Monday, 3 May 2010

How to Understand about Fertilizers

In order for a plant to grow and thrive, it needs a number of different chemical elements. The most important are Carbon, Hydrogen and Oxygen - available from air and water and therefore in plentiful supply.

Fertilizers are roughly broken up between organic and inorganic fertilizer, with the main difference between the two being sourcing of the chemicals, and not necessarily differences in nutrient content.

Organic fertilizers and some mined inorganic fertilizers have been used for many centuries, whereas chemically-synthesized inorganic fertilizers were only widely developed during the industrial revolution. Increased understanding and use of fertilizers were important parts of the pre-industrial British Agricultural Revolution and the industrial green revolution of the 20th century.

Plant life require thirteen essential nutrients. Three Major and three Secondary Macronutrients and seven Micro-nutrients, which are taken up from the soil or any other growing medium.

Major Macronutrients
  • Nitrogen (N) is required for healthy stems and leaves. It is an essential part of the amino acids which form the proteins and forms part of the chlorophyll molecule that has the main role in photosynthesis. 

    Large quantities are needed for vegetative growth, but too much will cause soft leafy plants which are vulnerable to pests and frost, and will delay flowering. It is taken up mainly as Nitrate (NO3-) and some as Ammonium ions (NH4+). The Nitrate is converted to Ammonium in the plant for use in the making of amino acids which are the building blocks of all living things. An excess at the roots can change the osmotic balance and water cannot be taken in by the plant.

  • Phospherous (P) is taken up as Phosphate ions (PO43-). It is found in the nucleic acids, DNA and RNA so is needed for reproduction and enzyme function. The ATP energy transfer process within plant cells requires P. It is moved around within the plant, being recycled from older parts to points of new growth.
  • Potassium (K) Is not involved as part of essential molecules but is used in the metabolic pathways such as protein synthesis and in maintaining water balance. It is required for healthy leaves, flowers and fruit, also making plants winter-hardy and improving disease resistance.
The macronutrients are consumed in larger quantities and are present in plant tissue in quantities from 0.2% to 4.0% (on a dry matter weight basis). Micronutrients are consumed in much smaller quantities and are present in plant tissue in quantities measured in parts per million (ppm), ranging from 5 to 200 ppm, or less than 0.02% dry weight.


Without the macronutrients, nitrogen, phosphorus and potassium, the plant simply cannot grow because it cannot make the pieces it needs. It's like a car factory running out of steel or a road crew running out of asphalt.
If any of the macronutrients are missing or hard to obtain from the soil, this will limit the growth rate for the plant. In nature, the nitrogen, phosphorous and potassium often come from the decay of plants that have died. In the case of nitrogen, the recycling of nitrogen from dead to living plants is often the only source of nitrogen in the soil.


To make plants grow faster, what you need to do is supply the elements that the plants need in readily available forms. That is the goal of fertilizer. Most fertilizers supply just nitrogen, phosphorus and potassium because the other chemicals are needed in much lower quantities and are generally available in most soils. Nitrogen, phosphorus and potassium availability is the big limit to growth.

The numbers on a bag of fertilizer tell you the percentages of available nitrogen, phosphorus and potassium found in the bag. So 12-8-10 fertilizer has 12-percent nitrogen, 8-percent phosphorous and 10-percent potassium. In a 100-pound bag, therefore, 12 pounds is nitrogen, 8 pounds is phosphorous and 10 pounds is potassium. The other 70 pounds is known as ballast and has no value to the plants. 


We show the percentages in our fertilizer articles by showing something like that below, which is the Growmore percentages:
  • NPK 7-7-7
Secondary Macronutrients
  • Magnesium (Mg++) required in small quantities for chlorophyll, where it forms the centre of the molecule. Also it is involved in the production of ATP, the molecule which facilitates energy transfer in living cells. Deficiency manifests as whitish stripes between the leaf veins. It is applied as soluble Magnesium Sulphate (Epsom Salts) or in a more slow release form as Magnesium Carbonate (Dolomitic Lime). Also it is available from Potassium Magnesium Sulphate and Nitrogen Magnesium fertilizers.
  • Calcium Ca taken up as (Ca++)ions, is required for the healthy growth of new stems as it is used to give cell walls their strength. It also helps in the uptake of negative ions. There is usually enough available in chalky soils and liming is the usual method of adding it, but this affects the pH of the soil. Too much can lead to Iron deficient chlorosis (see Fe below) - which usually shows on younger foliage. A deficiency can cause Bitter Pit on apples. It is slowly translocated in the plant so for quick action it is best to be applied as a soluble foliar feed such as Calcium Chloride which does not alter the soil pH. Gypsum (Calcium Sulphate) is a slow release source which does not affect the pH.
  • Sulphur (S) is taken up as Sulphate ions (S)4=. It forms part of all proteins, activates emzymes and is involved in the flavour factors of mustards and alliums. Legumes have higher requirements for S than most other plants.Deficient plants are very spindly and have an overall yellow appearance. It can be applied as soluble salts such as Potassium or Magnesium Sulphate. Ammonium Sulphate can be used where an acidifying effect is desired. It is found in rainfall particularly downwind of industrial regions where fossil fuels are burnt, and is the cause of acid rain.
Micronutrients
The seven Micronutrients are required in lesser amounts, but are just as essential for healthy growth. They are usually present in adequate amounts in most soils, but can be leached from sandy soils. Soil-less composts can run out of them if not supplemented, but if present at high levels they can be toxic to plants:-
  • Boron (B) taken up as H2BO3- ions. It is involved in the transport of sugars, in cell division and the production of amino acids. At a pH greater than 6.8 it is leached from peat-based composts. Borax is the usual compound to use as a supplement.
  • Chlorine (Cl) in the form of Cl- ions. It is needed for photosynthesis and keeping the cells turgid.
  • Copper (Cu) in the form of CU++ ions. It is involved with the enzymes in photosynthesis. Applied as Copper Sulphate or Copper Oxide.
  • Iron (Fe) in the form of Fe++ ions. Can be displaced by Calcium ions in chalky soils making them unavailable for uptake, leading to Iron deficient chlorosis as it is used in the making of Chlorophyll and in other metabolic pathways concerned with photosynthesis. Applied as a chelated form such as Fe-EDTA to the foliage.
  • Manganese (Mn) in the form of Mn++ ions. It is involved in the production of chloroplasts. Can be deficient in slightly acidic to neutral soil. At low pH it becomes more available and can reach toxic levels. Applied as a Manganese Sulphate foliar feed.
  • Molybdenum (Mo) taken up as molybdate (MoO4=) ions. It is needed for the conversion of Nitrate ions to Ammonia in the plant before it is included in amino acids. Also it is used in nitrogen fixation, so this is why it is particularly important to legumes which have a symbiotic retationship with bacteria on their roots that carry out this process. At low pH it is less available and liming usually solves the deficiency by raising the pH. Otherwise it can be applied to the soil as Sodium Molybdate or to the foliage as Ammonium Molybdate.
  • Zinc (Zn) in the form of Zn++ ions. It is used in enzymes particularly in the hormone balance of auxins. Applied as Zinc Sulphate or Zn-EDTA chelate.


Q. So inevitably we come to the question, "Why don't people need fertilizer to grow?" 
A. Because we get everything we need from the plants we eat or from the meat of animals that ate plants. So, our plants are factories that do all of the work to process the basic elements of life and make them available to us. So, look after your plants, because they look after us!

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