SOIL FERTILITY AND FERTILIZERS

• • Elements are the basic building blocks of chemistry and biology. 
• ELEMENTS - 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 ELEMENTS (carbon, hydrogen, and oxygen) are provided to plants through photosynthesis.
• These 13 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.
• THE ESSENTIAL ELEMENTS AND THEIR CHEMICAL SYMBOL:

  • PHOTOSYNTHETIC ELEMENTS 

                -Carbon, Hydrogen, Oxygen.

  • PRIMARY PLANT FOOD ELEMENTS

                -Nitrogen, Phosphorus, Potassium.

  • SECONDARY PLANT FOOD ELEMENTS

                -Calcium, Magnesium, Sulfur.

  • MICRONUTRIENTS

                -Boron, Manganese, Copper, Zinc, Iron,

                Molybdenum, Chlorine.
• NATURAL NUTRIENT SOURCES: AIR, WATER, SOIL
• Natural nutrient sources

  • Air
  • Water
  • Soil
• Carbon


  Used by plants in the photosynthetic process as carbon dioxide
• Water


  Source of hydrogen and oxygen, may supply some secondary and micronutrients
• Plant-availability of micronutrients and phosphorus


  Depends on soil pH or acidity
• Micronutrient availability at different pH levels


  • Molybdenum becomes very unavailable at low pH
  • Manganese becomes very available (to the point of toxicity) at low pH
• Knowledge of particular plant materials is necessary to set optimum pH levels
• pH (acidity or alkalinity) factor and total salts in water


  Also important
• Frequent light irrigations


  May cause salt accumulations in the roots and lead to serious plant injury
• 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-a positively charged ion (NH4+, K+, Ca2+, Fe2+, etc...) 
• ANION-a negatively charged ion (NO3-, PO42-, SO42-, etc...)
• 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. 
• EXAMPLES OF CEC VALUES FOR DIFFERENT SOIL TEXTURES ARE AS FOLLOWS:

   Soil texture                        CEC (meq/100g)

  Sand(light colored)----- 3-5

  Sand (dark) ------------- 10-20

  Loam     ---------------- 10-15

  Silt loam ---------------- 15-25

  Clay and clay loams           --- 20-50

  Organic soils        ---------- 50-100
• TWO FACTORS DETERMINE THE RELATIVE PROPORTIONS OF THE DIFFERENT CATIONS ADSORBED BY CLAYS:

  • First, cations are not held equally tight by the soil colloids.
  • Second, the relative concentrations of the cations in soil solution help determine the degree of adsorption. 
• 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.
• Thus 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.
• Thus 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 somewhat of an enigma. 
• Nitrogen is essential for plant growth, however the exact functions within plants are not well understood. 
• Nitrogen is known to be vital to photosynthesis, because it declines when potassium is deficient.
• Plants with adequate nitrogen have a dark green color because of high concentrations of chlorophyll.
• Conversely, nitrogen deficiency leads to reduced chlorophyll concentrations and thus chlorosis (yellowing) of leaves.
• Nitrogen deficiency is shown first on the oldest leaves and then younger leaves as the deficiency worsens.
• Some plants ( strawberries ) show a reddening of the older leaves on nitrogen deficiency.
• The first indication of phosphorus deficiency is an over-all stunted plant.
• In Phosphorus deficiency, leaves may be unusually dark green at some stages, may have distorted shapes, and may become purple.
• Lower leaves may turn yellow between the veins with Phosphorus deficiency.