• Although the term "fertilizer" may refer to human-applied plant nutrients, in the broadest sense fertilizers also include naturally occurring materials that serve as plant nutrients. Deer living in a forested area provide an example. Their droppings deliver organic matter and nutrients that improve soil conditions for plant growth. And their urine contains nitrogen, a major or primary plant nutrient. Plant physiologists divide plant nutrients into what they call primary nutrients, secondary nutrients and micronutrients. A micronutrient is one that is required by the plant in trace amounts.
  
  

Nitrogen

• Nitrogen is a primary nutrient that is important in many cell functions in plants. It is an essential component of amino acids, and amino acids are the building blocks of proteins. Further, nitrogen is an essential element in the chlorophyll molecule. Chlorophyll is the green pigment giving the plant its color, and it is the key molecule in photosynthesis. So nitrogen in one form or another---usually as a nitrate salt---is an essential component of plant fertilizers.
  
  

Phosphorus

• Phosphorus is another primary nutrient in plants. Among other actions, phosphorus is critical in the functioning of cell membranes. It is also a necessary component of the cell's DNA and is a critical constituent of the molecule called ATP. The "P" in ATP stands for phosphate and is essential as part of the so-called energy currency of the cell. Phosphorus promotes flowering and seed production in flowering plants. As such, it is a very important constituent in fertilizer for grain, vegetable and fruit crops.
  
  

Potassium

• Potassium is not as important to cell structure as are phosphorus and nitrogen. But it is nevertheless a primary and necessary element in plant physiology and nutrition. For example, potassium is involved in enzymatic activities that lead to the formation of proteins. Without efficient protein synthesis, other cell functions and overall plant health would be impaired. Any fertilizer, whether natural or synthetic, would be incomplete without potassium.
  
  

Minor Nutrients

• In addition to nitrogen, phosphorus and potassium, there are other nutrients that plant scientists consider secondary but still necessary for plant growth. Examples of these include magnesium, calcium and sulfur. The designation "secondary" doesn't deem these nutrients any less important. It really refers to the relative amounts demanded by the plant. An example of a secondary nutrient is magnesium. Although magnesium is required in relatively smaller amounts than other nutrients such as nitrogen, it is critical to the chlorophyll molecule. And, without the chlorophyll molecule, the rest of a green plant's functioning would grind to a halt.
  
  

Micronutrients

• A handful of elements are considered micronutrients. Examples are copper, iron and manganese. Micronutrients are essential for plant growth but in very small quantities. Deficiencies will cause symptoms in plants but excess amounts will cause toxicity. There is a relatively narrow band of optimal concentration for micronutrients in plant fertilizers.