How To Measure NPK In Soil

The soil nutrients are closely related to the growth of crops and agricultural yield. Therefore, it is necessary to focus on the changes in soil nutrients in farm production and management. Nitrogen, phosphorus, and potassium are three main elements in soil nutrients. So, how to measure the soil nutrients and nitrogen, phosphorus, and potassium in soil nutrients?

How to measure N P K in the soil in the laboratory

1. Determination of available potassium in soil

Weigh the dried sample passing the 2mm aperture sieve into the plastic bottle, add ammonium acetate solution (soil liquid ratio is 1:10), close the bottle stopper, shake well, shake at 15 ℃ ~ 25 ℃ by 150r / min ~ 180r / min for 30min, and dry filter. After appropriate dilution, we can measure the filtrate by the flame photometer or atomic absorption spectrophotometer.

2. Determination of soil available phosphorus

Weigh the dried sample passing the 2mm aperture sieve; place it in a plastic bottle, add phosphorus-free activated carbon; add sodium bicarbonate extractant at 25 ℃± 1 ℃; 

shake it evenly, oscillate at the frequency of 180r / min ± 20R / min for 30 ± 1min at 25 ℃± 1 ℃, and immediately filter it in a dry 150ml triangular bottle with phosphorus-free filter paper.

Suck the filtrate into the cuvette, add the developer, shake it slowly, discharge CO2, add water to volume to the scale, and shake it well. Place it at room temperature higher than 20 ℃ for 30min, use a blank solution as a reference, use a 1cm optical path cuvette to compare the color at the wavelength of 700nm, and measure the absorbance.

3. Determination of soil available nitrogen

Weigh the dried soil sample and ferrous sulfate powder passing through the 0.25mm sieve and evenly lay them in the outer chamber of the diffusion dish, and rotate the diffusion dish horizontally and gently to pave the soil sample.

In the inner chamber of the diffusion dish, add 2% boric acid solution and 1 titration nitrogen mixed indicator. Then apply alkaline glue on the edge of the outer chamber of the diffusion dish, cover the ground glass, rotate it several times to completely bond the ground glass with the edge of the diffusion dish, and carefully cross the two rubber bands into a cross ring.

Then slowly turn one side of the ground glass to expose a slit in the outer chamber of the diffusion dish, quickly add 1.2mnaoh solution into the outer chamber of the diffusion dish, and immediately rotate and cover the ground glass tightly.

Gently rotate the diffusion dish horizontally on the test bench to fully mix the solution with the soil; Then carefully place it in a 40% incubator. After 24h, take it out and titrate the amount of ammonia absorbed in the boric acid solution in the diffusion four chambers with 0.01M hydrochloric acid (or sulfuric acid) standard solution with a micro burette. The endpoint is that the color changes from blue to purplish-red. When titrating, use a fine glass rod to stir the solution in the inner chamber, and do not shake the diffusion dish to avoid overflow.

How to measure N P K in soil with a soil tester

Collection and treatment of soil samples

To make the measured samples represent the actual nutrient of the field, it needs to take multi-point mixed sampling. It should avoid taking samples at the edge of the field, roadside, ditch, manure pile, or the place where chemical fertilizer is put.

There are diagonal methods, five-point sampling methods, checkerboard sampling methods, etc., in the sampling method. Generally, we should take at least five sample points for each plot and more when the plot is large. The sampling depth is commonly about 0-20cm. We should take the samples from multiple points, then thoroughly mixed. We can reserve about half a kilogram and discard the excess part according to the quartering method after removing the dead branches and leaving residual roots, stone, and other impurities;

 generally, we should choose fresh soil samples to determine soil nutrients rapidly.

Determination of ammonium nitrogen in the soil

Pipette 2ml of extractant (as blank), 2ml of extractant + 1 drop of soil nutrient mixed standard stock solution (as standard), 2ml of soil to be measured solution into three small test tubes, and add them orderly:

• Soil ammonium nitrogen reagent 1 4 drops
• Soil ammonium nitrogen reagent 2 4 drops
• Soil ammonium nitrogen reagent 3 4 drops

Shake well, transfer to the cuvette after 5 minutes, and use the tester for measurement:

Turn the left wheel of the filter to set the value to 1, place the blank solution in the optical path, press the “colorimetry” key, the number switch to 1, and press the “adjustment +” key or “adjustment -” key to make the instrument display 100%.

Place the standard solution in the optical path, press the “colorimetric” key, the number switch to 3, and press the adjustment key to make the instrument display value 48.0.

Then place the liquid in the light path, and the instrument display is the ammonium nitrogen content in the soil (mg/kg).

Determination of nitrate-nitrogen in soil

Pipette 2ml of extractant (as blank), 2ml of extractant + 1 drop of soil nutrient mixed standard stock solution (as standard), 2ml of soil to be measured solution into three small test tubes, and add them orderly:

• 4 drops of nitrate-nitrogen -reagent 1 (gradually add and shake)
• 10 drops of nitrate-nitrogen – reagent 2
• 1 drop of nitrate-nitrogen –reagent 3 (shake before use or shake several times after hot water bath at about 70 ℃ for 3 minutes to entirely suspend the sediment)

Shake for one minute, and after standing for 15 minutes, transfer them to the cuvette respectively for tester measurement.

Turn the left wheel of the filter to set the value to 2, place the blank solution in the optical path, press the “colorimetry” key, the number switch to 1, and press the “adjustment +” key or “adjustment -” key to make the instrument display 100%.

Place the standard solution in the optical path, press the “colorimetric” key, the number switch to 3, and press the adjustment key to make the instrument display value 48.0.

Then place the liquid in the light path, and the display is the nitrate-nitrogen content in the soil (mg/kg).

Note:

Nitrate nitrogen rises and falls with the water, mainly distributed in the soil layer of 0 ~ 40cm. To make the test results more accurate, it is recommended to take the soil in the depth of 0 ~ 40cm and change the soil weight coefficient during calculation from 0.15 to 0.3.