PHYSICAL PROPERTIES OF THE SOIL 4.1 Te Texture: xture: Texture Te xture refers to the relative proportions of p particles articles of various sizes such as sand, silt and clay the soil. in The of theshown separates in classes commonly used inthe describing soils in are given theproportions textural triangle in Fig.1.5. In using the diagram, points corresponding to the percentages of silt and clay present in the soil under consideration are located on the silt and clay lines respectively. ines are then pro!ected inward, parallel in the first case to the clay side of the triangle and in the second case parallel to the sand side. The name of the compartment in which the two lines intersect is the class name of the soil in "uestion. For examples a soil containing 15# clay, $%# silt and &5# sand is sandy loam and a soil containing e"ual amounts of sand, silt and clay is clay loam. The percentages of sand, silt and clay in a soil could be determined in a soil laboratory by two standard methods ' hydrometer method and pipette method ()lac* et al., 1+&5a. )oth methods depend on the fact that at any an y given depth in a settling suspension the concentration of the particles varies with time, as the coarser fractions settle at a faster rate than the finer (Fig. 1.&. In the field, soil texture could be estimated by the following methods (Fig. 1.-. (i Feel method. In this method, the soil is moistened with water and rubbed between the thumb and fingers. The way the wet soil slic*s out/ gives a good idea of the clay content. The sand particles are gritty, the silt has a floury or talcum ' powder fell when whe n dry and is only moderately plastic and stic*y when wet. 0ccuracy of this method depends largely on experience.
Fig. 1.3. Volume Volume a! "eig#t $om%o&itio o' a &oil (Per$etage o' air a! "ater )arie& a$$or!ig to moi&ture &aturatio o' &oil*
Fig. 1.4. +ieralogi$al $om%o&itio o' &oil (area "it#i t#e 'igure !eote& t#e relati)e a,u!a$e o' mieral&*
Fig. 1.-. Soil textural triagle
Fig. 1.. T#e &ettlig o' %arti$le& i a &oil & oil &u&%e&io
Fig. 1./. Fiel! met#o!& o' &oil texture e&timatio (Feel met#o! a! 0all a! ri,,o met#o!* (ii )all and ribbon method The procedure of this method as described b by y 2oche and aughlin (1+35 is as follows Ta*e Ta*e a handful of soil and wet it so that it begins to stic* together without stic*ing to the hand. 0 ball of about 4 cm diameter is made and put down. If it falls apart it is sand. If it stic*s together roll the ball into a sausage shape & - cm long. If it does not remain in this form it i t is loamy sand. If it remains in this shape, continue to roll until it reaches 15 1& cm long. If it does not remain in this form, it is sandy loam. If it remains in this shape, try to bend the sausage into a half circle and if it doesn6t, it is a loam. If it does, bend the sausage to form a fullccircle and if it doesn6t it is heavy loam. If it does with slight crac*s in the sausage, it is light clay. If it does without any crac*s, it is a clay.
(iii )all throwing method The texture of the soil can be inferred by the wa way y a ball of soil acts when it is thrown at a hard surface such as a wall o orr a tree (Fig.1.3. The steps to be followed in this method as described by 2oche and aughlin (1+35 is as follows Throw a ball of soil to a tree or wall 4 m away. If the soil is good only for splatter shots when either wet or dry, it has a coarse texture (loamy sand. If there is a shot gun/ pattern when dry and it holds its shape against medium range target when wet, it has a moderately coarse texture (sandy loam. If the ball shatters on impact when dry dr y and clings together when moist but does not stic* to the target it has a medium texture (loam, sandy clay loam, silty clay loam. If the ball holds its shape for long ' range shots when wet and stic*s to the target but is fairly easy to remove it has a moderately moderatel y fine texture (clay loam. If the ball stic*s well to the target when wet and becomes a very hard missile when dr dry y, it has a fine texture (clay. The usual mechanical analysis of soils in the laboratory gives the percentages of the three size fractions, sand, silt and clay. For special uses, the same methods of laboratory analyses (pipette method or hydrometer method can provide a much more detailed analysis giving further brea*down of the relative amounts of soil particles for more size classes in the form of a table or graph. The data in the graphical form is given as a particle ' size fre"uency curve (78F curve. 78F curves for selected soils are shown in Fig. 1.+. The vertical axis represent the cumulative percentage of occurrance of the various particles sizes and the horizontal axis represents the logarithms of the particle size. The vertical axis in the left hand sidethe relate to the percentages particles passing through sieves ofofa particular size and vertical axis in the right of hand side relate to the percentages particles not passing through sieves of a particular size. The more vertical the 78F curve or part pa rt of the curve, the more uniform the particle size9 a vertical line represents a perfect uniform particle size. The more inclined the curve or part of it, the greater the difference between the particle sizes (i.e. smaller porosity and higher compaction. The inflexion point of the curve shows the most fre"uent particle size by weight. Fine textured soils have their curves towards the right hand ha nd side of the graph and the coarse textured soils to the left hand side. From the 78F curves, the percentages of silt, sand and clay can be calculated and using the textural triangular diagram the texture could be determined. 8oil texture is an important soil parameter determining the suitability of a site for a"uaculture. 0 clayey soil stabilises pond bottom besides the fact that it adsorb large "uantity of nutrients and release them slowly over a long period to the overlying water. The clayey soil normally holds higher amounts of organic matter than light textured soils and thereby increase the productivity of the pond. It should be noted that too clay a soil (very stic*y clay may not be very satisfactory as it may give rise to fixation of phosphorus and create other physico'chemical biological problems. 8uch soils may give rise to crac*s on draining the ponds, thereby increase seepage losses.
4. Stru$ture: The term texture is used in reference to the size of individual soil particles but when the arrangement of the particles is considered the term structure is used. 8tructure refers to the aggregation of primary soil particles (sand, silt and clay into compound particles or cluster of primary particles which are seperated by the ad!oining aggregates by surfaces of
wea*ness. 8tructure modifies the effect of texture in regard to moisture and air relationships, availability of nutrients, action of microorganisms and root growth. :.g. a highly plastic clay (&%# clay is good for crop product prod uct if it has a well developed granular structure which facilitates aeration and water movement. 8imilarly a soil though has a heavy texture, can have a strongly developed structure, thus ma*ing it not very satisfactory for a"uaculture as a result of this soil allowing high seepage losses. 8tructure is defined in terms of grade, class and type of aggregates. ;rade ;rade of structure is the degree of aggregation and expresses the differential between cohesion within aggregates and adhesion between b etween aggregates. These properties vary with the moisture content of the soil and it should be determined when the moisture content is normal ' not when unusually dry or unusually wet. The four ma!or grades of structure rated from % to 4 are a re listed below. %' no observable aggregation or no definite orderly arrangement of natural lines of 8tructureless wea*ness. escription of the consistence terms mentioned above can be obtained from ;uidelines for 8oil 7rofile >escription/ by F0C (F0C, 1+-A.
4.4 Partiole !e&it7 of soil is the mass per unit volume of o f soil particles (soil solid phase ' expressed in
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