Page 49 - Nutrient Management Planning Guide

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Chapter 2.2

Liming acid soils is an ion exchange reaction. It has the net effect of raising pH, restoring the buffering capacity, and increasing base saturation.

s i d e b a r

Acidifying Effect of Fertilizers and Manure

Aside from the obvious boost in crop available nutrients, fertilizer application can also impact soil pH (Figure 2.2.6). The nitrification of ammonium from fertilizers, plant residues or manure will acidify soil.

Source: McCauley et al. 2003

Figure 2.2.6 Effect of Ammonium Addition on Soil pH

Soil acidification can be beneficial on alkaline soils, but detrimental on acidic soils. It is important to select an appropriate form and rate of fertilizer based on the soil pH conditions.

The relative acidity of fertilizer refers to the amount of calcium carbonate (kg) required to neutralize the acid formed from the application of 100 kg of the fertilizer. Note, that the relative acidity is based on total weight of fertilizer applied and not weight of nutrient applied. Based on a weight of applied nitrogen the relative acidity of some fertilizers is: Ammonium Sulphate >>>> Urea = Anhydrous Ammonia

Table 2.2.3 Relative Acidity of Several Commonly Used Fertilizers

Fertilizer

Relative Acidity (kg CaCO

3

/100kg)

Anhydrous Ammonia 148 Urea 84 Ammonium Sulphate 110 Urea-ammonium Nitrate 52 Monoammonium Phosphate 65 Potassium Chloride 0 Potassium Sulphate 0 Gypsum 0

Source: McCauley et al. 2003

Salinity

Soil salinity describes areas where soils contain high levels of salt. In western Canada, compounds responsible for soil salinity include sulphate salts of sodium, calcium and magnesium (Na

2

SO

4

, CaSO

4

, and MgSO

4

, respectively). Soil salinity is a serious soil quality issue in Alberta affecting more than 640,000 ha (1.6 million acres).

Saline soils have high concentrations of soluble salts in the surface soil layers. Excess salt impairs the ability of plants to efficiently absorb water and nutrients from the soil. By keeping the ion concentration in the root sap higher than in the soil water, plants can maintain an inward flow of water into their roots. However, higher concentrations of salt ions in soil solution shift the concentration gradient creating osmotic stress (Figure 2.2.7). Plants in osmotic stress use more energy to maintain an inward flow of water into their roots. As a result, less energy is available for tissue growth and crop yields are reduced.