mol/L hydrochloric acid standard solution
  • Boric acid indicator
  • Standard solutions for atomic absorption spectrophotometer

Main Instruments

  • Centrifuge (speed ≥3000 r/min)
  • Kjeldahl nitrogen determination apparatus
  • Atomic absorption spectrophotometer
  • pH meter
  • Analytical balance (sensitivity 0.0001 g)

Determination Procedure

Determination of Cation Exchange Capacity

  1. Weigh 2.00 g of air-dried soil sample passed through a 2 mm sieve and place it in a centrifuge tube.
  2. Add 25 mL of 1 mol/L ammonium acetate solution, stir, centrifuge, and discard the supernatant. Repeat 3–5 times until exchange is complete.
  3. Wash the soil sample with 95% ethanol 3–4 times until no NH4+ is detected.
  4. Transfer the soil sample to the Kjeldahl nitrogen determination apparatus, add magnesium oxide, distill out NH3, and absorb it with boric acid solution.
  5. Titrate with 0.1 mol/L hydrochloric acid standard solution to the endpoint, and record the consumed volume.

Determination of Exchangeable Base Cations

  1. Collect the supernatant after the above ammonium acetate exchange and make up to volume.
  2. Measure the concentrations of K+, Na+, Ca2+, and Mg2+ respectively using an atomic absorption spectrophotometer.
  3. Calculate the content of each ion according to the standard curve.

Calculation of Results

The cation exchange capacity (CEC) is calculated according to the formula: CEC (cmol/kg) = (V × c × 1000) / m, where V is the volume of hydrochloric acid consumed (mL), c is the concentration of hydrochloric acid (mol/L), and m is the mass of the soil sample (g). The total exchangeable base cations is the sum of the individual exchangeable cations.

Precautions

  • The soil sample should be air-dried and sieved, avoiding excessive grinding that damages soil structure.
  • The pH of the ammonium acetate solution must be strictly adjusted to 7.0; otherwise, the exchange efficiency will be affected.
  • Ethanol washing must be thorough; otherwise, residual NH4+ will cause high results.
  • The distillation apparatus must be checked for airtightness to prevent NH3 leakage.
  • When using atomic absorption, attention should be paid to eliminating interference from interfering ions.

Original Standard Text

The following is the original PDF text of the NYT 295-1995 standard for detailed reference: