Page 51 - Nutrient Management Planning Guide

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Chapter 2.2

Canal seepage is a form of irrigation salinity caused when water seeps from irrigation canals or drainage ditches. Because many canals are located along a topographic break, canal seepage can exaggerate natural salinity.

s i d e b a r

Photo courtesy AAFRD

Figure 2.2.9 Saline Seeps with Saline Groundwater on Surface

Irrigation salinity occurs when the salts from irrigation water are not sufficiently leached from the root zone. This is a problem in soils with poor drainage. Irrigation salinity can also result from excess water applications that raise ground water and dissolved salts into the root zone. As with dryland seeps, dissolved salts are left behind as water in the surface soil evaporates.

Soil Fertility Implications of Salinity

There are several ways that soil salinity affects fertility:

Salinity reduces yield potential and therefore crop •

nutrient demand. The general reduction in crop yield on salt affected soils in Alberta has been estimated at 25%. Fertilizer and manure application should be adjusted to reflect this reduced yield potential. Salinity can cause nutrient imbalances as a result •

of high concentrations of salt ions in the soil. For example, excess sodium can lead to deficiencies in magnesium and calcium. Saline soils tend to have alkaline pH, which also •

affects nutrient availability. Sodic soils have structural problems that limit yield •

potential. Fertilizer and manure application should be adjusted to reflect these limitations.

Sodic Soils Quick Fact

Sodic soils contain high levels of exchangeable sodium. This reduces the ability of soil particles to cling together in stable soil aggregates. When wetted by precipitation or irrigation water, the soil aggregates in these soils easily break apart and puddling can occur. When the puddles dry, a solid crust develops on the soil surface. This crust can inhibit water and oxygen infiltration, as well as crop emergence, resulting in bare patches in fields.

Soil Salinity and AOPA

Manure has the potential to increase soil salinity because it contains 4 to 10% salt (depending on the species, diet formulation and salt content of the drinking water). This is a particular risk for fields that receive regular applications of manure and limited precipitation.

AOPA sets restrictions on manure application based on salinity. Manure cannot be applied in quantities that would raise the EC of the soil more than 1 dS/m after application. Furthermore, manure cannot be applied to soils with an EC measurement of 4 dS/m or more unless approved by the NRCB.

“Salt Effect” of Fertilizers

High rates of seed placed fertilizer can damage seeds and seedlings. One reason for this is the salt effect of fertilizer (i.e., fertilizer mimics the effect of soil salinity).

The potential of fertilizer to influence the salt level in soil solution is expressed as its salt index (SI). The higher the fertilizer SI, the greater the risk of salt burn to germinating seedlings (Table 2.2.4). The SI is based on equivalent product weights rather than actual nutrients supplied. For example, urea (46% N) has about half the salt effect of ammonium sulphate (21% N) when applied at equivalent rates of N.