Physical and mechanical properties of soil

Hardness

Hardness — the property of soil under natural conditions to resist the action of wedging forces. Hardness is affected by moisture, aggregation, and particle-size distribution. As soil dries, hardness increases. High hardness negatively affects plant root growth, increases energy costs for tillage, and increases wear of machine working elements.

The unit of measurement of soil hardness is N/cm² or kg/cm². To determine soil hardness, first use penetrometers to measure the resistance force of soil to vertical penetration of a device tip of various shapes (plunger, cone, ball, cylinder), and then divide this force by the cross-sectional area of the penetrating body.

Chernozems and aggregated soils have the lowest hardness. The optimal hardness of chernozems at moisture 0.7 LMC for cereal crops is 7–9.9 kg/cm², for maize — 5.2–7.2, for potato — up to 5 kg/cm².

With severe compaction during drying and an increase in hardness to critical values above 10 kg/cm² for cereal crops, root growth decreases and plant energy costs for overcoming soil resistance increase.

Cohesion

Cohesion of soils — the property of soil to resist loosening action. Heavy clay soils and solonetzes in a dry state have the greatest cohesion, which manifests itself in poor crumbling, cloddiness, and increased energy costs for tillage. In a moist state, these soils strongly adhere to machine working elements. Light and well-structured soils have the least cohesion, which allows them to be tilled over a wide moisture range.

Cohesion increases resistance to erosion.

Plasticity

Plasticity of soils — the ability of soil in a moist state to change and retain its shape under the action of external forces and to deform without crack formation. Plasticity depends on particle-size distribution, composition of the colloidal fraction and exchangeable cations, and humus content. Plasticity manifests itself within a certain soil moisture interval. The upper plasticity limit is determined by the lower liquid limit.

The lower plasticity limit manifests itself at the moisture content at which soil passes from a semi-solid to a viscoplastic consistency, for example, when it can be rolled into a rope. The difference between the upper and lower plasticity limits is measured by the plasticity index, equal for sandy loam soils from 0 to 7, for loamy soils 7–17, and for clayey soils more than 17. The most plastic soils include clayey, loamy, and solonetzic soils.

Stickiness

Stickiness of soils characterizes the ability of soil particles to stick together and adhere to working elements and wheels of machines. It manifests itself when a certain soil moisture level is exceeded.

Stickiness is measured in N/cm². To determine soil stickiness, the force required to detach a steel plate stuck to soil is divided by the adhesion area.

Stickiness depends on soil moisture and dispersion. At constant normal pressure, stickiness increases with increasing soil moisture to a maximum value, and then decreases as a result of increasing thickness of water films on the adhesion surface. As soil dispersion (pulverization) increases, sticking of implements increases.

Clayey soils have the greatest stickiness. In pulverized, i.e., structureless, soil, stickiness begins to manifest itself at relative moisture 40–50%, whereas in structured soil — at 60–70%. Therefore, it is necessary to preserve and restore soil structure, which creates optimal conditions fertility and reduces sticking of implements.

Measures aimed at increasing fertility and aggregation help reduce stickiness: application of organic fertilizers, lime application or gypsum application, drainage of waterlogged areas, coating working surfaces with polymer materials, use of slatted moldboards on plow bodies, etc.

Physical ripeness

Physical ripeness of soils — the optimal moisture interval for soil tillage at which physical and mechanical properties have the best qualities for technological operations.

For loamy soils, physical ripeness corresponds to 40–60% LMC, for light soils — 40–70% LMC. Due to compaction under the action of heavy machinery, tillage is usually carried out at 60–70% LMC. High quality of tillage at the lowest draft resistance is achieved at moisture 14–18%.

Tillage of dry soil is undesirable because of poor crumbling, severe cloddiness, pulverization, and compaction.

The best loosening quality is achieved at physical ripeness in spring at the depth of harrowing of autumn-plowed land and cultivation 6–10 cm, and for spring plowing — 16–20 cm. Tillage of immature soils increases draft and fuel consumption: on dry soils — due to increased cohesion, and on waterlogged soils — due to increased stickiness.

Moisture determines the choice of tillage implements. Disc and rotary implements are used for tillage of soils with moisture 2–3% higher, and units with sweep, flat-cutting, or chisel-shaped working elements — at lower moisture.

An increase in the speed of machine units, for example, during plowing, to 2.50–3.33 m/s expands the optimum moisture interval, and soil may be tilled at 18–20% LMC without deteriorating crumbling quality.

The optimal moisture corresponding to physical ripeness, at which the compacting effect of heavy agricultural machinery is minimal, for chernozem soils is in the interval 15–24%, for sod-podzolic soils — 12–21%, for gray forest soils — 15–23% LMC.

Sliding friction

The sliding friction of soil against the surface of a working element is called external. It is assessed by the force F of soil resistance to movement over the working surface. This force is proportional to the normal pressure force of soil on the working element:

The coefficient of proportionality f, or coefficient of friction, depends primarily on particle-size distribution and soil moisture. On loose sandy soils, the coefficient of soil against steel varies from 0.25 to 0.35; on cohesive sandy soils — from 0.5 to 0.7; on medium loamy soils — from 0.6 to 0.9.

From a production standpoint, friction during plowing is a negative factor. The friction force on the plowshare-moldboard surface is 30–40% of the total plow resistance.

Several methods are used to reduce the friction force:

• use of vibration and active working elements;
• creation of a boundary layer of water and air along the contact surface between soil and working element;
• polishing moldboards and coating them with various materials;
• changing the geometric shape of working elements;
• replacing sliding of soil by rolling over rollers.

Resistance to deformation

Resistance to deformation characterizes soil strength. During tillage by various working elements, soil undergoes compression, tension, shear, torsion, and combinations thereof. The ultimate strength of soil (before crumbling begins) under different deformation types varies widely. Thus, loamy soil at absolute moisture 21–28% has an ultimate tensile strength of 5–6 kPa, shear strength of 10–12 kPa, and compressive strength of 65–108 kPa. Therefore, loosening soil with minimum energy consumption is possible using working elements that ensure tension of the soil layer.

Soil abrasiveness

Soil abrasiveness is assessed by the content of physical sand with a large amount of rocky inclusions 0.25–3 mm in size, which cause increased abrasion (wear) of working elements.

According to the abrasive wear criterion, soils are divided into groups:

• with low wearing capacity, with sand content up to 80%;
• with medium wearing capacity, with sand content 80–95%;
• with increased wearing capacity, with sand content 95–100%.

Abrasive wear of plowshares when plowing 1 ha of soils of the first group is 2–30 g, of the second group — 30–100 g, of the third — 100–450 g.

Specific resistance

Specific soil resistance is a generalized characteristic of the difficulty of its tillage. The coefficient K_c of specific soil resistance during plowing is determined by measuring the plow draft P and dividing it by the cross-sectional area of the lifted furrow slice:

where

a

— plowing depth, cm;

b

— width of cut of a plow body, cm;

n

— number of plow bodies.

According to specific resistance, soils are divided into:

• light (K_c < 3 N/cm²);
• medium (K_c = 3-5 N/cm²);
• medium-heavy (K_c = 5-7 N/cm²);
• heavy (K_c = 7-12 N/cm²);
• very heavy (K_c > 12 N/cm²).

The specific resistance coefficients of soil during cultivation, harrowing, rolling, and other similar operations are determined by dividing the machine draft by its working width.

Interaction of a wedge with soil

In geometric form, the working elements of a plow and other tillage implements are made as flat or curved wedges. Flat wedges include plowshares, knives, cultivator shovels, and harrow teeth; curved wedges include spherical discs of harrows, stubble cultivators, plow moldboards, and hilling bodies. The wedge shape is also characteristic of seed drill and planter openers.

Flat wedge

Under the action of a flat wedge, soil deformation occurs; its character depends on the technological properties of the soil and the angle α of the working face of the wedge to the horizontal.

Soil deformation by flat (a...d) and curved (e) wedges

<b>Low-cohesion soils</b>. The main type of deformation of low-cohesion soils is shear. When the wedge moves from position I to position II, soil particles a, b (Fig. a) are pressed into the not yet deformed mass and pass into position a’, b’, i.e., the material is compacted. The crushing stress at point a is greater than at point b, since aa’ > bb’. As soon as the crushing stress exceeds the ultimate strength of the soil in shear, a shear plane OA appears ahead of the wedge blade, directed at angle ψ to the furrow bottom, and a prism-like clod OABa’ separates from the slice.

After shearing, the clods slide over the surface of the flat wedge without undergoing new deformations and therefore do not disintegrate. The sizes of the broken clods depend on the slice thickness, i.e., the tillage depth. A thin slice breaks into smaller clods than a thick one.

<b>Medium- and strongly cohesive (loamy and clayey) soils at optimal moisture</b>. At the very beginning of wedge penetration, a crack OC (Fig. b) forms, which widens, and element AOC breaks off from the slice. With further movement (from position I to position II), the wedge first cuts a slice of variable thickness along line OO’ (cleans the furrow bottom), then forms a new crack O’C’ and breaks off the next element of the slice.

<b>Hard and dry soils</b>. The fracture crack propagates downward (Fig. c), so the bottom is uneven, and the broken clod of the slice has an irregular shape.

Heavily sodded and wet loamy soils are torn by the wedge along the line of blade movement. Cracks arising during bending of the slice do not reach the surface; thus, the slice does not separate into individual elements and represents a continuous ribbon (Fig. d).

Dihedral wedge

Depending on the direction of movement and the position of the blade relative to the horizontal and vertical planes, the character of the dihedral wedge action on soil changes.

Interaction of dihedral (I-III) and trihedral (IV) wedges with soil

A dihedral wedge with angle α (Fig. I) separates the slice from the furrow bottom, lifts it, compresses it in the vertical plane, and splits it into individual clods.

A dihedral wedge with angle γ (Fig. II) separates the slice from the furrow wall, moves it aside, and compresses it in the horizontal plane.

When wedges with angles α and γ act simultaneously, the slice is broken in two directions. Further crumbling of the broken pieces as they move over the wedge surfaces ceases, since angles α and γ are constant. For more intensive crumbling of the slice, a series of simple wedges with gradually increasing angles α and γ is installed one after another, i.e., the simple flat wedge is replaced by a curved one.

A dihedral wedge with angle β (Fig. III) tilts the slice to the side. However, to turn the slice from a horizontal to an inclined position, not one but many wedges arranged one after another with angle β increasing from 0 to 90° are needed. To turn the slice over, the angle must be more than 90°.

References

1. Bazdyrev G. I., Loshakov V. G., Puponin A. I. et al. Agriculture. Textbook for universities. Moscow: Kolos Publishing House. 2000.

2. Niklyaev V. S. Fundamentals of Agricultural Production Technology. Agriculture and Crop Production. Moscow: Bylina. 2000.

3. Evtefeev Yu. V., Kazantsev G. M. Fundamentals of Agronomy: textbook. Moscow: FORUM. 2013.

4. Khalansky V. M., Gorbachev I. V. Agricultural Machines. Moscow: KolosS. 2004.