432 Organic waste reuse and recycling: technology and management
root zone would necessitate corrective action such as reduced hydraulic loading
or adding under drainage. The appearance of seeps or perched groundwater
tables might also indicate the need for corrective action.
Soil
In some cases, application of wastewater to the land will result in changes in soil
properties. Results of soil sampling and testing will serve as the basis for
deciding whether the soil properties should be adjusted by the application of
chemical and organic amendments. Soil properties that are important to land
treatment of wastewater include: pH, exchangeable sodium percentage or SAR,
salinity, nutrient status, and heavy metals.
Soil pH below 5.5 or above 8.5 generally is harmful to most plants. Below
pH 6.5 the capacity of soils to retain metals is reduced significantly, the soil
with pH above 8.5 generally indicates high sodium content and possible
permeability problems.
The levels at which salinity becomes harmful to plant growth depend on the
type of crop. Salinity in the root zone is controlled by leaching soluble salts to
the subsoil or drainage system.
Crop tissue
Plant tissue analysis is probably more revealing than soil analysis with regard to
deficient or toxic levels of elements. All of the environmental factors that affect
the uptake of an element are integrated by the plant, thus eliminating much of
the complexity associated with interpretation of soil test results. If a regular
plant tissue monitoring program is established, deficiencies and toxicities can be
determined and corrective action can be taken.