Question

What is fertilizer and why do plants need it? What is in fertilizer? What do the 3 numbers on most bags of fertilizer mean?

Answer

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
• Nitrogen, phosphorus, potassium (a.k.a. potash) - the three macronutrients, and the three elements you buy in most packaged fertilizers
• Sulfur, calcium, and magnesium - secondary nutrients
• Boron, cobalt, copper, iron, manganese, molybdenum and zinc - micronutrients.

• Every amino acid contains nitrogen.
• Every molecule making up every cell's membrane contains phosphorous (the membrane molecules are called phospholipids), and so does every molecule of ATP (the main energy source of all cells).
• Potassium makes up 1% to 2% of the weight of any plant and, as an ion in cells, is essential to metabolism.

If any of the macronutrients are missing or hard to obtain from the soil, their lack 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 are the big limits 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% nitrogen, 8% phosphorous and 10% 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.

So why don't people need fertilizer? Because we get everything we need from the plants we eat or from the meat of animals that ate plants. Plants are factories that do all of the work to process the basic elements of life and make them available to us!

Here are several interesting links:

• Fertilizers - the basics
• Correcting soil nutrient deficiencies with fertilisers
• Choosing a fertilizer
• World fertilizer situation and outlook

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