Gardening Tips: Fertilizing Periodically

Editor’s Note: We’re dipping into Michelle Smith’s archives to provide timely tips for gardeners.


What to do this week

As promised last week, here we go with a little bit of science. There are four main nutrients your plants look for in the soil, so you have to know about them. They are nitrogen, phosphorous, potassium and calcium. They are known as N, P, K, and Ca in soil science, their letters from the periodic table of elements. The first three, the N-P-K, are the numbers that you will see on any fertilizer you buy. Even natural fertilizer like blood or bone meal usually has their N-P-K numbers written on the front somewhere. In future weeks, we will talk about what kind of effects these three main nutrients have on the biology of plants. This week, I want to talk about calcium and soil acidity, because now is the time of year you should be thinking about soil acidity, and whether or not you should be putting lime on your garden.

Periodic Table (By 2012rc (Own work Notes and font fixed:The Photographer) [CC BY 3.0 (], via Wikimedia Commons)

Periodic Table (By 2012rc, own work, CC BY 3.0, via Wikimedia Commons)

All soils are made from rock that has been ground up. If it is not ground up very finely then it is sand. If it is very fine, it is clay. Clay is just rock that has been ground so finely that the particles of the soil have an electrostatic charge that makes them cling together. That is what makes clay sticky. Clay soils are also usually very acidic. Our Cape Breton soil tends toward clay, something that is very common in areas with high rainfall. When a soil is acidic or clay, the nutrients have a hard time moving through it and up into the plant. Adding compost can help, and is very important, but you may have to raise the pH of your soil even further. This means to make it less acidic or more alkaline. Remember, if the soil is too acidic, the nutrients don’t move properly through it to the plants where they are needed. You can fix this with lime.

Liming the soil is not absolutely necessary every year, or on a large scale, especially if you have good organic matter because you add lots of compost. But generally speaking, in the Maritimes, you are going to have to add some agent to additionally raise the pH toward alkaline.

Some people use hardwood ash instead of lime. That is what people used to use to make soap, so you can imagine that it is quite alkaline and will raise the pH in your soil, but the potassium hydroxide you find in wood ash is very water soluble. Wood ash will really spike the pH in your soil, but the next heavy rain will wash it down again. These sudden changes can be a shock to all the microorganisms in the soil so wood ash is okay to use once in a while in the garden, but it is not a good long-term strategy. It is a good way to add potassium, if you need potassium, but it isn’t a good way to manage the pH of your soil. The best way is to add garden lime.

There are two main kinds of garden lime. There is what I call regular garden lime. You can get that pelleted or powdered. Powder is usually cheaper, but it is more difficult to spread evenly. You can put the pellets in your fertilizer spreader and easily spread them over your garden or lawn. But better than that is what is called “dolomitic limestone.” This has the calcium that is needed to raise your pH, but it also has magnesium in the correct proportion needed to match the calcium. Plants have a biology that requires they get calcium and magnesium in a certain proportion in order to take nutrients up properly. Because dolomitic limestone has this balance between the two, I highly recommend you use it to raise the pH of your soil. Unfortunately, it only comes in powder form. You can get it at the Farmers’ Co-op, or at a good garden store. Just don’t spread it on a windy day!

A bag of dolomitic limestone is quite heavy, probably 30 kg, so be careful lifting it. One bag should be all you need for a backyard garden.  If you like your measurements to be more precise, check the bag for the recommended coverage. One bag should cover 1,000 square feet.

I am telling you about lime now because the best time to lime is when you are preparing your garden in the Spring, well before you plant. It will take time, and even a few good rainfalls, for the lime to mix in well with the soil. It is very slow release, and so will have little effect on this year’s crop if you wait too long into summer to add it. You could also lime in the fall. I recommend that you add some lime every year, rather than a lot every four or five years, which is often recommended for larger operations.

Next week I will get into the N-P-K elements, and how you can learn what your plants are needing as they grow just from looking at them.

Featured image: Dolomite limestone (Source:

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Market gardener, farmer, workshop leader, seed-saver, political candidate and mother, Michelle Smith has spent over 30 years coping with the challenges of our bioregion and in the process has built a store of practical and technical knowledge. The Inverness resident has served on the board of Seeds of Diversity Canada and represented Alternative Producers with the Federation of Agriculture but can do nothing about her hair. She is pictured with a head of Club Wheat, a seed that shares her approach to hairdressing.



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This article was first published on 5 April 2017.