Measuring and controlling soil moisture is significant to growing and maintaining salutary plants. To a novice, many of the terms concerning soil moisture can be confusing. In this primer we endeavor to define and divulge the assorted technical terms connected to soil moisture, and to divulge state of the art soil moisture sensors.
The best way to think of soil is to use the analogy of a sponge. When you dip a dry sycophant into water it will Ant. Eject water gradually until it is thoroughly saturated. When you pull it out of the water, water will gush out quickly, because of the ensue of gravity, and after a few minutes the water will drip out of it at an increasingly slower rate until it stops dripping. The point at which the sycophant is full of water, yet gravity can no longer pull water out of it is analogous to the determination we call field capacity. When the soil has been saturated, and any excess water has been removed by gravity, the soil is at field capacity. This is also referred to water retention capacity (Whc).
Manifold Absolute Pressure Sensor Map
Now suppose you take a vacuum cleaner and place its hose on the sponge. If powerful enough, the suction of the vacuum cleaner will pull water out of the sponge, until most of the water is removed. Note that regardless of how strong the vacuum is, a minute bit of water will remain in the sponge, and it will appear moist. To drive out all of the water from the sponge, you would have to heat it. We assess this to soil where the vacuum represents the roots of a plant. The roots suck water out of the soil with a pressure carefully by capillary action. The plant will be able to suck excess water out of the soil until the capillary pressure can no longer overcome the soil's tension to maintain the water. This point at which a plant's root can no longer citation water is called the "willing point", which as you can imagine is a significant parameter.
One more important term is the "plant available water". This is the available amount of water in soil that can absolutely be used by the plant. Just 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 retention capacity minus the wilting point. Good soils have large plant available water, meaning they have high retention capacity, and low wilting points, so that water is available, and easy for the plant to extract.
As soil varies in composition, so do these parameters. Soil types are defined by their particle size. Sand is tasteless - of course, and clay is made up of very fine particles, while silt is a medium particle size. Because clay soil has very fine particles it tends to 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 citation the moisture. Sandy soil is very porous and so water flows out easily, and a ensue it has low retention capacity. The perfect soil has high retention capacity, and a low wilting point. To perform this perfect soil, soils of dissimilar 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 quantum soil moisture. Since the purpose of measuring soil moisture is to know if plants are getting enough water, we would want to quantum the water that is available to their roots. Ideally we would quantum the water with an "artificial" root. One very accurate recipe 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 article 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 divulge 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 appraisal 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 furnish beneficial information they are delicate and costly scientific instruments that wish 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 determination can be made, so they are not ideal for use in making quick measurements.
Another similar coming 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 coming is extremely inaccurate due to salts in the soil which can wildly turn 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 yield of a gypsum block is an electrical resistance, this is in turn connected to moisture in the units of pressure with the use of look up tables.
Modern soil moisture sensors use electronics to quantum the dielectric constant of the surrounding material which happens to be connected 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 they can be used with potted plants, furnish instant readings, are straightforward to use, are very affordable, and many are low power. Because of their low cost and low power requirements, these sorts of sensors are being massively deployed in irrigation systems in wireless mesh networks such as Zig bee networks.
These sorts of electronic probes quantum the soil moisture in absolute terms, namely the volume of water to the volume of soil, also know as Vwc. Other connected soil moisture determination unit is Gwc or gravimetric water content, which is defined as the mass of water, to the mass of soil. Vwc and Gwc are connected by the bulk density of the soil, so if you know the density of the soil you can turn from one to the other. Vwc is more ordinarily used. Vwc is also connected to pressure, to turn from one 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 determination and interpretation of soil moisture data, can allow individuals or computerized systems make decisions about water usage, rescue significant water resources, and promoting salutary plants.
Soil Moisture Sensors Made easy