Greenhouse Garden Tips: Soil Amendments & Nutrients

Different Varieties of Tomatoes

Importance of Soil Amendments & Nutrients in Greenhouse Gardening

A few years back my greenhouse tomato leaves were purple, yellow and spotted. Not all at once, but over time and on different leaves. It was the second year I was growing in my greenhouse and the commercial potting soil I bought was low on phosphorus, nitrogen and potassium. The leaves on the plants were telling me what was going on. And it didn’t look good.

Brown spots on tomato leaves means potassium shortage. Time to fertilize even if you have a super soil.

Did you know - Brown sports on tomato leaves mean potassium shortage. It's fine to fertilize even if you have a super soil.

Why "New" or "Natural" Soils are not Always the Best Solution

Is it best to throw away old soil and buy everything new when you have nutrient deficiency troubles?

After reading my last post you’ll realize it is impossible to buy “new” or “natural” soils because real soils are ancient and ever-developing. Plus, natural soils are hard to buy and not always the best solution. Building on what you can buy as “potting soil” or “soil-less mix” is a good start towards building your own super soils.

Building Your Own Super Soil: Ingredients and their Effect on Plant Growth

But what are the components of super soils and what effect might they have on plant growth? Soil is just there to support the plants, give them an anchor if you will. With super soils you can provide nutrients and keep the plants fed between fertilizer applications. Here are some of the ingredients I have seen added to soils to make them super. I have also included a suggested blend below.

Soil Meal: Amino Acis and their Role in Plant Hormones and Enzymes

These are the amino acids left in the seed once canola, soya, alfalfa and other seeds are crushed. Amino acids have long chain molecules with nitrogen and other minerals. During my podcast interview with scientist Dr. Chris Trobacher, I discovered the amino acids in seed meals go into the plant like pieces of LEGO, ready to be put together into plant hormones and enzymes. Seed meals are available from feed stores, but they are also included in specialty fertilizers like Gaia Green and Ferticare’s Organi-X.


Two ferticare options

Fertilizers with kelp and micronutrients help plants thrive even when the soil is already super!

P.S. The nitrogen in most slow-release commercial fertilizers has to be converted to plant-usable forms by micro-organisms in the soil. Blood meal is an exception. It is ready to be used by plants right away.

When to add:

Initially and every spring

Biochar: The Benefits of this Amendment Made from Organic Matter

An amendment made from carefully burned organic matter. This is not the same as fireplace ash. Biochar is charcoal. It helps the soil hold water, nutrients, carbon from the air and micro-organisms. A great explanation of biochar from the maker of Soil Matrix Biochar explains how it helps soil hold nutrients. By the way,  fireplace ash does contain high levels of potassium, but the ash is very soluble in water and quickly washes away in the soil water.

Biochar stays in the soil forever; it is stable and long-lasting. Biochar is permanent and doesn't decompose or break down. 

Image of Biochar in a garden

It's common practice to add manure or composted manure to beds in fall. Better yet add compost to soil as they are built and again as a light dusting in spring

When to add:

Initially and with annual additions to compost

Worm Castings: Adding Micro-Organisms and Nutrients to the Soil

Also known as worm poop, these are found naturally in organic garden soils and in compost. They are also available commercially as a stand-alone product. The newest source I have found is called Annelida Premium Earthworm Castings. Worm castings inoculate the soil with micro-organisms and also add organic matter and nutrients to the soil. The suggested level of worm castings added to super soils is 10% by volume but this amount adds various amounts of nutrients and micro-organisms to the soil depending on the way it is made and stores.

Various supplements available to boost greenhouse soils.

Various supplements are available to boost greenhouse soils: I depends on worm castings, kelp, biochar and Agricultural lime.

Worm castings continue to break down and release nutrients, so they need to be topped up annually. Also, if you use commercial fertilizers you may destroy some of the micro-organisms added with each addition of castings.

When to add:

Initially as 10% of mix and a sprinkle on top of soil or pot each spring.

Calcium: Essential for Cell-Strengthening and Plant Growth

Bone meal is one source of calcium for low pH soils. Some gardeners add it into the hole near the roots of plants to provide phosphorus and encourage rooting. However, like most fertilizers, it needs to be converted to an available form by micro-organisms before the plant benefits. Luckily bone meal is not the only way plants get this cell-strengthening ingredient that triggers brown edges on lettuce in the fall and blossom-end-rot on tomatoes as they ripen.

Tomato with blossom-end-rot in a Calgary greenhouse

I spotted this tomato with blossom-end-rot in a Calgary greenhouse. Luckily I added calcium to my soil as I built it and have never had this problem.


Calcium is also available as calcium carbonate (Agriculture Lime), Calcium Magnesium Carbonate (Dolomite lime) and Calpri. I add Agriculture lime when I first mix soils and then I sprinkle Calpril on my greenhouse soil before I plant each spring. It is quickly available because it is water soluble. I also finely grind chicken egg shells in my Vitamix food grinder and add those to my compost to add calcium to compost. By the way, Dolomite adds lime and magnesium together. Unless you know you need magnesium, stick to adding AG Lime or Calpri.

If you don't regularly get soil tests in your greenhouse just assume you are low on Calcium and add a bit each spring. Gypsum is a better source of calcium where natural soils' pH tests high, indicating it is basic. Gypsum is Calcium Sulfate and it adds a pH neutral source of Calcium and Sulfur.

When to add:

Initially and again each spring on soil surface so it gets worked in with planting.

Bat Guano: Boosting Nitrogen 

This is similar to chicken manure, but it's poop from bats. Go figure. It boosts nitrogen and has been deemed organic. It is in a lot of the super soil recipes. Animal manures are usually composted before they are added so I consider them as part of compost. I do not add bat droppings separately into my soil.

When to add:

No need to add ever. Use compost or composted manure to soil.

Epsom Salts: Magnesium to Help Photosynthesis

Dr. Trobacher, research scientist at Nutriag, tells me Magnesium is the centerpiece in the giant molecules involved with photosynthesis. Various sources of Epsom salts (Magnesium Sulfate) and fertilizers like K-Mag provide this element to the soil and the plants as they grow and photosynthesize. Plants low in Magnesium have spotty yellow patches on the leaves and can't grow as well or as quickly.

B careful adding magnesium. If you have natural soil containing clay in your soil mix it could "tighten" or become hard with too much magnesium. I like to use fertilizers with micronutrients, like magnesium, as I water plants instead of adding Epsom salts.

When to add:

Only if it is absent on plant fertilizer labels you like

Kelp: Improve your micro-nutrients

A magical algae from the ocean that is so similar to plants on land but somehow different too. Dr. Trobacher says commercial fertilizer mixes containing kelp as well as micro-nutrients help plants grow better than nutrients alone.

I have also bought and applied dry kelp products and mixed them in with greenhouse soil. A maintenance fertilizer to enhance the soil during the season may be all you need. Check fertilizer with kelp.

When to add:

Initially and again with seasonal fertilizers

Rock Dust: Is this amendment needed?

These products come from the dust created when rocks are mined. They are variable depending on the source and will sometimes contain excess heavy metals like aluminum. I have stopped buying rock dust for food gardens because so little research has been done on these products and, as mentioned, they are so variable. Instead I am watching crops closely and adding missing minerals as needed during the grow cycle with fertilizer.

When to add:

No need to add rock dust. Trust known and proven items like finely ground Agriculture lime and Gypsum.

Peat and Coir: Holding Water Better

Peat is surface mined and possibly becoming limited. Coir is a byproduct of coconut groves in tropical forests where Orangutan habitat has been stripped in favour of commercial farms. I see this as a no-win situation yet am continuing to buy commercial soil mixes even though I know they contain some or both of these products. I no longer add them as stand-alone products but am aware they are part of the existing soils I buy. Luckily, Biochar both holds water, helps with drainage and lasts forever. It may eventually become a larger part of the soils I grow in.

When to add:

Initially as part of original soil-less mixes used to create super soils

Compost: Organic Matter You Can Make

Purchased or homemade compost will always add organic matter. The truth is that every compost is different and while it always boosts organic matter levels, it also adds different amounts of nutrients and micro-organisms. It is a basic component of the original soil mix but is a bit pot-luck because so many kinds are on the market. I continue to make my own, use it in the base soil mix and add it as a topdressing in spring to pots and beds.

When to add:

Initially and again annually as a small 1/2 inch topdressing. I compost now with biochar so the micro-nutrients become embedded in the charcoal fabric.

The Super Soil Recipe

While Some soil recipes call for 60% soil and 40% compost, others are called triple mix because they have 33% soil, 33% peat and 33% compost. I believe if you are starting with a commercial soil mix you already have some peat or compost so consider the following mix instead: (By the way, I suggest mixing in part to simplify the math. Simply use the same size bucket as part and mix as you add product!)

  • 6 parts (i.e. 6 - five gallon pails) soil-less mix from supplier Promix , Sea soil  , Sungro
  • 2 parts (2 five gallon pails) compost (homemade or purchased)*
  • 1 part (1 five gallon pail) biochar
  • 1 part (1 five gallon pail) worm castings

*Original Sea Soil contains composted bark so if using this formula as your base use 1 part compost and 2 parts biochar in the recipe.

For every 100 litres of soil add 500 ml high protein seed meal (protein percentage is listed on bag), 250 ml Agricultural lime, 250 ml Gypsum and 500  ml dry kelp meal (loosely adapted from Steve Solomon’s work and his book The Intelligent Gardener.) Watch carefully and fertilize as your plants grow with a product that also supplies chelated micronutrients such as magnesium and zinc.

Tiny Jasper cherry tomatoes from the greenhouse roasted for dinner are so tasty because the soil they grow in supplies most of their nutrients.


Final Thoughts: Creating and Maintaining Super Soil for Greenhouse Gardens

When I saw the signs of low phosphorus, potassium and nitrogen in my greenhouse I was using soil-less soil mixed with the pure natural sand we call soil in this part of the world. I wasn’t adding back the elements the plants were removing from the soil as I harvested them. There wasn’t enough compost to keep the soils supplied with nutrients and I wasn’t timely with fertilizers.

That has all changed now. I have built super soil and am also fertilizing as the plants grow and are harvested. Meanwhile we are going into the low light season and microbial action is limited in cold soils so don’t despair. Nutrients will be waiting for you in the spring.

Sweetness in tomatoes is a measure of mineral in soil and variety. Jasper tomatoes grown in my greenhouse have a naturally higher Brix level and are sweeter


How do you get the best flavor in your food and brightest colors in your flowers? Start with super soil and add fertilizer as needed because signs of nutrient deficiencies in the greenhouse are tragic and unnecessary.

By the way, purple leaves are low in Phosphorus, brown spots on leaves are a sign of potassium deficiency and lower leaves turning yellow desperately need nitrogen.

Raspberries growing in my greenhouse are supplying a late season of sweetness
Raspberries growing in my greenhouse are supplying a late season of sweetness
Tiny Jasper Cherry Tomatoes from my greenhouse

Tiny Jasper Cherry Tomatoes from my greenhouse

Sweetness in tomatoes is a measure of minerals in soil and variety

Sweetness in tomatoes is a measure of minerals in soil and variety