How to measure soil pH

Portable Test Kits

You can check soil pH fairly accurately right in the field using a good quality liquid indicator kit or a portable electric tester Read and follow the instructions carefully and be sure to measure both topsoil and subsoil pH since they're usually different. Even if the field's soil appears uniform in color and texture, check pH at several locations. Readings from the better quality kits like the Hellige-Truog are accurate within 0.2-0.3 pH units. Litmus paper kits don't work well. Soil labs measure pH as a routine part of soil testing.

CALCULATING HOW MUCH LIME IS NEEDED

Role of the Soils Lab

Portable kits won't tell you the full story. They can be be useful for troubleshooting, but if you do find a soil whose pH appears to be too acid, the kit won't tell you how much lime to add or even if it's needed after all. Here's why :

• The amount of lime needed to raise soil pH by one unit varies greatly from soil to soil. In fact, one soil may may require up to 5-10 times as much lime as another to achieve the same rise in pH, even though both have the same initial pH. This buffering capacity varies directly with the amount of negative charge (cation exchange capacity a soil has. Only a soil lab can measure this. (Buffering capacity is explained in more detail below; exchange capacity Was covered in Chapter 6.)

• pH isn't the only criterion for deciding if timing is needed. The soil's content of soluble aluminum (called exchangeable aluminum) is often even more important, and a portable pH test kit can't measure this. The amount of harmful soluble aluminum increases as pH drops. but some soils reach this point at a higher pH than others (i.e. 5.5 instead of 5.0). The soils lab can determine this.

Soil Exchange Capacity and timing Requirements

This is a concept worth understanding so let's go through it step by step:

• Soils have troth an active and a reserve acidity. The active acidity is produced by the those hydrogen ions (H+) floating around free in the soil's water, and it's what a pH test kit measures.

• However, for every free-floating H+ ion, there may be thousands or more H+ ions held (adsorbed) to the soil's clay and humus particles (remember, they're the only soil particles with a negative charge). These H+ ions make up a soil's reserve acidity and can't be measured with a pH test kit.

• As the applied lime neutralizes the free-floating H+ ions (active acidity), the huge reserve of H+ ions held by the clay and humus particles start" supplying H+ ions to take the place of the neutralized ones.

• The higher the amount of clay and humus a soil has, the greater its negative charge and buffering capacity, and the more lime will be needed to obtain a given rise in pH.

• An important point: Don't get confused! In terms of true soil pH (active acidity), clayey or high humus soils aren't any more likely to be acid than sandy soils; but, they're more resistant to changes in pH (either upward or downward), due to their greater buffering capacity.

• "Tropical" v". "temperate" soils: True tropical soils (old, highly-weathered red or yellow soils) usually need less lime than "temperate" soils (see Chapter 1) of the same texture to obtain an equal rise in pH. That's because true tropical clay minerals have a much lower negative charge than temperate types (see Chapter I) and, therefore, less buffering capacity. Remember that both tropical and temperate clay minerals occur in the tropics.

What to Do if No Soil Lab is Available

As explained later in this chapter, applying too much lime to an overly acid soil can be worse than not liming at all, so a farmer needs to be somewhat precise. If reliable soil testing isn't available, there are 2 alternatives:

• Check with your local extension service to see if liming recommendations have been developed for the area's soils based on prior experience, soil type, and lab testing.

• Use a generalized liming recommendation table such as Table 11-3.