All vegetables, flowers, and herbs that we grow use photosythesis from sunlight and the air, and NPK and trace elements from the soil, in order to survive and grow.

But how does all this really work?

Here are a couple of responds from two of our GardenWeb organic gardening experts and friends:

Kelly_Cassidy:

"The common use of the term "feeding" to mean application of NPK is misleading. Animals get their nutrients and energy from the same source, i.e., they eat other organic matter. Plants get their energy from the sun: they convert the energy in sunlight to stored chemical energy in the form of carbon compounds such as sugars. They take up carbon dioxide from the air and water and combine them, using solar energy, into things like sucrose (table sugar) which has only carbon, hydrogen, and oxygen. Starch and cellulose are made by linking sugars together. The bulk of plant material is carbohydrates (sugars, starches, cellulose, etc.)

But, in order to put those carbohydrates together, plants need a cell structure and enzymes that drive the reactions. Enzymes are a specialized type of proteins. Proteins are also major structural components of cells. In fact, it is proteins that ultimately control everything that is made in a cell and everything that goes in and out of a cell. Proteins are chains of amino acids. All amino acids contain at least one N molecule. Plants make all their proteins from scratch. They pick up the N (in inorganic form, with a few very rare exceptions) as nitrate or ammonia, assemble the amino acids, then link the amino acids into the proteins they need. Animals eat other organisms and use the amino acids and proteins in those organisms to make their proteins.

DNA is a "dictionary" of an organism's proteins. It stores the information about which amino acid to link to which to make a particular protein. DNA has a sugar-phosphate backbone. The plant needs P (in the form of phosphate) to make more DNA. It also needs P for ATP (adenosine triphospate), which is sort of the immediate currency cells use to drive chemical reactions. P exists in all cells, but is also concentrated in vertebrate bones, which are made of calcium phosphate.

Plants use K (potassium) mainly to control opening and closing of stomates and (according to some new ongoing research) probably also to control the flow of water in xylem. Thus, K is very important in controlling water uptake.

In short, plants are mostly C, H, and O in the form of sugar, starch, and cellulose, but those small amounts of N, P, and K are critical for the plant to function."

Field:

"Just to add a little more to what Kelly has said, here is the typical chemical analysis, by weight, of a plant:

Oxygen - 45%

Carbon - 44%

Hydrogen - 6%

Nitrogen (N) - 2%

Phosphorous (P) - 0.5%

Potassium (K) - 1%

All other nutrient elements combined - 1.5%

Notice that N, P, and K are in the ratio of 4-1-2, which is the reason why so many agronomists recommend that ratio for mineral fertilizers."

It always helps to better understand how plants grow, in order for us as gardeners to do our jobs better.
Happy Gardening!