Total Organic Carbon

Technical Note DOER-C2

May 1999

plants; and >16, satisfactory yield only in plants that are extremely salt-tolerant. The information

from salinity and electrical conductivity tests are somewhat similar but used for different purposes

by different individuals. Either test can be used to meet the specific needs of the user.

Total Organic Carbon. Soil organic carbon is the fraction of total carbon that is derived from

the organic matter in the soil and consists of plant, animal, and microbial residues (fresh and in all

stages of decomposition) as well as the humus. Organic matter normally contains many of the

nutrients required for plant growth: 95 percent of the dredged material nitrogen, 50 percent of the

phosphorus, and when iron sulfides are not present, ≥80 percent of the sulfur. Organic carbon

comprises 48-58 percent of the organic matter content of soil. Conversion factors can be used to

obtain an estimate of the organic carbon (organic matter 1.724 (surface soils) or 2.0 (subsurface

soils)).

Total Phosphorus/Orthophosphorus. Phosphorus is an essential nutrient for all forms of

life. Plants take it up primarily as the orthophosphate ion (H2PO4 ) from fertilizers or as it is released

from organic matter decomposition. The other ionic forms, monophosphate ions (HPO4 ) or

phosphate ions (PO4 ), are less available for plant uptake. Most metal phosphates are insoluble

under neutral and alkaline conditions (except those of alkali metals) but are soluble under acidic

conditions. The orthophosphate ion (H2PO4 ) is generally the soluble form of phosphorus occurring

in dredged material/soils, but it can react quite rapidly with soluble iron or aluminum to form

insoluble phosphates. pH affects phosphorus availability through its effect(s) on microbial growth

and the solubility of calcium, iron, or aluminum. The optimum pH for phosphorus bioavailability

to plants is 6.5 in mineral soils and 5.5 in organic soils.

Carbon:Nitrogen (C:N) Ratio. The C:N ratios present in dredged material/soil help determine

if conditions are optimal for the growth of soil microbes, as well as plants. Bacteria require four

pounds of carbon for every pound of nitrogen (4:1) in order to have optimum growth and metabolism

(decomposition/recycling of organic matter). Decompositional activities of microbes can be

increased by the addition of more nitrogen. Materials with a wide C:N ratio are low in nitrogen

content. Bacteria are more abundant and in closer contact with soil particles than the root surfaces

of plants. Therefore, if nitrogen is low, bacteria will use up available supplies before it ever becomes

accessible to plant roots, resulting in nitrogen deficiency in plants.

Nitrogen. Nitrogen is the nutrient most likely to be limiting for plant growth. It can be lost from

soil/dredged material by leaching, volatilization, denitrification, or immobilization. Ammonium

nitrate is often used as a fertilizer because of its low cost. Half of its nitrogen content is in the form

of ammonium and half is nitrate. The nitrate ions are quite mobile and bioavailable to plants when

ammonium nitrate is added to soil. The ammonium cations tend to adsorb to the cation exchange

sites and are bioavailable to plants but less mobile than the nitrate.

Potassium. The availability of potassium in the dredged material needs to be determined if

vegetation establishment is the potential beneficial use. Most of the potassium requirements of

vegetation is supplied by the exchangeable potassium ions in the soil CEC and from soluble

potassium ions in the soil solution. If the CEC of the dredged material is low, as in sandy material,

it may need to be amended with potassium fertilizers. Since potassium forms a positive ion, it has

limited mobility through the soil and should be placed where it is most accessible for growing roots.