pH

pH is a measure of the level of acidity or alkalinity in the soil. The pH scale ranges from 0 to 14 and reflects the hydrogen ion concentration in the soil. A pH value of 7.0 is neutral. Values below 7.0 are acidic and those above 7.0 are alkaline or basic.

Soil pH has an impact on the availability of most nutrients. Elements such as nitrogen, calcium and molybdenum are less available at pH levels below 6.0. The availability of other nutrients, such as manganese, zinc, phosphorus, and potassium, decreases at pH levels greater than 7.0 in mineral soils.

Vegetables grown on mineral soils have a target pH of 6.1 to 6.5. Due to differences in nutrient availability relative to pH, the target pH is 5.1 to 5.5 on muck soils.

pH also affects the activity of soil micro-organisms. In natural soil ecosystems, fungal activity is stronger in acidic soils, while bacteria dominate at intermediate and higher pH levels. These organisms build soil structure, cycle organic matter, and fix nitrogen in legume nodules.

Soil pH can have a drastic effect on the performance and breakdown of some pesticides. The efficacy of soil applied triazine herbicides like atrazine and metribuzin is often decreased on acidic soils. pH may also affect the breakdown of residual herbicides used in field crops. Imazethapyr, flumetsulam and cloransulam degrade very slowly at a soil pH less than 6.0 to 6.5, potentially causing problems with vegetable crops grown following crops treated with these products. Chlorimuron-ethyl degrades slowly at a pH greater than 7.

Identification of Soil pH Problems

Soil testing is the only reliable way to determine whether the soil pH needs adjusting. However, some areas of a field may show symptoms of low pH even though the average pH or even the grid sampled pH for the entire field may be acceptable.

Sandy knolls often have a lower pH than the rest of the field. These areas should be sampled separately.

A very low pH can cause some crop roots to appear stunted and burnt. Plant establishment in low pH areas may be poor and the crop may be stunted and delayed. Crops such as barley and peas are very sensitive to low pH.

Checking soil pH

Soil pH is usually measured with a standard lab test using an electrode, using the saturated paste or 1:1 water method. There are also several hand-held meters available. Although there is a wide range in cost and accuracy amongst models, there are several that, when used correctly, can give results very similar to that of the standard lab test. Generally, any meter that is directly inserted into the soil is not adequate. This is due to the variation in soil moisture over the season. Since soil moisture carries hydrogen ions, pH is very difficult to measure accurately in a dry soil. A representative soil sample is crucial to the accuracy of any pH test.

Management Notes

Plant species differ in their requirement and tolerance of different pH levels in soil. Soil texture also has an impact. For example, liming is considered beneficial below a pH of 6.1 for many crops, including beans, peas, and tomatoes on coarse- and medium-textured soils; in fine-textured soils, liming is beneficial only below pH 5.6 for these same crops. And while Cole crops and alfalfa require liming when pH falls below 6.1, corn, rye and pasture grasses can tolerate pH as low as 5.1 (in fine-textured soils).

Buffer pH is used to calculate the amount of liming material required to adjust pH to a desired level based on the ability of the soil to resist changes in pH. Lime will not change soil pH overnight. Agricultural limestone does not dissolve quickly, particularly if it has a below average Agricultural Index value. The full effects of liming may take up to 3 years. It is important to ensure that lime is applied well in advance of sensitive crops being grown