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Soil Moisture Sensors Made Simple

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Measuring and controlling soil moisture ‘s very important to growing and maintaining healthy plants. To a novice, many of the terms regarding soil moisture can be soil pH sensor confusing. In this primer we attempt to define and relate the various technical terms related to soil moisture, and to describe state of the art Soil Moisture Sensors.

The best way to think of soil is ty trying the analogy of a sponge or cloth. When you dip a dry sponge or cloth into water it will absorb water slowly until it is completely saturated. When you pull it out of the water, water will gush out quickly, because of the effect of gravity, and after a few minutes the water will drip from the jawhorse at an increasingly slower rate until it stops still dripping wet. The point at which the sponge or cloth is full of water, yet gravity cannot pull water from the jawhorse is analogous to the statistic we call field capacity. When the soil has been saturated, and any excess water has been removed by gravity, the soil is in field capacity. This is also referred to water holding capacity (WHC).

Now suppose you take a hoover and place its hose-pipe on the sponge or cloth. If powerful enough, the suction of the hoover will pull water out of the sponge or cloth, until most of the water is removed. Note that regardless of how strong the vacuum is, a little bit of water will remain in the sponge or cloth, and it will appear rainy. To drive out all of the water from the sponge or cloth, you it is fair to heat it. We compare this to soil where the vacuum represents the roots of a plant. The roots draw water out of the soil with a pressure determined by capillary action. The plant will be able to draw excess water out of the soil so that the capillary pressure cannot overcome the soil’s tension to retain the water. This point at which a plant’s root cannot get water is called the “willing point”, which as you can imagine is a critical parameter.

One more important term is the “plant available water”. This is the available amount of water in soil that can actually be applied by the plant. Because soil may have water in it doesn’t mean that the plant has enough “suck” to pull it out. So the definition of plant available water is the holding capacity minus the wilting point. Good soils have large plant available water, meaning they have high holding capacity, and low wilting points, so that water is available, and simple for the plant to get.

As soil varies in article, so do these variables. Soil types are defined by their particle size. Sand is coarse : of course, and clay comprises of very fine dust, while silt is a medium particle size. Because clay soil has very fine dust it can hold moisture well, but it also holds on to it so the wiling point of clay is quite high, making it difficult for plants to get the moisture. Soft sand soil is very porous and so water flows out easily, and a result it has low holding capacity. The perfect soil has high holding capacity, and a low wilting point. For this purpose perfect soil, soils of different particle size are mixed together with organic matter such as humus.

Now that we have discussed how soil holds water, we can discuss how to measure soil moisture. Since the stage that measuring soil moisture is to know if plants are getting enough water, we would want to measure the water that is available to their roots. Ideally we would measure the water with an “artificial” root. One very accurate method of doing this has been a tensiometer, which measures the water as a function of pressure. Since it measures pressure or tension its units are also in terms of pressure. The tensiometer doesn’t tell you what the absolute moisture content of the soil is, but hearkening back to our soil moisture analogy, informs you how much pressure it takes to draw water out of the soil.

Many technical articles describe results from tensiometers and give units in pressure such as bars, etc. Now in addition know what kind of soil the tensiometer is measuring, then you can compute the absolute soil moisture or at least get an estimate today. A clay soil may have high moisture content, and at the same time have a high pressure, portrayal the moisture useless to the plant. While tensiometers are accurate, and provide useful information they are delicate and expensive scientific instruments that want specialized knowledge to operate and misinterpret. They are also slow in the sense that they must come into stability with the surrounding soil before a statistic can be made, so one of these are not ideal for used making quick measurements.

Another similar approach to the tensiometer is the gypsum block. This is essentially 2 chrome steel electrodes that are encased in plaster. As moisture absorbs into the gypsum resistivity decreases. The gypsum serves as a salt barrier. Many cheap Soil Moisture Sensors consist of two chrome steel fishing rods that insert into the soil. This process is highly inexact due to salts in the soil which can significantly change the resistance of the soil, and thus give inexact parts of moisture content.

The gypsum block sensor to some extent overcomes salinity issues with the gypsum barrier. The main disadvantages with gypsum blocks is that they are typically slow and heavy. After having a block is put in the soil, there is a lag before the gypsum comes to the same moisture level as the surrounding soil. Because they are large and obtrusive they can not be applied in potted plants. The output of a gypsum block is an electrical resistance, this is in turn related to moisture in the units of pressure with the use of look up tables.

Modern Soil Moisture Sensors use electronic devices to measure the dielectric constant of the surrounding material which happens to be related to moisture content. These sensors are also known as capacitive Soil Moisture Sensors, or TDR Soil Moisture Sensors. These sensors are small and unobtrusive so as to be applied with potted plants, provide instant parts, are user friendly, are very affordable, and many are low power. Because of the low cost and low power requirements, these types of sensors will be massively working in irrigation systems in wireless fine mesh networks such as Zig bee networks.

These types of electronic probes measure the soil moisture in absolute terms, which is the quantity of water to the volume of soil, also know as VWC. Another related soil moisture statistic unit is GWC or gravimetric water content, which means the mass of water, to the mass of soil. VWC and GWC are related by the bulk thickness of the soil, so if you know the thickness of the soil you can convert collected from one of to the other. VWC is more commonly used. VWC is also related to pressure, to convert collected from one of to the other the type of soil must be known. As was mentioned, a clay soil may have a high VWC, but a plant may have a hard time extracting water from it.

Accurate statistic and handling of soil moisture data, can allow individuals or computerized systems make decisions about water usage, saving valuable water resources, and promoting healthy plants.

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