Fertilizer 101

So far as we know, all living things, plants, animals, fungi, etc., need various nutrients in order to complete their life cycle. We’ve read the packages that our food comes in and at least tried to understand what they are.

Plants are a bit different. They need elemental nutrients (like us), but we tend to divide these into two major groups – the major nutrients and the minor nutrients. The minor nutrients are those that are needed in very small amounts or only at certain times of their life. The major nutrients are those (usually only three) that are needed in relatively large amounts and constantly.

The next time you purchase some plant fertilizer check to see if there is a strange number on the bottle or bag. This is usually a series of three numbers, for instance, 10-10-10, 5-10-10, 8-12-22. These have great meaning for whether the plants you are going to give the fertilizer to will merely survive or will really thrive.

Major Nutrients 

These three numbers refer to the individual percentages of the bag contents that are made up of Nitrogen, Phosphorous and Potassium (NPK).  [The chemical symbol of Nitrogen is N; the chemical symbol for phosphorous is P and the one for potassium is K] If we use the common type of fertilizer that is designated on the bag as 12-5-5, we know that 12% of that bag of fertilizer is made up of nitrogen, 5% is phosphorous and 5% is potassium.  Likewise, if the numbers on the bag are 0-0-12 you will know there is no nitrogen and no phosphorous and 12% is potassium.

So, in a 40-pound bag of fertilizer a 10–10–10 fertilizer, 4 pounds are nitrogen and the same for both phosphorous and potassium.  And 28 pounds are filler and carriers designed to carry the fertilizer.

OK, so what good are these numbers to me, a simple gardener?  Perhaps the best thing is to understand what each of the 3 major nutrients is used for by the plant.  

Nitrogen (N) is primarily used for the development of leaves and leaf parts on the plant.

Phosphorous (P) is used by the plant for root development and flower and fruit production.

Potassium (K) is used for the overall functions of the plant so it can perform optimally.

If you are interested in leafy veggies, like spinach, lettuce, chard and cabbage and you want the best of the best, you would add more nitrogen with your fertilizer to get the best leaves.  Likewise, if you are interested in growing tomatoes, okra, cucumbers, etc., you would like more phosphorous in your fertilizer.  And a good mediator for the plants, no matter what you’re growing, is to add some potassium.

If you are not sure what to use, select a balanced fertilizer such as 5-5-5 or 10-10-10.  But before you select something, a good idea is to have your soil tested.  For about $8.00 you can get the results of a soil sample.  Using the soil test box provided by the University of Tennessee, Hamilton County Extension Office [(423) 209-8560)].  They will process the sample and report the results directly to you.  Then you’ll know what the soil is deficient in and you’ll be able to adjust accordingly.

For instance, if the soil test shows you need 1 pound of nitrogen per 1,000 square feet of yard in your soil, you can add a 10-10-10 fertilizer at the rate of 10 pounds to restore your soil (10% of 10 pounds is 1 pound).  Remember to err on the side of lower amounts so as not to harm your plant; fertilizers can “burn” your plants.

Minor Nutrients

Needed in very low concentrations, but necessary. Iron (Fe), Boron (B), Molybdenum (Mo), and Zinc (Zn) are the ones most often needed in our area.   These are usually found in low amounts in the soil.

If all the leaves on a plant are a yellowish green, your plant may have chlorosis, which can be cured by adding iron to the soil.  And if your soil is slightly basic (e.g., between pH of 7.2 to 9.4) the above nutrients cannot be used by plants. When the pH is that high, the plants cannot take up the nutrients.  

Soil testing will show how acidic or basic your soil is and you can adjust it accordingly.  We recommend having your soil tested in the late autumn so that any amendments applied can work into the soil over the winter and be ready for your veggies, lawn, or landscape plants in the early spring when they break their dormancy and begin growing.

With too little boron in the soil, plants may appear healthy but will never bloom or set fruit.  Boron cannot be absorbed by plants if the soil pH is above 7.4.  

Molybdenum must be available in the soil otherwise several critical enzymes will not be present and continued life will be curtailed.

Zinc also impacts the production and use of enzymes within the plant cells in particular the production and use of chlorophyll.

While these nutrients and their effect on plants are difficult to understand, an easy two-step process can be followed for the best growth results. First, test your soil every year to determine just how many nutrients are needed. Second, apply the needed nutrients in the amounts shown on the soil test results. And lastly, enjoy the fruits of your labor.