MEASURING OF SOIL PH 1

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    Biology
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    Article
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    Undergraduate
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    2
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    1187

Measuring of Soil PH

Measuring of Soil PH

Introduction

Soil PH has an impact on various properties of soil such as chemical, physical and biological compositions. It has a direct impact on the ability of the roots to absorb a number of elements such as nutrients and toxins. Among the characteristics that can be measured, soil PH is one of the most significant. On the contrary, there are a number of challenges encountered during its measurement (Rayment, 2011).

Soil PH can be defined as the concentration of hydrogen ions in a soil sample mixed with water. PH is usually expressed in the form of a logarithm to base of 10. When there is a lower concentration of hydrogen atoms, the PH becomes high. Soil hydrogen ions are in equilibrium with various phases of solids and liquids in hydrogen ions and have enabled understanding whether soil is acidic or basic.

The measurement of PH of a sample of soil takes place in a dilute salt solution such as 0.01 mol L-1 of CaCl2 or 0.005 mol L-1, KCL at a soil: water ratio of 1:2 or 1:5. Before the use of pH(CaCl2), soil PH was measured by use of soil to water sample of 1:5. However, there are a number of advantages of pH(CaCl2) in comparison with pH(H2O) such as reduced seasonal changes in the soil solutions, strength of ions of the soil solutions, strength of ions, and reliability in the relation with plant growth (Sposito, 1989). In addition, it has the advantage of less change when subjected to storage for a long duration of time.

Another characteristic of a soil sample measured is the electrical conductivity (EC) of a soil sample. This is a measurement used to determine the concentration of soluble salts in the soil. The main compositions of soluble salts include cations such as: Na+, Mg2+, and K+ and a combination of anions such as Cl,SO42- and HCO3.

Background

The practice of measuring soil PH can be achieved in the field by use of a test kit or sending a sample of soil to the laboratory in order to obtain accurate outcomes. The outcomes of the kit test are a soil PH on water scale and can be used as a guide to understanding the actual value (White, 1969). Most accurate analyses of soil PH have been achieved by conducting laboratory analysis. It has acted as the basis on which acid soil management activities should commence. The standard method of measuring soil PH has been the use of calcium chloride. It involves mixing an air dry soil with five times its weight of a dilute Calcium Chloride and the PH is determined by means of electrodes.

The main activities during the experiment included making a suspension of soil in a solution, mixing and determining the PH of the suspension using a combined glass electrode. The main aims of the experiment included:

  1. To gain familiarity with the procedures for measuring the soil PH by use of water and 0.01M CaCl2 and 0.01M KCl.

  2. To make a comparison of the effects of strengths of ions during solution extraction while measuring soil PH

  3. To compare the capabilities of samples of sand and clay loam to buffer

  4. To make a comparison of soil PH as illustrated using a combined electrode composed of 1:5water, 1:5 CaCl2, and 1:5 KCl.

Three soils samples were used where they were labeled as samples A, B, and C. Varying quantities of lime were added to the soil samples. The source of the soil samples was Cranbourne or Kinglake. 3 lots of 5.0g were weighed to tubes A, B, and C. 25 mL of deionzed water was added to tube A, while 25mL of 0.01M CaCl2 was added to tube B and finally 25mL of 0.01M KCL was added to tube C. all the tubes were mechanically shaken for 1 hour followed by centrifugation for 5 minutes.

The PH of the resulting PH was measured in all the solutions. In order to obtain an accurate PH, the electrode was held vertically and readings were taken only when equilibrium was reached. For soils whose PH was greater than 7, they were allowed to stay for longer time durations so that equilibrium could be reached. The reading of the PH was done to a one decimal place and the results recorded in a sheet of paper. The electrical conductivity of the deionized water was measured using EC meter and electrode. The soil PH of the same soil was determined with a test kit and porcelain. This value was compared with the measured value.

The following graphs were obtained from the data collected during the experiment

Kinglake

Solution

4000 mg/kg

1:5 CaCl2

1:5 CaCl2

Table 1. Outcome of PH for various Soil samples

SOIL PH and EC

Figure 2. Conductivities of Soils samples with different substances

SOIL PH and EC 1

Figure 3. Soils sample solutions from different locations

SOIL PH and EC 2

Figure 4. Class Results of ionic strengths of various soil samples

Discussion

When the PH of all soil samples were measured, it was established that the sample containing CaCl2 had the lowest value, followed by KCL and finally water solution had the highest value. When the samples from various origins were compared, there was no variation in the PH characteristics from different origins.

From the experiment, it was established that that in solutions with higher concentrations of positive ions, the PH was low indicating acidic medium. Solutions that contained ions with lower concentrations of negative ions, the PH was higher indicating and alkaline medium.

The curves on the graphs provided differ between two soils with reference to buffer capacity because the higher concentration of positive ions in one solution prevents the buffering capacity compared with another solution where the concentration of positive ions is low. It easier to change the PH of sand compared with clay because sand has larger particles and allows easy dissolution of its water content into the solution compared with clay that prevents penetration of the solution through its particles.

From the graphs, the amounts of CaO required neutralizing the exchangeable acidity in the top 10cm with a bulk density of 1.33g/cm3 is:

Mass of CaO = 20+8 = 28g

Volume of CaO required is obtained by, Vol = Mass/Density = 28/1.33g; Vol. = 21.05cm3

The corresponding quantity of Ca (OH) 2 required is: 20+ (8+1)2 =38g

Volume = Mass/Density = 38/1.33 = 28.571 cm3

Conclusion

This paper presents an experiment involving the measuring of PH of a sample of soil. It also presents the understanding of conductivities of various samples of soil. The study is of great importance in establishing the level of acidity of the soil so that actions can be taken to regulate it to the right level that supports plant growth. It also enables understanding of the suitability of the soil for the survival of various microorganisms and other living organisms that depend on the soil for their survival.

References

Rayment GE, Lyons (2011) Soil Chemical Methods Australiasia. (CSIRO Publishing, Collingwood, Vic).

Sposito G (1989) The Chemistry of Soils. (Oxford University Press: New York)

White RE (1969) On the measurement of soil PH. Journal Australian Institute of Agricultural Science 35,3-14.