Gardening 101.5

Carbs? Protein? Polyunsaturated fats? French fries? Tofu? What should you feed plants to make them grow up strong and healthy? Herein, all is revealed.

Plant nutrition

In nature, as in your garden, plants get their moisture and nutritional elements from the soil. In the case of plants growing in the wild, we are seldom concerned with the fruits they produce, or their size, or the productivity of the plant. In some cases, as with wild berries, we are in fact concerned, but it’s impractical to do anything to help.

There are rare exceptions. When I was growing up in Nova Scotia, there were some families who owned a piece of scrubland, which had long since been cut over for firewood and, for whatever ecological reasons, grew wild blueberry bushes much better than more trees for firewood. These pieces of land would become the family’s blueberry supply, and it was traditional to burn them over every few autumns. The burning didn’t kill off the blueberry plants, and the crop would indeed improve over the next few years. No one could ever explain to this curious youngster why burning helped. Eventually, I figured it out.

The holy trinity

There are three main nutritional elements that plants need. They are nitrogen, phosphorus, and potassium – often abbreviated to NPK, their abbreviations in the periodic table. NPK are always spoken of in that order. So when people speak of a fertilizer as being a ‘6-12-12’, it’s understood to mean six per cent nitrogen, and twelve per cent each of phosphorus and potassium.

As I remember it from my organic gardening course in the 70’s, NPK correspond in terms of plant development to ‘leaf, root, and fruit’.

In other words, nitrogen feeds leaf growth, which is why nitrogen is the main concern in fertilizing lawn grass. Plants whose green leaves and stalks we eat, such as celery, lettuce, spinach, and cabbage depend on nitrogen for proper growth.

Phosphorus feeds root growth, which is why it’s often recommended that a bit of bone meal (which is high in phosphorus) is sprinkled in the soil where you set out transplants – to encourage rapid and vigorous new root development. Apart from that, turnips and carrots are examples of plants where the root is the part of the plant we’re most interested in. They depend on an adequate supply of phosphorus.

Potassium feeds fruit, and that’s why the burning of a blueberry range is beneficial, because ash is a good organic source of potassium. The old fellows didn’t know how that worked, they just knew it did. Fruiting plants like apple, cherry and plum trees, tomatoes, zucchini, pumpkins, peas, and beans are examples of those which depend on a good supply of potassium.

I’ve read that potassium is also important to flowers, and that makes sense, since the flower is just the beginning of the fruit. But I’ve also read that phosphorus is important to flowers, so I’m somewhat confused, and for now, I’m going to stick with my ‘leaf, root, fruit’ belief.

It’s not critical to remember or understand all this stuff, apart from the mid-term, for reasons you’ll see as you read on. I just wanted to demystify the NPK and 6-12-12 stuff.

Standard fertilizers

In practise, we tend to buy fertilizers in a mix for specific applications. Lawn fertilizer will be something like 30-0-3, and now you know what that means and why that’s good for grass. Bone meal is 4-12-0, highest in phosphorus. Tomato fertilizer is 15-15-30. You get my drift.

The 50-odd pound bags of commercial fertilizer used by farmers is usually 6-12-12. This granular stuff has been around for ages, and for good reason – it does its job. I add it to my soil, along with a nice bit of lime, when I turn over the soil in the spring before planting. About as much 6-12-12 as you’d sprinkle salt or pepper on your meal.

In addition to the 6-12-12, all I have is a tub of 20-20-20 water-soluble plant food, and a tub of 15-15-30 water-soluble tomato food. Those three are all I use, apart from bone meal and lime, for everything I grow, including flowers.

Container caveat

The two water-soluble ones, you mix in your watering can according to instructions, and with that I water everything growing in containers. The reason for the extra feeding of stuff in containers is that in the garden, the plant is free to spread roots far and wide in a continual search for new sources of water and nutrition. In a pot on the deck, even a big pot, there comes a point where the root spread is stopped by the inside edges of the container, and the nutrition in the soil gets used up. The plant begins to starve.

Lots of people get a gorgeous hanging basket of wave petunias or million bells from the nursery, with hundreds of flowers, take it home, and water it faithfully. A month or two in, it starts to look a little peaked, not quite so lush any more, and people wonder what’s going on. Should I call the nursery and complain? No, it looks like that because it’s starving. There’s only so much nutrition in a potful of soil, even gorgeous black fluffy potting soil, and once that’s used up, the plant starves. Nurseries go through a LOT of plant food. That’s why their stuff looks so lush when you go there to shop.

I’ve grown 8 or 9 foot Mammoth Russian sunflowers in pots a foot across at the top. When they’re full-grown, they remind you of a daisy growing in a thimble. That would never happen if I didn’t regularly give them fertilizer along with their water.

Once a week, by the way. Not every time you water.

I have special tomato food because if you fertilize tomatoes with 20-20-20, they grow many times the foliage needed and put relatively less effort into producing fruit, which of course is what you want. In my experience, even with the special 15-15-30 food, tomatoes put out a shocking amount of foliage, and I usually end up pruning quite a lot of it off later in the season. But at least that food will encourage them in the direction of good fruit production.

I mentioned lime in passing. Virtually all natural soil on the Island is acidic; more so than most plants like to grow in. Lime ‘sweetens’ soil; makes it less acidic; draws the pH towards the level plants thrive in. But you only need to apply it to soil in the ground; potting soil bought in bags tends to be okay, pH-wise.

How much? As much as you can afford. There’s practically no such thing as too much lime in Newfoundland. I use the pelletized limestone, made from ground limestone. Dolomitic lime, in my opinion, is less effective; less value for money.

In addition to NPK, there are a host of other minerals and trace elements that are required by plants, but generally only in small amounts. In practise, these tend to not be much of a concern. The water-soluble fertilizers like 20-20-20 have a lot of these elements in them in addition to the main three. I think of them as complete, balanced nutrition for plants, and experience bears that out.


I haven’t talked about compost, which will usually supply all of your garden’s nutritional needs. And it’s ‘green’, of course, but takes a lot more effort and technique than just throwing your kitchen waste into those black Darth Vader composters the MMSB was selling. Composting is another column. If you’re big on using compost, I recommend the Pisces stuff made in St. Shott’s. I get good results with that. The composted sheep and cow manure available all over the place, I don’t find worth, well, you know what. Ordinary manure has been used for generations, of course, but it HAS to be well rotted. Manure is one commodity where freshness is not a good thing.

Okey doke. Thus endeth the lesson on nutrition. NPK. Leaf, root, fruit. 6-12-12. 20-20-20. Piece of cake.

Previous gardening columns -> 101.4 101.3 101.2 101.1

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