Soil texture is an approximation of the relative quantities of sand, silt and clay particles in a soil. Soil structure is a measure of the arrangement of these soil particles and the spaces between them.
Soil structure is somewhat dependent on soil texture. Good soil structure is present when the soil forms stable aggregates or cohesive groups of particles. This produces numerous pore spaces, which encourage root penetration and easy passage of water, nutrients and air and which also assist the growth of micro-organisms.
There are two main types of structureless or non-structured soil:-
a) single grain _ like sands, and
b) massive _ like compacted clays.
There are four main types of soil structure:-
a) crumb structure _ which has small rounded aggregates of soil particles loosely adjoining other aggregates. This soil is therefore porous and permeable, yet retains moisture. It is the most ideal soil structure.
b) prismatic structure _ which forms aggregates in columns with flat tops and separated by deep cracks. Aggregates usually form larger units called clods. Permeability is variable _ better around the deep cracks and poorer inside columns.
c) blocky structure _ aggregates are blocky and soil is moderately permeable.
d) platey structure _ flat horizontal laminated aggregates like alluvial floodplain soil. Drainage and permeability are poor.
Good soil structure is one of the major factors for soil health and therefore, sustainable soil fertility. Sustainable soil fertility can be defined as the correct balance of chemical, organic, biological and structural conditions within the soil that will produce high yields over an extended time frame, using minimal inputs.
Soil texture is not easily changed whereas soil structure can degrade or improve very quickly through various agricultural practices. The tendency of the soil structure to become unstable is related to soil type, texture (finer texture - higher tendency), water content and soil chemistry. Some soil chemistry factors that adversely affect soil structure include soil sodicity, acidity and salinity. The decline of soil structure will exacerbate the decline in soil health and fertility. Soil pores become smaller or less numerous which restricts water, air and nutrient movement. Therefore porosity, drainage and plant root growth are reduced. This can lead to an increase in either soil density or structural instability or both, especially in clayey soils. Sometimes a surface crust may form, inhibiting seedling growth, preventing water penetration and increasing erosion.
The good news is that all these changes are reversible. The actions required to improve soil structure depend on the individual soil conditions including stability of the soil structure. There are various options to improve soil structure, including some physical and chemical techniques such as:-
a) maintaining continued plant cover on land by using appropriate stocking rates;
b) use of cultivating discs to elevate dispersive subsoils to the surface;
c) deep ripping of compacted soils or layers;
d) use of minimum tillage practices especially when soil is wet;
e) stubble retention and green manuring to increase organic content and reduce compaction and erosion;
f) use of gypsum on sodic soils.
FARM MONITORING HANDBOOK by Hunt, N and Gilkes, B. (1992) Farm Monitoring Handbook. The University of Western Australia. (contact: firstname.lastname@example.org)