1. Soil Structure

2. Soil Structure
In many soils the individual particles of soil are quite separate as in sand. In other soils the particles join together to form lumps or peds which have very distinct shapes. The way soil particles clump together to form these peds determines the structure of the soil.

3. The structure of a soil is important because if a soil has good structure, water can pass slowly down through the soil profile and air can get into the spaces between the peds.

4. Structure can be changed by agricultural processes
Structure can be changed by agricultural processes. Poor structure indicates that something is wrong with a soil, e.g a platey structure indicates that a soil has been compacted and is often found around gateways and lanes. It is difficult to establish and maintain a healthy pasture in these areas

6. Good soil structure Allows easy water movement through the soil.
Allows easy gas flow through the soil and avoids anaerobic conditions ( those where there is no air).
Allows for easy root penetration and firmly rooted plants.
Makes cultivation and the production of a tilth easier.
Takes time to develop.

7. Soils that usually have good structures are those found under pasture
Soils that usually have good structures are those found under pasture. Poor structure develops when soils are over cropped. The small crumbs are bashed by the heavy machines. The gummed joints holding individual grains of sand or silt or clay, are weakened and broken. This results in soil that looks like flour. Water and air cannot penetrate easily. Soils in this condition are easily eroded by the wind. However, if the soil is sown down in pasture for a few years soil structure will begin to improve.

8. Soils with good structure will warm quickly in spring
Soils with good structure will warm quickly in spring. This is due to having less water to warm up and the fact that warm air can move into the soil.

9. Ways in which Soil Structure can be improved
Man cannot build soil structure directly, but he can enhance it.
Till (cultivate) soils only at the proper moisture contents. Never till when the soil is too wet. This can cause the soil to become cloddy.

10. Add the proper amounts of lime
Add the proper amounts of lime. Without the proper amounts of lime present many beneficial soil organisms cannot grow and help form soil structure.
Grow grass and legumes that contribute organic matter to the soil. This organic matter can stabilise unstable aggregates.
Turn under crop residues.

11. Formation of Soil Structure
Soil structure formation takes place as a result of three distinct processes:
Chemical Processes
Some soil particles (clay and humus) have a negative electrical charge. They ‘attract’ other soil particles that have a positive electrical charge. In this way, separate soil particles can be bought together to form aggregates.

12. Microscopic Processes
Clay and humus can coat sand and silt particles, and acting like a ‘glue’, bind sand and silt together to form aggregates

13. Macroscopic Factors
The roots of plants and parts of soil fungi can hold small aggregates together. They achieve this by wrapping around the aggregates and binding them into larger aggregates.

14. Development of Soil Structure
At any given time, the structure of soil is in various stages of development. A soil can have either a strongly, moderately or weakly developed structure:
Weakly developed- soil breaks easily, few complete aggregates.

15. Moderately developed- soil harder to break, a large number of complete aggregates and a little unaggregated material.
Strongly developed- soil breaks cleanly into complete aggregates with no broken aggregates and unaggregated material.

16. Questions 1. Soil Structure describes: components of a soil
arrangement of soil components
the size of soil components.
2. Which of the following is influenced by soil structure?
soil component size
water content
air content.
3. A soil’s structural type is determined by
its colour
its texture class
the dominant shape of aggregates in the soil.

17. 4. The best soil structure for growing plants is
Spheroidal
Prism
plate.
5. Plate structure is often found in
a. soils used in forestry
b. lower horizons
c. sandy soils.
6. The best soil for growing plants would have a
a. weakly developed structure
b. moderately developed structure
c. strongly developed structure.

18. 7. Good soil structure aids the growth of plants by supplying them with adequate
minerals and micro-organisms
air and water
sand, silt and clay
lime and humus.
8. Care should be taken when using cultivating machinery on wet soils because it may affect soil
a. minerals b. structure
c. texture d. temperature
9. The presence of a clay pan in a soil can
a. cause water-logging above the pan
b. cause soil acidity levels to increase
c. improve root penetration through the soil
d. prevent erosion of the topsoil.

19. 10.Soil air is important to plants because
a. their roots require oxygen for respiration
b. it increase the availability of minerals
c. it contains a very small amount of carbon dioxide
d. roots need space to grow.