Determination Of Soil PH

DETERMINATION OF SOIL PH The availability of nutrients for plants is highly dependent on the pH of the soil. Ph is formulated as follows: pH = - log aH+ i.e. the negative logarithmic value of the activity of hydrogen ions in soil solutions. + For dilute solutions apply : aH = [H+].fH+ = [H+ ], because + → 1. fH The pH value ranges from 0 – 14. The higher the concentration of H+ in the soil, the lower the pH of the soil, and vice versa, the lower the concentration of H+ the higher the pH of the soil. In relation to the pH value of the soil, three possibilities are found, namely: sour, neutral, and alkaline. At pH = 7.0 means that the concentration of H+ is equal to the concentration of OH-, this state is called neutral. When the pH value is less than 7.0, it means that the concentration of H+ is higher than the concentration of OH-, and this state is called sourness. When the pH value is greater than 7.0 then the concentration of H+ is lower than the concentration of OH-, and this state is called alkaline. Soil salinity is differentiated into active salinity and potential salmonium. Active acidity is caused by free H+ in soil solution, whereas potential acidity (reserve) is caused by interchangeable H+ and Al3+ on the colloidal surface of soil. The H+ and or Al3+ ions in the soil solution are in equilibrium with H+ squeezed. When the concentration of H+ in the soil solution decreases, then the H+ in the adsorption complex (colloidal soil) will enter the soil solution. The same is the case with Al3+. Before the pH increases, the H+ and Al3+ in this adsorption complex must be neutralized first. On the surface of the complex, the concentration of H+ is higher and the farther away from the complex, the concentration decreases. The effect of pH on plant growth depends on the type of plant. Plants that grow well at low pH are tea, azaleas, rhododendrons, while plants that grow well at medium pH are soybeans, tobacco, tomatoes, corn and wheat. Good plants that grow at a rather high pH are alfalfa and asparagus. Effect of H+ concentration on plants and availability of nutrients for The plant has been extensively researched. Disruption of plant growth at low soil pH is caused by: (1) direct destruction of root cells by H+, (2) disruption of nutrient absorption, (3) increased solubility of Al, Fe and Mn so that it poisons plants, (4) reduced availability of Mo and P, and (5) low alkaline content such as Ca, Mg, K. Similarly, the pH of too high, the growth of the plant is disturbed. At high pH, microelements such as Zn, Cu, B, Fe, and Mn become less available to plants, while the solubility of P decreases because it is precipitated by Ca and Mg. 13 Soil pH measurement Soil pH determination can be done using the colorimetric method (based on color) and by using a pH-meter. In the colorimetric method, an indicator or litmus is used whose color changes according to the pH value. The colorimetric method is generally done in the field because it is more practical. The pH value of soil set by colorimetry (pH-paper) is usually slightly different from the pH of soil using a pHmeter. The results of measurements with a pH-meter vary greatly depending on the precision of the soil preparation to be analyzed. Factors that affect the soil pH determination value include: (1) the ratio of the medium to the soil, (2) the content of salts in the soil solution, and (3) the balance of airCO2 with soilCO2. The ratio between water and soil used, usually is 1:1; 2.5:1 or 5:1. The higher this ratio, the higher the soil pH measurement value. If this comparison is too low, the contact between the soil solution and the electrode is not perfect, resulting in disturbed pH measurements and less thorough results. As a medium for determining pH, water (known as pH H2O), 1 N KCl (pH KCl), or 0.01 M CaCl2 (pH CaCl2) is used. When the KCl solution is used as a medium, the K+ ions from KCl will pushing H+ ions from the soil adsorption complex so that generally the pH KCl will be 0.05-1.5 pH units lower than the pHH2O. In this practicum, pH H2O and pH KCl will be determined. For acidic sulfate soils, in addition to pH H 2O and pH KCl, soil pH with a medium of H2O2 of 30% (ratio 3:1) will also be determined. How it works: The stages of the work are as follows: 1. Weigh 10 g of soil, put in a whisk bottle, 2. Add 10 ml of destilate water (pH H2O 1:1), 3. Shake for 30 minutes using a whisk, 4. Let stand for 5 minutes and measure using a pH-meter. Don't forget that the pHmeter must be calibrated first using the pH 4.0 and pH 7.0 standards. In the same way, use 10 ml of 1 N KCl instead of stillate water. The other procedure is the same as determining the pH ofH2O. 14