 # Name

Name: Christopher Deonarine
Student ID number: 816013062
Course: Soils and the Environment
Course Code: AGSL 1001
Course Coordinator: Dr. Mark Wuddivira
Title: Soil Texture

Introduction
Soil texture refers to the composition of the soil in terms of the amounts of small (clays), medium (silts), and large (sands) size particles. (Ranjan, Nivetha and Nivetha 2014). To add, soil texture can be classified into 4 main categories clay, sand, loam and silt. However, these categories are further broken down into sub-categories such as sandy loam soil, clay loam soil etc. The function of soil texture is important as it influences nutrient availability. To increase the soil nutrients organic matter such as leaves, manures and other organic matter can be added to the soil texture to aid with plant growth. Soil texture determines the rate at which water drains through a saturated soil and also its erodibility. The pH buffering capacity of the soil is also influence by its soil texture. A soil textural triangle is used to determine the type of soil sample there is by matching the increased percentage of the soil. Stokes’ law applies and it states mathematical V=kr2 where K is constant related to the density and viscosity of the water and the acceleration due to gravity. Stokes’ law focuses on the three forces that act on the soil particle buoyancy force, drag force and the gravitational force. From his law it can be rearranged to find the radius of the particles as they settle as well as the percentage of each size. From this data it can be used to identify the soil textural class. (Brady, Neil and Weil 2008). However, we assume that the density of the water, particles and viscosity remains constant but due to their mineralogical and chemical composition the law can be misleading. Two methods that was used to determine the soil texture are hydrometer method and soil feel method. The hydrometer method is one of the simplest and fastest methods however any large soil particles such as sand settles to quickly below the plane affecting the reading on the hydrometer as it relies on buoyancy of the soil particles. On the other hand, soil feel method is also a quick and easy method as no equipment is being used to determine the type of soil, due to this there is low accuracy.

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Methodology

Hydrometer Method
50g of dried air 2 mm sieved soil was placed in a 600 ml beaker.

5% of 50 ml sodium hexametaphosphate (Calgon) was added. This was a dispersant. The Na+ had replaced Ca2+ on the exchange sites. The PO3- anion had precipitated the exchangeable and soluble cations such as Ca2+, which may cause flocculation.
The mixture was shaken until well mixed and it was allowed to stand for 15 minutes, which had allowed time for the hydration of the particles and the slaked of aggregates.
The contents of the flask were transferred to a dispersing cup of an electric mixer and it was ensured that all the suspension was transferred and that the cup was not too filled.

The mixer was attached to the cup and it was stirred for 2 minutes.

The soil was transferred from the dispersing cup to a 1-L graduated cylinder. The content in the cylinder was bought to the 1000 ml mark in which distilled water was used. The original concentration (g/L) of the soil particles in the suspension was established.

The soil was mixed with a plunger until a uniform suspension is obtained. The plunger was gently removed and the time was noted immediately. The plunger was inserted gently into the suspension in which it was never raised above the surface while it was being mixed, to avoid loss of sample.
A hydrometer was placed gently into the suspension after the plunger was removed and a reading was taken at the end of 90 seconds. 10 cm was measured by the hydrometer since it was the density of the suspension. All the sand particles were assumed to have fallen below this depth after 90 seconds in which the density of the hydrometer was measured (Rs 90-s) of the silt clay in the suspension at this depth.

The cylinder was kept with its content undisturbed for 90 minutes. At the end of the 90 minutes, it was assumed that silt particles would have settled. The hydrometer was gently inserted and the density (Rs 90-m) of the suspension was recorded. This reading was a measured of the clay separated.
The hydrometer correction factor was determined for the density of the dispersing solution referred to as the Blank (Rb) as the hydrometer was gently inserted into the solution provided for this purpose. The Blank was the cylinder that contained the dispersing solution, made up to the mark with distilled water, without any soil sample. The hydrometer value of the Blank (Rb) was recorded.
After the temperature was measured (T) a temperature correction (RT) was performed where RT= (T oC- 19.5) x 0.3.

The percentage of the various soil separates were calculated and the textural class of the sample were determined by use of the textural triangle and the results were tabulated.
Soil Texture by Feel
A handful of soil was taken up into the palm of your hand.

The ball of soil was pressed between your thumb and forefinger to form a ribbon.
If a ribbon was formed the length was taken and recorded.
A pinch of soil was taken to form a texture ball in which water was added.
The soil was rubbed and a muddy puddle was made in your palm. The grittiness was recorded.

Results
TABLE 1 SHOWING THE HYDROMETER READINGS AND TEMPERATURE READINGS FOR THE SOIL SAMPLES 1, 2 AND 3 AFTER 90 SECONDS AND 90 MINUTES.

Soil Sample # Hydrometer Value (g/L) Temperature oC90 seconds 90 minutes 90 seconds 90 minutes
Blank 0 0 24 24
1 8 2 24 23
2 23 12 24 23
3 41 36 24 23
Symbols
Rtb- Corrected Reading for the Blank
Temperature Correction for Blank at 90 seconds
Rtb 90-s (g/L) = (T0C – 19.5) x 0.3
Rtb = 0+ (24-19.5) x 0.3
= 1.35 g/L
Temperature Correction for Blank at 90 minutes
Rtb 90-m (g/L) = R + (T0C – 19.5) x 0.3
Rtb = 0 + (24-19.5) x 0.3
= 1.35 g/L
Soil Sample # 1
Temperature correction for soil sample #1 at 90 seconds.

Rts 90-s (g/L) = R + (T0C- 19.5) x 0.3
Rts = 8 + (23-19.5) x 0.3
= 9.05 g/L
Final corrected hydrometer reading for soil sample #1 at 90 seconds.

RCs 90-s (g/L) = Rts 90-s – Rtb 90-s
RCs 90-s (g/L) = 9.05-1.35
=7.7 g/L
Percentage of Sand for soil sample #1
% silt + clay = RCs 90-s (g/L) ÷ 50 (g/L) x 100
% silt + clay = (7.7 g/L ÷ 50 g/L) x 100
= 15.4%
% sand= 100% – (% silt + clay)
% sand = 100%-15.4%
% sand = 15.4 %
Temperature correction for soil sample #1 at 90 minutes.

Rts 90-m (g/L) = R + (T0C-19.5) x 0.3
Rts = 2 + (220C-19.5) x 0.3
= 2.75 g/L
Final corrected hydrometer reading for soil sample #1 at 90 minutes.
RCs 90-m (g/L) = Rts 90-m- Rtb 90-m
RCs 90-m (g/L) = 2.75-1.35
=1.4 g/L
Percentage of clay and silt for soil sample #1
% clay = corrected 90-m (g/L) ÷ dry weight of soil (g) x 100
% clay = RCs 90-m (g/L) ÷ 50 (g/L) x 100
% clay = (1.4÷50) x 100
% clay = 2.8 %
% silt = (% silt + clay) – % clay
% silt = 84%-2.8%
% silt= 81.2%
Soil Sample # 2
Temperature correction for soil sample # 2 at 90 seconds
Rts 90-s (g/L) = R (T0C- 19.5) x 0.3
Rts = 23+ (24-19.5) x 0.3
= 24.35 g/L
Final corrected hydrometer reading for soil sample #2 at 90 seconds
RCs 90-s (g/L) = Rts 90-s – Rtb 90-s
RCs 90-s (g/L) = 24.35-1.35
= 23 g/L
Percentage of sand for soil sample # 2
% silt + clay = RCs 90-s (g/L) ÷50 (g/L) x 100
% silt + clay = (23÷50) x 100
=46%
% sand = 100% – (% silt + clay)
= 100-46
% sand = 54 %
Temperature correction for soil sample #2 at 90 minutes
Rts 90-m (g/L) = R + (T0C- 19.5) x 0.3
Rts = 12+ (22-19.5) x 0.3
=12.75 g/L
Final corrected hydrometer reading for soil sample #2 at 90 minutes
RCs 90-m (g/L) = Rts 90-m – Rtb 90-m
RCs 90-m (g/L) = 12.75-1.35
=11.4 g/L
Percentage of silt and clay for soil #2
% clay = corrected 90-m (g/L) ÷ dry weight of soil (g) x 100
% clay = RCs 90-m (g/L) ÷ 50 (g/L) x 100
% clay = (11.4÷50) x 100
% clay = 22.8 %
% silt = (% silt+ clay)- % clay
% silt = 53.4-22.8
% silt= 30.6 %
Soil Sample # 3
Temperature correction for soil sample # 3 at 90 seconds
Rts 90-s (g/L) = R + (T0C – 19.5) x 0.3
Rts = 41 + (24-19.5) x 0.3
=42.35 g/L
Final corrected hydrometer reading for soil sample #3 at 90 seconds
RCs 90-s (g/L) = Rts 90-s – Rtb 90-s
RCs 90-s (g/L) = 42.35-1.35
= 41 g/L
Percentage of sand for soil sample #3
% silt + clay = RCs 90-s (g/L) ÷50 (g/L) x 100
= (41÷50) x 100
=82%
% sand = 100%- (% silt+ clay)
= 100-82
% sand = 18 %
Temperature correction for soil sample #3 at 90 minutes
Rts 90-m (g/L) = R+ (T0C- 19.5) x 0.3
Rts = 36 + (23-19.5) x 0.3
= 37.05 g/L
Final corrected hydrometer reading for soil sample #3 at 90 minutes
RCs 90-m (g/L) = Rts 90-m-Rtb 90-m
RCs 90-m (g/L) = 41.3-1.35
= 39.95g/L
Percentage of silt + clay for soil sample #3
% clay = RCs 90-m (g/L) ÷50 (g/L) x 100
% clay= (39.95÷50) x 100
= 79.9%
% silt= (% silt+ clay) – % clay
% silt= 17.4-79.9
%silt= -62.5%
TABLE 2 SHOWING THE CORRECTED HYDROMETER READINGS FOR SOIL SAMPLES 1, 2 AND 3 AFTER 90 SECONDS AND 90 MINUTES
Soil Sample Number Corrected hydrometer reading after 90 seconds g/L Corrected hydrometer reading after 90 minutes g/L
1 9.05 2.75
2 24.35 12.75
3 42.35 37.05
TABLE 3 SHOWING THE FINAL CORRECTED HYDROMETER READINGS FOR SOIL SAMPLES 1,2 AND 3 AFTER 90 SECONDS AND 90 MINUTES
Soil Sample Number Final hydrometer reading after 90 seconds g/L Final hydrometer reading after 90 minutes g/L
1 7.7 1.4
2 23 11.4
3 41 39.95

TABLE 4 SHOWING THE PERCENTAGE OF SAND, SILT AND CLAY PRESENT IN THE SOIL SAMPLES 1,2 AND 3 ALONG WITH THE TEXTURE OF EACH SOIL
Soil Sample Number % Sand % Silt % Clay Soil Texture
1 15.4 81.2 2.8 Sand
2 54 30.6 22.8 Sandy loam
3 18 -62.5 79.9 Silt loam
Discussion
References
Brady, Nyle C, and Ray R. Weil. 2008.The Nature and Properties of soil. Fourteenth ed. Pearson Prentice Hall
Prakash, Nitishrarjan. “soil texture and its impacts on plant growth”, 2014: 1427628.http:// www. Authorstream.com/presentation/ nitishrarjanprakash-1427628-soil-texture-its-impacts-on-plant-growth.

Tarahaat, “texture and soil properties and plant growth”, 2007. http://www.tarahaat.com/ Soil_Texture.aspx

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