It’s fall, which means it’s soil sampling season for many farmers. Or maybe not. It’s a busy time and soil testing can fall to the bottom of the list of things to do.
Measuring pH in the field is easy with a reliable, inexpensive pH meter. This farm is suffering from low pH. Photo credit: Ray Weil
It can also add up as an expense if you have a lot of fields, even though most soil testing labs offer reasonable rates. This post is NOT meant to discourage you from sending away your soil samples. However, there are some aspects of your soil that you can test yourself to supplement your soil test results. You can measure more frequently (if you don’t send in those samples ever year, for example…) or in more locations and depths throughout your fields.
Soil pH is one of the most important pieces of information (some might argue the most important) that comes back with soil test results, but you don’t actually need a lab to measure it. Measuring soil pH on your own requires a small investment in time and money, and can quickly tell you if your crops may be suffering from common maladies associated with non-optimal pH or if your management practices (adding manure, compost, or fertilizers) are changing the pH of your soil. Monitoring pH is an easy and meaningful way to keep tabs on your soil. But first, a bit about logarithms.
Logarithms?!
Logarithmic scales are important to understand whether they’re for decibels, earthquakes or pH. Because pH is a logarithmic scale, a soil with a pH of 6.0 has ten times as many hydrogen ions (acid) in solution as a soil with a pH of 7.0. A soil with a pH of 5.0 has 100 times more hydrogen ions than a soil with a pH of 7.0.
Color indicator kits can be very inexpensive and effective tools to get a good idea of a soil’s pH range, but there’s a big difference between 6.0 and 6.6 so interpret carefully! This sample is close to 6.0. Photo credit: Ray Weil
But pH isn’t just measured in whole units. What about a pH of 6.3 vs. a pH of 6.6? A 0.3 unit decrease in pH represents approximately a doubling of hydrogen ions in solution–twice as acidic. You could easily see a fluctuation in pH this large throughout the growing season or from spot to spot in a single field, which is one reason why it’s important to check the pH under similar conditions (time of year) if you only measure once a year. If you’re measuring on your own and not sending samples to the lab, you can measure the pH as often and in as many places in one field as you like! The more measurements you take, the better sense of any trends that might be occurring in your fields.
Why does pH matter?
The chemistry and biology of soil and soil-plant interactions are strongly influenced by pH– so much that it’s called a “master variable” in soil science.
Phosphorus deficiency can be a result of inadequate soil pH. Photo credit: R.L. Croissant www.bugwood.org.
Some reactions are primarily chemical; as pH changes, certain compounds are formed or break apart and this makes nutrients available or unavailable for crops.An example of this is phosphorus (P). At low pH, P binds tightly with aluminum or iron and at high pH, it binds tightly with calcium; only in the middle range of pH is P readily available for plants.
Some microbes and plant roots are able to change the pH in their immediate vicinity, giving them a way to access P. Despite this, the pH of the bulk soil (as opposed to the immediate vicinity around the root or microbe) has a very strong influence on the availability of P for plant uptake.
Phosphorus availability is only one example of how pH affects nutrient availability, but it is often the most important for farmers. In the east, we have predominantly acid soils so we don’t frequently think of problems associated with high pH, but micronutrients like Zn, Fe, and Mn commonly become unavailable to plants if pH is high (>7). Even in a soil that began as an acid soil, this can occur with overuse of some kinds of manure, compost, and over-liming.
An important biological effect of pH is that soil pH is the main factor determining the fungi : bacteria ratio in soils. Higher pH favors bacteria, lower pH favors fungi and sometimes plant pathogens. Changing the soil pH (along with your tillage regime) is one of the most effective ways to influence this ratio.
Measuring pH in the field
Color indicator test kits as in the picture above are inexpensive and pretty good for obtaining ball-park numbers, but a bit more investment (<$50 total) can get you a decent glass electrode pH meter and calibration solutions. Don’t feel dread at the word “calibrate.” These things are easy-peasy and way faster and less messy to calibrate than a manure spreader (on a logarithmic scale, a pH meter calibration might be a 1 and a manure spreader would be a 5).
A “pH meter” that isn’t a pH meter at all. Reliable field pH meters have glass electrodes, not metal probes. These data show the incorrect values given for a range of soils from this meter. If you don’t want to buy a glass electrode meter, color methods like LaMotte are reasonable. R. Weil & K. Bertaut.
Don’t be tricked into buying a cheap “pH meter” that does not have a glass electrode. There are many of these in stores and online, and they really should be taken off the market. What they measure is not based on pH at all (see graph).
If you splurge and go for a glass electrode version (I got mine for $27), there are a few things you should know before going out and measuring your soil’s pH with it.
First, you will need buffer/calibration solutions for pH 4.0 and pH 7.0. I got mine from a hydroponic supply store online. If you’re in an area with typically alkaline soils, you may want to get a pH 10 buffer solution as well.
The Hanna glass electrode meter is easy to calibrate. The actual glass electrode is protected at the end of the plastic rod.
Before using your pH meter, you need to make sure your glass electrode is conditioned; sitting out in the open air is bad for the electrode, so storing it in the pH 4 solution should keep it conditioned. If it has dried out, just stick it in the solution for a few hours to recondition it before measuring anything. For the model I use (a Hanna meter as pictured–this is not a produce endorsement, but it’s the least expensive glass electrode we’ve found in our search and it’s what we use), you will also want to get a tiny screwdriver for calibrating it. I got mine in an eyeglass repair kit at Rite Aid for less than $2.
Each model of meter is slightly different to calibrate. With the Hanna meter pictured, a tiny screwdriver is used to adjust the reading until it reads 7.0 in the 7.0 calibration solution (green) and 4.0 in the 4.0 calibration solution (pink).
Have you ever thought about how they measure pH in the lab?
When pH is reported on most soil tests, it’s just reported as “pH.” They don’t tell you how much soil they added and how much water (or if it was even water- some labs use a salt solution). How you measure pH can make a difference in the results you get. Eeek! I know that’s annoying. So here’s a quick how-to for measuring your soil’s pH using our field method with distilled water, which is probably very close to what they are measuring in the lab.
Measuring pH in the field
I probably should mention that it’s really important to get a representative soil sample before measuring pH. Choose the area to sample and the depth, and make sure you get at least a dozen cores that represent that area.
With your well-mixed, representative soil sample, your calibrated, conditioned pH meter, and some distilled water (I had to buy some since I don’t own an iron and have no other reason to have distilled water) you’re ready to measure pH.
These meters will “drift” a bit over time, so take the reading after it has stabilized for four straight seconds.
What to do to change your soil’s pH (for the better)
Although there’s something nice about simple answers and solutions, I’m hoping readers of this blog will embrace the myriad ways you can amend your soil to adjust for pH BEYOND the typical recommendation of lime (for raising pH). But this post is getting very long, so I will save a discussion of lime, calcium silicate, organic materials, gypsum, sulfur and more for the next post, which will follow at the heels of this one.
I’m too lazy for canning, but bacteria have naturally lowered the pH of my sauerkraut to around 4.0 making it safe (and delicious!) for me to eat. Another use for the pH meter (after washing it off).
p.s. If you’re a home canner, you might be wondering if a pH meter would also work for that. Indeed! Check out this fact sheet from University of Wisconsin Extension on selecting a pH meter (the Hanna Checker I used is on there). I find canning laborious, but I like to make sauerkraut. Check out the pH on my sauerkraut!
