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One cannot harvest where no seed was planted! This is logic. But what do you do when the heavy implements used to till the lands is actually the cause of a smaller harvest; when the combined weight of tractors and implements is the cause of the soil being compressed and thus having a negative influence on plant growth?
Martiens du Plessis, NWK soil scientist, has a recipe to manage this problem.
The mechanics of soil compaction
The total volume of soil in its natural state consists of just below 50% pores and slightly more than 50% mineral particles. The pores are filled with air and water. Pores are divided into three groups based on their size – namely macropores, mesopores and micropores. When soil is compressed mechanically, it becomes compacted. This results in various changes in the physical characteristics of the soil.
In the process of soil compaction the total pore volume decrease; the number of macropores decrease and the mesopores increase. It is precisely the macropores that are important for water conduction, root development and gas exchange. Compaction results in delayed root development, water conduction and air movement.
When soil is compacted, the gross density of the soil is increased because more mineral particles per volume unit are compressed, resulting in less pore volume remaining. Soil with a higher gross density requires more energy to be transformed such as when a plant root grows through it. Soil with an abnormally high gross density has to be tilled to loosen it.
Reasons for soil compaction
Agricultural implements are the main reason for soil compaction, tractor wheels being the main single cause. Other implements, such as harvester combines and fully laden transfer trailers are secondary causes. The least soil compaction is caused by soil cultivating implements. Certain soil characteristics has an influence on the compaction potential of soil. The two most important aspects are distribution of particle size and water content.
There are major cohesion and internal friction forces between particles in dry soil. The soil can then accommodate heavier loads and it presents resistance against deformation and consequently compaction. When soil is saturated with water, pores are filled with water and compaction is thwarted. Many soil types transform easily because the cohesion forces and internal friction between soil particles are very low and the soil particles easily glide over others with the result that the load carrying capacity is also low. Soil compacts easiest between dry and wet extremes.
The volumetric content of sandy soil where it is most prone to compaction, is approximately 14%, while clayish soil compacts easiest at approximately 30%. Most soil tilling operations are undertaken during this optimal water content period.
Where there is a compaction zone in the soil profile, a distinction is made between ploughsole compaction and a sub-ploughsole compaction. Ploughsole compaction takes place in the cultivated zone and is normally within the top 250 mm of the profile. It is normally caused by land traffic during secondary tillage such as seedbed preparation, cultivator work, fertiliser spreading and planting.
’n Sub-ploughsole compaction forms directly below the normal tilling depth, namely 250 mm to sometimes more than 600 mm. Keep in mind that a tractor wheel moving over soil can compact sandy soil up to 400 mm deep resulting in root limitation. Transformation of the soil takes place up to a depth of 700 mm, but not together with root limitation at that depth. Here the main culprits are tractor wheels which run in the open furrow and causes the compaction from ploughing depth (normally about 250 mm) to more than 600 mm.
It can be expected that tractor wheels moving on the topsoil can compact soil over time up to 350 mm and even 400 mm deep. When this soil is tilled up to 250 mm there still remains a section of the compacted soil which is, technically speaking, a subsoil ploughsole compaction.
The influence of soil compaction on root development
When a plant root grows through soil, the soil particles are physically pushed out of the way and into adjoining pores. When the soil is compacted and the pores are not big enough to receive the soil particles, the roots are forced to slower and laborious growth. When the soil’s mechanical resistance against transformation is higher than the inherent force of the root to push soil particles out of its way, root growth will stop completely and the roots will transform to thickened ends in the compaction zone.
Usually the development of side roots is also limited detrimentally by compaction. A root that does grow through the compacted zone will typically form few or short side rootlets, or they will be totally absent. Some roots will reach the compacted zone and then turn and grow horizontally.
How is soil compaction identified?
Several techniques are applied to identify soil compaction:
Root studies in profile holes showing unsatisfactory root development, are often the first indication of a compaction zone, but there are also other signs indicative of the problem:
• A practiced hand with a sharp-pointed hammer can often ‘feel’ the compaction zone as a variance in density. The density of the soil is influenced by moisture and the clay content, which hampers the accuracy of this method. However, it is difficult (if possible at all) to identify the bottom limit of the compacted zone.
• A penetrometer is the correct instrument to identify compaction. With this instrument a standardised conical meter is pushed into the soil and the resistance of the soil against penetration is measured electronically or indicated on a mechanical clock-like face. Penetrometer data can be illustrated graphically to indicate the presence of a compacted zone. The lowest limit of the root penetration can be determined relatively accurate from the graph. It is important to wet the soil a few days before such tests are undertaken to the level of veld water capacity.
• A simplified form of the penetrometer is a steel probe which is pushed into the ground slowly by hand. The relative force required to push the probe into the soil will also provide an indication of a possible compacted zone.
Breaking up soil compaction
Compacted layers have to be broken up by mechanical means. Compaction in the plough zone (zero to 250 mm) can be broken up by less aggressive methods such as a heavy duty ripper, share plough or disc plough. The most general method, however, is the use of ripper ploughs because they do not mix the upper and lower soil. A trench plough (Nardi) is often used for this purpose but it has the disadvantage of mixing the upper and lower soil layers, which has a diluent effect on plant nutrients and humus in the topsoil. Tine spacing should not be too wide and a spacing of 70% of the working depth is recommended.
Pro-active management of soil compaction
Compaction can be managed by loosening the soil annually or pro-actively by keeping the growth zone soil loose and not allowing it to compact.
During conventional tilling traffic on lands normally move arbitrarily in different directions across the land and uncontrolled compaction occurs, resulting in the soil having to be tilled all over every year to prepare for the next crop. Secondary tilling and planting direction differ, resulting in planting taking place on compacted tracks.
The other method is a primary loosening of the soil and marking tracks for the new season; then having all land traffic of the season move on those tracks. With this system it is ensured that no planting takes place on compacted tracks.
In this system cultivation is done according to a pre-determined fixed traffic grid. All land traffic is limited to perennial tracks and traffic directions are not changed. These tracks are never cultivated and has the advantage of increasing traction and lowering rolling resistance. The planting zones are initially tilled as a whole and all compaction is broken up with, for example, tine ripper ploughs.
It makes sense not to till the tracks at all from the beginning. Plant zones can then be tilled merely to control weeds and manipulate plant rests so that planting can be done again. This system is ideal in combination with minimum tillage. It is known as sustainable tillage. Soil compaction is detrimental for growing grain, significantly so in the sandy soils of South Africa.
By identifying it correctly and then discontinuing the practice, it can be managed successfully. The ideal is to manage it pro-actively in a permanent traffic grid system.
Article supplied by Martiens du Plessis, GWK.