Soil Chemistry PDF

SOIL CHEMISTRY SOIL CHEMISTRY Soil is the top layer of the earth’s crust in which organic matter grows. There are many components which determine a soil type such as pH, nutrient level and organic content. These factors can vary depending on the type of plant or crop which grows in the soil...

SOIL CHEMISTRY SOIL CHEMISTRY Soil is the top layer of the earth’s crust in which organic matter grows. There are many components which determine a soil type such as pH, nutrient level and organic content. These factors can vary depending on the type of plant or crop which grows in the soil and also on geographic location. The best way to determine soil quality is by conducting a soil test. SOIL Gardeners know that you cannot grow vegetables just anywhere. The quality of vegetables grown depends on the condition of the soil. Soil is the top layer of the Earth’s surface that is suitable for plant growth. Although it is often referred to as dirt or ground, there is a lot more to soil than meets the eye. pH One of the most important components of soil is the pH. The pH of soil can be modified by adding different chemicals. Soil pH indicates how acidic or alkaline the soil is. The pH of a soil is crucial because crops grow best in a narrow pH range which can vary among crops. For example, blueberries and a few types of flowers grow best when the pH is 5.5 or less. Potatoes, a more familiar crop, grow best with a soil pH range of 5.5 to 6.0. Most garden vegetables, shrubs, trees and lawns grow best when the soil pH is over 6.0 or 6.5. The range between 5.5 and 7.5 is favourable for two reasons. It allows sufficient microorganisms to break down organic matter. It is also the best range for nutrient availability. LIMING The pH of soil can be increased by liming. This is why people sometimes spread white powder on their lawns or gardens. This white powder is lime. Calcitic limestone (CaCO3) provides a good source of Calcium (Ca) and helps neutralize soil acidity. Dolomitic limestone functions similarly but also adds Magnesium (Mg). The best limestone will have the greatest calcium and magnesium content and will be ground into very tiny particles. The smaller particles allow the limestone to correct soil acidity more rapidly. The chemistry to liming is quite simple. Hydrogen ions (H+) are attracted to soil and organic material which have a negative (-) charge. When lime is applied, these hydrogen ions are exchanged for calcium or magnesium (Ca2+ or Mg2+) ions which have a greater positive charge. This helps to neutralize the acidity of the soil. The free hydrogen ions are taken out of solution. This also helps to increase the pH. This reaction demonstrates the process of liming: In some cases, the soil may have very high pH and need to be made more acidic. This can be done by using sulfur, aluminum sulfate, or ammonium sulfate. NUTRIENTS Each year the soil undergoes a series of cycles in which materials are added and then taken away. Organic matter and nutrients, in various forms, are constantly being added to the soil. Nutrients are the minerals required by plants to survive. It is very important that plants receive all the required nutrients. There are a total of sixteen elements required for plant growth. Each is required in different amounts. The most important nutrients are called macronutrients. Nutrients which are essential, but only needed in small quantities are called micronutrients. Carbon, Hydrogen, and Oxygen are the big three macronutrients. These are obtained in almost unlimited amounts from the atmosphere and from the water around the plant. The ENGR. JONAYKA A. TAPIADOR Page 1  SOIL CHEMISTRY other macronutrients are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). These nutrients should be available for plant uptake from the soil. Micronutrients are obtained from the soil. They are boron (B), copper (Cu), chlorine (Cl), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn). They are all essential because the absence of any one of these will cause the plant to grow poorly or develop disease. MACRONUTRIENTS MINERAL 1. HYDROGEN, OXYGEN AND CARBON ✓ Plants and animals are primarily made up of carbon, hydrogen, and oxygen. Plants obtain carbon and oxygen from the air (as CO2 and O2) and hydrogen and oxygen from water (H2O). With the help of light energy, they recombine these three elements into carbohydrates. This happens in the leaves of plants during photosynthesis: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2 ✓ Carbon, hydrogen, and oxygen also combine to form hydrocarbons, the long molecular chains that make up fats, and the same three elements combine with nitrogen to form the main structure of proteins. Overall, these three elements are key components of the large organic molecules that comprise all living beings. ✓ Carbon also plays a key role in global climate change, as increased levels of CO2 and CH4 (methane) in the atmosphere (along with water vapor and a few other gases) reflect infrared radiation to the earth, overall increasing the average surface temperature. Ecologists and soil scientists have been examining the potential for building up soil organic matter as a way to sequester C, removing it from the atmosphere and maintaining it in the soil. NON MINERAL 2. NITROGEN (N) ✓ Physiological role in plant development Plants take up nitrogen either as the ammonium ion (NH4+) or nitrate (NO3-). Most organic compounds in plants contain nitrogen, including amino acids, nucleic acids, many enzymes and energy transfer materials such as chlorophyll, ADP, and ATP. N is necessary for the production of sugars such as is found in sweet ripe fruit. Growing plants must have N to form new cells, so it is essential for plants. ✓ Nitrogen deficiency symptoms in plants include: o Slow growth, stunted plants o Yellow-green color (chlorosis) o Firing (burnt look) of tips and margins of leaves beginning with more mature leaves o Low protein content of crops ENGR. JONAYKA A. TAPIADOR Page 2  SOIL CHEMISTRY ✓ Symptoms of nitrogen excess include: o Dark green, succulent, vegetative growth at the expense of seed production in grain crops, the expense of fruit production in tomatoes and some tree crops, and the expense of sugar content in beets o Watery potatoes o Frost damage if there is too much succulent growth when frost hits o Weakened stems (lodging) o Delayed flowering or fruiting o Boron or copper deficiency due to inhibited uptake of these nutrients ✓ Forms of nitrogen in the soil: o Nitrogen gas in the soil air (N2) o Nitrate (NO3-) o Nitrite (NO2-) o Ammonium (NH4+) o Ammonia (NH3) 3. PHOSPHORUS (P) ✓ P is present in all living cells, including as nucleic acids (DNA and RNA), as part of phospholipid cell membranes, and as molecules for energy storage and transfer (ATP). P also stimulates early growth and root formation, hastens bloom time, and promotes seed production and size. It is used in protein synthesis and is found in legume nodules. ✓ P must be balanced with N both in the plant and in the soil. In the soil, P and N compete to be taken up. Because N is highly mobile and P is one of the least mobile nutrients, excessive N availability can cause a P deficiency, even if there is enough P in the soil for the crop. ✓ Phosphorus deficiency symptoms in plants include: o Slow growth, stunting o Purplish coloration on foliage of some plants o Dark green coloration with tips of leaves dying o Delayed maturity 4. POTASSIUM (K) ✓ Potassium plays a role in several key processes in plants: o Regulating the rate of photosynthesis (by activating enzymes used in photosynthesis and by helping in the production of the energy storage molecule ATP) o Opening and closing stomata (openings on leaves) to allow CO2 in and O2 out and to regulate water loss o Transporting sugars within plants, again by its role in ATP production o Starch formation, by activating the enzyme responsible for this process o Plant growth, by helping to produce proteins (the building blocks) and enzymes that regulate growth ✓ Potassium deficiency symptoms in plants include: o Slow growth o Tip and marginal “burn” starting on more mature leaves and progressing toward the top of the plant o Weak stalks, plants lodge (fall over) easily o Small fruit or shrivelled fruit and seeds o Reduced disease and pest resistance o Increased sensitivity to drought, frost, and salts o White or yellow spots develop along the edges of clover leaves; in severe cases these join to give a scorched appearance ✓ Excess potassium can cause: o Magnesium deficiency o Calcium deficiency in acid soils ENGR. JONAYKA A. TAPIADOR Page 3  SOIL CHEMISTRY 5. CALCIUM (Ca) ✓ Calcium is an essential part of cell wall structure and must be present for the formation of new cells throughout the plant. Calcium also helps control movement into and out of cells, including by reacting with waste products to precipitate them or to render them harmless to the plant. ✓ Calcium is not mobile in plants. Young tissue is affected first when there is a deficiency. ✓ Deficiency symptoms in plants include: o Death of growing points, including on the root tips and shoot or leaf tips o Abnormal dark green appearance of foliage o Premature shedding of blossoms and buds o Weakened stems because cell membranes lose permeability and disintegrate o Blossom-end rot of tomatoes o Short, thick, bulbous roots ✓ Plants take up Ca as an ion (Ca2+). Calcium is normally so abundant that it usually only needs to be added to very acidic soils where lime is required. However, excessive irrigation can leach Ca from the soil enough to cause deficiency symptoms in plants. Excess Ca can lead to a deficiency of Mg or K. 6. MAGNESIUM (Mg) ✓ Magnesium is the central atom of chlorophyll molecules, so it is required for photosynthesis. It also helps activate key enzymes for converting CO2 gas into carbohydrates, as well as many plant enzymes required in growth processes. Magnesium also activates enzymes necessary for P transfer within plants. ✓ Magnesium is mobile within plants and can be translocated from older tissue to younger tissue during conditions of deficiency. ✓ Symptoms of Mg deficiency include: o Chlorosis (yellowing) between the veins in older leaves; marginal yellowing with a green fir-tree shape along the big midrib of the leaf o Upward curling of leaves along their margins o Stunted growth o Ripe fruit is not sweet ✓ Plants take up Mg in its ionic form (Mg2+). Magnesium is generally available throughout the dry-climate Western states but it is often more deficient than Ca. Like Ca, Mg is easily leached, and soils with low CEC have low Mg content. It is important to have a balance of Mg, K, and Ca ions so that no one of these elements dominates the CEC sites. 7. SULFUR (S) ✓ Sulfur is part of two amino acids (cysteine and methionine) that are incorporated into proteins. Sulfur is also essential for nodule formation by N-fixing bacteria on the roots of legumes. It is present in oil compounds that give plants such as garlic and onions their characteristic odor. (Vidalia onions, known for their sweetness, come from an area that has low S soils.) ✓ Sulfur deficiency problems can occur if growers rely on fertilizers that are concentrated with other nutrients (e.g., N, P, and K) but are free of S. ✓ Symptoms of S deficiency in plants include: o Pale young leaves, light green to yellowish in color, sometimes with veins lighter than surrounding tissue. In some plants older tissue may be affected also. o Small and spindly plants o Slow growth rate and delayed maturity ✓ Plants take up S as the sulfate ion SO42-. Sulfur is also sometimes absorbed from the air through leaves in industrial areas where S is in high concentration. ENGR. JONAYKA A. TAPIADOR Page 4  SOIL CHEMISTRY MICRONUTRIENTS 1. BORON (B4O72-) ✓ Boron is needed in plants for: o Synthesizing protein o Transporting starches and sugars o Regulating N and carbohydrate metabolism o Root growth o Fruit and seed formation o Water uptake and transport o Boron contributes more than any other micronutrient to the quality of produce. ✓ Boron is required in minute quantities by plants, but may be insufficient in some soils. Boron can also become toxic in amounts not much higher than that needed by plants. Boron toxicity is most frequently a problem in soils formed at the bottoms of enclosed basins in arid areas where groundwater evaporates upward through the soil, leaving salt concentrations near the surface. 2. COPPER (Cu2+) ✓ Copper is a catalyst for respiration (combusting sugars for energy in plants) and an activator of several enzymes. It is important for carbohydrate and protein synthesis. ✓ Copper is fairly abundant and deficiencies rarely occur. It is found as impurities in the structures of clay particles and other soil compounds. As these materials weather the copper is released, and then adsorbed onto CEC sites, from where it may be taken up by plants or leached from the soil. Consequently, soils formed from highly weathered materials may be deficient in copper. However, since copper can be highly toxic at low levels, amendments should not be used except where the need for it has been established. 3. IRON (Fe2+, Fe3+) ✓ Iron plays several critical roles in plants. It is used in chlorophyll synthesis, during respiration, and as a constituent of some enzymes and proteins. It also activates nitrogen fixation. ✓ Plants require Fe in larger amounts than any other micronutrient. Iron is very abundant in the soil, but some of its forms are so insoluble that plants may suffer a deficiency in spite of its abundance (this would be like being stranded in the ocean yet being thirsty for want of fresh water). This is particularly true at pH levels above 7; where there is a high content of lime or manganese; or where there is poor aeration. Treatment may consist of adding iron in a form that won’t be bound up in the soil or by lowering the soil pH. 4. MANGANESE (Mn2+) ✓ Manganese is part of multiple enzymes and is a catalyst of other enzymes, and so is used in the metabolism of N and inorganic acids; for the formation of vitamins (carotene, riboflavin, and ascorbic acid); for the assimilation of CO2 during photosynthesis; and in the breakdown of carbohydrates. ✓ Similar to Fe, high pH (over 6.5) may make Mn unavailable, as can soils very high in organic matter (muck soils). High Mn levels may induce iron deficiency. Improving soil structure can improve Mn availability. 5. MOLYBDENUM (MoO42-) ✓ Molybdenum is necessary for nitrogen fixation and for converting nitrate-N taken up by plants into a form the plant can use to build amino acids and thus proteins. Because of this a Mo deficiency can cause an N deficiency in plants. ✓ As with boron, molybdenum is needed only in minute quantities and is toxic at levels above what plants require. ENGR. JONAYKA A. TAPIADOR Page 5  SOIL CHEMISTRY 6. ZINC (Zn2+) ✓ Zinc activates enzymes that run photosynthesis, helps regulate and combust carbohydrates, and is part of the synthesis of the plant hormone auxin. It is also a key for seed and grain maturation and production. ✓ Soils formed from highly weathered materials may be deficient in Zn, while soils formed from igneous rocks tend to have higher levels of Zn. Warm soil temperatures improve Zn availability, as does a well-aerated soil. High levels of available P can cause Zn deficiency in plants. 7. CHLORINE (Cl-) ✓ Chlorine is required for photosynthetic reactions in plants. However, the quantities needed are so small that deficiencies are rare and usually in places with high rainfall and sandy soils, where Cl anions would leach out. ORGANIC MATTER Many pleasant drives in the country have been affected when the passengers of a car are hit with an unpleasant, but familiar odor. Many people complain about the smell without questioning why it exists. There is actually a very good reason for this practice. Although they are often given more unpleasant names, these animal wastes are known as organic fertilizers. These fertilizers have a very high content of organic matter. Organic matter is simply dead decaying matter that originated from a living source. It prevents nutrients from being lost from the soil by binding these nutrients. Therefore, the best soil for crop production will have a very high organic content. Most organic fertilizer originates from livestock such as cows, pigs, and poultry. Compost is also an organic fertilizer. Compost can be made up of grass clippings, table scraps, ashes, seaweed, and many other types of food products. Organic fertilizers contain high levels of Nitrogen and moderate levels of Phosphorus and Potassium. The nutrient content of organic fertilizer can vary according to the animal that produced it. The process of spreading organic fertilizers gives farmers the opportunity to rid themselves of accumulated livestock waste. It also provides farmers with a free source of fertilizer which is sometimes sufficient to meet the needs of the desired crop. Organic fertilizers are also less harmful to the environment. This may be one of the first recycling practices that ever developed. ENGR. JONAYKA A. TAPIADOR Page 6