1579 Fanshawe park Road east, London - (519) 455-8873

Weekly Gardening Guide: Fertilizers!

April 23, 2015

There are so many types of fertilizers out there, so how do you choose what is best for your specific plants?  Organic vs. synthetic, root boosters, lawn fertilizers, liquid, granular. The list goes on! Before you choose one, it's important to first learn what it is that fertilizers do, and what they are made of. Then you can decide which type is best for your specific case. 

You’ve all learned your ABC’s, now it’s time to learn your NPK’s!

You may or may not already be aware that the numbers on a bag of fertilizer are the “analysis”. These three numbers represent the percentage *by weight* of N (nitrogen), P (phosphorus) and K (potassium). For example, a lawn fertilizer that contains 21-3-18 contains 21% Nitrogen, 3% Phosphorus and 18% Potassium. Always in that order! If there are additional nutrients, they will be specifically noted on the package. N, P and K are the 3 major nutrients required by all plants to grow. These are called “macro nutrients”. This means that they are required by plants in relatively large quantities – in most cases, these needs are over and above what the soil can provide on its own, so that’s where FERTILIZERS come in! 


We’ve all heard of nitrogen and we all know it’s important, but do you know why? Well to put it simply, Nitrogen is crucial for plant growth and colour. It is a component of chlorophyll, which is the compound responsible for the green colour of leaves and tissues (and can also produce other colours in plants), moreover it plays a major role in photosynthesis. Photosynthesis (another word we all know, but maybe don’t understand) is the process by which the plant utilizes the sun, water and carbon dioxide to produce “food” known as ATP.

Nitrogen plays a crucial role in this process. A deficiency or lack of nitrogen can lead to a complete shutdown of plant growth and eventual death. Nitrogen is a mobile nutrient, meaning that it can move through the soil with air and water, making it easier for the plant to access following surface application. But be aware, this also means some nitrogen can be lost to leaching – nitrogen moving through the soil below the reach of the root system. 


What about phosphorus? Phosphorus is another crucial component for plant growth. While it plays an active role in cell division and production of DNA in plants, in simple terms its most important job is in carbohydrate storage and allocation within the plant. Like humans, plants use carbs for energy and growth. Phosphorus helps the plant with this process. For example, when the plant is waking up from winter, phosphorus is important in helping the plant reallocate stored energy from root preservation into foliar growth. Phosphorus is an immobile nutrient, meaning that often bonds with soil compounds and does not readily move through the soil. Roots must be touching phosphate compounds in the soil in order to utilize them.

An important note about phosphorus: While it is an essential nutrient, it is also quite abundant in most soils. Once plants are established, they are generally not deficient in phosphorus. Governments and environmental agencies have been pushing for an increase in the number of “phosphate free” fertilizers on the market, since excess phosphorus running into waterways encourages troublesome algae blooms. Phosphorus is typically only required on very sandy soils, or when a new plant (one with little or no roots) is planted or transplanted as their roots have not grown enough to access soil phosphates. 


Potassium, is known as the “root nutrient”. Among other roles within the plant, such as its role in the sodium/potassium exchange within cells, potassium’s most important job is in building healthy and deep root systems. For this reason, potassium is especially important in the fall when the plant needs to prepare for winter, and the reason most fall fertilizers will contain a “high last number” in an analysis. Potassium (commonly found as Potash) should be applied anytime you are laying sod, ground covers, or anywhere a weak root system is suspected. It should also be part of regular lawn and garden maintenance. Potassium is immobile, so the best time to apply it to your lawn or garden is following aeration or cultivation so it can be moved right into the soil/rootzone. 

What about other nutrients?

Other nutrients, mostly known as micronutrients are just as important for plant metabolism. However, they are used in such tiny amounts, that most soils have abundant quantities to sustain plant growth for years, assuming other cultural practices such as composting, mulching and fertilizing are followed. Some examples of essential micronutrients include zinc, boron, magnesium, calcium, and manganese. 

Which fertilizer is best? 

There are so many fertilizer options on the market. From Roses, to trees and shrubs, to lawn fertilizer; granular, to spikes to liquids; from organics to synthetics. The differences between these types of fertilizers could fill an entire text book. To put it simply, every plant has different nutrition requirements, and every soil and location will behave differently with every type of plant! So fertility issues become a bit of a trial and error game. Certainly there are soil tests that can be done to help guide you, but at the end of the day following the general requirements for each plant will act as a baseline, and your plant will help tell you the rest! 

Organic vs. Synthetic

As far as the organic vs. synthetic debate, they truly both have a time and place. Synthetic fertilizers such as Scott’s Turf Builder or Miracle Gro are incredibly accurate and will give you exactly what they promise on the bag every time. However, they do have a tendency to be very 1 dimensional. They do very little to “feed the soil” and focus only on N, P, K (and sometimes Iron or other micros). Organics on the other hand are much more complex by nature, and as a result they sometimes break down too quickly, or unevenly BUT they greatly add to soil structure, stability and can provide a complete range of micronutrients and micro organisms. 

A balanced fertilizer program which utilizes both synthetic and organic fertilizers is recommended by the author (Bryan). When in doubt, think of my motto “Feed the soil not the plant”.

Thanks for reading, and again, my apologies for my inability to be concise. Please feel free to contact us with any questions!

- Bryan

← Back to Blog