The Importance of Measuring pH for Farms and Gardens
In gardening, knowing the soil pH is very important because it has such a strong influence on how well your plants will grow. It influences root development, microbial activity, fungi, symbiotic nitrogen fixation by legumes and the availability of nutrients.
What pH actually represents is the ratio of Hydrogen ions (H+) to Hydroxyl Ions (OH-). If the soil is acidic (less than 7pH) then there will be free Hydrogen ions and if the soils is alkaline (higher than 7pH) then there will be free Hydroxyl Ions in the soil. The closer the pH is to 7, then the closer the soil is to being neutral with all the free H+ ions and OH- ions being bonded together forming H2O (water). Because pH is measured on a logarithmic scale, a drop in a pH from 6.00 to 5.00 for example represents a tenfold change in H+ concentration. A soil with a pH of 4.00 is 100 times more acidic than soil with a pH of 6.00.
Below is a table from (Sartoretto, 1991) that shows how the availability of various plant nutrients is affected by different soil pH levels.
You’ll see that while most plants can tolerate a wide range of soil pH, they tend to grow best between 6.5 and 7.0 because that is when most of the important nutrients are available. As always, there are exceptions to the rule. Radishes, for example, prefer an acid (5.5 to 6.5) soil, while asparagus do best in 6.5 to 7.5 soil. It is therefore best to determine the ideal soil pH for a particular variety and test the soil levels before planting. In some gardens, pH testing must be done on a regular basis. Remember also that bacteria in the soil is also affected by the pH range and different cultures will thrive in different pH ranges, but as a rule of thumb, a range between 6.00 and 7.00 will provide an ideal environment for bacteria to breed, feed and flourish.
How to Measure pH
The first step in measuring pH is to make a soil extract that we can test. There are various methods for doing this and pH measurements can vary depending on the extraction procedure and the technique used to measure the pH, so it is important to use the same method every time so that you can make meaningful comparisons of your results.
Now some laboratories use a distilled water with a small 0.01 molar Calcium Chloride solution as an extract, and others use a 1 N KCL solution. However the most common method and by far the safest method to use if measuring with a pH meter is to simply take a 1:1 ratio by volume of soil to distilled water in a glass beaker. The reason I like this method is because it most closely resembles how plants feed in nature and the way that rainwater percolates through the soil and gives up the essential plant nutrients into the rainwater which the plant then drinks.
Below is the method we recommend for measuring pH.
Gather a fresh soil sample in a plastic zip-loc bag. Try to get a profile from the top 6” of soil that the plants will grow in and take care not to contaminate the sample by touching with anything.
Open the bag and let it air-dry for a few hours until it is mostly dried.
Mix the soil in the bag to ensure a homogenous sample and then use a sieve with approximate 2mm spacing to remove any large soil clumps.
Measure out ½ of a cup of the dried soil and put into a glass beaker.
Measure out ½ of a cup of distilled water and put this into the glass beaker with the soil.
Stir the mixture gently for 30 seconds. Do not mix to harshly as you may destroy the humus structure and the soil may give up elements that it otherwise would not do in nature.
Let the soil-water suspension stand for 30 minutes.
Stir water gently again before taking the measurement for 30 seconds.
Insert the pH meter or litmus paper into the beaker and swirl it gently around in the soil-water extract.
After approximately 30-60 seconds or after pH has stabilized, read the pH to the nearest 0.1 unit.
Using this method should give you a good indicator of the soil pH at the location from where you took the sample. If you have a large garden you may want to meaure numerous samples, or you could take several samples and mix them together to get an sample you can measure that represents the average pH for your entire field.
If my pH is too high or too low, how do I adjust it?
What we recommend you do if it is below 5.5 or above 7.5 is to get a soil test performed to identify the cause of the soil imbalance. It could be indicative of a serious problem that would impair any plant growth until corrected.
Generally however To raise the pH when soil is below 5.5, we do what is known as “liming” the soil. Calcium Carbonate (limestone) is most commonly used to lime soils. To lower the pH, when the soil pH is over 7.5, sulfur is most commonly used.
So as you can see, soil pH is extremely important in determining the quality of the soils that you grow your crops in. It is also one of the easiest things you can do to detect a problem and will tell you so much about the biological and chemical reactions that are taking place in the soil
Please take a look at our new plant and soil testing category to see a range of instruments including pH meters and lab supplies that can be used to help you test the quality of your soils.
Agriculture Solutions LLC