Measuring and controlling soil moisture is very important to growing soil moisture sensor and maintaining healthy plants. To a novice, many of the terms regarding soil moisture can be 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 with the analogy of a cloth or sponge. When you dip a dry cloth or sponge 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 it at an increasingly slower rate until it stops leaking. The point at which the cloth or sponge is full of water, yet gravity still cannot pull water from it is analogous to the rating we call field capacity. When the soil has been saturated, and any excess water has been removed by gravity, the soil are at field capacity. This is also referred to water holding capacity (WHC).
Now suppose you take a dyson and place its garden hose on the cloth or sponge. If powerful enough, the suction of the dyson will pull water out of the cloth or sponge, until most of the water is removed. Note that regardless how strong the vacuum is, a little bit of water will remain in the cloth or sponge, and it will be humid. To drive out all of the water from the cloth or sponge, you must heat it. We compare this to soil where the vacuum represents the roots of a plant. The roots stink water out of the soil with a pressure determined by capillary action. The plant will be able to stink excess water out of the soil prior to the capillary pressure still cannot overcome the soil’s tension to retain the water. This point at which a plant’s root still cannot remove 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 taken by the plant. Simply 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 garden soil have large plant available water, meaning they have high holding capacity, and low wilting points, so that water is available, and straightforward for the plant to remove.
As soil varies in arrangement, so do these guidelines. Soil types are defined by their particle size. Sand is coarse — of course, and clay courts comprises very fine allergens, while silt is a medium particle size. Because clay courts soil has very fine allergens it will hold moisture well, but it also holds on to it so the wiling point of clay courts is quite high, making it difficult for plants to remove the moisture. Remote 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. To make this happen perfect soil, garden soil 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 intention of measuring soil moisture is to know if plants are getting enough water, we would want to measure the water which can be found to their roots. Ideally we might measure the water with an “artificial” root. One very accurate method of doing this has 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, lets you know how much pressure it takes to stink water out of the soil.
Many technical articles describe results from tensiometers and give units in pressure such as bars, etc. Now if however, you know what type of soil the tensiometer is measuring, then you can figure out the absolute soil moisture or at least get an estimate of computer. A clay courts soil may have high moisture content, and at the same time have a questionable, object rendering the moisture useless to the plant. While tensiometers are accurate, and provide useful information they are delicate and expensive scientific instruments that need specialized knowledge to operate and think of. They are also slow in the sense that they need to come into harmony with the surrounding soil before a rating can be made, so they really are not ideal for utilization in making quick measurements.
Another similar approach to the tensiometer is the gypsum block. This is essentially 2 steel electrodes that are surrounded 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 steel the fishing rod that insert into the soil. This method is highly wrong due to salts in the soil which can quite change the resistance of the soil, and thus give wrong tellings of moisture content.
The gypsum block sensor somewhat overcomes salinity issues with the gypsum barrier. The main disadvantages with gypsum blocks is that they are typically slow and cumbersome. Following a block is defined 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 be taken 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 consumer electronics to measure the dielectric constant of the surrounding material which happens to be related to moisture content. These receptors are also known as capacitive Soil Moisture Sensors, or TDR Soil Moisture Sensors. These receptors are small and unobtrusive for them to be taken with potted plants, provide instant tellings, are easy to use, are very affordable, and many are low power. For their low cost and low power requirements, these kinds of receptors are increasingly being greatly stationed in irrigation systems in wireless nylon uppers networks such as Zig bee networks.
These kinds of electronic probes measure the soil moisture in absolute terms, that is the actual of water to the volume of soil, also know as VWC. Another related soil moisture rating unit is GWC or gravimetric water content, which is understood to be the mass of water, to the mass of soil. VWC and GWC are related by the bulk solidity of the soil, so if you know the solidity of the soil you can convert from to the other. VWC is more commonly used. VWC is also related to pressure, to convert from to the other the type of soil must be known. As was mentioned, a clay courts soil may have a high VWC, but a plant may have a hard time taking out water from it.
Accurate rating and model of soil moisture data, enables individuals or computerized systems make decisions about water usage, saving valuable water resources, and promoting healthy plants.