How does plant food work?

:(Yet again as a school project we have to make our own brand of plant food, i need to know what is in plant food and also how plant food works.:(

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Not sure about making your own plant food unless you mean formulating a mix of the raw chemical ingredients.

Different propriety plant foods will contain varying proportions of the essential major chemical nutrients.
The major elements (macronutrients) are Nitrogen; Phosphates; Potassium; Calcium; Sulphur, and on a container of fertiliser you will see the letter N P K with figures after them denoting the ratio of N (nitrogen) P (phosphates) and K (potassium) in the mixture. What trace elements are added is somewhat of a mystery; in most cased probably none.

Trace elements or (micronutrients) are Iron, Copper, boron, zinc, Manganese, Molybdenum, Boron and are usually plentiful in cultivated soils that are kept fertile. There are also very fine traces of several other elements obtained from soils.
Appropriate chemicals can be purchased in the case of a deficiency of a particular trace element.

The chemical elements are taken up by the plants natural function of transpiration which draws water up from the roots by a process called Osmosis (put very simply). The chemical elements within the plants water are utilised during the plants process of Photosynthesis to manufacture substances which go to building plant cells (plant growth).

The amateur gardener often makes plant food using natural ingredients such as sheep droppings and the leaves from the Comfrey plant. Exactly what percentage of which plant nutrients the resulting liquid contains can only be determined by analysis.

Andrew, this a only an extremely short answer to your question and to gain an understanding of such a subject you will need to do some reading - possibly the local library; or learned papers on the www

http://www.raffia.plus.com

 

Basically plant food supplements nutrients naturally found in the soil but there are many types of plant food, do you need to discuss artificial fertilizer? like Plantsman said NPK or do you also need to talk about organic fertilizer's such as growing green crops or legume type plants (peas basically) and then digging the plants into the soil at the end of the year, or even natural types of fertilizer such as leaf fall in autumn