Tomato Red Face: What’s the Deal with Compost?

When I started this blog, I figured I was a better-than-average tomato grower. I’d picked up a lot of tricks over the years, I said. I also said that tomatoes are super resilient and hard to ruin – which is kind of true. You can do a lot of things wrong and still get some tomatoes, but they may not be very good tomatoes, or the plants may not produce to their full potential – and it’s pretty clear that this is what happened to me this summer.

As I mentioned I’ve been taking some classes through the U of S’s Prairie Horticulture Certificate program and having multiple “eureka” moments about what I may have screwed up this summer, usually based on common but bad advice. While this is embarrassing for me, someone who thought they were so good at growing tomatoes that I could have a whole blog about it, I owe it to you, my 11 or so readers, to talk about the practices I used that were not based in sound science. These posts focus on four specific topics I didn’t understand very well (photosynthesis, plant hormones, nutrient uptake, root development) and why my well-intentioned but misguided practices may have led to my 2020 gardening woes.

Regarding sources – unless otherwise specified, the majority of general plant science information in these posts is based on what I learned in the University of Saskatchewan Prairie Horticulture Certificate courses HORT 13 Applied Botany and SLSC 14 Soils for Horticulture.

For this post, I’m going to focus on a plant that I desperately wish I’d taken a photo of but never did – sorry, worst blogger. I had a few too many seedlings to fit in my raised bed so I put one of them (from my “surprise pack” of mystery cultivars) in a pot on my deck, where it would get a lot of heat and a lot of sun. Thinking I could be the ultimate organic frugal gardener, I filled the pot half with reused potting soil, and half with City of Saskatoon compost. Using the common “wisdom” that compost is “the best fertilizer”, I figured this would provide all the nutrients the plant needed for the full season – set it and forget it, and no scary-sounding synthetic chemical fertilizers needed!

The plant grew very spindly, with thin leaves that remained a light green colour for most of the summer, and only bore four cocktail-sized fruit on the same cluster – two matured in late August, and the other two finally ripened indoors during the last week of October(!). (I will note that I did not excessively prune this one, or cover it in red plastic, and these were some of my best-tasting tomatoes.) I identified the plant as the Italian cultivar ‘Principe Borghese’ – the official sundried tomato plant, which should be a high producer and mature in 70 days. When I pulled the plant out of the pot to compost it in late September, the roots were only a couple inches deep.

What the heck happened? It got a ton of sun and plenty of water. But via my soil science class I have posthumously (or post-HUMUS-ly? Get it? I’ll see myself out) diagnosed this plant with nearly every nutrient deficiency in the book – that’s not an exaggeration. It is highly likely this plant was deficient in ALL of the essential nutrients that tomatoes need. But isn’t compost “the best fertilizer”? In short, no. At least not for annual food crops like tomatoes.

We are likely all aware that compost is the product of the decay of organic materials – in the case of City of Saskatoon compost, mainly yard and organic food waste (I mean organic as in plant material, not “certified organic”). While these materials contain some nutrients, they need to break down or “mineralize” in order to release these nutrients in forms that are available for other plants to use. Plant roots soak up nutrients that are in mineral (inorganic) form only – for example, to get a banana to release its precious potassium for tomatoes to use, it needs to be fully decayed! (On that note PROMISE ME that you won’t go Googling banana fertilizers, so much awful and wasteful advice.)

Plant matter mineralizes as soil microbes decay it. Microbes are living things and they also need nutrients for their own energy as they break down these organic materials. I don’t want to overcomplicate this but I think I should mention the concept of the C/N (carbon to nitrogen) ratio of organic materials for improved understanding. Soil microbes need both nitrogen and carbon to decay organic matter. This is a very simplified explanation, but the ratio of carbon to nitrogen in organic materials will essentially tell you how quickly the soil microbes are able to decay the material, and thus release the nutrients for use by the plants. If something has a high C/N ratio, it will take a lot longer to mineralize than material with a low ratio. Manure generally has a lower C/N ratio, and things like wood chips and straw have higher C/N ratios.

(The extremely stupid mnemonic I have come up with to remember which is which is that the CN Tower is really high up in the air, a high C/N ratio takes a long time to break down, it also would take a long time to walk all the way down the CN Tower, this has been a Very Special Insight into how my brain works.)

All composts and manures are NOT created equal (despite the common advice to “throw on some compost or manure”), so you cannot simply say that “compost” or “manure” definitively has a specific C/N ratio. If you and I both have a backyard compost pile, our C/N ratios may be very different, based on what we’re putting into the pile and how decomposed it is. For manure this also varies based on type of animal, what it was eating, and the age of the manure.

Below is a photo of compost one gets from the City – notice it is quite chunky and contains a lot of only partially decomposed wood mulch and tree material. I would bet money that this particular compost has a high C/N ratio (i.e., slow to mineralize). There was absolutely no way that this material was going to supply my tomato plant with all of its required nutrients over a full growing season. It would have been far better to use a synthetic fertilizer like Miracle-Gro, in which the nutrients are immediately available in their inorganic, mineral form for the plant to use.

Once I learned how nutrients become available to plants, I realized what a big mistake I’d made in my soil and fertilization choices. Unfortunately it was the end of the season so there was nothing I could do to remedy it other than chalk it up to a lesson learned, and share it all with you in hopes that you were all smarter than me in the first place.

But wait – if compost isn’t “the best fertilizer” then what is the point of using it? In the future I will think of compost more as a soil amendment that can be used to slowly improve nutrient content (for my perennial plants) and soil structure. For example, I have very high clay soil in my yard, and incorporating compost into my in-ground garden beds has been noticeably helpful for improving the soil texture.

I don’t have the time or space to expand on this too much, but I’ll also mention that the rabbit manure I put in my raised beds, expecting it to provide bountiful nutrients to my tomatoes over the full season, also likely created some additional nutrient issues (both deficiencies and toxicities) which led to the plants being pretty stunted and late to produce. I won’t ramble on, but the biggest lesson I’ve learned about all of this is: “natural” fertilizers like compost or manure are REALLY hard to apply properly if you don’t know what you’re doing, and you can definitely screw up your plants with improper “natural” fertilizer application. There is nothing wrong or environmentally unfriendly with using a synthetic fertilizer if you follow the instructions on the package and don’t overuse it, and it will take a lot of the guess work out of trying to decipher the nutrient content and mineralization timetable of manure or compost.

While I’m not proud of any of this, I’m excited to be able to learn from these mistakes. I’m writing this down for myself a) (of course) to fulfill the requirements of my Applied Botany class but also b) so I can identify and remember the bad practices I used so I don’t repeat them in the future. And I am sharing it with you, even though it is a bit humiliating to do so, in case it is helpful for anyone. If you followed any of my bad advice this summer and similarly screwed up your tomatoes, I hope that the hours of effort that I’ve put into these posts might also serve as a sufficient apology.

Let’s grow some better tomatoes in 2021!

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