Page 110 - Nutrient Management Planning Guide

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Chapter 3.4

pH

Soil pH (or reaction) indicates acidity or alkalinity of the soil. Soils below pH 6.7 are acidic and soils above pH 7.3 are alkaline. A pH near 7.0 is considered neutral. A more descriptive classification of soil pH is based on the ranges described in Table 3.4.3.

Table 3.4.3 Qualitative and Quantitative Descriptions of pH for Alberta Soils

3.0–5.6 5.6–6.2 6.2–6.7 6.7–7.3 7.3–7.9 7.9–8.5 >8.5

Strongly Acidic

Moderately Acidic

Slightly Acidic Neutral

Slightly Alkaline

Moderately Alkaline

Strongly Alkaline

Source: Kryzanowski et al., 1988

Under low pH conditions, some nutrients bind tightly to soil particles and as a result are unavailable to plants. In addition, chemical structures of some nutrients, particularly P, can change under low pH making them less available to crops. Low pH conditions also impact the growth and survival of soil microorganisms, some of which are instrumental in releasing nutrients bound in organic matter for crop use.

Crops vary in their acidity tolerance (Figure 2.2.5) which is strongly influenced by the sensitivity of crops to various levels of soluble aluminum (Al 3+ ). Aluminum solubility increases substantially under strongly acidic conditions. Crops produced in soils more acidic than their tolerance level will result in reduced yields. Fertilizer recommendations should be adjusted for reduced yield potential on the basis of crop type and pH.

To contend with soil acidity, select acid tolerant crop types or consider liming the soil to correct the high pH condition. Before applying lime, request a lime requirement test which will provide a recommendation for an appropriate rate. Liming acid soils can be an extremely costly procedure so the potential return on investment should be carefully assessed. More information about soil pH and acid soil conditions can be found in Chapter 2.2.

Salinity

There are two soil parameters used to characterize soils as saline, sodic or saline-sodic. These are electrical conductivity (EC) and sodium adsorption ratio (SAR). Only EC is part of routine agricultural soil analysis.

EC

Soluble salts are present in soils at all times; however, when the concentration of salts is high, the soil is considered saline and crop growth can be reduced. EC is a measure of the total soluble salt concentration in a soil (i.e., salinity). It is determined by measuring the ability of a small current to be transmitted through saturated soil between two electrodes of a conductivity meter that are a fixed distance apart. The units commonly used to express EC are decisiemens/metre (dS/m). Soils are classified on the basis of salinity according to the EC ranges specified in Table 3.4.4.

more info

For more information on liming acid soils can be found in the factsheets below, which can be ordered from the AF Publications Office or searched by Agdex number on Ropin’ the Web:

AF. 1996. Liming acid •

soils. Agdex 534-1.

AF. 2002. Wood ash: •

An alternative liming material for agricultural soils. Agdex 534-2.