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Ministry of Agriculture, Food and Rural Affairs

pH - Soil Diagnostics

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; 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.

Vegetables grown on mineral soils have a target pH of 6.1 to 6.5. On muck soils the target pH is 5.1 to 5.5.

pH also affects the activity of soil micro-organisms. These organisms build soil structure, cycle organic matter or 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 after 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, or the crop may appear 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 and a saturated paste. There are also a number of hand held meters available. There is a wide range in cost and accuracy but 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 (due to the variation in soil moisture over the season – soil moisture carries the hydrogen ions, so pH is very difficult to measure in a dry soil). A good 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. The buffer pH is used to calculate the amount of liming material based upon the ability of the soil to resist changes in pH. Lime will not change soil pH overnight. Agricultural limestone does not dissolve quickly. 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.

pH Scale Stunted Sweet Corn on a Low pH Soil
Click to enlarge.