How to measure soil moisture memory?

By continuing on reading the instructions for and completing the soil moisture measurement tasks as described, you agree that you have read the consent form (adult, children, and parents) before you start the project and collect data, had an opportunity to ask any questions about your participation in this research and voluntarily consent to participate. If you would like to keep a copy of this information for your reference, please print or save one now before continuing on.

In this project, we will work through four science exploration tasks to measure soil moisture memory. For each task, there is a demonstration video. Click on the video and try it yourself.

Task 1: Monitor soil moisture

There are many soil moisture meters designed to monitor soil moisture conditions. Unlike the expensive research-grade soil moisture meters that provide soil moisture reading with three digits (e.g., 0.252 or 25.2%), most of the low-cost soil moisture meters only give a scale (e.g., 1-10). Such soil moisture meters are relatively affordable to monitor soil moisture conditions for gardening activities or other purposes. Watch Video 1 to learn how to measure soil moisture with a low-cost soil moisture meter. 

Note: Select a monitoring site close to your home and measure the soil moisture at five different locations (we need more than one measurement to reduce the errors) close to the site once per day (you can choose the same time of the day, like 10 AM). When measuring the soil moisture, make sure the metal rod is inserted into the soil at the 2-inch (~5 cm) depth. Start your measurements one day after a high rainfall event and repeat the measurements for 14 consecutive days (two weeks). Record your data on a piece of paper with the date. Task 1 usually takes less than 5 minutes per day to complete.

Task 2: Identify soil texture

Calculating soil moisture memory requires us to obtain the "true" soil moisture (soil water content expressed in a volume fraction) instead of a scale between 1 and 10. When we do not have access to a research-grade soil moisture meter (sometimes costs over $1,000) and do not want to spend a long time using the oven-drying method (like the NASA Citizen Science project), we can use another way to figure this out.

The basic idea is to build a relationship between the low-cost soil moisture meter readings and true soil moisture collected from a research-grade soil moisture meter. This relationship could be expressed in a linear formula:

True soil moisture = a + b ✕  soil moisture meter reading

Here, a and b are so-called coefficients that convert the readings from your soil moisture meter (a number between 1 and 10) to true soil moisture (a number between 0 and 1, often less than 0.5). Our lab has collected soil moisture readings using both a low-cost off-the-shelf soil moisture meter (the one with a 7-inch rod) and a research-grade soil moisture meter (HydroGO Portable Soil Sensor System, Stevens Water Monitoring Systems, Portland, OR) to obtain coefficients a and b.

The coefficients a and b are not the same for different soils and depend on a soil property named "soil texture". In the US, people classify soils into 12 soil texture classes (see figure above) and each class has a different ability to hold water (and nutrients) and a unique set of coefficients a and b. Some people even believe that soil texture controls the quality of the wine, along with climate, exposure to sunlight, and grape varieties.

Soil texture can be measured using a quick "dirty" ribbon test or estimated using a web-based App (Web Soil Survey, watch Video 2 to learn how to use it). You can select either method or compare if both methods give you the same soil texture class for your measurement site.

Task 3: Obtain rainfall data 

We need to know if there is any rain during the 14-day period. If there is no rainfall over the 14-day period, write down a value of zero. If there is rain on any of the 14 days, we need to find out how much it rains. Two methods are introduced in this project to help us know the rainfall amount. The first is to build a rain gauge yourself. Check this webpage for detailed instructions. The second one is to use a web-based App (“Visual Crossing”). Watch Video 3 to learn how to get past rainfall data from the web-based App. You can select either method or compare if both methods give you the same rainfall data.

Task 4: Calculate soil moisture memory

Now you have collected all the data needed to figure out soil moisture memory. Submit your data using this form after you complete the 14-day experiment and you will visualize soil moisture memory on the website. Watch Video 4 to learn how to submit your data and compare your soil moisture memory with those from other places in the US.

Questions:

1. Can you explain why your soil moisture memory is longer or shorter compared to other places?

2. What factors control the soil moisture memory at your site and can you think of a way to increase your soil moisture memory? 

Check the Knowledge page for more information about soil moisture memory and a selection of Books for more interesting stories about soils.