What Plant Nutrients Do and How to Fertilize

You’ve probably already planted much of your garden and things are starting to grow. How can you make sure that your plants continue to grow, stay healthy, and taste good? This is where fertilizer comes into play. While plants make 90-95% of their food themselves through photosynthesis, they get the other 5-10% through nutrients in the soil.

Most people know that plants need nitrogen, phosphorous and potassium – these are the three major nutrients that help a plant grow. But what do they actually do for your plants?

  • Nitrogen is primarily used to create chlorophyll, a key driver of photosynthesis, which is what gives plant foliage its green colour. Any time you want more green vegetation to grow, nitrogen is required. However, for plants where you don’t eat the foliage, only add nitrogen in the early stages of growth and cut back when the plant matures as it may produce foliage instead of the fruits/roots that you want.
  • Phosphorous helps stimulate cell division and, therefore, helps drive plant growth in all areas. It also helps in forming sugars and starches, which is important in fruit and root development. Phosphorous is needed at all stages of plant growth.
  • Potassium helps build proteins within the plant, it helps the plant take in CO2 and emit water vapour, and it assists in the movement of nutrients throughout the plant. It is needed in greater quantities as plants mature.

There are several other elements that plants use in smaller quantities. These are beyond the scope of this post but, suffice to say, plants need all of these nutrients for health as well as for improved flavour of the harvest.

As a basic starting point, use a balanced fertilizer that will release slowly – compost or worm castings are great. This alone will enable your vegetable plants to grow, but here are some important fertilizer factors to ensure that your veggies grow really well.

As a general summary for which fertilizer to use and when:

  • Young seedlings require mostly nitrogen and phosphorous
  • Leafy greens require additional nitrogen and phosphorous as they grow
  • Fruiting and rooting plants require potassium and phosphorous as they mature
  • Many fruiting vegetables also require calcium and magnesium as they bear fruit
  • Vegetables that have a more distinct flavour profile will benefit from sulfur, copper and boron

In terms of the amount, vegetables are divided into heavy feeders (e.g., tomatoes, potatoes, and squash), medium feeders (onions, carrots and chard) and light feeders (peas, beans and lettuce). Ideally, heavy feeders should get approximately 3″ of compost each year, medium feeders should get 2″, while light feeders require less than 1″. Of course, it also depends on how fertile your soil already is.

Finally, let’s discuss where to apply the fertilizer. To start with, almost all fertilizers should be fed to plants through the soil. Fertilizers are best worked into the soil (without disturbing plant roots) so that they don’t leach away when it rains. As exceptions, some sickly plants enjoy a spray of compost tea, while immobile nutrients (e.g., calcium and boron) are often applied as a foliar spray when the plant needs them quickly (e.g., tomatoes with blossom-end rot need calcium in their leaves immediately).

Now that you know which fertilizers plants need and when, how do you buy the right one? The N-P-K indicators on most fertilizers tell you the percentage of the fertilizer’s weight that is comprised of nitrogen, phosphorous and potassium respectively.

With this info in mind, take a few minutes to classify your plants based on food requirements (high, medium and low-feeder) and the type of vegetable (leaf, stem, fruit, flower and root) so that you can choose the right fertilizer in the right amounts, and apply them at the right times and in the right places!