Soil Nutrient Pools in the Geosphere

The primary pools or reservoirs of nutrients in a soil are:

With the exception of carbon, hydrogen and oxygen, essential plant nutrients originate primarily from soil pools. The importance of each of these pools varies greatly between elements. For example, the primary pool of nitrogen is organic matter while plant-available calcium originates predominately from the exchangeable cation pool. Furthermore, the plant availability of nutrients varies greatly between the different nutrient pools. Nutrients contained in the soil solution are readily available for plant uptake while nutrients contained in organic matter and primary minerals must first undergo mineralization and chemical weathering, respectively.

Nutrient pools are generally reported in terms of kg/ha. Chemical analyses of solid-phase nutrient concentrations are determined on a weight basis (g nutrient per kg of soil) for each soil horizon. To convert nutrient concentrations on a weight basis to a unit area basis (i.e., kg/ha), the effective volume of the rooting zone and the mass of soil contained within this zone must be determined. The soil mass of each horizon is calculated by measuring the thickness of each horizon and multiplying by the bulk density. The bulk density can be obtained by the coring method, paraffin-coated clod method, or by digging quantitative pits. Soil analyses are typically performed on the <2-mm soil fraction and it is assumed that the>2-mm fraction has a negligible nutrient supplying capacity. Thus, to determine the effective nutrient storage volume of each horizon, the coarse fragment volume (>2-mm) is subtracted from the total volume of soil within a given horizon. Based on the nutrient concentrations (g/kg) and the weight of a particular horizon (kg), the nutrient pool (kg/ha) associated with a given horizon can be calculated. The nutrient pool associated with a given soil is then determined by summing the nutrient pool associated with each soil horizon within the rooting zone as shown in the following equation:

Nutrient pool (kg/ha) = SH1 n[[T x Db] * V<2-mm ] * nc * 105. where:

SH1n is the summation of each individual soil horizon within the rooting zone

T is horizon thickness (cm)

Db is bulk density (g/cm3)

V<2-mm is the less than 2-mm volume percentage (%)

NC is nutrient concentration (g/kg) and

105 is the conversion factor to obtain the final unit of kg/ha

Because of the great spatial variability of soil properties on a given landscape, a large number of replicate samples is required to estimate nutrient pools on a landscape scale. Thus, while it may appear a simple matter to estimate soil nutrient pools, it realistically requires a tremendous effort due to the large number of replicate samples and analytical measurements required to produce reasonably rigorous estimates.