Moisture Meters

Overwatering is the most common cause of death for houseplants and holiday plants. Under-watering can be just as devastating, indoors or out. So, how much water is enough, and how much is too much? And how can you tell?

Handheld soil moisture meter (Sealman) CC BY-SA 4.0

Moisture meters measure the amount of available water in a soil sample. You can buy a simple moisture meter at any garden supply store. You can also install a multi-million dollar moisture sensing system throughout your landscape, but that’s beyond the scope of this blog (and my budget).

The trick to using a moisture meter is understanding the reading, and using the information properly to avoid water-stress. Before we learn how to read those results, let’s see what types of moisture meters are available.

Types of moisture meters

Handheld moisture meters measure soil moisture using a bimetal tip that measure electric conductivity. Other types of moisture meters use a variety of methods to measure soil moisture and its availability to plant roots. These other tools include gypsum blocks, tensiometers, watermarks, and neutron probes. [A neutron probe is a radioactive tool used by professionals who have been trained and licensed. If you get your hands on one of these, please donate it to your local County Extension Office as soon as possible.] Moisture meters either measure the amount of water in a soil (content), or how tightly the soil holds that water (tension).

​The most commonly available handheld moisture meters do not require any real understanding about soil moisture. You simply stick the probe(s) into the soil, look at the display, and water accordingly. 

These inexpensive moisture meters are very useful tools, though the probes tend to corrode rather quickly. Since each type of soil has unique properties, it is important to be able to calibrate your moisture meter for your soil texture. Salt levels in soil can have a big impact on moisture meter readings. If your soil has high salt levels, you will need to take that into account, as well. Also, where you place your moisture meter probe has a big impact on the usefulness of the information.

If you want to know more about soil moisture, read on. Otherwise, skip to the bottom section on caring for your moisture meter.

Soil moisture

Each soil has its own water holding capacity. For example, if you have sandy soil that is saturated with water, gravity will pull any excess water downward, away from plant roots. If you have clay soil, as we do here in San Jose, water may be present, but much of it will be unavailable to plant roots. This is because clay has many small spaces with which to hold tightly to water molecules. [Adding organic material to clay soil increases the availability of that water.] The extent to which soil holds onto water is called its soil moisture tension. Soil moisture tension is measured in centibars (cbar). Most plants perform best between 40 and 80 cbar:

• sandy or loamy soil  has 40-50 cbar
• sandy loam has 50-70 cbat
• loam has 60-90 cbar
• clay loam or clay has 90-120 cbar

The amount of moisture available to plant roots is called a soil’s plant available water. Plant available water (PAW) can be measured as a percentage of weight, a percentage of volume, or by depth, as inches of water per foot of soil. Soil moisture content varies widely between different soil types:

• sand 7%
• loamy san 12%
• sandy loam 15%
• loam 20%
• silt loam 23%
• silty clay loam 28%
• sandy clay 27%
• silty clay 30%
• clay 31%

When plant roots are unable to pull water from a soil, it will have reached its wilting point. The permanent wilting point occurs at 15 to 20% for clay soil, 10 to 15% in loamy soil, and at 5 to 10% in sandy soil.

​How much water do plants need?

There is no correct answer to that question. [Sorry.] There are simply too many variables at play: plant type, age, size, and developmental stage, soil structure, soil texture, sun exposure, microclimate, time of year, air temperatures, recent weather… You get the idea. In California, UC Davis recommends one inch of water each week during the peak of summer. To fill a one foot by one foot area with water to a depth of 1 inch uses 0.623 gallons. In winter, your plants may not need any water, assuming it rains.

​A plant’s need for water varies throughout its lifecycle. The biggest demand for water occurs during vegetative growth and initial fruit production.

​Since this normally occurs during summer, when evaporation is at its peak, maintaining the proper moisture level makes the difference between healthy, productive plants, and plants that are struggling for survival. As temperatures drop or senescence (preparation for death) begins, the need for water drops dramatically. Adding more than is needed creates a different sort of life-threatening set of conditions. [Can you say fungal disease?]

Caring for your moisture meter

The sensor found at the end of most moisture meter probes is sensitive to damage and corrosion. To keep your moisture meter operating properly, be sure to wipe it off after each use and do not force it into dry, compacted soil. If a reading is needed under those conditions, create a starter hole with a screwdriver. Also, watch out for rocks, which can damage the probe.

As an added benefit, many moisture meters can also provide you with soil pH information. This is one tool every gardener should have on hand and use regularly. Just remember, you get what you pay for.