5. Mechanical properties of mineral soils and their management.

Soil plasticity is one of its mechanical properties. Plastic soil is capable of being molded or deformed continuously and permanently, by relatively moderate pressure, into various shapes. Only wet soil can be plastic, but wet within certain limits, lower and upper. The lower wetness of soil plasticity is close to what is popularly known as soil maturity (зрелость) to tillage. At such as wetness the soil shows its best performance in granulation (formation of grainy aggregates).

Soil consistence is the resistance of the soil to deformation or rupture. It is determined by the cohesive and adhesive properties of the entire soil mass. Consistence is important for tillage and traffic considerations. Dry sand exhibits minimal cohesive and adhesive properties. But it is easily deformed and vehicles can easily get stuck in it. Clay soils may become sticky when wet and thus make hoeing and plowing difficult.

In Ukraine it is customary to speak about soil resistance to tillage. For dry sands it is low and for dry clays - very high (over 1 kg/cm²). When soil is mature for tillage its resistance to plowing is not high. For any loam and clay soil at field maturity to tillage this resistance is near its minimum. Other things being equal, the soils resistance to tillage increases with increasing clay content.

Soil hardness or penetrometer resistance (penetrability) is the ease with which a probe can be pushed in to the soil. In our country it is measured by special tools (penetrometers). The force of pushing (prodding) is divided by the horizontal area of a probe and soil hardness is determined in kgs of force per sq. centimeter. If penetrometer resistance exceeds 80 kg/cm², the roots of plants can penetrate the soil only along the cracks.

Stickiness is the property of a wet soil to stick to other bodies. It is measured in grams per sq. cm. It may be close to zero or higher, reaching sometimes 10-20 and more g/cm².

Some soils rich in clay swell on wetting and shrink on drying, forming cracks through which water evaporates. Illuvial horizons of sodic and saline-sodic soils rich in absorbed sodium become sticky when wet, increase in volume (swell) and decrease in volume (shrink) on drying.

Minimum tillage, chemical amendments, fertilizing systems with sufficient rates of manure or residues and the planting of perennial grasses - all contribute to the improvement of the mechanical properties of soils.

LECTURE SEVEN.

Soil Water and its Management.

Study outline:

1. Soil Water Content and Potential.

2. Soil Water Movement.

3. Plant and Soil Water Relations.

4. Soil Water Regime.

5. Soil Water Management.

1. Soil Water Content and Potential.

The variable amount of water contained in a unit mass or volume of soil and the energy state of water in the soil are important features affecting the growth of plants. Large quantities of water must be applied to satisfy the evapo-transpiration requirements of growing plants. This water must be available when the plants need it and most of it must come from the soil. Water acts as the solvent which together with the dissolved nutrient make up the soil solution.

The movement of water into and within the soil, the moisture – storing capacity of soils and the availability of soil moisture to plants are related directly or indirectly to the size and distribution of soil pores and to the attraction of the soil solids for moisture.

Today the various forms of soil water are classified much simpler than they used to be:

1.Adhesion water.

2.Cohesion water.

3.Gravitational water.

Adhesion water. Water molecules are strongly polar and attract each other through H – bonding. Soil particles have sites both electrically negative and positive. The mutual attraction between water molecules and the surfaces of soil particles results in strong adhesive forces. Water molecules will spread themselves over the surfaces of the soil particles to form a thin film of water. It may be several water molecules thick. It is called adhesion water. It is so strongly adsorbed that it moves little. Adhesion water has the lowest energy level , is the most immobile water & is generally unavailable for use by plant root & microorganisms.

Cohesion Water. The attractive forces of soil particles decrease inversely & logarithmically with distance from the particle surface. Beyond the sphere of strong attraction of soil particles for water molecules of the adhesive water layer , cohesive forces operate between water molecules because of hydrogen bonding. This results in increase in the thickness of a water film. The cohesive water is slightly mobile & generally available to plants. The outermost water molecules have the greatest energy, the greatest energy, the greatest tendency to move & the greatest availability for plant roots. As roots absorb water overtime the water films become thinner, roots encounter water with decreasing energy content or mobility. At some point, water may move so slowly that plants wilt because of the lack of water. If placed in a humid chamber wilted plants fail to recover, the soil is at the permanent wilting point (PWP). Most of the cohesive water has been absorbed.

Field capacity. When soil particles are close to or touch adjacent particles, water films over lap each other. Small interstices & pores become water filled. The pores in field soils that are small enough to remain water filled, after soil wetting & downward drainage of excess water has occurred, are the capillary or microporses. At this point, the soil retains all possible water against gravity (adhesive plus cohesive water) , & the soil & considered to be at field capacity. The aeration or micropores at field capacity are air filled. Good, if a loamy soil at field capacity has a volume composition of about 50% solids, 25% water, & 75% air.

Gravitational water. Soil water that exists in aeration pores, & that is normally removed by drainage because of the force of gravity, is gravitational water. Gravitational water in soil is detrimental when it creates oxygen deficiency. When water-filled macropores are allowed to drain, the water in the center of the macropore has the wait energy & is the most mobile. It leaves the pore first. It moves at the fastest speed. When all the pores in the soil are filled with water, the soil is saturated with water (full capacity).

Forces of retention. The two facts are obvious: 1) Moisture is held in the soil with a suction of tension & work must be done to remove this water. 2) The tension with which the water is held depends upon the amount present. The smaller the amount the greater the tension. The tension with which the water is held in the soil is called suction. At the field capacity the tension is 0,1 to 0,5 atmosphere. At the field capacity the wilting coefficient (wilting point) it is 15 atmospheres. The pressure of soil water (an indication of tendency for soil water to move, is expressed by the soil water potential. Technically it is defined as the amount of work that must be done per unit quantity of water to transport or mole reversibly (without energy loss due to temperature change) an infinitesimal quantity of water from a pool of pure water at specified elevation & atmospheric pressure to the soil water at the point under consideration. The amount of work needed to move water from a reference pool to another point. The symbol for it is psi, ψ. The total water potential is made up of several subpotentials, including the gravitational, metric, & osmotic. The gravitational potential ψg is due to the position of water in a gravitational field. The important forces affecting water movement are adhesion and cohesion. These are affected by the size and nature of particles and peds. Matrix is the physical arrangement (расположение) of surfaces and spaces. The interaction of the soil matrix with the water produces the matrix water potential. The osmotic potential, ψo, is caused by the forces involved in the adsorption of water molecules by ions (hydration) from the dissolution of soluble salt.