Soil Moisture, pH, and Light Sensor (Yina Yang)

Author

Yina Yang, Alliance Gertz-Ressler High School

Sensor / Probe

The 3-in-1 sensor measures soil moisture, pH, and light (Vigoro brand from Home Depot). The

meter has 2 prongs attached to the meter. The

meter has a switch that measures the soil moisture, pH and light individually. The meter cannot measure all three elements of the soil at the same time. No other software or program is needed to be used in conjunction to the sensor. No battery is needed to operate this sensor. Soil Moisture Meter

The moisture meter does not have units of measurement. The moisture level is simply measured on a scale of 0 (dry) to 10 (moist). The meter has three different color zones. Accordingly to Hydro Farm, if the reading registers between 0 and 1.5 in the red zone, then it is time to water. When the reading registers in the green zone (middle of scale), between 5 to 7, it is also time to water. But, if the meter reads between 8 to 10 in the blue zone, there is no need to water.

pH Meter

The effect of soil pH is great on the solubility of minerals or nutrients. Before nutrients can be used by plants, the nutrients must be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid soils than in neutral. Extremely and strongly acid soils (pH 4.0-5.0) can have high concentrations of soluble aluminum, iron, and manganese which may be toxic to the growth of some plants. A pH range of approximately 6 to 7 promotes the most ready availability of plant nutrients.

Light Meter

Unfortunately on this meter, there is no units of measurement for light. Light, generally, is measured using the lux (lx) as the unit of measurement. An article titled Lights and Plants stated that "the illumination is measured in "lux", a measurement of how many lumens falls on each square meter of surface." When the sensor is switched to the light mode, the meter detects the amount of light illuminating onto the meter. Due to photosynthesis, it is important that plants receive the appropriate amount of light. And, yes, there are some plants, such as the Azalea, that does not require much sunlight.

Sample Investigation (Soil Moisture)

The soil moisture/pH/light sensor can be use to conduct an investigation on the rate of evaporation of soil. In this investigation a plant in a pot (Azalea found at home depot) was used. The plant was watered. The sensor was set in the soil. The meter registered exceeding 10 indicating that the soil is beyond moist. Therefore, let the pot set and drain out the excessive water. Insert the meter into the soil testing the soil to make sure the starting point reads 10. Then, set the timer. Wait until the meter reads 9 and stop the timer. Calculate the evaporation rate.

Sample Data & Analysis and Interpretation

From the soil moisture scale of 10 to 9, it took 3,958 seconds (65.97 minutes) to reach. In other words, it took approximately 65.97 minutes for the soil moisture meter to detect 1 scale of change in the Azalea plant soil. The rate of evaporation inside my apartment may have taken a lot longer due to limited amount of sunlight. Often time, the temperature in my apartment unit is lower than that of the temperature outside due to limited amount of access to sunlight. This may have been the cause to the slower rate of evaporation relative to the plant being placed in the sun.

Photos

The meter has three different readings. The top row shows the light scale in terms of lux, although not indicated on the meter. The

second row is the soil moisture scale from 0 (dry) to 10 (moist). The third row, not seen from the angle of the photo, is the pH scale from 4.5 to 8. As recommended to not leave the

meter in the soil, the meter is subject to damage. The meter was left in the soil of the Azalea plant in this investigation and damaged result is indicated on the image on the right.

Due to the nature of the Azalea plant not requiring a lot of sunlight, this investigation was conducted inside my apartment unit, which does not have much sunlight. Therefore, the amount of time that it took the soil moisture to change may have taken a lot longer than if this investigation was conducted outside in the sun.