adminEvans S Zininga Harare Bureau
There are a number of various forces of soil compression by agricultural equipment, nature, animals and even rainfall, among other things, that can causes soil particles to become compacted closer together into a smaller volume.
As these particles are compressed together, the space between particles (pore space) is reduced, thereby reducing the space available in the soil for air and water.
This force causes the crushing of soil aggregates, which destroys the general soil structure and texture. In a nutshell, this condition, factor and/or scenario qualify to describe or explain SOIL COMPACTION! It is indeed, an enemy to plant and crop production.
Most Zimbabwean soils are composed of about 50 percent solids (sand, silt, clay and organic matter) and about 50 percent pore spaces. Of all the results of soil compaction, they will all zero in on depressed crop yield.
Across the broad agriculture platform, from field crops, plantations, grasslands, velds and grazing lands — the destroying effects of soil compaction yield just as bad results.
It is however unknown to most farmers due to stagnancy in information spreading and sharing amongst farmers and technical knowhow administering agencies across the whole country.
Effects of soil compaction
Soil compaction can have a number of negative effects on soil quality and crop production — These are the most common and pronounced ones.
Causes soil pore spaces to become smaller slowing down or stopping, completely, air movement.
- Reduces water infiltration rate into the soil.
- Decreases the rate that water will penetrate into the soil root zone and subsoil. This disturbs the uptake of nutrients by plants, delaying growth and development.
- Increases the potential for surface water logging, water runoff, and surface soil water logging and soil erosion.
- Reduces the ability of a soil to hold water and air, which are necessary for plant root growth and function.
- Reduces crop emergence as a result of soil crusting, germination rates and plant populations are affected.
- Impedes root growth and limits the volume of soil explored by roots.
- Limits soil exploration by roots and decreases the ability of crops to take up nutrients and water efficiently from soil.(Translocation)
- Above all the effects — outrightly there is a massive drawback on crop yield potential.
In wetter than normal years, soil compaction can decrease soil aeration and lead to the increased loss of nitrate nitrogen by denitrification, which is the conversion of plant available nitrate-nitrogen into gaseous nitrogen forms that are lost to the atmosphere.
This process occurs when soils are in an anaerobic condition and soil pores are mostly filled with water. Reduced soil aeration can affect root growth and function, and lead to increased risk of crop disease. All these factors result in increased crop stress and yield loss.
Types of soil compaction
Identifying types of soil compaction
Soil compaction can occur at the soil surface in the form of soil crusting, or it can occur in the subsoil. Soil compaction is sometimes blamed for reduced crop productivity, but it is important to correctly diagnose the cause or causes of reduced crop production. Poor plant growth can be caused by a number of factors, including soil compaction.
The first step is to correctly diagnose if a soil compaction problem exists and then, develop short and long-term management practices to prevent further damage.
Soil compaction can occur at different times of the year through different mechanisms. Careful observations can help diagnose the problem:
- Is there poor crop growth in all years, with all crop types in the same area of the field?
- Is there a spatial pattern to the crop growth (associated with wheel tracks, windrows, equipment widths, haul trails)?
- Does the soil surface appear smooth and crusted?
- Has there been a change in equipment size, weight or operations?
- Are there soil types in the field with naturally dense horizons such as eroded knolls?
- If you scrape away the surface soil with a shovel or trowel, can you see dense layers and/or horizontal root growth’
Soil tillage that removes the protective residue from the soil surface, leaving the soil prone to natural environmental forces or excessive soil tillage that causes surface soil aggregates to break down or degrade, can lead to soil crusting, causing the surface soil layer to become hard and compacted. Soil tillage implements can induce soil compaction just below the depth of tillage, particularly when soils are wet.
The weight of large farm equipment (tractors, seed carts, combines, trucks, manure spreaders) can cause wheel traffic compaction to a considerable depth within the root zone. As soil moisture content increases, so too does the depth of soil compaction.
Surface Soil Crusting
Compaction by combination of soil tillage and raindrop or irrigation water impact
Causes – Soil tillage can bury much of the protective residue cover on the soil surface and degrade the granular structure of surface soils (mechanical crushing or breaking of larger soil aggregates). The impact energy of rainfall or irrigation droplets can also cause considerable degradation and breakdown of soil aggregates, causing soil particles to become suspended in water, flow together and then dry into a hard surface soil crust. The crusted soil can restrict water infiltration into soil and emergence of germinating crops.
Correction and Prevention — A short-term emergence solution to soil crusting after seeding might be a light harrowing or rolling with packers to gently fracture the soil crust after seeding, to aid in seedling emergence through the crust.
SUBSURFACE COMPACTION
Hardpan tillage-induced compaction
Causes – A tillage-induced compaction layer is sometimes referred to as a “hardpan’’, and occurs in the layer of soil just below the depth of tillage. It occurs when soils are cultivated repeatedly at the same depth. The weight of the tillage equipment, such as discs or cultivators can cause compression of the soil and smearing at the base of contact between the soil and tillage implement. Compaction will increase when soil moisture conditions are wet at the time of tillage and/or if soils have a higher silt and clay content.
Correction and Prevention — It has generally been assumed that compacted soil layers break down naturally with annual freeze-thaw and wetting-drying cycles. When compacted soil persists, the soil may need some form of tillage to physically break up the hardpan.
When soils are relatively dry, use heavy-duty cultivators with spikes to penetrate just below the hardpan to fracture and break it up. Generally, this tillage should take place in the dry season when soils are dry; however, great care is needed to maintain residue on the soil surface to prevent soil erosion and not intermix subsoil with topsoil.
WHEEL TRAFFIC-INDUCED COMPACTION
Causes — Heavy farm equipment, including tractors, Scotch carts, combines, trucks, Vicons Spreaders and wheels of Centre pivot irrigation systems, can exert considerable weight onto the soil surface and, consequently, into the subsoil. The effect of equipment weight can penetrate down to 60 cm when soils are moist.
Correction and Prevention — Wheel traffic-induced compaction can be managed by using good agronomic practices, deep tillage or a combination of both. Ideally, it is best to use agronomic practices both to prevent and correct wheel traffic compaction. A good agronomic option is to plant a deep-rooted crop, such as Alfalfa, to penetrate a compacted soil layer and utilize natural wetting-drying and freeze-thaw cycles to mellow the soil.
OTHER SYSTEMS OF CONTROLLING SOIL COMPACTION
CONTROLLED DEEP TILLAGE
Some rippers cause greater mixing of surface soil with subsoil, which results in the deterioration of soil structure, reduction in soil organic matter, reduced soil fertility and increased potential for surface soil crusting. These conditions can be much worse than minor soil compaction problems, so there is a serious need to control tillage depth in our fields.
DIRECT SEEDING
It is best to manage soils and crop rotations to prevent the development of compacted soils by implementing basic soil conservation practices. Direct seeding will eliminate the need for soil cultivation, which is the primary cause for development of a hardpan. Direct seeding will also reduce the amount of wheel traffic and, therefore, the amount of wheel traffic compaction on soils.
CROP ROTATIONS AND MANAGEMENT
Use diverse crop rotations, which include forage, cereal, oilseed and legume crops that vary in rooting depth and type (fibrous versus taproot).Good cropping practices promote plant roots to grow through and break up compacted soils. They increase soil organic matter and improve soil structure, water infiltration and penetration into soil. They also promote biological diversity.
SUMMARY
Soil compaction is not considered a widespread, serious problem in Zimbabwe; however, it can be a serious and unnecessary form of soil degradation. Preventing soil compaction is far better than trying to correct a compaction problem after it occurs. A number of management options can be implemented to minimize the risk of soil compaction. The practices expressed above, if maintained and followed amicably, can curb, control and eliminate soil compaction from your lands.
- Evans Zininga is an Agronomist by profession. He sits on the Board of Animal Farm Consultancy (Pvt) Ltd; he is a past national board member of SOFECSA-A University of Zimbabwe Innovation Platform for Farmers in Zimbabwe. He writes for the Farmer’s Magazine – a column called Agri Business Forum. He brings a lot of experience in Research, Development and Project Management. He can be contacted on 04 -2934147, 0731 892 673 or [email protected]
derick.khumalo