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Competency Area 2: Basic Concepts of Soil Fertility

PO 11. Distinguish each macronutrient as mobile or immobile in the soil.

As discussed in PO 4, nutrient mobility in the soil affects the ease of its uptake by plants, and the likelihood of its leaching through the soil.

Nitrogen mobility depends on the form it is in.  Nitrate (NO3-) is very mobile in soil water, and can be easily leached. Ammonium (NH4+) can be held on cation exchange sites, and is not susceptible to leaching.

Phosphorus is typically immobile in soil, unless soil levels rise above the soil's ability to bind it.

Calcium, magnesium, and potassium are considered immobile, since they are held onto cation exchange sites. Only when excess cations are added to replace the nutrients' place on the CEC are these released.

Sulfur is commonly found in the anion sulfate form (SO4-), which does not bind to cation exchange sites and is thus mobile in most soils.