soil physics and fertility

8/6/2019 Soil Physics and Fertility

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Soil physics and fertility

Texture (see Particle-size distribution)

The texture of a soil refers to the size-composition of elementary grains in a soil. The

texture affects productivity in several ways. Sandy soils are said to be light and clays

heavy. These terms originated from the number of horses needed to pull a plow. Thetexture is an important factor determining the amount of pores and the pore size

distribution, two properties that are of fundamental importance for water relations,

aeration and root penetration and thus for soil fertility. Sandy soils have large pores sothat infiltration rates and permeabilities to water are high, and they retain little water

(Table S8). In contrast, clays have low infiltration rates, low permeabilities, retain much

water in available as well as in unavailable form, and may be poorly drained. Specific

surfaces and ionexchange capacities (CEC) of sands are low as compared to clays although clay

mineralogy is another factor to be considered for the CEC. Aeration is good in sandy

soils and fair to poor in clays, depending on their structure. Roots penetrate sands more

easily than clays. Soils of intermediate textures such as loams are also intermediate inporosity, water retention, and drainage. The general tendency is for productivity to be

better on medium-textured soils consisting of a mixture of sand, silt,and clay than on soils that are light, heavy or mainly silty. The texture is one of the most

important soil properties affecting fertility. It is easily estimated by feeling the soil: sandy

soils feel gritty, intermediate soils more silky and clays are cloddy or plastic. More

accurate laboratory methods involving sieving and sedimentation of elementary particlesare also used. Management of texture is usually not economical. However, consequences

of poor texture can often be partially alleviated by improving soil structure.

Structure

The arrangement of elementary particles into aggregates is considered structure, which,in combination with texture, governs porosity of the soil and thus affects water relations,aeration, root penetration, and the metabolic activities of soil flora and fauna. The binding

among elementary particles occurs through unbalanced charges at broken edges of clay

particles, through bi- and polyvalent ions that neutralize charges of two different clayparticles and thus form a bridge to hold the particles together through iron, calcium

carbonate or silicate cements and through organic matter. Water too is a potent glue

due to the fact that water in small pores is under a lower pressure than atmospheric.

Therefore, the atmospheric pressure acting on the particles presses them together andgives them stability. There is no agreement as to how to describe and measure structure

(see Structure). In a large number of soils it is important to manage soil structure. Many

difficulties of extremely light and extremely heavy soils can be overcome. For example,poor water infiltration and permeation of heavy soils can be overcome by an adequate

soil structure allowing for enough pores for an adequate flow of water and air.

Application of solid animal manure is a good way to improve soil structure. Syntheticorganic chemicals that have an effect on soil structure have appeared on the market.

Water: Plants require much larger

amounts of water than mineral nutrients. Most of the water simply passes through the

plant and is lost into the atmosphere through stomatal openings on the leaf surface. These


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openings are necessary for the entry of CO2, which is utilized in photosynthesis. Only a

small quantity of the water is used for

biochemical and other processes such as maintenance of turgidity. Important aspects ofsoil water are the availability and its buffering capacity.

The state of water in soils is in modern terminology described by the soil water

potential which is equivalent to the chemical potential of water and is measured in energyunits per unit mass (e.g., J kg1). Incorrect pressure units (Pa) are often used,

corresponding to energy per unit volume. The water potential is relative to the standard

state of pure free water at a standard pressure and a reference elevation. The potential isaffected by the matrix, which gives the matric component, by solutes, defining an

osmotic component, and by the pressure,

Soil physical properties can be divided into 3 groups:

_ relatively stable (texture, mineralogy, cementing agents, profile

development, topography), resulting from geological and

pedological processes,_ dynamic (structure, bulk density, pore size, aggregation,

profile characteristics),

_ highly variable (water, air, temperature, mechanical resistance).Every physical property and process in question is modified

by natural conditions (mainly weather phenomena such as precipitation

and radiation) and human manipulations (managementpractices such as tillage, plowing, chemical additives,

irrigation and drainage), which drastically change physical

properties of upper soil layers.

Soil physical properties and processes are more or lessclosely interconnected, which is shown on Figure P47. A very

good relationship exists between soil, water, air, temperature

and mechanical resistance, which directly affect plant roots.They are by turn, affected by texture, structure, bulk density,

aggregation, pore size distribution and profile characteristics.


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figureSoil-root relations are very complex and dynamic. Theserelations occur at the soil-root interface, which is defined as aboundary between the outermost root tissues and the particlesof soil, which are closest to them. Within this boundary is azone of microbial activity called rhizosphere. Root environmentis rapidly changing especially in respect to soil physicalconditions such as mechanical properties, moisture, aerationand temperature.


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Soil structureSoil pore size distribution, and hence the liquid and gas phaseimpedance factors, are related to soil texture and packing.A comparison of the data for pairs of contrasting soils inFigures D6 and D7 illustrate the effect of soil texture on the

variation of fL with yL, r and pF. The larger the average poresize the less tortuous is the diffusion pathway, however it isnot possible to make precise predictions. The pore size distributionsof sieved, repacked soils differ from undisturbed coresand this will be reflected in the impedance factors, howeverthe effect is generally small (Pinner and Nye, 1982). Soil structurecontributes to the formation and stability of gas bubblesthat may in turn interfere with the diffusive flux of soil gasesincluding methane (Rothfuss and Conrad, 1998).

Bulk densityThe liquid phase impedance factor has been found to be dependenton the bulk density. This probably arises largely fromthe dependence of the geometric tortuosity on the pore sizedistribution. Few data are available at constant moisture content.Figure D6b illustrates the decrease in the liquid phaseimpedance factor with increasing bulk density for two soils,each maintained at constant moisture content. In general, theeffect of bulk density, for a given soil, is of minor importancewhen compared to that of water content (Barraclough andTinker, 1981; So and Nye, 1989). Similar trends are foundfor gaseous diffusion, as increasing bulk density usuallyimplies decreasing air-filled porosity.

POROSITYThe property of being porous. Operationally, the total volume

in a soil or sediment, not occupied by solid materials. Theinterconnectivity of pores is a principal determining factor inhydraulic conductivity

Soil water potentialsMovement of water occurs from locations in the soil where thewater in the soil has a high energy to locations where thatenergy is lower. The energy of soil water must be expressedin terms of the total potential, which is energy per unit quantity.The total potential of water c is defined as the mechanical

work required to transfer unit quantity of water reversibly andisothermally from a standard reference state, where c is takenas zero, to the situation where the potential has the definedvalue. A pool of pure water at an elevation that can be arbitrarilyspecified and that experiences a gas pressure of 100 kPa istaken as the standard reference state. Many forces may affectthe total potential of water in the soil. The gravitational potentialZ is due to the attraction by the Earth and increases withheight from the Earth's surface. The pressure in soil water ata particular point will be greater than one atmosphere if that


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point is submerged beneath a free-water surface, which canbe the level of the groundwater. The potential due to this causeis the submergence potential S. If the soil is unsaturated, thepressure in the soil water is less than that of the local atmosphere.It is convenient to refer to a pressure less than atmospheric asa suction or tension (see Wetting front). The potential dueto this cause is the matric or capillary potential M. From their

character it follows that the S andMpotential are mutually exclusivepossibilities. Another possible cause of pressure change insoil water is a change in the pressure of the air adjacent to it. This may be expressed bythe term pneumatic potential G. It is convenientto define the pressure potential P: P M (or S) G,which sums the effect of all pressure changes on the potentialof soil water, whatever the cause of these pressure changes maybe. In addition, osmotic effects of solutes in soil water mustbe considered because they affect the thermodynamic propertiesof the soil water. The effect of solutes on the total potential of soilwater, expressed as the osmotic potential O, becomes of primarysignificance if the water is separated by a membrane whose permeabilityto water molecules differs from that to solute molecules.This aspect is relevant for water movement into andthrough plant roots. The total potential C of soil water is thesum of all the component potentials:C Z P OIt is often convenient to define potentials on a unit weight basisrather than on a unit mass basis. For example, for the gravitationalpotential Z, it follows that Zw W/rg(dv) z (cm) where Wequals r(dv)gz; representing the work needed to transfer the quantitydv over a distance z (r equals density of water; g equals accelerationof gravity). For many flow problems, only the Z and Ppotentials are relevant. Therefore a hydraulic potential f hasbeen defined as follows: f Z P. When potentials areexpressed per unit weight, it follows that f H (cm). The heigh H is called the HydraulicHead.

soil physics and fertility

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