Testing Soil pH

Plants obtain nearly all their nutrients from soil, and pH is crucial to nutrition as it can affect plant growth through the modification of nutrient availability, soil structure, or toxic elements. Since not all the nutrients present in the soil are accessible to plants, only a pH determination will indicate whether soil will need to be treated to adjust the pH level.

Generally speaking, soil pH indicates acidity (‘sourness’) or alkalinity (‘sweetness’), and is measured in pH units. The scale ranges from 0 (very strong acid) to 14 (very strong alkaline or base), with 7 being neutral. As a master variable in soils, the pH optimum ranges from 5.5 to 7.0 for the majority of plants, as this range promotes the most ready availability of required plant nutrients. Some plants, such as pines and firs, require and even create acidic soils (5.0 to 5.5), whereas spinach and cabbage thrive in a more alkaline environment (6.0 to 7.5). Therefore, having an ideal pH is one of a number of environmental conditions for the quality of plant growth and avoiding plant stress, which increases susceptibility to insect and disease damage.

Equipment:

plastic beaker, extraction solution A (Calcium chloride solution (CaCl₂, 0.0125 mol/dm³) for soil determination), metal spatula, folded filters, funnel, measuring cylinder (100 ml), HE comparator block plus 4.0 – 10.0 colour disk and 2 test tubes, 4 drops of pH 4-10, watch, weighing scale

Procedure:

The soil extract is produced from a non-dried soil sample, but it should not be too wet. After removing all coarse and main organic material, weigh out 100 g of your sample in a plastic beaker using the weighing scale. Add 100 ml of extraction solution A, and stir it rigorously with a metal spatula. Leave the solution to stand for 15 minutes, stirring again several times during this period. Insert a folder filter in the funnel, and place it onto a measuring cylinder. Pour the suspension into the folded filter, and fill it up to the ring mark with the soil extract. Add 4 drops of pH 4-10, and mix it. Insert the 4.0 – 10.0 colour disk into the HE comparator block, and place the measuring cylinder in it (to the right side). Put another measuring cylinder containing tap water in the comparator block (to the left side). Look through the glasses from above, and compare the colours by turning the disk until they match. By reading off the results, intermediate values can be estimated.