Fertilizer – What do those numbers mean?

When talking about plant nutrition, we are spoiled for choice. But what do we choose? And what do the numbers on the packages mean?

Ingredients

First, let’s understand the major ingredients in fertilizer. Most of the commercial fertilizers have a 3-part number on the package, separated by dashes – for example, 3-4-2 or 5-8-10. These numbers refer to the weight percentage of the three main ingredients in fertilizer – nitrogen, phosphorus, and potassium. So, if you have a 1.36 kg bag of fertilizer labelled 3-4-2 that bag contains 3% nitrogen, 4% phosphorus, and 2% potassium. The rest of the bag is minor plant nutrients and fillers.

Nitrogen

Nitrogen is a major component of chlorophyll which is instrumental in photosynthesis. It has an important role in this critical function for plants, aiding in producing protein. When plants don’t have enough nitrogen they turn yellow, they don’t grow as large, and they produce smaller fruit.

Phosphorus

Phosphorus is a component of ATP (adenosine triphosphate) which is the molecule responsible for energy storage and transfer in cells. ATP’s structure contains phosphorus and plants need this nutrient for photosynthesis, energy storage and transfer, respiration, and other functions. Plants that don’t have enough phosphorus have thin, weak stems. Their leaves turn a bluish-green and they are less likely to produce flowers, fruit, and seeds.

Potassium

Potassium helps in regulation of carbon dioxide intake, activates enzymes, and is needed in the production of ATP. Plants that lack potassium do not grow properly, their roots are poorly formed, and their stems are weak. Lacking potassium, they cannot use water efficiently.

When choosing fertilizer for your indoor garden, you want a complete fertilizer that has all three of these main ingredients. While the three main fertilizer needs are listed on the front of the bag, reading the ingredients label on the back will show you the other ingredients in your fertilizer. These will likely be things such as magnesium, iron, calcium, organic matter, and fillers.