How to Troubleshoot Compacted Soil

Compacted soil is one of the problems that considerably reduces productivity, but which can also be solved simply. To do this, you just need to know what the causes are and when is the best time to carry out the unpacking.

Soil is a natural body that has several definitions, depending on the area of ​​knowledge that studies it, however, for the area of ​​agricultural sciences the best definition is that it is a three-phase natural body, that is, it is made up of liquid and solid phases. and gaseous, formed by mineral and organic materials containing living matter that can be vegetated in nature (Embrapa, 2006).

The soil constitution close to ideal would be 5% organic matter, 45% mineral material, 25% water and 25% air. The liquid phase and gaseous phase are competitors, when one increases, the other decreases, therefore, although for the plant it is interesting that the soil is always at field capacity, it is important that the gaseous phase is close to 25%, because below 15% the plants begin to have oxygen deficiency and, below 10%, they do not develop.

The soil morphologically has four characteristics to be evaluated (color, texture, structure and consistency), however, in the study of compaction what matters is the basic knowledge of structure and consistency.

Soil structure is the aggregation of primary soil particles, which are separated from neighboring aggregates by fracture surfaces, being divided into macrostructure and microstructure.

However, in the study of compaction the essential part of knowledge about structure is soil porosity. Between the aggregates there are porous spaces, which are divided into micropores (porosity within the aggregates <0,05mm) and macropores (porosity between the aggregates <0,05mm). 

The soil's gaseous phase is located within the macropores. The greater the number of macropores, the greater its capacity to carry out gas exchange and the lower the water retention. The micropores are where the liquid phase of the soil is stored, and the greater the number of micropores, the greater the soil's water retention. It should be noted that sandy soils have a greater number of macropores, that is, they have a lower water retention capacity and a greater gas exchange capacity. Clay soils have a greater number of micropores and total porosity (total porosity is the sum of macropores and micropores), having a greater water retention capacity and a lower capacity for gas exchange.

Consistency is understood as the manifestation of the forces of cohesion (attraction of particles to each other) and adhesion (attraction of particles to another body), present in the soil, the action of these forces occurs according to the humidity. According to the amount of water in the soil, the consistency can be classified as tenacious, friable, plastic, sticky and fluid.

When the soil has a tenacious consistency, resistance to the rupture of the aggregates is found; when it is friable, the soil structure is easily crumbled; in plasticity, the soil is easily molded; in stickiness, the soil has a high adhesion to other bodies. and finally in the fluidity there is more water than soil, with neither of the two forces (cohesion and adhesion) acting at the moment.

When preparing the soil with a tractor in conditions of high humidity (plasticity, stickiness and fluidity), physical damage to the soil structure (future compaction) and greater adherence to agricultural equipment may occur, especially in clayey soils, up to the point of making the desired operation unfeasible, as in addition to requiring greater tractor power, it will also consume greater fuel. At this consistency, the soil can be molded, but it does not return to its original condition, the micropores are filled, accelerating the process of degradation and compaction.

The soil with excess water after the machine has been used has a glossy and flat appearance, this is due to the filling of the pores, when it dries it will form a more resistant crust, which can eventually evolve into a compacted layer. Furthermore, the adhesion force of the soil is high, so soil particles tend to adhere to other objects, causing these particles to adhere very easily to equipment, requiring greater tractor force to pull this equipment. , when it does not make the activity completely unfeasible.

The identification of the tenacious consistency in the soil is similar to the previous ones, however, the sample does not shape itself or when it is in the form of a clod, its cohesion strength is so high that a greater amount of force is needed to break up this aggregate.

Preparing the area with a lack of moisture, tenacious consistency, does not immediately cause any physical damage to the structure, however, it is necessary to pass a greater number of passes with the machine over the soil to leave it broken up for sowing. This increases fuel costs and can also lead to the appearance of compacted layers, however, not as quickly as when soil preparation is carried out with excess moisture.

Preparation with implements that cause high disintegration (rotary hoe) can destroy the soil structure, superficially, pulverizing it and facilitating the wind and water erosion process. In the case of the second, soil particles are carried by water in the leaching process, filling the soil's micropores and contributing to its compaction.

Another factor when working with soil in tenacity is that, due to the soil's cohesion strength being very high, the soil is very hard, so the equipment is often unable to penetrate it. As a result, the equipment practically only scratches the ground, making the operation almost unfeasible due to the number of passes required to prepare the area.

COMPACTION

Using this information, the term compaction can be defined as the increase in soil density in the same volume with the reduction of its porosity that occurs when it is subjected to great effort or continuous artificial pressure, that is, when it is through the incorrect use of soil by man, whether through the incorrect use of agricultural equipment or poor sizing in animal husbandry.

Compacted layers are mistakenly called densified, there is a difference between compaction and densification. Compaction is something done by man or animals as a result of inadequate management. Densification is a natural process, the dense layers arise from soil pedogenetic processes (addition, loss, translocation and transformation). It is a non-anthropogenic process that takes years to occur, such as the soil wetting and drying cycle, the natural translocation of clay between the profile, the freezing and thawing cycle in cold areas, among other situations. Another factor that differentiates both is the difficulty of correction, with dense layers being easier to correct than compacted layers.

Among the soil classes, those considered clayey, that is, with a higher percentage of clay and clay with high activity, are the most susceptible to compaction. This occurs, firstly, due to the specific surface of the clay, which is very high, and secondly, because clays with high activity are more expansive than those with low activity, that is, when you drive a tractor over clay soil, it will expand. Furthermore, when it dries it will not retract, as it was “crushed” by the equipment. Furthermore, clayey soils have greater total porosity and microporosity, making them easier to fill than macropores.

The consistency of the soil to enter with agricultural equipment is friability, as plasticity (excess moisture) worsens the compaction process. However, this goes against the ideal situation for plants, generally irrigating the soil. Therefore, if you are going to carry out any activity in the area, it is recommended to stop irrigation so that the soil becomes friable and then, from there, enter the area with the machine.

PROBLEMS IN COMPACTED AREAS

Soils with compacted layers have greater resistance to root penetration, which can grow deeper growing laterally or even close to the surface. As a result, there is also a reduction in the volume of the root system, as it was reduced by the compacted layer, which is terrible for tubers whose development is compromised.

As the compacted layer has its pores filled, rainwater or irrigation does not infiltrate correctly into the soil profile. It infiltrates until it reaches the compacted layer and then flows superficially and, as a result, there are water deficit problems for crops.

Another problem is water erosion, caused by surface runoff of water that cannot penetrate, taking part of the soil with it and reducing its fertility, since the material carried is generally organic matter and the A horizon, which are the richer in nutrients in the soil.

Once surface runoff has passed, water tends to stagnate at certain points in the soil, as it cannot infiltrate. As a result, there are problems with soil aeration, as the macropores, the pores where most of the gas exchange occurs, will be saturated with water. With stagnant water and low aeration, the biological activity of the area will decrease, considering that with the lack of oxygen in the soil, aerobic organisms (their respiration only occurs in the presence of oxygen), responsible for the decomposition of organic material, will give place to anaerobic organisms (breathing in the absence of oxygen).

Due to the reduction in fertility, aeration, biological activity, area explored by the root system, among the other problems mentioned above, vegetation will tend to have a reduction in its production, reducing the profits arising from the activity. Another problem is the increase in fuel consumption in the area, since, in addition to periodic preparation, it will be necessary to correct areas with compacted layers, increasing the number of passes in the area and consequently fuel consumption.

COMPACTING AGENTS

Compacting agents include agricultural equipment, tractors and overcrowded animals. The animal, more than the tractor, if we consider pressure exerted per unit area with the same weight, that is, if the tractor had the same weight as the animal, it, the tractor, would compact less, since the wheel-ground interface is greater than the animal's paw-soil interface, causing the pressure exerted on a point on the ground to be lower, as it is distributed over a larger area. Still comparing the two, the compaction exerted by the animal is superficial, easier to correct, whereas the tractor, due to its weight and misuse, can cause compacted layers at greater depths, which is more difficult to correct, with the need of using specific equipment that demands a greater energy requirement from the tractor and, consequently, greater fuel consumption.

Comparing types of wheelsets (track and tire), track wheelsets compact much less than tire wheelsets, as their interface with the ground is greater, which distributes the pressure over larger areas of soil, unlike tire wheelsets. tires that focus on four points. Another factor that differentiates them is that tracked wheels compact more superficially when compared to wheeled tractors.

Comparing tire wheelsets, wider wheelsets will have a greater tire-ground interface than narrow wheelsets, being less harmful to the ground.

COMPACTION PREVENTION

Some actions help prevent soil compaction, minimizing or even preventing the occurrence of this problem. One of them is to ensure that the soil is at the point of season, that is, friability, which is the ideal point of humidity whenever you prepare it.

The addition of organic matter is always welcome, in this situation even more so, since with organic matter on the surface there is an improvement in soil structure, improving total porosity and reducing erosion processes, among other benefits inherent to organic matter.

The evolution towards a conservationist system, such as minimum cultivation and direct planting systems, is a good solution to try to avoid compaction, as equipment will no longer be used for soil preparation, such as plows and harrows which, when misused, contribute greatly to compaction. Through conservation systems, organic matter will be added to the surface and machine traffic and soil mobilization will decrease in the area.

Alternating soil preparation equipment and working depth also prevents compaction. Every time you work with equipment such as a plow and harrow at a certain point, a filled glossy surface is formed. If the soil is always prepared at the same depth for several production cycles, this surface will evolve into a compacted layer. So, it is best to always carry out preparations at different depths and, if possible, rotate crops with different root systems, some shallower and others deeper.

Crop rotation is of great importance to work the area at different depths and have a root system at different depths. Among these crops in the rotation, it is good to include decompacting crops, these are crops that have a strong pivoting root system that tends to reduce the resistance of compacted layers. It is also necessary to include crops in which green manure is applied and the subsequent formation of straw, thus accumulating organic matter on the surface. If it is not possible to travel in friable conditions, the least immediately harmful situation for the soil is in dry soil conditions, but this does not mean that it will not compact the soil.

Driving the machine in the area is another factor to avoid compaction, the faster the work is done, the shorter the time the tractor will spend on the ground, thus the shorter the time it takes for this tractor to compress the area. It is also recommended that tractors be used that are compatible with the activity to be carried out and that they are suitable so that they have sufficient ballast to carry out the operation, with the ideal ballast for that type of soil and operation.

Finally, it is interesting that all areas are divided into sub-areas and between these smaller areas predefined spaces are placed for carrying out maneuvers (carriers), thus reducing the number of passes with the tractor over the production area.

How to quickly identify soil consistency

To enter any equipment into the ground, the ideal consistency is friability, where the adhesion and cohesion forces are practically equal, with no excessive interference from one to the other. In this condition, the operator can operate with minimal tractor effort, giving better results in the services performed, this point is also known as the soil saturation point.

To check whether the soil is friable, you must take a portion of soil and try to mold it. If it is molded easily and also falls apart easily after the pressure stops, the soil is friable.

The method of identifying plasticity or stickiness is similar to friability, differentiating the result; it is possible to mold the soil, but it does not fall apart, or the soil can be molded, but when the pressure is removed it remains adhered to the fingers.

Soil compaction assessment methods

The first and most accurate method consists of soil analyzes in the laboratory. You must go to the field to collect undisturbed samples at different depths, generally at the depths at which the root system of the crop of interest will operate. This sample will have a known volume and will undergo physical analysis to find density, macroporosity, microporosity and total porosity.

With the values ​​in hand, the density of the sample should be checked. If the soil is sandy and the density is between 1,2-1,4kg/dm³, the soil will be at the limit, above these levels it can be considered compacted. For clayey soils, the value is 1-1,2kg/dm³.

This method is field and consists of opening a trench in the area where it is suspected to be compacted. With a sharp object (knife, box cutter, etc.) you must pierce the soil until you find it difficult to penetrate. When you encounter greater resistance, you must mark the area and measure the actual depth at which it is located.

Another important factor is the response of the plant and soil to compaction, these factors are checked visually. Among the symptoms observed, we can mention the slow emergence of seedlings, plants of varying size, plants with poor color, shallow, curled and tortuous root system, soil with surface crusting, presence of water puddles, erosion and increased need for tractor power.

Of the existing electronic equipment, the most used is the penetrometer, it does not measure compaction directly but rather the resistance to penetration to which that soil subjects any body that wants to drill into it. It indirectly measures compaction because it does not have a penetration resistance value associated with compaction. The root system begins to find it difficult to penetrate when resistance values ​​are between 2MPa and 3MPa.

Deivielison Ximenes Siqueira Macedo, Leonardo de Almeida Monteiro, Viviane Castro dos Santos, Carlos Alessando Chioderoli, Enio Costa, Maria Albertina Monteiro dos Reis, Renata Fernandes de Queiroz, UFC

Article published in issue 152 of Cultivar Máquinas.