Fertilizers provide plants with the nutrients that they need, outside of what they cannot get from the environment around them. There are 3 major nutrients and 6 minor nutrients that plants need for their health. The major nutrients are the N-P-K nutrients – Nitrogen – Phosphorus – Potassium. The 6 minor nutrients are calcium, magnesium, sulfur, iron, manganese, and zinc. Deficiencies in these nutrients will result in poor growth and even death in plants.
At the same time, the gardener needs to understand that an excess of nutrients is just as bad as any deficiency. Too much nutrients can lead to what is termed “fertilizer burn.” Just like vitamins, some are needed by plants in large quantities and others, in only minute amounts. Too much fertilization, particularly chemical fertilizers, can also reduce the quality of your soil or upset its ecological balance in the long run.
Nitrogen
The main nutrient needed by plants is nitrogen. Nitrogen is not a mineral, and therefore, is not found in the soil. Plants can only obtain nitrogen from the air, or from organic material added to the soil. Although nitrogen is plentiful in the atmosphere, it is reliant on agents to transfer it into the soil, where plants need it the most. These agents include:
- Rain
- Bacteria
- Decomposing organic material
In order for plants to utilize nitrogen, it first needs to be in nitrate form. Nitrogen in nitrate form can be easily absorbed by plants for immediate use. Nitrogen in ammonium form on the other hand, cannot be broken down so easily, and needs the action of microbes or other organisms to do the job.
A good nitrogen fertilizer should contain part ammonium-nitrogen and other nitrates to simultaneously release nitrogen slowly and rapidly. Organic nitrogen like urea nitrogen which release nitrogen slowly, should only be used in conjunction with faster acting nitrogen sources.
Phosphorus
Phosphorus is a mineral, and therefore, it does not dissolve or break down as easily as nitrogen molecules. Phosphorus ions bind together with other elements to form phosphate compounds which may or may not be absorbable by plants, depending on which other ions have bound themselves together.
Phosphorus fertilizers should be applied to the soil as close to the roots as possible, or in anticipation of their growth path. The reason for this is because of the highly reactive nature of phosphorus, which will bind onto other mineral ions at the slightest opportunity.
As a starter fertilizer, phosphorus and nitrogen are the two most important components which should be applied in the soil around seeds before they sprout, usually about a couple of inches between them and the seed.
Phosphorus fertilizers are commonly available as either water soluble, citrate soluble, or citrate insoluble forms (citrate is a form of citric acid). For seed planting, water soluble or citrate soluble phosphate are most suitable. Be careful with diammonium phosphate (read the label) because of its high ammonia content which can injure germinating seedlings; monoammonium phosphate is the better choice.
Potassium
Potassium is one of the most important nutrients for plants; plants need it for manufacturing starch and stomata activation. As such, plants absorb and use it in large quantities every day – nearly as much as nitrogen. While potassium is found in abundance in many soils with the exception of sandy or peaty soils; like phosphorus, it is not easily available to plants unless it is in a form called exchangeable potassium. The main bulk of soil potassium is locked up in mineral formations that are resistant to weathering.
Exchangeable potassium is found as colloids on clay particles or humus particles, where the K ions trade places with other ions that replace them once they get taken up by plants. This exchangeable form of potassium is best obtained from fertilizers, since freely available potassium is naturally scarce in the soil, and easily washed or leached away by rains.
Potassium and phosphorus fertilizers should be split into several applications a year, because plants will take in potassium regardless if they need it or not. So splitting up the applications will save valuable potassium from going to waste, or improve its efficiency.
Potassium should be applied as close to the roots as possible, because like phosphorus, it doesn’t “travel,” and will get bound to other ions before the roots even get near. Manure, compost, or potassium chloride will all provide potassium to the soil, while improving its structure. For fruit trees and vegetables, try using potassium nitrate instead, which has a lesser potassium content than potassium chloride, and provides some nitrogen as well.
