Page 50 - Nutrient Management Planning Guide

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Basic Soil-Plant Interactions

From Seelig 2000

Figure 2.2.7 The Impact of Soil Salinity on Water Uptake by Plants

The measure of soil salinity is electrical conductivity (EC), which is measured in decisiemens per metre (dS/m). Electrical conductivity reflects the total soluble salt concentration in the soil. Soil salinity can be determined in a laboratory by taking a water extract of a soil sample and measuring the conductivity in the extract using an EC meter. Salinity can also be measured in the field using an EM38 apparatus.

Salt affected soils are classified as saline, sodic or saline-sodic based on soluble salt content in the soil and the percentage of exchange sites occupied by sodium ions. Saline soils have an excess of soluble salts and sodic soils have high levels of exchangeable sodium. Saline-sodic soils are characterized by both problems.

Salinity is often placed into one of two categories: dryland salinity and irrigation salinity. Dryland salinity is caused by groundwater redistributing salts and accumulating these at the soil surface. When groundwater moves from upland to lowland areas, it accumulates salts and raises the water table in the lowlands (Figure 2.2.8). When the water table comes within two metres of the soil surface, capillary action raises the groundwater to the soil surface. When the water evaporates, salts accumulate in the root zone and topsoil (Figure 2.2.9). Dryland salinity is further influenced by activities of agriculture and land management.

Solonetzic soils are not classified as saline, but are characterized by excessive levels of exchangeable sodium.

s i d e b a r

Adapted from Wentz 2000

Figure 2.2.8 Generalized Saline Seep Formation

Recharge area Discharge area

Low permeability

Evapotranspiration

Water table

Groundwater

Salty water moves upward from the water table by capillary action

Salts remain behind

Water evaporates

Water table

Soil surface

Excess water

Infiltration

Deep percolation