Importance and determination of Soil Texture in relation to soil fertility

Rajan BhattAssistant Professor (Soil Scinece)Krishi Vigyan Kendra,Kapurthala

rajanbhatt79@rediffmail.com(98159-63858)

Soil comprises of the three important primary partners, viz. sand (2-0.02 mm), silt

(0.02-0.002 mm) and clay (<0.002 mm). These primary particles are generally clustered

together to form secondary particles or aggregates which further binds together with

organic matter and some other materials in different proportions to form different types

of the soils viz. loamy soil has 40% sand , 40% silt and around 20% clay. Sand feels

gritty between fingers and the particles are generally visible to the naked eye. As sand

particles are relatively large, the bigger pores in between them promote easy drainage of

water into and out of soil profile and exchange of gases with the atmosphere. Silt

particles are smooth, silky like flour. The pores between them are smaller and many,

thereby retaining water for a longer period of time. The clay particles have a very large

specific surface area and have a tremendous capacity to adsorb water and other

substances and make the soil sticky or plastic.

Importance

The mechanism of water infiltration, retention in soil and drainage out of the root

zone are a phenomenon associated with soil texture. The coarseness of fineness of

soil determines the rates of these processes and hence water availability to the

plants.

The structure of soil, an important parameter determining the physical fertility of

soil is a function of soil texture. The fine textured soils have a more stable

structure but a plenty of micropores, which help retain more water and less air in

the soil, but coarse-textured soils have more macropores, which conduct water

very fast.

Soils with finer particles get waterlogged during excessive rains or irrigation and

result in aeration stress to plants with the result that they are not able to take up

water and nutrients, which are present in the soil in sufficient quantities.

The fine-textured soils help in retaining more nutrients on their surfaces through

adsorption and hence lesser losses through leaching etc. The cation exchange

capacity of soils, which is very important in determining the availability of

nutrients to plants, is a function of soil texture.

The workability of the soils is a direct function of soil texture. In fact, the heavy

or light texture means the force required at drawbar during cultivation.

The erodibilty of soil-resistance of soil towards the erosion process is also a

function of soil texture.

In fact most of the soil physical and chemical properties are a function of soil

texture either directly or indirectly.

Relation with soil fertility

Since weathering takes place at the surface of mineral particles, releasing

constituent elements into the soil solution depends upon the surface area of the

minerals. Greater the surface area, the greater the rate of release of plant nutrients

from weathering minerals.

The retention of nutrients and water on the soil solids is a function of surface area

of the clay minerals. Also the leaching of nutrients is a function of soil texture.

Thus finer soils tend to be more fertile than the coarse textured ones.

The fertility of soil being a function of soil moisture, fine textured soils, which

retain more water for a longer period of time, are more fertile than the coarse

textured soils. At the same time a balance between soil moisture and soil air is a

must for better uptake of nutrients. Soils with appreciable amounts of different

primary particles are more fertile.

Soil fertility is maintained through various chemical reactions in the soil, which

are influenced by microorganisms. These tend to grow and colonize particle

surfaces. Microbial reactions are thus greatly affected by the specific surface area.

Methods of determination

Feel method

International pipette method

Hydrometer method

The feel method is a crude method by which we are able to judge broadly the classes of

soil texture viz. sandy loam, loamy sand, silty loam, clay loam etc. The experience of the

person matters in accurately judging the texture.

The pipette method involves the dispersion of soil sample into ultimate particles and then

separating the coarse sand particle through sieving and fine through sedimentation, which

is based on Stoke’s law. The differential settling of particles is a function of their size.

The bigger particles settle first, followed by smaller particles. The sample of soil-water

suspension taken after a certain pre-calculated time corresponding to a particular size

group of particles, may contain those particles and after drying the sample at 105oC, the

percentage of that group of particles can be found out.

The hydrometer method also involves dispersion of soil into primary particles, sieving

coarse sand particles and subsequently the differential settling of particles. But instead of

actually taking a sample for different fractions, this method is based on the measurement

of density of soil-water suspension at different times and relates it with the particle size

groups. A hydrometer reading at 4 minutes means measuring density for silt and clay

particles and that at 2 hours means density for clay particles at a temperature of 68oF. At

temperatures above and below this value needs temperature correction in the hydrometer

readings.

For determining the texture of a soil, the relative proportion of different primary particles

(sand, silt and clay) needs to be found out and procedure is called Particle size analysis. It

is an index of physical and chemical properties of soils. The methods used for this

analysis are Pipette method and Hydeometer method. Both these methods are based on

Stoke’s law for the fractionation of finer particles but differ in the mode of recording.

Pipette method is laborious and time consuming whereas hydrometer method is simple

and rapid.

Procedure

The particle size analysis by Hydrometer method involves two important steps:

dispersion and fractionation of soil particles.

Dispersion

Take 50 g of 2-mm sieved oven-dry soil in a 500 ml beaker.

Add 20 ml of H2O2 solution into it and swirl the contents well and place it on a

hot plate. Continue digestion, stirring the contents all the time with a glass rod to

minimize frothing, till the reaction completely subsides. Cool the beaker.

Detach the soil particles from the inner sides of the beaker by rubbing with a

policeman and with a jet of distilled water.

Add 25 ml of 2N HCl solution and allow the contents to react for an hour. Filter

the contents through a Whatman No. 50 filter paper and discard the filtrate. Wash

the soil with distilled water till the filtrate is free from chlorides.

Transfer the soil from filter paper to a 500 ml beaker with a jet of distilled water.

Make the volume to 300-350 ml with distilled water and add a few drops of

phenolphthalein indicator. Add N/10 NaOH solution till the whole suspension

shows a pink colour and stir the contents of the beaker with an electric stirrer for

5-10 minutes.

Fractionation

Transfer the contents of the beaker to a 1-litre cylinder of standard dimensions

and make the volume to 1 liter. Place the cylinders in a controlled temperature

room at 68oF (otherwise note down the temperature of the suspension and correct

the subsequent hydrometer readings for this temperature).

Shake the suspension with a plunger for 20-25 times or till all the soil particles

come into suspension. Take the hydrometer readings at 4 minutes and 2 hours for

silt + clay and clay, respectively as shown below.

Carefully put the hydrometer in the suspension 30 seconds before the actual

timing and let it become stable. Note down the hydrometer reading at 4 minutes

after the plunger was taken out of the suspension. Take the hydrometer out of

suspension and wash it with distilled water and take reading at 2 hours after the

plunger was taken out of the suspension.

In case the temperature of the suspension could not be maintained at 68oF, add 0.2

to the reading for each degree above 68oF and subtract 0.2 for each degree below

68oF within 60 to 75oF.

Determination of soil textural class

The texture of soil is determined from the relative proportions of sand, silt and

clay that it contains. Triangular classification suggested by ISSS is in common use. It

makes use of an isolateral triangle whose area is divided into 12 compartments each

representing a texture. The differences between the two are primarily due to differences

in size ranges of sand and silt fractions. For the determination of the texture of a soil,

locate the clay and silt %ages on the respective sides of a triangle. Draw a line parallel to

the sand axis in the former case and to the clay axis in the later case. The compartment in

which the two lines intersect is the texture of the soil.

Soil fertility management tips

1. Must apply fertilizers as per the soil testing report.

2. Go for green manuring after wheat preferably with cowpea and sun-hemp. Green

manuring with Dhaincha crop is somewhat discouraged because of it’s high water

demand and sensitivity toward attack of insect-pests which further required

insecticides spray. However, green manuring with moong preferably var. SML-

668 (Sathi moongi-as it took around 60 days) because moong crop roots had

nodules having rhizobium bacteria which further fixes atmospheric nitrogen in

soil. Inspite of increasing the inherent soil fertility, moong grains provides an

extra income to the farmer.

3. Apply well rotten farm yard manure to fields @7 -10 t ha-1.

4. Apply urea as per leaf colour chart because higher dose of urea invites more

attacks of insects-pests where by applying urea as per LCC we apply urea as per

crop demand.

5. Shifting of the cropping pattern from wheat-paddy to wheat- maize as on one side

it will improves the soil health and on other hand uplifts the declining water table

as maize water requirements is quite low as compared to the paddy.

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