How To Measure Soil Nutrients & Fertility

Soil nutrients are closely related to plant growth. So, how to measure the content of nutrients in the soil?

Test items of soil nutrients

The test items of soil nutrients mainly include pH value, organic substance content, total nitrogen and inorganic nitrogen content, nitrate-nitrogen (NO3-N), total phosphorus and available phosphorus, total potassium and available potassium, as well as soil electrical conductivity (EC), water and soil trace elements.

Many determination methods include chemical analysis, photoelectric colour separation, the indirect determination method of soil conductivity, near-infrared spectroscopy, etc.

What are the ten items of soil measurement?

The top ten items of soil include soil total nitrogen determination, soil alkali hydrolyzable nitrogen determination, soil available phosphorus determination, soil available potassium determination, soil slow available potassium determination, soil organic substance determination, soil pH value determination, soil available sulfur determination, soil cation exchange capacity determination, plant nitrogen, phosphorus, potassium and water content determination.

The top five contents of trace elements are organic substance, nitrogen, phosphorus, potassium and pH value.

There are PH 4 ~ 8, pH 7 ~ 9, pH 4 ~ 11 and other indicators in the pH mixed indicator colourimetry. According to the type of soil pH grade, pH < 4.5 is strongly acidic soil, and pH > 8.5 is strongly alkaline soil. Therefore, we can select pH 4.5 ~ 8.5 mixed indicators with apparent colour scale and tone changes.

1)The determination of soil water content by alcohol combustion method.

2) To determine soil pH, we can measure it using mixed indicator colourimetry. The pH range of the colourimetric card is 4.5 ~ 8.5, including 9 different colour scales. The pH value difference between adjacent colour scales is 0.5 with a noticeable difference. Compared with the conventional analytical acidimeter method, it can save time by about 10 times.

3)Soil organic substance was determined by potassium dichromate sulfuric acid oxidation colourimetry. It does not need heating equipment due to using sulfuric acid dilution heat, no calibration and titration. It has the advantages of simple operation, apparent colour scale, noticeable color change, and easy distinction. The determination range of organic substance is 0.5 ~ 5.0mg/kg, including 9 color scales, and the difference between adjacent color scales is 0.5mg/kg. The standard color scale is made of glucose. Compared with conventional analysis and determination, it can save about 5 times of time, and reduce the use of the oven, analytical balance and other instruments.

4)For the determination of soil ammonium N, sodium reagent colorimetry is used, with the determination range of 5.5 ~ 80 mg/kg, including 6 color scales used for paddy soil determination.

5)For the determination of soil nitrate-nitrogen, the colorimetric method of nitric acid test powder is adopted, with the determination range of 2.5 ~ 80 mg/kg, including 6 color scales used for dryland soil determination.

6)For the determination of soil available phosphorus, molybdenum blue colorimetry is adopted, with the determination range of 3 ~ 50 mg/kg, including 7 color scales, saving about 8 times of time. Compared with conventional analysis, and reduces the use of analytical balance and spectrophotometer.

7) To determine soil available potassium, sodium tetraphenylborate turbidimetry is used, with the determination range of 25 ~ 200 mg/kg, including 6 turbidimetric levels. Colorimetric analysis saves about 5 times of time than conventional analysis, and it does not need expensive instruments such as a flame photometer.

8)The soil nutrient tester is mainly used to determine the content of water, salt, pH value, total nitrogen, ammonium nitrogen, alkali hydrolyzable nitrogen, phosphorus, potassium, calcium, magnesium, and shed and nitrogen, phosphorus and potassium in the fertilizer.

How to use a soil nutrient tester?

The soil nutrient tester can fast measure them independently in the village or quickly in the laboratory, directly print the test results, and give scientific formula fertilization suggestions according to the test results.

Sample collection and processing

In order to make the measured samples represent the nutrient status of the field, it needs to take multi-point mixed sampling. Avoid taking samples at the edge of the field, roadside, ditch, manure pile or the place where chemical fertilizer points. We can get samples by diagonal method, five-point sampling method, checkerboard sampling method, etc. Generally, it should take at least five sample points for each plot and more when the plot is large. The sampling depth is in the plough layer (0-20cm).

When we take the samples from multiple points, they should be thoroughly mixed, the excess part should be discarded according to the quartering method, and retain about half a kilogram after removing the litter, residual roots, stone and other impurities. If there are soil blocks, they should be crushed as the samples for analysis and test. Generally, fresh soil samples are suitable for rapidly determining soil nutrients.

Determination of soil organic substance

1. Preparation

Preparation of soil organic substance extractant:

• Take a bag of soil organic substance extractant powder.
• Put it into a 500ml volumetric flask or plastic bottle.
• Add distilled water to fix the volume to 500ml.

2. Operation steps: weigh 4g of air-dried soil sample into the extraction bottle, add 20ml of soil organic substance extractant, shake well for 5 minutes, then filter. We can determine soil organic substance by the filtrate.

3. Determination method

Suck about 3ml of the solution into the cuvette and test it on the computer.

Select the measurement item, put the cuvette into the No. 2 cuvette, press the OK key, and the instrument will read the data and display the results automatically.

Determination of water content

weighing method

Soil moisture content not only affects the growth of crops but also needs to be used to calculate the nutrient content in soil nutrient determination.

Step:

1)After wiping the aluminium box, weigh it on balance and record it as W1.

2) Put about 5g of fresh soil sample with impurities removed into the aluminium box, weigh it together with the aluminium box and record it as W2.

3)Use a straw to suck 5-10ml of burning alcohol into the aluminium box, mix it with the soil and ignite it. After the flame is extinguished, add 5ml of alcohol to ignite it. After the flame is extinguished and slightly cold, weigh it on balance and record it as W3

4)Calculation:

soil water content (%) = (W3-W1) / (W2-W1) × 100%

Determination of pH value of soil

1. Preparation of soil sample:

Remove large particles from the soil sample, weigh 25g and put it into a small beaker. Add distilled water to 1:1 times the weight of air-dried soil (2:1 times for acid soil) depending on the soil water content, stir to disperse the soil particles fully, and stand for half an hour for determination.

2. PH test paper determination method

Insert pH5 ~ 9 test paper into the apparent solution of the soil sample, take it out after half a minute, observe the test paper’s colour, and compare it with the colorimetric plate.

pH7. 0 is neutral, ph6 5 ~ 7.0 is slightly acidic, pH 6.5 0 ~ 6.5 is weakly acidic, pH 5.5 5 ~ 6.0 is acidic, and pH < 5.5 is strongly acidic; pH7. 0 ~ 7.5 is slightly alkaline, pH 7.5 5 ~ 8.0 is weakly alkaline, pH 8.0 0 ~ 8.5 is alkaline, pH 8.5 more than 5 is strongly alkaline.

3. PH electrode method

Calibration

A. Calibration before measurement: press the “temperature / pH” key to switch the function number to 5, wash the pH electrode probe with distilled water, suck up the water with filter paper, and then it is the standard buffer solution with put pH4.01, press the “adjust +” key until “E” disappears and the displayed value is stable, and this calibration must be done before each measurement.

B. Periodic calibration: after pH4.01 for the calibrated electrode, rinse the probe with distilled water, suck up the water with filter paper, and then put it into the standard buffer solution with PH9.18, press the “adjust -” key until “E” disappears and the displayed value is stable, and this calibration should be done once a week.

Measure

Wash the calibrated pH electrode probe with distilled water, dry it with filter paper, put it into the bathymetric solution, and wait for 2-3 minutes. After the display value is stable, the displayed value is the pH value of the solution.

Note:

After using the pH electrode every time:

• Wash the probe part with distilled water and dry it with filter paper.
• Soak it in saturated potassium chloride solution.
• Install a 50 Ω terminator on the electrode interface to prevent electrostatic interference.

For the determination of salt content, the preparation of soil samples is the same as above. Measure the salt in the soil by a salt meter. If the salt content of the liquid exceeds the range, please dilute the liquid proportion, and the test value multiplied by the dilution ratio is the test value of the liquid content.