It is common for people to say that they are going to “feed their plants” with fertilizers. As with many aspects of our relationship with the plant world, we often find ourselves thinking that plants live and grow the same way animals do. We need to remember that plants do things very differently.
Fertilizing a plant is not the same as feeding a pig or a cat or ourselves. Animals take in carbohydrates, proteins and sugars and during digestion, break them back down into their component parts. The body then uses these elements to build muscle, organs and other tissues. The energy that is released in this process came originally from the sun…through plants. Remember that the steak you are eating came from a cow that ate plants. That fish may have been a predator that ate other fish but somewhere down the food chain, it began with something that was a vegetarian.
On the other hand, plants take the basic elements such as carbon, hydrogen, oxygen, nitrogen, potassium, phosphorus and others directly into their systems. Most of the elements come in through the roots while others enter in gaseous form through the stomata. These nutrients are combined with water, carbon dioxide and the energy from light in the presence of the green pigment, chlorophyll to produce complex compounds such as sugars and carbohydrates. This is the process called photosynthesis that we all learned about in science class.
In the darkness of night, most plants take the carbohydrates and sugars and break them down again. This process is called respiration. The plant uses the end products plus some oxygen to build complex tissues such as roots, stems, leaves, flowers, bark, seeds and other plant parts.
The practical implication of all this bears on how and why we apply fertilizers. The key is to remember that an application of nitrogen will do absolutely no good unless the plant has the proper mix of nutrients, water, light, temperature and chlorophyll necessary to complete photosynthesis.
If I eat a bunch of donuts, cokes and potato chips all the time, I will gain weight no matter how much time I spend in the sun. A plant will not grow unless it has the proper balance of all of those factors needed for photosynthesis. The minimum levels of each input required, however, will vary from genus to genus of plant. That is why some plants can grow in the shade while others require full sun all day long.
Sometimes people think that poor growth in plants can be overcome simply by giving them a shot of fertilizer. They want to fertilize full-sun turfgrass situated in the deep shade to make it grow better. Without adequate light, the plant will not be able to use the nitrogen no matter how much is applied. That is why it is often recommended to fertilize grass in the shade only during the spring or the fall before or after the leaves have dropped from the tree and the grass can receive full sun. Even then, this is a case of trying to make the most of a bad situation.