📘 NPK Sensor User Guide

Product Overview

The multi-parameter soil monitor can measure eight soil parameters, including soil temperature, soil moisture, soil EC, soil salinity, soil nitrogen, phosphorus, and potassium, and soil pH value. It features high measurement accuracy, excellent stability, and convenient operation.

Suitable for water-saving agricultural irrigation, meteorological monitoring, environmental monitoring, greenhouse cultivation, flower and vegetable cultivation, grassland and pasture management, rapid soil testing, plant cultivation, scientific experiments, and other fields that require measurement of soil temperature and humidity, electrical conductivity, salinity, nitrogen, phosphorus, potassium, and soil pH.

Precautions

1. Please check if the packaging is intact and verify if the sensor specifications match the product you have purchased.
2. The installation site should be kept away from chemical corrosive environments, high voltage electricity, heat sources, etc.
3. Sensors are precision instruments and should be stored in a dry, ventilated, room-temperature indoor environment.
4. Sensors are precision components — please do not disassemble them yourself during use, to avoid product damage.

Measurement Parameters

Soil Nitrogen, Phosphorus, and Potassium (NPK)

Effect of N, P, K Content on Crop Growth

Nitrogen (N) can promote crop growth and increase yield. If excessive, it can lead to overgrowth of crops and affect their quality.

Phosphorus (P) can promote the root growth of crops, increase the transport and storage of photosynthetic products, and improve crop stress resistance. If excessive, it can lead to the accumulation of soil phosphorus and affect the activity of soil microorganisms.

Potassium (K) can promote photosynthesis in crops and improve their stress resistance. If excessive, it can affect the absorption and balance of other elements in the soil.

In summary, reasonable fertilization needs to be carried out according to the standards of nitrogen, phosphorus, and potassium content in the soil, to ensure that the needs of crop growth are met and to improve crop yield and quality. At the same time, attention should also be paid to avoiding soil pollution and crop quality decline caused by excessive fertilization. Therefore, fertilization should be carried out according to the soil nitrogen, phosphorus, and potassium content standards to achieve scientific fertilization and improve crop yield and quality.

Regional Differences

The distribution characteristics of soil nutrients show a general pattern of "high in the north and low in the south":

• Available nitrogen (alkali-hydrolyzed nitrogen): highest in black soil in the north, lowest in the northwest and other northern regions. Paddy fields generally have higher levels than dry land.
• Available phosphorus: total phosphorus content in northern soil is significantly higher than in southern red soil.
• Quick-acting potassium (available potassium): northern soil far exceeds southern red soil; potassium deficiency is mainly concentrated in southern regions.

Soil Nitrogen, Phosphorus, and Potassium Standards

The standards for soil N/P/K content vary depending on soil type, region, and crop demand, but the national soil survey provides a universal grading standard: available nitrogen (30–150 mg/kg), effective phosphorus (3–40 mg/kg), and available potassium (30–200 mg/kg).

Laboratory Methods for Measuring N, P, K in Soil

Determination of Available Nitrogen (Alkali-Hydrolyzable Nitrogen) in Soil

Weigh a sample of air-dried soil that has passed through a 0.25 mm sieve, together with iron sulfate powder, and spread them evenly on the outer chamber of the diffusion dish. Gently rotate the diffusion dish horizontally to flatten the soil sample. In the inner chamber, add a 2% boric acid solution followed by 1 drop of titration nitrogen mixed indicator. Apply an alkaline adhesive solution along the edge of the outer chamber, cover with a glass plate, rotate several times to ensure the glass plate is fully adhered to the edge of the dish, and secure with two rubber bands crossed in a cross shape.

Slowly open one side of the frosted glass to expose a narrow slit in the outer chamber. Quickly add 1.2 mol NaOH solution to the outer chamber, immediately rotate the frosted glass to seal it tightly, and gently rotate the diffusion dish horizontally on the lab bench to ensure thorough mixing of the solution with the soil. Place it in a 40°C constant-temperature incubator. After 24 hours, remove the diffusion dish and use a microburette to titrate the amount of ammonia absorbed by the boric acid solution in the inner chamber with a 0.01 mol standard solution of hydrochloric acid (or sulfuric acid). The endpoint is when the color changes from blue to purplish red. During titration, use a fine glass rod to stir the solution in the inner chamber — do not shake the diffusion dish, to avoid spillage.

Determination of Available Phosphorus (AP) in Soil

Weigh an air-dried sample that has passed through a 2 mm sieve, place it in a plastic bottle, add phosphorus-free activated carbon and NaHCO₃ extraction agent at 25°C ±1°C, shake well, and shake at 180 r/min ±20 r/min on a shaker at 25°C ±1°C for 30 ±1 min. Immediately filter through phosphorus-free filter paper into a dry 150 ml conical flask. Transfer the filtrate to a cuvette, add the color developer, gently shake, allow CO₂ to escape, then dilute with water to the mark and mix thoroughly. Let stand at room temperature above 20°C for 30 minutes. Using a blank solution as reference, measure the absorbance with a 1 cm pathlength cuvette at a wavelength of 700 nm.

Determination of Available Potassium in Soil

Weigh an air-dried sample that has passed through a 2 mm sieve into a plastic bottle, add ammonium acetate solution (soil-to-liquid ratio of 1:10), seal the bottle tightly, shake well, and shake at 15–25°C on a shaker at 150–180 rpm for 30 minutes, then filter. Measure the filtrate directly on a flame photometer, or dilute appropriately and measure using an atomic absorption spectrophotometer.

Soil pH Value

What is pH Value?

Soil pH value — i.e., soil acidity or alkalinity — is primarily determined by the concentration of hydrogen ions and hydroxide ions in the soil solution. It is expressed as the negative of the common logarithm of the hydrogen ion concentration (activity) in the water solution, i.e., -log[H⁺].

Soil contains various organic acids, inorganic acids, bases, and salts. Different concentrations of these substances result in varying degrees of acidity or alkalinity. Soil with a pH between 6.5 and 7.5 is neutral; below 6.5 is acidic; above 7.5 is alkaline. Soil acidity is generally classified into seven levels:

Reference Values for Suitable pH Levels by Crop

Vegetables

Grain, Oil, and Medicinal Crops

Fruit Trees

Soil Moisture

This product measures soil moisture as soil volumetric water content, using the internationally advanced FDR (frequency-domain reflectometry) principle — the most scientific and precise method for rapidly measuring soil moisture.

Soil volumetric water content refers to the percentage of water volume in a unit volume of soil relative to the total volume of the soil. Specifically, it is defined as the ratio of the volume of water in the soil to the total volume of the soil (including solid particles, water, and air).

Calculation formula: Soil volumetric water content = (volume of water ÷ total soil volume) × 100%. The normal range is generally between 5% and 40%, with a maximum typically not exceeding 50%. The sensor reads 0% in air and 100% in pure water.

Note: the relationship between soil volumetric moisture content and soil weight moisture content is: Soil volumetric moisture content = Soil weight moisture content × Soil bulk density.

Soil EC Value

What is Soil EC Value?

Soil EC value, or soil electrical conductivity, is an indicator of soil water-soluble salts. These salts are an important indicator of mineral nutrients in the topsoil that can be rapidly utilized by plants, and they determine whether salt ions in the soil limit crop growth. High concentrations of soluble salts can damage plants or cause root system death. Normal soil EC values typically range between 100 and 1000 µS/cm.

Soil EC Value Reference Standards

The EC value of general irrigation water should be less than 800 µS/cm. The ideal soil EC value for plant growth is typically between 100 and 1000 µS/cm, with a maximum of no more than 2500 µS/cm.