Soil

How to Measure Soil Moisture

Published:

August 28, 2024

Jason Blalock

Key Takeaways:

The soil moisture metric shows the amount of water in the soil, which is interconnected with water potential and the accessibility of water.
Moisture content affects your plants' health as well as soil chemistry and biological makeup, including the soil's long-term health.
Both under- and over-watering can be harmful, so aiming for optimal moisture levels is essential.
Special tools for remote monitoring of soil moisture save time and help better understand plants’ health and plan irrigation more precisely.

Quick Summary

To measure soil moisture, you can use the gravimetric method:

Collect soil samples from multiple locations and depths.
Weigh each sample while wet.
Dry the sample in an oven or microwave in five-minute sessions until the weight stabilizes.
Calculate moisture content using the formula:
Gravimetric Water Content (%) = [(wet soil mass - dry soil mass) / dry soil mass] × 100

Numerous factors determine the health of your crops — but soil moisture is among the most important. The amount of water available for crops determines how well they thrive, and either too much or too little water can substantially reduce your harvest. If the soil moisture status is too far off, it could even cause issues with the soil's microbiome or the accumulation of unwanted chemicals in the soil.

Cropler wants to help optimize your fields with high-tech solutions to age-old problems. Our visual monitoring systems can already help you manage your plants' health, and with our upcoming soil sensors, you'll be able to evaluate soil moisture from the comfort of your own computer or smartphone.

So let's talk about soil moisture, how to measure the water content of soil, and ideas to optimize your irrigation management for great harvests!

What Is Soil Moisture?

On its face, the concept of soil moisture is simple: it's how much water is in the soil. If you could squeeze out every drop of water in the dirt, how much is there?

However, the topic is more complicated than that, because it's not possible to simply squeeze the water out. Extracting water takes energy, plus the water will be continually evaporating, percolating downward, or flowing away. So, proper irrigation management also involves knowing the field's capacity to hold water, as well as the soil's water potential.

Water potential relates to how much energy it takes for a plant to extract water from the ground. Water that has been absorbed into the soil isn't free; it takes effort — energy — to unbind it from the dirt, minerals, and other components in the soil. So a plant will always derive less net energy from the water than the theoretical maximum, due to the effort needed to extract that water. This should be factored into your irrigation.

Understanding these basic principles of moisture evaluation will help ensure your plants receive enough moisture in the end, as well as support the overall health of your farm.

Other Factors Affected by Soil Moisture

Soil moisture doesn't only affect how much water a plant receives. It will have an effect on numerous other aspects of your fields, even having a long-term impact.

Soil moisture can alter:

The chemical content of the soil, including aeration, chemical buildup, pH balance, and salinity, as well as soil microbial and biochemical activity.
The weight of the soil, its thickness, and the overall soil texture and ground structure.
Ground temperatures, as well as the heat capacity of the soil.
Longer-term trends, such as the amount of chemical runoff or other forms of leaching in the soil.

In short, constant moisture evaluation is necessary for any farm. It helps improve the health of your plants today, as well as affecting the long-term viability of your land.

How Soil Moisture Affects Plant Growth

Plants can be harmed by both over- and under-watering. The latter is obvious: a plant in drought conditions that is starved of water will have no way of pulling nutrients out of the dry soil, and will quickly wilt.

Over-watering is more subtle. If the plant's roots are swamped, they will struggle to pull sufficient oxygen out of the ground—the plant equivalent of drowning. It can also lead to a buildup of toxins within the plant, which harm its development. Worse, over time, changes to oxidation reactions in the soil may even lead to an overall loss of nitrogen and a buildup of soil toxins, because the plants are no longer helping to maintain the soil's health.

So a field that has become waterlogged during a season may need extra attention and fertilizing before it is ready for the next planting.

What Affects Soil Moisture

Numerous factors in your soil and the environment will affect the overall measurement of soil moisture content, as well as the field's capacity and water potential. The most prominent of these are:

Structure and texture: Looser soil with plenty of soil pores will have better water penetration and retention.
Density: As soil density increases, water will have more difficulty penetrating into it — which can cause a problem for plants with deep roots.
Temperature: High temperatures prevent water from penetrating deeply, as well as encouraging evaporation.
Salinity: Salt naturally absorbs water, so high salinity levels will reduce water available to plants.Robust security systems can add to the overall costs and complexity of adoption.
Organic content: Organic matter holds water but keeps it available for plants, increasing the overall water capacity of the soil.

When you have questions, we'll help you find the answers you need to take control of your fields and the harvest you care about. Contact Cropler to discover more about our products and the ways we help you proactively protect your operations from the effects of crop diseases. Discover More

Measuring the Moisture Content of Soil

With all that in mind, you can start conducting moisture evaluations and optimize moisture levels for the specific plants you're growing.

The most common method of estimating soil moisture is the gravimetric method, which relies on weighing the soil before and after drying to determine water content. A simple version of this would look like:

Choose several places to take soil samples that are representative of the field as a whole.
Take samples from multiple depths in each spot, which should be immediately placed in sealed and labeled zip-loc style bags.
Once you're ready to analyze, mix the soil from each spot thoroughly on a paper plate. Be sure to weigh the plate beforehand, and subtract it from weights going forward.
Weigh the wet soil on the paper plates.
Start microwaving the soil sample in five-minute sessions, weighing it after each session.
Once the sample goes through a five-minute session without its weight changing again, it's fully dry. Take that as the final dry weight.
Calculate the water content with the following formula:

Gravimetric Water Content (%) = [(mass of wet soil (g) − mass of dry soil (g)) / mass of dry soil (g)] × 100

This can then be compared against the water needs of each individual plant, to determine whether they're receiving an appropriate amount of water.

The Future of Measuring Soil Moisture: Remote Sensors

Evaluating soil moisture is difficult work — but it's becoming much easier! Modern technology is coming to fields in the form of "Internet of Things" (IoT) sensors which are planted in the ground, and monitored by smart AI systems. Products are already on the market, with more coming soon that can continually measure soil moisture, temperature, and more.

In turn, this data is gathered in an online dashboard which allows you to remotely check your fields' moisture levels at any time, automatically compared against optimal levels for the plants. Your work measuring soil moisture will be slashed while allowing much more precise micro-management of moisture levels. This will be of particular interest to those looking to adopt climate-smart agricultural practices that limit ecological damage from farming.

Summing Up

Optimizing your moisture levels will lead to better harvests now, and a healthy farm in the long term. While measurement by hand is difficult, investing in smart sensors can streamline the process by providing instant data.

Cropler can help make this happen! Our current agri-camera and a web platform can monitor your plants visually for signs of over- and under-watering. Our soil monitors are coming next, and will greatly improve your irrigation management.