Fertilizer 101 by the Numbers
How to Understand and Make the Best Use of Plant Food
Easy to follow information on how to select, compare and use fertilizer that every gardner needs to know.
All fertilizers are labeled with three numbers that represent the primary nutrients that plants need: nitrogen, phosphorous and potassium. They are always listed as a percentage of those nutrients and always in that order. For example, a 4-1-2 fertilizer contains 4 percent, by weight, nitrogen, 1 percent phosphorous and 2 percent potassium. These can occur in different ratios and combinations, but they all represent the strength or concentration of the fertilizer. So an 8-2-4 would be twice as concentrated as a 4-1-2. This is important when comparing prices because it would take twice as much of the second fertilizer to equal the same amount of nutrient as the first.
What do the Three Primary Nutrients do? - Nitrogen is very important in plants for good foliage growth and dark green color. Phosphorous is important for rooting and also for blooming, and potassium is important for cold hardiness and plays a role in fruiting and blooming. It is possible to use a balanced fertilizer, one where the three numbers are equal or close to one another (15-15-15), throughout the growing cycle. A better plan is to look for a fertilizer brand that has at least two formulas to accommodate different stages in the plants growing cycle and/or different varieties of plants.
For vegetables grown mostly for their leaves (salad or other greens) a fertilizer with more nitrogen than anything else (a higher first number) is best. For vegetables that flower first like peppers and tomatoes, a fertilizer with a higher proportion of phosphorus (a higher middle number) works well. Alternatively, some gardeners produce great results by using a high growth (high nitrogen) formula for the beginning of the growth cycle and then switch to a high bloom (high phosphorus) fertilizer when the plants begin to flower.
Micronutrients - The three primary macronutrients have been discussed, but what about the other micronutrients that plants need to stay healthy. Fortunately the better fertilizer companies have added these micronutrients to their products as well. It is a good idea to look for a brand of fertilizer that includes additives to address necessary micronutrients. The following is a list of micronutrients and their functions:
Calcium (Ca)
- Strongly influences proper soil pH
- Essential to strong cell wall structure and cell division
- Can improve soil structure and water retention
Magnesium (Mg)
- Plays an important role in photosynthesis and chlorophyll production
- A necessary component in many essential enzyme systems within plants
- Important in aiding the plants use of phosphorous
Sulfur (S)
- Works with nitrogen to produce new protein for plant growth
- Plays an important role in the utilization of oxygen
- Influences the level of activity of soil microorganisms
Iron (Fe)
- Necessary for the formation of the chlorophyll
- Aids in the activation of a number of biochemical processes within the plant
Manganese (Mn)
- Important to the formation of chlorophyll and the activation of the initial growth process
Zinc (Zn)
- Necessary for the production of chlorophyll
Copper (Cu)
- Important in the synthesis of certain plant growth substances
- Serves as an activator for several essential enzymes
Chlorine (Cl)
- May help in the regulation of osmotic pressure within the plant cell
Both organic and chemical fertilizers are available in dry and liquid forms. Once a decision is made concerning which type to use, the information in this article can be applied to compare and select the most economical brands. Regardless of which brand is selected it is a good idea to have a few favorites and switch fertilizers every few months.
The following articles provide additional useful information:
What is Fertilizer?: Nutrients for the soil.
What is Slow-Release Fertilizer?: Fertilize for longer periods
