The terms "plant food" and "fertilizer" are often used interchangeably. Indeed, it's true that home gardeners use fertilizer to help their plants get the essential nutrients they need to grow and bloom properly. Scientifically speaking, however, fertilizer is not the same thing as plant food. Most plants get hydrogen, oxygen and carbon from water and from the air, but plants also need other nutrients that they usually get from the soil. The most important of these are nitrogen, phosphorous and potassium. These are called macronutrients. Fertilizer contains these and other nutrients for plants, as well as fillers. Plants use these nutrients in fertilizers (and in the environment) to produce their own food.
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Plant fertilizers are composed of macronutrients, micronutrients and ballast, or filler. Some fertilizers contain equal amounts of the three macronutrients. These are usually labeled as 10-10-10 or 20-20-20. The numbers represent the percentage of that nutrient in the fertilizer. The first number represents nitrogen, the second represents phosphorous, and the third represents potassium. Some fertilizers contain more of one of the nutrients. Nitrogen, for example, is used to promote foliage growth. So, fertilizer made for foliage plants may be labeled 20-5-5. Fertilizers also contain micronutrients such as calcium and iron. The bulk of the fertilizer, however, is made up of fillers which are designed to help distribute the nutrients and increase their ability to be absorbed by the soil.
Fertilizers are used to feed plants nutrients, but the plants themselves make their own special food with those nutrients and air, sunshine and water. Carbon dioxide from the air enters the plant through the foliage. As it enters the leaves, it comes into contact with chlorophyll. This substance absorbs and stores energy from the sun. Chloroplasts within the chlorophyll react with the carbon dioxide to produce a simple sugar, which is then distributed through the plant. Water helps move the sugar through the plant. In addition, as water moves through the roots and up into the plant, it also takes with it minerals found in the soil, which are vital for the process of photosynthesis to work properly. Water is also vital to retain the turgidity of the plant's cells. If a plant lacks water, the cells will not be as turgid, and the plant will wilt.
Organic vs. Chemical
Nutrients in fertilizers can be organic or chemical in form. Organic nutrients come from natural sources such as manure, compost or fish meal. Chemical nutrients are purer in form. Organic nutrients take longer to break down into the soil, but they are also sometimes less expensive, especially for home gardeners who have their own compost piles. Chemical nutrients are usually water-soluble and can be absorbed by the plants immediately.
The pH level of the soil is vital when it comes to the ability of plants to absorb fertilizers in order to create their own food. Soils with a very high pH (above 7.0) or low (below 5.5) pH are not as hospitable to nutrients. In these soils, the nutrients in fertilizers are either too soluble or not soluble at all. In effect, the plant can't absorb the nutrients, or alternately, the nutrients become toxic. Soil can be amended with lime or elemental sulfur to lower or raise the pH level.