Getting your Raised Bed Garden Soil Right, Part I

Growing in raised beds can be really rewarding. A common question that arises for people interested in growing in a raised bed for the first time is “what type of  soil and soil amendments do I put in it?”

You want to make sure your garden soil has enough nutrients in it to sustain your crops throughout the growing season, that it isn’t too acidic or too alkaline, and that you have good drainage and soil density. Figuring out how much soil to put in your bed is also important (we will cover how to do those calculations in Part II of this Blog. Stay tuned!) 

First, let’s talk a bit about what soil does. Soil can be a wondrous web of life, full of complex inter-dependencies and processes. We know that in a teaspoon of healthy soil lies billions of lifeforms. We rely on our soil to provide our crops with nutrition. To feed our plants, we need to feed the soil.

Our plants rely on many nutrients and minerals in the soil, but the ones most often talked about are the primary nutrients, Nitrogen (N), Phosphorus (P), and Potassium (K) = (N-P-K), whose ratios you will see in that order on soil & fertilizer bags at the store (i.e. if you see 5-5-5 on the side of your compost bag, those numbers are the percent amounts of each nutrient. An NPK value of 5-5-5 means that the fertilizer contains 5% nitrogen, 5% phosphorus and 5% potassium.)

• Nitrogen provides growth and leaf-greening.
• Phosphorus supports plants as they flower and fruit.
• Potassium supports healthy root system development which is important when plants may face drought or disease.
• Trace minerals and micronutrients found in kelp meal and worm castings and other organic soil amendments will provide additional nutrition and soil conditioning to your garden, resulting in more nutrient dense food, which means TASTIER food.

The higher the nutrient density in your food, the sweeter and more flavourful it will be! This is one of the reasons why it is so easy to tell if you are eating a garden carrot or tomato vs a grocery store carrot or tomato.

It is good to remember that you are feeding your soil, not your plants. Your plants will take what they need from the soil, when they need it. When you do a soil test- if your results show that your soil is low in Phosphorous, then you would add a fertilizer that is higher in Phosphorous. If your results indicate a lack of Nitrogen, then you would add more Nitrogen. Each vegetable needs its own specific levels of N-P-K, but we must trust that they will take what they need when needed. As long as there are adequate levels of NPK in your soil, you should be just fine. You can purchase home Soil Test Kits that test for Primary Nutrients and pH from most garden centres. 

When it comes to the pH of your soil, it is important to understand what’s going on as the pH can affect what nutrients will be available to your plants.

• The pH scale ranges 0-14, where a pH of 7 is neutral, lower is acidic and higher is alkaline.
• A good pH range for garden vegetables is around 6.1-6.9. Check out this super informative article on soil pH and how to raise or lower it naturally. Changing the pH of Your Soil | Home & Garden Information Center (clemson.edu)

Your soil's density will affect drainage & water retention. Heavy clay soil is often too compact for the delicate roots of several plants, and sandy soil will be so loose that it will not hold on to moisture very well. Topsoil, the top 2-8 inches of soil, contains the highest concentration of organic matter and nutrients and is a desirable consistency for the base of your raised bed. 

Read the upcoming Part II of this blog post to learn how to calculate how much soil to put in your raised bed, and some helpful top soil to compost to peatmoss ratios to consider! What do you put in your raised bed or growing pots?? Let us know in the comments below!