Keeping a houseplant or potted plant alive is good. Keeping a plant healthy with shiny leaves and multiple blooms is better. Using the right fertilizers will help you reach your garden goals and carefully tend your house or potted plants. But the fertilizer choices are so confusing! Here are the highlights and uses of fertilizers and soil amendments.
What is Fertilizer?
Plant food actually comes in the form of light, captured by plants and converted to carbohydrates through photosynthesis. The sun provides the energy the plants need and stores it within the plant.
But the molecules containing the elements plants need to bloom (P & K), form strong cell walls (Ca), develop healthy roots (P) or a dark green colour (N & S) is often sold as fertilizer. If you are growing a plant indoors or in a confined pot there is a good chance your plant needs fertilizer.
Spinach in my greenhouse this November is glossy and green even though I am not heating yet and evening temperatures are down to minus 10. Is this because of the greenhouse, the wise use of fertilizer or just good luck? It’s a combination I guess! Fertilizers can make plants more cold-hardy and the glossy leaves suggest these plants are fertilized to perfection.
Fertilizers help build a heathy, not just living plant. I used to be in the tough love group of gardeners adding zero or limited fertilizer and then I realized my plants could be so much stronger, bigger and more full of blooms if I went to the dark side of adding supplements or at least commercial fertilizers to my house plants and potted plants.*
(*The topic of fertilizers for outdoor plants is so much more complicated because our soil often supplies the needed elements. But – our garden soils vary so much I subscribe to the “feed the soil, not the plant” theory of outdoor plant growth in my rich clay soil in Alberta, Canada. In my sandy soil on Vancouver Island I subscribed to the heavy organic matter plus slow release fertilzer program.)
A fertilizer lists the main nutrients it will provide for plant growth but it is not a source of energy. Energy comes from the sun or from grow lights.
If your plant can separate out the elements in the fertilizer it needs from the complex package it is offered so much the better. If it is tough for the plant to access the actual available ion (or charged particle) because of some complexity in your soil or water or other nutrients offered – meaning you will get very slow or zero response. See the Khan Academy for excellent videos about what is an ION.
An eggshell , for instance, is a possible source of Calcium. Bonemeal, for instance, is a possible source of phosphorus. But a water soluble fertilizer labelled as containing either Ca or P is probably more readily to plants. Water soluble fertilizers are designed to go right to work, providing the plant with the needed item.
Egg shells, such as these shown in a museum in Quebec can take a long time to break down and become available to plants. These eggshells are from the 17th century and still look like egg shells. Chances are they are not available to plants yet.
In my podcast “Helping Gardener’s Grow” #17, Dr. Chris Trobacher, research scientist for a fertilizer manufacturer in Canada, told us why fertilizer works and how to make sure you buy a source of fertilizer that works more effectively for your plants. Listen to the podcast HERE.
In an earlier podcast and in his book, Steve Solomon (author of The Intelligent Gardener) told us how to get “vibrating vegetables” by combining your own nutrient sources such as seed meal, feather meal, kelp, blood meal, fish bone meal and more. Warning: I love Steve and his information but I had to reduce our 3 hour interview greatly because it wandered off topic frequently and there were sound issues because Steve’s house in Tasmania, Australia was being renovated as we spoke. Consequently, unless you read his book you will likely miss a lot of the details in our interview. Also, like egg shells, I suspect many of the items used in Steve’s recipes take ages to break down and become available to plants. His recipes rely on a biologically active natural soil and are not going to work as well in a naturally lean sandy soil or for houseplants in “sterile” potting mix.
One more complication is that the different nutrients plants use can cancel or reduce another nutrient as seen in the mulders chart below.
In this version of the Mulders Chart you can see how different nutrients compete or confuse plants. Nitrogen can cancel the effect of Potassium. This is why the fertilizer I use to encourage blooming doesn’t contain nitrogen.
I have also found it difficult to source all the raw materials Steve loves so only one piece of information he told me really sticks: If your plant leaves are healthy and shiny, the plant is receiving the ideal nutrients it needs. I use this concept even when shopping for groceries. Only shiny leaved spinach comes home when I am out buying greens and my own home-grown spinach is glossy or I adjust the nutrients I am using. Listen to my interview with Steven HERE.
Strawberry plant is healthy and the leaves are glossy – when plants look like this there is probably no need for fertilizer.
Because some people learn by listening and others want to read and see pictures, I have included photos from my garden in this blog post about fertilizers.
Role of Fertilizer Elements:
Magnesium is part of the Chlorophyll molecule so the most common deficiency symptom in plants lacking magnesium is inter-veinal chlorosis on older leaves (or dark green veins and light green space between. In spruce or firs, needle growth is stunted or leaves turn yellow at tips when Magnesium is unavailable. (High levels of K will also aggravate the problem by reducing Mg uptake)
Magnesium deficiency in this lemon leaf may also be a sign of high potassium (K). I have started watering my lemon plants quarterly with Epson salts in water because this reduces the pH and allows the magnesium to be more available.
Iron, similar to Magnesium, is important to photosynthesis. And it is also linked to inter-veinal chlorosis, but especially when soils are high in pH – like the natural high pH clay soils in my Calgary location.
Amur maple growing in high pH soil shows signs of Iron deficiency. But is iron actually lacking? Probably not .The plant just can’t access the iron because of the high soil pH.
Fertilizers, like those containing Nitrogen, promote rich, lush growth and a dark green colour. I have noticed that gardeners growing in poor outdoor soils or in pots often have stunted or dwarf plants. At very least, container-grown plants have leaves lacking in rich green colour.
This zucchini leaf is nice and green after fertilizing with a nitrogen containing fertilizer because fertilizer is very mobile in plants. Anything grown in containers usually needs extra fertilizer because potting mixes are notoriously low in nutrients.
These pale green zucchini leaves were grown in a container where the gradually used up all the available nitrogen. I fertilized them and they turned dark green (photo above) because nitrogen is very mobile in plants.
Other elements, like Phosphorus, “help develop strong root systems (especially on root plants like beets) but also help improve flowers, seed development early maturity and a normal, healthy green colour.” A symptom of red in leaves often means phosphorus deficiency – possibly because leaves are not able to pull up nutrients from limited roots or because soil is acidic and this means it is not as available?
Young lemon cutting with limited roots. There may have been more roots if I had fertilized the pots at some point after taking the cuttings.
Severe phosphorus deficiency in my sandy acidic soil on Vancouver Island inside my greenhouse. I guess I should have fertilized this tomato more! Yikes
A minor phosphorus deficiency shows as very pale red parts of these raspberry leaves probably due to acidic soil in my Qualicum Beach garden even though I did add Aglime almost every year to neutralize the soil and topdressed with compost every spring. Heavy rains in winter lead to acidity and leaching of nutrients.
Potassium has a balancing effect and promotes strong stems, good roots and resistance to disease. It is also essential for translocation of sugars in plants.
Strawberry leaf is shiny indicating it is healthy.
Shiny leaves on my Romaine lettuce indicates it is nutrient packed and healthy
Sulfur, like nitrogen, promotes dark green leaves, and a lack of it may result in yellow leaves or the failure of some plants to set seed. Sulfur may be of particular importance to cole crops like broccoli.
Calcium is a major component of cell walls (may also be deficient in overly acidic soils) and is related to nutrient transport within a plant. Anything interrupting normal calcium movement (such as uneven water supply) will result in calcium deficiencies even if there is enough Calcium added via fertilizer or showing in soil tests.
When the “blossom” end (not the stem end) of a tomato turns brown and become sunken this is often blamed on a shortage of Calcium. But it might be due to sloppy, uneven watering. When there are sudden hot conditions or when the gardener is away and the soil goes dry, calcium shortages show up as sunken brown ends. Some types of tomato are more susceptible than others.
Optimum nutrition puts a a glossy coat on leaves, making them more resilient to disease, insects and trouble of all kinds. This is why if plants are grown in potting mix or bagged “soils” instead of natural soils there is a need to add fertilizer.
Bringing Plants into Bloom
It is one thing to have a lemon as a houseplant in a northern garden. It is a completely different thing to have your healthy lemon houseplant bloom and produce lemons. And this is where specialty fertilizers come in. They are not a big dump of equal parts Nitrogen, Phosphorus and Potassium. The are specialized to balance the nutrients so you trigger bloom. I have a water soluble fertilizer (1-25-35) with added kelp and micronutirents and when I am ready to stimulate blooms I use it once or at most twice.
Keeping a houseplant alive is one thing. Bringing a lemon into bloom in your northern home or garden is as simple as using the right fertilizer and making sure the plants receive good light so their energy is high!
Role of Soils
“Poor gardens and lawns are usually due to poor physical or chemical condition of the soil, or both. Of course, even with the best soils, gardens and lawns still require adequate, but not too much, moisture if they are to grow well.” From Soils and Fertilizers for Gardens and Lawns by C.F. Bentley and J. A. Robertson, Department of Soil Science, The University of Alberta, 1977
Notice Drs. Bentley and Robertson don’t mention fertilizer on page one of their book about soils and fertilizer. They mention soil physical and chemical composition. And water. And although their book is almost 50 years old, and only 48 pages long, it was an essential primer when I was a student and is still relevant from the view of soil physical and chemical condition.
One factor affecting soil chemistry is pH. Soils high (basic) or low (acidic) in pH will affect specific organisms in the soil and this affects nutrient availability. According to the very excellent book Elements of the Nature and Properties of Soils high calcium and near neutral pH generally result in the largest, most diverse bacterial populations. Also, the authors stress soil fauna and flora are indispensable to plant productivity. One example they give is that nitrates are present in the soil due to microorganisms transforming them from ammonia. Likewise phosphorus, sulfur, Iron and manganese are more available because of microbial activity.
Soils in every garden will differ as the natural soils in the area differ but the importance of soil management to keep the acidity neutral is paramount. Adding organic matter very positively affects soil chemistry and soil physical condition.
Another very important effect on microorganisms is water. If there is stagnant or sitting water, available oxygen levels are lower and the natural levels of beneficial microorganisms are also lower. Again, the amount of organic matter in natural soil affects the drainage of the soil and the air spaces retained in the soil.
Excellent friable soil that drains well and is neutral in pH is the goal. Sadly, all native soils are not so perfect, which means fertilizers or the elements they contain are not always accesible to plants.
Role of Soil Amendments
Peat moss, compost, worm castings, perlite, humic acids, bone meal, eggshells, gypsum and biochar are all amendments that have various effects on the soil, some chemical and some physical.
Most people know, for instance, that compost is slightly acidic and is an excellent addition to clay soils, especially high pH clay soils because it makes their physical structure easier to work and also changes their chemical structure as a neutral pH encourages microorganisms to multiply and convert nutrients into available particles for plants.
Sifting my own home-made compost so I have plenty of home-grown ammendments for my soil. In my sandy soil on the west coast, compost helped by holding nutrients. In my current clay soil in Alberta, compost helps to soften the soil, making it drain faster and easier to work.
Gypsum is sold as a soil conditioner. According to Brady and Weil in their book “Elements of the Nature and Properties of Soils”, Gypsum (Calcium sulphate) provides enough ions to flocculate soil and inhibit surface crusting.This allows water to enter soils better and erode less. It also allows greater root penetration where there is a subsoil layer. Bacteria may do the same thing but gardeners always need a faster answer than just building soil biology!
On the west coast, Biochar increased my CEC (cation-exchange-capacity) and made me feel good because nothing else seemed to help my sandy soil hold nutrients.
Peat moss used to be an annual addition to my clay soils in Edmonton, but we now know it is a limited natural product so I only use it to start seedlings, not to amend soil. Perlite is an expensive amendment and I limit it to houseplants that need better drainage, like citrus.
I am a huge believer in worm castings and both make my own and buy bags of castings from local producers. It adds a special something to soil the way compost does and also adds microbes so makes soil softer over time by adding the beneficial bugs but it does not make a meaningful addition of fertilizer to soil.
Worms I have raised because it is fun, builds my soil and adds beneficial microbes. But it is a mess, takes space in my basement and adds tons of work to my schedule so sometimes I just buy the finished “worm poop” or castings as they are called.
Bone meal and egg shells don’t make the cut in my garden because there are better ways to add Calcium or phosphorus to boost plants. But I do save shells, blend them when dry and add the powdered shells to my compost heap just in case it helps.
I buy but never remember to use humic acids. Brady and Weil also mention humic acids, marketed for their soil conditionaing effects when incorporated at low rates, but in their 3rd edition they had no results to share on effectiveness even though many studies have been done.
And To Close (Although this discussion is never over!)
Fine tuning outdoor, natural soils by adding organic matter, watering as needed, boosting beneficial natural soil microbes with worm castings, modifying pH with organic matter added or using Aglime (instead of dolomitic lime) if pH needs upping is important.
In other words, with proper soil management few, if any, additional fertilizers will be needed to grow plants in rich natural soils.
Many “secret” products or ingredients are actually kelp (see back side of this label below). So watch the price. So many forms of kelp are available and I would hate it if you paid for water with just a bit of kelp and in fact I prefer the dry forms or to buy fertilizer with kelp included. Just add water is an easier idea.
The label side of the Thrive Alive product shows it is really is only kelp (with vitamin B1) Kelp is a good thing, but I prefer the dry over the liquid forms and I mix it as I go.)
For planters or in any potted plant such as a raised bed for vegetables, the conditions are so different from natural soils that fertilizers are most definitely needed. And when you add fertilizer don’t forget to add a source of Kelp when you can because “(when) you add a little bit of kelp (along with fertilizer) it seems like the plant’s able to use it’s food and combat the stresses in its environment a lot better than (with) nutrition alone,” says Dr. Trobacher. And he’s the doctor, not me.
Kelp is available as a liquid and as a dry product in more than one type of fertilizer and it is also sold as a single item in products such as Thrive Alive. Mix a bit (according to label instructions) when adding other fertilizer for best effect!
