How to Test Soil pH cheaply

A farmer in soil sampling process 

Defining the pH of the soil is crucial for a profitable farming. Soil pH helps farmers to know the types of fertilizers and other chemicals they should use in their farms. Furthermore, it assists them in deciding on the best crops that can grow well in their farms. Sadly, most farmers do not determine soil pH before they plant. Failure to carry out a soil pH test before planting always results in poor farm production. But why many farmers are not interested in conducting a soil pH test? The answers to this question hinge on the inadequate information on the soil pH testing and unavailability of soil pH testing kits. As a result of these answers, farmers should equip themselves with the right information to uncover the obscured facts on soil pH testing. Testing soil pH is not demanding. Any farmer can perform it using readily available materials. This article illustrates how to test soil pH using cheap materials. It entails the brief description of soils based on pH, Soil sampling and soil pH testing.

A brief description of soil based on pH values

The soil can either be alkaline or neutral or acidic. The alkaline soils refer to soils with high pH values (pH values greater than 8.5). The alkalinity of the soils is mainly contributed by the presence of sodium carbonate (Na₂CO₃) and sodium bicarbonate (NaHCO₃). On the other hand, acidic soils are soils with lower pH values less than 7 while neutral soils have pH of 7. The presence of either sodium carbonate or sodium bicarbonate is exploited in the determination of soil pH.

Soil sampling

Before you conduct soil pH test you have to collect soil from various spots in your farm. Soil sampling enables a farmer to have a good representative of the farm. A farmer should adhere to the following:

Timely sampling

The best time to sample the soil is when the soil is moderately wet. Not too wet. Moderately wet soils easily break when pressed into balls. Thus allows homogenous soil mixture.

Farm division

Divide your farm into different managing areas. This approach applies when a farmer has a big farm. However, if the farm is small then there is no need of dividing the farm into small areas. Farm division ensures a good soil sample.

A good soil sample

Poor soil sample leads to inaccurate results and eventually misleading information. Therefore, farmers should follow an appropriate method of soil sampling. A good soil sample is achieved by collecting soil from at least twenty spots in the farm. One can use Zig-zag across  method where you take soil in a zig-zag manner in the field (See Figure 1). A farmer should avoid taking a soil sample from unusual spots (See Figure 1). Spots like old back furrows, roadbeds, small depressions, eroded places, limed areas, old fence rows or where manure have heaped as demonstrated in Figure 1. These areas are likely to give incorrect results.

Figure 1: Zig-zag across method 

Suitable sampling tools

A farmer should use tools made of stainless steel when sampling soil. Tools such as soil auger, spade, and chrome-plated soil tube are commonly used in soil sampling. One should avoid tools made of brass or bronze or galvanized tools since they contaminate soil samples with zinc or copper. The depth of soil sample should be between six and eight inches for cropland and two and four inches for pasture lands. Always use clean container when sampling soil.

Mixing of soil sample

Use a transparent plastic container to prevent soil contamination. From your twenty or more spots thoroughly mixed the soil to have uniform soil sample. Remove all foreign materials such as plant roots, stalks, rocks etcetera. Set it aside for the next step (soil pH testing).

Soil pH testing

You need to assemble the following to enable you to conduct a soil pH test: 

• Well mixed soil sample
• Baking powder (sodium bicarbonate or sodium hydrogen carbonate (NaHCO3))
• Acid (you can use organic acids or mineral acids)
• Distilled water
• Two plastic beakers/containers (1- liter beaker/container or ½-litre beaker/container is recommended)
• Two tablespoons
• Three plastic stirrers
• Two 100ml-measuring cylinders

Procedure

Thoroughly rinse plastic beakers/containers (transparent ones are commended) of your desired size, tablespoons and plastic stirrers. Take one tablespoonful soil sample and add it to one beaker/container. Scope another one tablespoonful soil sample and add it to the second beaker/container. Put one stirrer in one container and another in the second beaker. Prepare baking powder solution (sodium hydrogen carbonate) by scoping two tablespoonful baking powder. Transfer it into a clean 1/2 – liter beaker. Add 200ml of distilled water into the beaker and stir the mixture using plastic stirrer until it completely dissolves. Put it aside.

If you are using mineral acids or synthesized organic acids (diluted or concentrated) then directly measure 100ml acids and add it to one of the beakers containing soil sample and stir. If there is no observation, measure 100ml of baking powder solution to the second beaker and stir. Record your observation. Clean your apparatus and discard unwanted reagents.

Alternatively, you can extract organic acids from fruits such as lemon (citric acid), etc. In this case, you will use fruit blender or you can extract it manually extract it and follow the same procedure. Vinegar which is mainly acetic acid is used in most websites. Though, this does mean it is the only acid for the soil pH test. You can use any acid.

Result

When acid is added to the soil, the soil fizzes if the soil is alkaline. However, acidic soil does not release a gas (fizzing sound) when an acid is added. For this reason, baking powder solution is added to confirm this. When baking powder is added to acidic soil, it produces fizzing sound. If there is no fizzing sound after adding an acid and baking powder solution to the beakers then the soil is neutral. These chemical reactions take place upon adding an acid or baking powder solution to the soil sample in the beakers:

Sodium carbonate (Na₂CO₃) + Acid = Salt + CO₂ (a gas that produces fizzing sound) + H₂O (water)

Let us use acetic acid to show the actual chemical reaction.

Na₂CO₃ (aq) + 2 CH₃COOH (acetic acid) = 2 CH₃COO^-Na⁺ (Sodium ethanoate) + CO₂ (carbon (IV) oxide) + H₂O (water)

Or

Sodium hydrogen carbonate (NaHCO₃) = Salt + CO₂ (a gas that produces fizzing sound) + H₂O

NaHCO₃ (aq) + CH3COOH (acetic acid) = CH₃COO^-Na⁺ (Sodium ethanoate) + CO₂ (carbon (IV) oxide) + H₂O (water)

Conclusion

This method gives you a rough idea of the status of the type of soil in your farm. Though, it does not indicate the exact value of pH of your soil. For exact values, you may consider using the electronic pH meters. Nonetheless, it is a simple method that can be carried out by any farmer.