LAB 8. DETERMINATION OF ph AND LIME REQUIREMENT OF SOIL

Transcription

1 LAB 8 DETERMINATION OF ph AND LIME REQUIREMENT OF SOIL Learning outcomes The student is able to: 1. determine soil ph and its nature 2. measure lime requirement of a soil Introduction We are interested in soil ph because it plays an important role in plant growth. Soil ph influences many facets of crop production and soil chemistry, including availabilities of nutrients and toxic substances, activities and nature of microbial populations, solubility of heavy metals, and activities of certain pesticides. The soil ph is easily determined and, like taking your temperature when you are sick, it gives us some quick, valuable information that will enable the "Plant Doctor" to prescribe corrective procedures. ph is defined as the negative logarithm of the hydrogen ion (H + ) concentration. When water ionizes to H + and OH - (a neutral solution), both H + and OH - ions are in equal concentrations of moles per liter. That is a very small concentration. HOH < > H + + OH - [H + ] = [OH - ] = 1 x 10-7 moles/liter. The H + ion and OH - concentrations in water are very small. The ph scale has been devised for conveniently expressing these small concentrations by expressing ph = -Log [H + ]

2 When the hydrogen concentration is greater, such as moles per liter, the ph is 4; when it is smaller, such as , the ph is 8. One thing to remember is that when the ph changes from one unit to another, the change in the hydrogen ion concentration is a ten-fold change, not just one. So a ph of 5 is ten times more acid than a ph of 6 and 100 times more acid than a ph of 7. Causes for Acid Soils The ph of a soil is dependent on the parent material, the climate, the native vegetation, the cropping history (for agricultural soils), and the fertilizer or liming practices. The ph range for most mineral soils would be from 5.5 to 7.5. This is also the range for most soils found in Malaysia. Exchangeable hydrogen is the principal source of H + until the ph of the soil goes below 6. Below 6, exchangeable aluminum becomes the source of hydrogen ions, due to the dissociation of Al from clay minerals. For simplicity, we will use the term "exchangeable H" for the cause of acid soils. Soils tend to become acidic as a result of: (1) rainwater leaching away basic ions (calcium, magnesium, potassium and sodium); (2) carbon dioxide from decomposing organic matter and root respiration dissolving in soil water to form a weak organic acid; (3) formation of strong organic and inorganic acids, such as nitric and sulfuric acid, from decaying organic matter and oxidation of ammonium and sulfur fertilizers. Strongly acid soils are usually the result of the action of these strong organic and inorganic acids. Sources of H + ions in the soil: 1) dissociation of carbonic acid, which forms readily in soils when CO 2 is present; 2) organic acids formed during the decomposition of organic matter;

3 3) the burning of coal in electrical power plants releases sulfur to the atmosphere which is added to soils during precipitation as sulfuric acid, and fertilizers containing sulfur, which adds H + ; 4) the conversion of NH to NO 3 releases H + during the nitrogen cycle or when nitrogen fertilizers are added to soils. ph is < 4.0 =indicates that the soil contains free acids probably as a result of sulfide oxidation. ph is < 5.5 =indicates that the soil's exchange complex is dominated by Al ph is < 7.8 =soil ph is controlled by a range of factors ph is > 7.8 =indicates that the soil contains CaCO 3 Where leaching is minimal, the concentration of basic cations (Ca ++, Mg ++, K +, and Na + ) on the exchange complex will be large. These basic cations will come from the weathering of rocks and minerals, from dust blown on soils, from irrigation water or runoff water. When basic cations dissociate in the soil solution, they will produce hydroxyl ions (OH - ). This will raise the ph of the soil. The "ph of the soil" refers to the concentration of hydrogen ions in the soil solution--not on the exchange complex. Descriptive terms commonly associated with certain ranges in soil ph are: Table 8.1 : The descriptive terms associated with certain ph Ranges. Acidity ph Examples Extremely acid <4.5 Lemon(pH=2.5); vinegar (ph=3.0); stomach acid(ph =2.0); soda (ph = 2-4) Very strongly acid Tomatoes (ph=4.5) Strongly acid 5.1 5,5 Carrots (ph = 5.0); asparagus (ph = 5.5), boric acid )ph = 5.3); cabbage (ph=5.3) Moderately acid Potatoes (ph = 5.6 Slightly acid Salmon (ph = 6.2); cow milk (ph = 6.5) Neutral Saliva (ph = ); blood (ph=7.3); shrimp (ph=7.0) Slightly alkaline Eggs (ph= ) Moderately alkaline Sea water (ph = 8.2); sodium bicarbonate (ph = 8.4) Strongly alkaline Borax (ph = 9.0) Very strongly alkaline >9.1 Milk of magnesia (ph=10.5); ammonia (ph=11); lime (ph=12)

4 Significant of Soil ph The effect of soil ph is great on the solubility of minerals or nutrients. Fourteen of the seventeen essential plant nutrients are obtained from the soil. Before a nutrient can be used by plants it must be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid soils than in neutral or slightly alkaline soils. Phosphorus is never readily soluble in the soil but is most available in soil with a ph range centered around 6.5. Extremely and strongly acid soils (ph ) can have high concentrations of soluble aluminum, iron and manganese which may be toxic to the growth of some plants. A ph range of approximately 6 to 7 promotes the most ready availability of plant nutrients. There are of course exceptions, such as azaleas, rhododendrons, blueberries, white potatoes and conifer trees, which tolerate strong acid soils and grow well. Also, some plants do well only in slightly acid to moderately alkaline soils. However, a slightly alkaline (ph ) or higher ph soil can cause a problem with the availability of iron to pine oak and a few other trees causing chlorosis of the leaves which will put the tree under stress leading to tree decline and eventual mortality. This is also a problem on beans and strawberries. The soil ph can also influence plant growth by its effect on activity of beneficial microorganisms. Bacteria that decompose soil organic matter are hindered in strong acid soils. This prevents organic matter from breaking down, resulting in an accumulation of organic matter and the tie up of nutrients, particularly nitrogen, that are held in the organic matter. Changing Soil ph The ph of the soil is dependent on the quantity of hydrogen ions in the soil solution. If we want to raise soil ph, we need to increase the quantity of OH- ions in solution. However, when more H + ions are removed from the solution, they are replaced by hydrogen ions that were held on the cation exchange sites. This ability of the soil to withstand rapid changes in ph is important for plant growth. However, it means that the total amount of bases needed to raise the ph is dependent on the total amount of hydrogen ions held on the reserve. This is referred to as buffering capacity.

5 This diagram shows a convenient way to think of the capacity of the soil to resist changes in ph (the soil's buffering capacity). The active acidity (soil solution hydrogen ions) is in the small outside spout. The reserve acidity is in the big tank. When we remove some active acidity by adding a base such as CaCO 3, we remove some hydrogen ions from the active acidity. However, the original soil acidity of the soil solution is very rapidly restored. So, in order to change soil ph, we need to change the percent base saturation. It is common practice in soil testing labs to make direct measurements of lime requirements by using a buffer solution. A buffer with a ph of 7.5 is mixed with a known quantity of soil. The exchange acidity is replaced from the exchange sites, and the depression in ph from 7.5 is a measure of the total acidity Liming Soils Raising soil ph requires a quantity of agricultural liming material that is determined by the amount of acidity in the soil and the quality of the liming material. Soil acidity is measured by soil testing; the quality of agricultural liming material is determined by its purity and particle size distribution. An agricultural liming material is defined as a material containing calcium (Ca) and/or magnesium (Mg) compounds capable of neutralizing soil acidity. These materials include: limestone (both calcitic and dolomitic), burned lime, slaked lime, marl, shells, and by-products such as sugar beet sludge, and sludge from water treatment plants. Fluid lime is a term that is generally used to describe the concept of suspending liming materials of various types in either water or fertilizer solutions. Frequently, the liming material in fluid lime is finely ground agricultural limestone with a high neutralizing value. Advantages include rapid availability and application with existing fluid fertilizer equipment. Drawbacks are low rates of application and relatively high cost for the lime applied.

6 Lime Neutralizes Soil Acidity When added to the soil, calcium and/or magnesium dissolved from the liming materials displaces hydrogen (H + ) from the clay particles. Remember it is the hydrogen ion (H + ) that makes soils acid. The displaced hydrogen then reacts with carbonate. Carbonate dissolved from the limestone materials forms carbonic acid. Carbonic acid is not stable in soils and quickly forms carbon dioxide and water. With this chemical process, the hydrogen (H + ) has been converted from an ion on a clay particle to a neutral molecule of water, thereby reducing soil acidity. The chemical reaction for this process is H + Clay + CaCO 3 (Limestone) > Ca ++ Clay + H 2 CO 3 (Carbonic Acid) Then: H 2 CO 3 > H 2 O (Water) + CO 2 (Carbon Dioxide) Everything that contains calcium or magnesium is not necessarily a liming material. Gypsum, for example, is calcium sulfate (CaSO 4 H 2 O). When added to the soil, the calcium in the gypsum can displace the hydrogen on a clay particle. The hydrogen, however, would remain in the soil solution and the ph would not change because of the absence of carbonate Materials Plastic vials Distilled water 1 M KCl solution 0.01 M CaCl 2 solution ph meter 0.04N (0.02M) Ca(OH) 2 solution

7 Activities Soil ph Determination One of the methods to determine soil ph is by using ph color indicator. It is simple but not so accurate. The most accurate determination can be made using a ph meter and glass electrode. The electrical conductance of the solution is measured using the meter. The conductance is correlated in the machine to ph values which are read directly. There are three main internationally accepted methods available for measuring soil ph. All of them rely on shaking (or stirring) soil with a solution for 1-2 hours and then determining the ph of the resultant soil slurry. 1. Weigh out 10 g of soil into labeled 50 ml plastic (polypropylene) vials 2. Add one of the following 3 solutions a) 25 ml of distilled water. (This is the simplest method and normally OK for most soils. It doesn't remove H + from the exchange sites and is not very good for soils with high salt content) or b) 25 ml of 1 M KCl (used to mask differences in soil's salt content). Useful if determining exchangable cations as both cations and ph can be done on the same sample. It does displace H + from the soil's cation exchange sites, so the results are usually slightly lower than obtained with methods (a) and (c). or c) 25 ml of 0.01 M CaCl 2. This is an intermediate between methods (a) and (c) and masks small differences in the soil's salt content. 3. Shake for 1 hr at room temperature (25 C) 4. Let the soil settle for a few minutes (e.g. 3 min) and measure the ph after a two point (ph 4 and ph 7) calibration of the ph meter. 5. Normally 2 replicates are performed for each soil sample 6. Field moist soil (store at 5 C) should preferably be used..

8 Lime Requirement Determination 1. Weigh 10 g of soil in each 6 plastic vials. 2. Add in distilled water and 0.04 N (0.02M) Ca(OH) 2 solution according to the volume ratio in Table 8.1. Table 8.2 : Ratio of distilled water and 0.04 N Ca(OH) 2 solution in each vial. Vial No. Distilled water (ml) 0.04N Ca(OH) 2 (ml) ph Shake the vials for 30 minutes and let it settle down for another 1/2 hour before ph is measured. 4. Draw a graph of ph vs. ml 0.04N (0.02M) Ca(OH) 2 on a graph paper and determine lime requirement for the soil if we want to raise its ph to a certain level. Worked Example Suppose that our soil has a ph of 5.5 in water and we want to raise its ph to 6 according to crop or plant requirement. From the graph that we have sketched, it was found that our soil needs 10 ml of 0.04N (0.02M) Ca(OH) 2 to raise it. Calculate the lime requirement of this soil if: 1. the bulk density of the top 15 cm of the soil is 1.5 g cm -3, and 2. and use calcite (CaCO 3 ) with 90% purity is used. For every hectare of the land. 1. Calculating the amount of Ca(OH) 2 that are needed Milliequivalent (me) of Ca(OH) 2 needed = Normality x volume (ml) = 0.04 x 10 = 0.40 me Ca(OH) 2 /10 g soil

9 or = 0.40 me CaCO 3 / 10 g soil 2. Calculating the weight of Ca(OH) 2 needed Weight of Ca(OH) 2 needed = 0.40 me /10 g soil 1 me Ca(OH) 2 = (16 + 1) mg 2 = 37 mg Thus, 0.40 me Ca(OH) 2 = 37 x 0.4 mg = 14.8 mg/10 g soil 3. Changing to unit of part per million (ppm) 10 g (10 4 mg) soil needs 14.8 mg of Ca(OH) 2 Thus, 10 6 mg (a million mg) soil needs = 10 6 x 14.8 mg Ca(OH) = 1480 mg Ca(OH) mg soil = 1480 ppm 4. Calculating the amount of Ca(OH) 2 in the top 15 cm soil in a hectare of land. Soil bulk density is 1.5 g cm -3 (1.5 Mg m -3 or 1500 kg m -3 ) Weight of 1 hectare soil is = 1500 x 100 x 100 x 0.15 kg = 2.25 x 10 6 kg But 10 6 kg of soil needs 1480 kg of Ca(OH) 2 (see 3 above) Therefore, 2.25x10 6 kg soil needs = 2.25 x 10 6 x 1480 kg of 10 6 Ca(OH) 2 = 3330 kg Ca(OH) 2 /ha = 3.33 metric ton Ca(OH) 2 /ha 5. Calculating the lime (CaCO 3 ) used in the field 1 me CaCO 3 = 50mg of CaCO 3 Thus, 0.4 me CaCO 3 = 0.4 x 50 mg CaCO 3 = 20 mg CaCO 3 /10 g soil That is, 10 4 mg soil needs 20 mg CaCO 3 Thus, 10 6 mg soil needs 2000 mg CaCO 3

10 Thus, 1 hectare soil needs = 2.25 x 10 6 x 2000 kg CaCO = 4500 kg CaCO 3 But the purity of CaCO 3 is only 90%, and thus the actual quantity of CaCO 3 needed is = 4500 x 100 kg CaCO 3 90 = 5000 kg CaCO 3 /ha soil = 5 metric ton CaCO 3 /ha.