7. SOIL STRUCTURE
Aggregation of soil particles can occur in different patterns, resulting in different soil structures. The circulation of water in the soil varies greatly according to structure, Therefore, it is important for you to know about the structure of the soil where you plan to build a fish-farm. Although you may not be able to assemble all this information yourself, the specialized technicians from the soil testing laboratory will be able to provide it after examining your undisturbed soil samples. They will be able to tell you if your soil has bad structure or good structure (pores/capillary canals, network, etc.). They will also be able to tell you about the degree of water circulation or permeability.
7.1 Description of soil structure
Soil structure is most usefully described in terms of grade (degree of aggregation), class (average size) and type of aggregates (form). In some soils, different kinds of aggregates may be found together and they are then described separately. The following paragraphs will briefly explain the various terms which are most commonly used to describe soil structure. This will help you to judge better the quality of the soil where you plan to construct fish-ponds. It will also enable you to learn how to define the soil structure yourself when examining a soil profile. For descriptions of soil structures, see Table 17A.
Note: the characteristic structure of a soil can be recognized best when it is dry or only slightly moist. When you are studying a soil profile to determine the grade of structure, make sure you examine a fresh profile.
By definition, the grade of structure is the degree of
aggregation, expressing the differential between cohesion*
within aggregates and adhesion* between aggregates. As
these properties vary with the moisture content of the
soil, grade of structure should be determined when the
soil is neither unusually moist nor unusually dry. There
are four major grades of structure rated from 0 to 3 as
1 Weak structure is poorly formed from indistinct aggregates that can barely be observed in place. When removed from the profile, the soil material breaks down into a mixture of very few entire aggregates, many broken aggregates and much unaggregated material;
2 Moderate structure is well formed from distinct aggregates that are moderately durable and evident but not distinct in undisturbed soil. When removed from the profile, the soil material breaks down into a mixture of many distinct entire aggregates, some broken aggregates and little unaggregated material;
3 Strong structure is well formed from distinct aggregates that are durable and quite evident in undisturbed soil. When removed from the profile, the soil material consists very largely of entire aggregates and includes few broken ones and little or no non-aggregated material.
By definition, class of structure describes the average size of individual aggregates. Usually, five distinct classes may be recognized in relation to the type of soil structure from which they come. They are:
By definition, type of structure describes the form or shape of individual aggregates. Generally, soil technicians recognize seven types of soil structure, but here only four types are used. They are rated from 1 to 4 as follows: