Determination of available chlorine in soil

1. Method principle

This method uses silver chloride turbidimetry to determine chloride ion in solution. When excessive Ag+ is added to the solution, the content of Cl- in the solution is directly proportional to the content of AgCl white precipitate.

2. Drugs that need to be prepared by users themselves

1) soil effective sulfur and chlorine extractant: weigh 2. 12g soil effective sulfur and chlorine extractant solid, dissolve it in proper amount of water and transfer it.

In a 500 ml volumetric flask, measure the volume with distilled water and shake well.

3. Determination of available chlorine in soil

Preparation of soil nutrient solution to be tested

Weigh 5.0g of air-dried soil sample or 5.0×( 1+ water content) g of fresh soil sample, put it in a 100mL triangular flask, add 25mL of soil available sulfur and chlorine extractant, shake it on an oscillator for 5min at room temperature, and quickly filter it with quantitative filter paper. If the filtrate is turbid, it needs to be filtered again. The filtrate is the solution of available chlorine in the soil to be tested, and it can also measure available sulfur in the soil.

2) Determination of available chlorine in soil

Suck 2mL of extractant (as blank), 2mL of extractant+1 drop of soil chlorine standard stock solution (as standard) and 2ml of soil solution to be tested into three small test tubes, and add them in turn:

2 drops of soil available chlorine 1 reagent

2 drops of No.2 soil available chlorine reagent

1 drop of soil available chlorine No.3 reagent (each reagent needs to be shaken before adding the next one), and then transfer it to a colorimetric cup for determination.

① Turn the left wheel of the filter, set the value to 6, put the blank solution into the light path, press the "Colorimetric" key, and the function number will be switched to 1, and press the "Adjust+"key or "Adjust-"key to make the instrument display 100%.

② Place the standard solution in the light path, press the "colorimetric" key, and the function number will be switched to 3. Press the adjustment key to make the instrument display 50.00.

(3) Place the liquid to be measured in the light path, and the displayed reading is the content of available chlorine in the soil (mg/kg).

Note: 1. Before filtration, shake the solution and soil sample in the triangle bottle evenly, and then pour it on the filter paper for filtration. If the filtrate is turbid during filtration, it can be started.

When filtering, the filtrate drips back to the original triangular bottle, and then it is filtered into a clean triangular bottle after the filtrate is clarified.

2. The linear range of available chlorine in soil is 5 ~ 200mg kg-1.

4. Critical value and abundance index of available chlorine in soil

When the content of Cl- in soil and irrigation water is too high, it will often cause toxicity to crop growth. The tolerance of different crops to chlorine is obviously different. Beet, corn, sorghum, millet and other crops have strong tolerance to chlorine, and the critical value of chlorine in soil is about 800mg/kg, while spring wheat, cucumber, potato, peanut and other crops have general tolerance to chlorine, and the critical value of chlorine in soil is generally 350~600mg/kg. However, sugarcane, Chinese cabbage and strawberry have poor chlorine tolerance, and the critical value of chlorine in soil is only 250~300mg/kg. The chlorine tolerance of tobacco is even worse, and the critical value of chlorine in soil is only 45mg/kg.