Soil Fertility and Fertilizers

• Elements are the basic building blocks of chemistry and biology.
• Of the over 100 which have been identified, about 80 occur in nature, and only 16 are required for plants to complete their life cycle.
• Three of the 16 (carbon, hydrogen, and oxygen) are provided to plants through photosynthesis.
• The 13 other elements are usually present as part of larger chemical compounds, but are generally able to divide into charged particles called ions which are used by growing plants.
• Photosynthetic elements are carbon, hydrogen, and oxygen
• Primary plant food elements are nitrogen, phosphorus, and potassium
• Secondary plant food elements are calcium, magnesium, and sulfur
• Micronutrient elements include boron, manganese, copper, zinc, iron, molybdenum, and chlorine
• The plant uses carbon as carbon dioxide in the photosynthetic process.
• Water is also a source of hydrogen and oxygen, and may supply some secondary plant food elements and micronutrients.
• The plant-availability of many micro-nutrients and phosphorus (and many other soil processes) depends on the pH or soil acidity.
• Molybdenum becomes very unavailable at low pH, but manganese

  becomes very available (to the point of toxicity) in the same conditions.
• Frequent light irrigations may cause salt accumulations in the roots and lead to serious plant injury.
• The pH (acidity or alkalinity) factor and total salts in water are also important.
• To avoid salt damage, water applications should be less frequent, but longer in duration to wash excess salts below normal root depth.
• Plants obtain the primary and secondary nutrients and some micronutrients solely from the soil.
• Cation-exchange capacity is defined as the degree to which a soil can adsorb and exchange cations.
• Cation is a positively charged ion.
• Anion is a negatively charged ion.
• Soil particles and organic matter have negative charges on their surfaces.
• Mineral cations can adsorb to the negative surface charges or the inorganic and organic soil particles.
• Once adsorbed, these minerals are not easily lost when the soil is leached by water and they also provide a nutrient reserve available to plant roots.
• Cations are not held equally tight by the soil colloids.
• The relative concentrations of the cations in soil solution help determine the degree of adsorption.
• When the cations are present in equivalent amounts, the order of strength of adsorption is Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+.
• Very acid soils will have high concentrations of H+ and Al3+.
• In neutral to moderately alkaline soils, Ca2+ and Mg2+ dominate.
• Poorly drained arid soils may adsorb Na in very high quantities.
• In contrast to CEC, Anion-Exchange Capacity (AEC) is the degree to which a soil can adsorb and exchange anions.
• AEC increases as soil pH decreases.
• Mineral anions such as nitrate (NO3- and Cl-) are repelled by the negative charge on soil colloids.
• Mineral anions such as nitrate (NO3- and Cl-) are repelled by the negative charge on soil colloids.
• Nitrogen is part of every living cell and usually increases plant growth more than any other element.
• Inside the plant, nitrogen is part of amino acids, which in turn make up proteins.
• Nitrogen is an important component of DNA.
• Nitrogen is also part of the chlorophyll molecule, thus, it is important in photosynthesis.
• Phosphorus serves as the currency of energy exchange within the plant itself.
• Phosphorus plays roles in photosynthesis, respiration, cell division, cell enlargement, and many other processes within the plant.
• Phosphorus promotes early root formation, and improves the quality of many fruits and vegetables.
• Potassium is essential for plant growth, however the exact functions within plants are not well understood.