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How to Measure pH in Soil

Updated: Sep 12, 2022



If you are serious about growing and maximizing your crop yield, you've figured out that soil pH is very important.

In this article we will cover the following topics:

Why soil pH is important and what is an 'ideal' soil pH?

The pH reflects the acidity or alkalinity of a sample. The pH scale ranges from 1 to 14. Neutral sample will be around pH7 while acidic sample have a low pH (< 6.5) and alkaline sample have a high pH (> 7.5).

Each type of crop thrives at their own optimal soil pH level, and pH is usually the first parameter growers want to measure. It's essential to control regularly your soil pH and adjust it to the optimal level for the best crop development.

Want to know the optimal pH for your crop? Our NutriCheck guide (comes with all Agronomist kits) covers 1000+ data points ranging from pH reference for 50+ crops to optimal range for nutrients like Nitrate, Phosphorus, Calcium and Potassium.
 

How can soil pH influence my crop?


Soil pH is directly related to many biological and chemical mechanisms, and it influences:

➢ The activity of micro-organisms responsible for breaking down organic matter

➢ Most of the chemical transformations happening in the soil.


For most crops, an “ideal” soil pH is close to neutral (pH 6.5 - 7.5), ranging from slightly acidic to slightly alkaline.


Soil pH affects nutrient availability and solubility. Even if you have enough nutrients in the soil, they may not be available for the plant. If the soil pH is not optimal, then the nutrient cannot be absorbed. The table below illustrates this point well.


High pH (alkaline soil) reduces the absorption of both macro and micronutrients especially the availability of Phosphorus and Calcium since they tend to bind together at high pH. This stimulates the formation of Calcium Phosphate molecule, which is less soluble and less available for the plant. High pH impacts micro-organisms in the soil.

Low pH means that your soil is acidic:


➢Soils that have a pH below 5.5 generally have a low availability of Calcium, Phosphorus, and Nitrogen.

➢Low pH also impacts the soil bacteria responsible of the nitrification which will reduce the availability of nitrate. Additionally, it affects microorganisms involved in reducing humification and mineralization of organic matter.

 

How to collect soil sample?

To get the most accurate soil test, it is critical to collect representative samples of the field to be analyzed. Since soil composition is highly variable, we recommend collecting different soil samples in a random pattern across the field to have a homogeneous representation.

Best Practices:

  • Make sure that all soil samples are collected within the same day

  • Sampling consistency: always take the samples in the same manner (depth and distance from the plant)

  • Collect at least 10 samples per acre at randomly selected locations

  • Collect samples at the roots level of your crop

  • Use composite sample: once you collect all samples, mix them into one composite sample


If you perform sampling in the field, consider working with Sampling Expert kit: it will streamline your sampling process, and you can use it as a worktop while performing sampling procedures right next to your crop.


Soil Sampling Protocol

  1. Using a soil sampling probe, collect the sample at a depth of 6 inches (note that this is a typical root depth, but it can change depending on the crop type. Make sure you verify the roots' depth for your specific crop).

  2. For each sample, collect around 1 ounce (around 30g.)

  3. Place all the samples collected in a plastic bag.

  4. Once all the soil samples have been collected, mix the soil well inside the plastic bag in order to obtain one composite sample.

 

How do I prep my soil for pH testing?


As with soil testing for EC, once you have your soil sample, there are 4 commons ways of testing soil pH:


Soil solution is absorbed via a vacuum tube installed in the soil​

Dilute 1 volume of soil with 2 volumes of water​

Soil sample is saturated by added water, the volume added depends on the soil moisture​ level

4. Direct measurement (see below)

If you're not familiar with these sampling methods, you can review quickly the pros and cons of each one in the table below.

Additionally, we will cover how each method compares to each other with your particular crop or soil type at the end of the article.


 

How to measure pH of my soil samples?


​​1. Prepare a soil solution by choosing your preferred sampling method: Soil Slurry (Dilution 1:2), SSAT or Saturated Media Extract method.

2. Before you measure your soil solution, make sure to calibrate your pH meter to ensure high precision - daily calibration is considered best practice.

3. Place the solution onto the sensor of the meter (HORIBA LAQUAtwin models) or place the solution in a beaker to immerse the pH sensor in the solution for Hanna pH meter.

4. Wait for the meter to display stable reading before recording the results.

5. We recommend taking 3 measurements of the same sample and taking the average result for best accuracy.

6. After the measurement clean the electrode, rinsing it gently under the running water.

measuring soil pH (using a drop of soil slurry) with LAQUAtwin meter

Place soil slurry or soil solution onto the sensor. Do not use dry soil, as the meter won't be able to read it.

 

How to measure pH directly in soil


There are few direct testers on the market, but I had experience working with Hanna one, so we will briefly cover this option too.

Hanna HI981030 Soil pH Tester is designed for direct soil pH testing. It allows skipping the soil sampling and preparation steps, as the pH is tested right in the ground.

  • Calibrate the pH probe prior to measurement.

  • Make a hole in the soil (depth ~ 20 cm). The hole needs to be the same depth each time to avoid pH discrepancies.

  • Add some distilled or deionized water to the hole, (soil should be damp but not saturated).

  • Insert the electrode insert into the hole.

  • Wait for stabilization of the reading

  • After measurement, gently wash off leftover soil (avoid using a cloth).

Note:

  • Direct soil pH testing is not recommended if your soil is rocky or hardened, as the electrode can be damaged. In this case we recommend other soil preparation methods. (like SSAT or soil slurry method)

  • Remember that this method will only allow measurement in the superficial (soil surface) layer of the soil as far as the electrode would go. (around ~5cm)

  • If your crop has deep root system, direct measurement is not recommended for the reason mentioned above

 

Final Thoughts & Recommendations

pH testing will help you manage your soil fertility better and today there are many methods that allow you to test right in the field. This way you can have an almost instant result without shipping the samples and waiting for laboratory testing times.

Remember that these two points play a role in the accuracy of your testing:

1) consistency of the sample collection,

2) frequent calibration and good maintenance of the instruments

 

Now we have covered the basics of soil testing for pH, you may have questions as to which method could work better for you.

As a rule of thumb, SME method ticks all the boxes, unless your crop is hydroponic. (In this case it's super easy to perform direct measurement right in the nutritive solution). It is also the easiest method that requires less equipment.

If your soil is extremely dry or stony, then it's more likely that you will need to dilute your soil sample and dilution method 1:2 (soil slurry) may work better for you.

I would say from observation that a lot of growers end up using soil slurry method because it's versatile, accurate, relatively quick and also it allows to test various samples in the field in 30 minutes.

See recommendation table below to compare the methods based on your soil type, crop type and other factors of importance.


Comparison between different soil preparation methods based on crop type and soil condition

Do you have any questions on the article? Feel free to drop me a line via the 'contact us' page.


-Seb









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