The Spruce Sensor’s soil moisture is expressed as volumetric water content (%). Simply stated, this is the quantity of water contained in the soil (with the soil itself, water, and air comprising 100%). Generally, soil moisture will range from 10% to 45%, but can be higher during and after watering.
The water in your soil is stored on the surface of the soil particles, as well as in the pores, which are holes or gaps between individual soil particles. Pores will contain both water and air/oxygen. The amount of moisture in your soil will depend on weather (rain, heat, sun exposure, wind), runoff/drainage, and soil type (which dictates the size of the pores).
There are 2 important characteristic values for soil moisture:
- Field Capacity is the amount (%) of soil moisture or water that a soil can hold after excess water has drained away. It also represents the ideal balance of air and water in the soil pores. Above field capacity, the pores are oversaturated with water and oxygen levels are restricted.
- Wilting point is the moisture level, below which, plants will wilt and eventually die. Water in the soil below wilting point is not available to the plant.
Field Capacity, Wilting Point, Available and Unavailable water vs. Soil Texture
Soil type will determine field capacity and wilting point, as the texture and structure of the soil will dictate how much and how fast water can infiltrate the soil. For example, sandy soil has large pores which allow water to move freely, and therefore also has a low field capacity. Conversely, the pores in clay are small, causing water to move slowly. Clay also forms a sticky mass when it gets wet, which slows drainage and increases water-holding capacity.
Why does the soil feel dry but the moisture reads wet?
The very top surface of the soil dries much faster than the soil below the surface. This is partially because it is directly exposed to the sun and heat but also because of drainage. Just 2 inches below the surface, the soil will normally be much wetter. This is where the Spruce sensor measures and is also where your plants get their water from.
Why does my clay soil change slowly?
Clay soil has very dense will small pores. Checking the chart above, we see that the ‘available water’ region is at the top of the scale, meaning it can hold a lot of water. Clay soil holds water for a much longer time compared to typical soil. Because of this and the typical frequency of watering, it will not show much of a change in moisture value.
The simplest solution to seeing more dynamic changes in moisture and give the system a better soil moisture range to work with is to add a small layer of better draining soil around the probe. This is done by enlarging the location where the sensor is placed by a small amount.
- Remove the Sensor from the ground.
- Use a stake or similar object that is just a little bigger than the sensor probe to enlarge the hole where the Sensor was located.
- Now fill the hole with potting soil or a sand/soil mix.
- Re-install the sensor.
The new layer of soil will act in a similar manner to a sponge. When the clay soil increases in water content the more porous potting soil will absorb water and show a higher value. In turn, when the clay starts to dry, it will absorb the water from the potting soil, showing a lower moisture value. The effect will amplify the changes and give the measurement a more dynamic range.
What should I set my moisture setpoint to?
The above information gives good context for ideal case situations. In the real world, it is difficult to suggest a moisture setpoint value because there are many variables. For example, the moisture reported by Spruce Sensors can vary depending on how well the probe is contacting the soil, and soil type. As well, soil is often a mix of several different types, making it difficult for users to know where their zone falls in the field capacity chart above.
Soil Moisture Example
There is some trial and error since each soil type holds water moisture differently and each area dries out at a different rate. The recommended procedure to find the watering setpoint based on soil moisture is:
- Install the sensor in the area to be watered and ensure water will reach the probe. So the sensor should be in range of the sprinkler head spray, or the drip emitter should be very close to the sensor body.
- Water the soil around the sensor very well. The sensor should report increased moisture, unless the area was already very wet.
- Wait several days monitoring the soil moisture each day. You are looking for a steady drop and then the value should stabilize.
- Set the Soil moisture setpoint just above the low value observed.
An example would be to place the sensor in a raised bed with a dripper.
- The moisture reads 22% when installed, and it is placed right next to a drip emitter.
- The drip irrigation is turned on for 45 minutes and the value goes up to 28%. We then use a hose to further water the area and it goes up to 30%. This is the field capacity or max value of moisture the soil will hold.
- If it is warm and sunny out, the soil moisture may drop down to 25% the next day
- The 2nd day, it drops to 23%
- The 3rd day, it drops to 21%
- The 4th day, it remains around 20-21%.
- We now set our soil setpoint to 22 or 23%.
Most plants do best when the soil is permitted to dry out between water. But each time they are watered, they should be watered well. This is why the soil moisture should be permitted to drop.
You can use Learn Mode to let Spruce learn what setpoint to use.