To achieve this 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 purpose of 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 is with 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, tells you how much pressure it takes to suck water out of the soil.
Many technical articles describe results from tensiometers and give units in pressure such as bars, etc. Now if you happen to know what sort of soil the tensiometer is measuring, then you can compute the absolute soil moisture or at least get an estimate of it. A clay soil may have high moisture content, and at the same time have a high pressure, rendering the moisture useless to the plant. While tensiometers are accurate, and provide useful information they are delicate and expensive scientific instruments that require specialized knowledge to operate and interpret. They are also slow in the sense that they have to come into equilibrium with the surrounding soil before a measurement can be made, so they are not ideal for use in making quick measurements.
Another similar approach to the tensiometer is the gypsum block. This is essentially 2 stainless 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 stainless steel rods that insert into the soil. This approach is highly inaccurate due to salts in the soil which can wildly change the resistance of the soil, and thus give inaccurate readings of moisture content.
The gypsum block sensor partially overcomes salinity issues with the gypsum barrier. The main disadvantages with gypsum blocks is that they are typically slow and bulky. After a block is placed 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‘t be used 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 electronics to measure the dielectric constant of the surrounding material which happens to be related to moisture content. These soil sensors are also known as capacitive soil moisture meter, or TDR soil moisture sensors. These sensors are small and unobtrusive so they can be used with potted plants, provide instant readings, are simple to use, are very affordable, and many are low power. Because of their low cost and low power requirements, these sorts of soild sensors are being massively deployed in irrigation systems in wireless mesh networks such as Zig bee networks.
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