I have been making "fermented plant juice" (FPJ) as explained in Nigel Palmers book - The Regenerative Growers Guide to Garden Amendments. Basically, you mix plant material with sugar, 50/50 by weight. The sugar draws out the juices and with that the minerals etc. Then after a week or so you'd strain the liquid off. For a soil drench you would dilute with water, 500:1, or 1000:1 for a foliar spray.

I've made my own potting mix made with soft sand, leaf mould and plenty of finished home made compost without much success. I've recently discovered on this forum that I likely needed to add fertilizer to the mix, as compost itself isn't that high in nutrients and what's more the nutrients are, in the main, locked up and will only be released slowly over time. Who knew?

So it seems that I need to !charge! the whole mix with fertilizer rather than just apply a top up or amendment.

I keep reading places that you can't really over do natural organic fertilisers and that it's only the synthetic fertilisers that would burn the root tips. Is this true to a degree? Either way I'm thinking to charge my potting mix with fermented comfrey juice (plus a few other plants/grasses in the mix) and am wandering if anyone knows of quantities to use for a given volume of potting soil. I'd like also to try charging the existing, failed potting mix which is host to 40 strawberry plants which were bareroot back in the spring.

  • How can too much of a good thing not be too much? And don't you need soil in your soil mix, like a clay ingredient? Commented Jun 28, 2023 at 23:24
  • Well, I'd have to agree that too much of anything would, by definition be "too much". No, I believe clay isn't needed in a potting mix. Commented Jun 29, 2023 at 6:32
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    50% sugar sounds a lot to me. Unless, all the sugar has fermented, you will be attracting a whole lot of wildlife such as ants. Commented Jun 29, 2023 at 13:04
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    In what way does your potting mix not work now?
    – Jurp
    Commented Jun 29, 2023 at 15:22
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    @Jurp 24 tall spindly, crumpled and twisted leaves, slight necrosis, pale and red leaves/stems. They are somewhat better now as I've been foliar feeding and soil drenching for 3 or 4 weeks now with calcium, phosphurus and general FPJ, but still young leaves are coming up slowly exhibiting similar issues. This is why not having "charged" the potting mix makes sense as it would be a wholesale remedy to all the issues. You can see a couple of cropped piccys in my last question posted this morning. Commented Jun 29, 2023 at 15:58

2 Answers 2


You answered my question asking for details on how your potting soil is failing with the following (bullet points mine):

  • Tall spindly [stems]
  • Crumpled and twisted leaves
  • Slight necrosis,
  • Pale leaves and stems
  • Red leaves and stems.

You are currently feeding with "calcium, phosphorus and general FPJ" (another name for anaerobic compost tea, I think).

My only recommendation is to get a soil test done on your potting mix because you're simply adding a witch's brew of stuff which you have no idea will work or not. For example:

  1. You should stop adding phosphorus, as P deficiency manifests as stunted smaller stems, not taller, and leaves that are darker green than normal. Your tall and spindly stems and pale leaves contradict this, meaning that the phosphorus content is fine. Nitrogen deficiency also acts the same as phosphorus deficiency, so your nitrogen is at least normal, but could be high.
  2. If by "necrosis" you mean a bit of leaf scorch around the edges of leaves, then your plants could be suffering from potassium deficiency, although this is rare.
  3. Adding calcium is likely messing up the soil because too much calcium can deplete magnesium under alkaline conditions; this is an essential chemical for photosynthesis (this could be causing the pale leaves, perhaps). Too little magnesium can also lead to a phosphorus shortage even in soils that have adequate phosphorus.
  4. Dead spots on leaves can be caused by a number of factors, including manganese deficiency (probably not in this case) and zinc deficiency, both are more prevalent on limed soils - or soils with too much calcium! They can also be caused my more than a few fungi.
  5. The crumpled and twisted leaves sound more like a fungus or disease than something deficient in the soil. This makes sense since you're not sterilizing your potting soil before using.

Comfrey juice will have N,P, and K in it, but if your soil isn't deficient in these elements (and it certainly doesn't appear to be deficient in any of them), then adding it won't "charge" your soil.

EDIT: Also, the sugar in the "FPJ" could be an excellent source of food for non-beneficial bacteria and fungus, which could also be a factor in the poor performance of your plants. END EDIT

This analysis is, of course, incomplete and may be inaccurate because I don't have all the information needed, but I hope it shows you why you need to get your soil tested.

Without knowing the EXACT COMPOSITION of your soil you cannot accurately add amendments to it. Unless you get lucky, your attempts to fix your problems are unlikely to end well.

In answer your initial question: "Is it true that you can't add too much natural fertiliser... to a potting mix as opposed to synthetic fertiliser?" the answer is No, that is not true. The source of the fertilizer is irrelevant; it's the contents of the fertilizer that matters. Get a soil test.

  • yes, a soil test would mostly likely help get to the root of the problem and yes it does sound like a witches brew :) Commented Jun 30, 2023 at 20:23
  • I believe a compost tea is mainly a microbial drench. I'm sorry I omitted to say that I didn't start the amendments until after the symptoms had appeared. The witches brew has mostly been very dilute and as said in my comment seems to have improved things somewhat. From the info I'm getting and finding it looks like a general lack of available nutrients/fertiliser so I have no reason to think that i added to the problem. Synthetic is in instant form and organic still needs breaking down I believe. That would make a difference no? - much like an apple versus pure sugar in our diet. Commented Jun 30, 2023 at 20:51
  • There are timed-release synthetic fertilizers that release their nutrients over the course of several months. And whether you add too much phosphorus in one "quick shot" or over two months is irrelevant because either way you end up with too much phosphorus in the soil.
    – Jurp
    Commented Jun 30, 2023 at 23:41

It's easier to accidentally over-fertilize with synthetic fertilizers because they are so much more concentrated, but you can over-do anything. Healthy soil is a balance.

Some fertilizers are added to put nutrients directly in the soil (charge it). The main nutrients are nitrogen, phosphorus, and potassium. These fertilizers will show you how much of each they contain on their NPK label. A natural fertilizer might be rated 4-4-4, where a synthetic fertilizer might be 20-20-20, and your leaf mould might be something like 2.2-0.8-1.6. You'd need almost ten times more leaf mould than synthetic fertilizer to add the same amount of nitrogen, so a mistake would have to be ten times bigger to cause the same nitrogen damage too. And at some point, the bulk of natural fertilizer also helps make mistakes feel like mistakes, so you catch them. Depending on what you made it from, your FPJ probably contains nutrients, although probably not enough NPK to be relevant. Plants just need too much.

The other addition your FPJ makes is a dump of microscopic life into the soil. This is the same idea as compost tea. The main danger from overdoing this kind of fertilization is probably over watering.

Another answer is exactly correct in suggesting a soil test. That will tell you exactly what is and is not in your soil, and then it's easy to look up what strawberries need specifically. From there you can rule out your soil as the problem, or you'll know exactly what to add, and how much. I pay $10 for a soil test through the mail.

  • @stormy I miss you, you'd have a better answer.
    – MackM
    Commented Jun 30, 2023 at 0:58
  • @MachM OMG! - so now not only is a mix of rich compost and leaf mould deficent in nutrients and not available enough, but a heavy ratio of fermented plant juice as a fertiliser is....... "not relevant". So, there's no such thing as an organic potting mix, no? This is so counter intuitive. What am I missing. Commented Jun 30, 2023 at 21:18
  • @MachM big thanks by the way. Commented Jun 30, 2023 at 21:19
  • @MichaelSherpa Spot on about the FPJ, it's a soil probiotic first and adds minor nutrients as a bonus. But that also means it's hard to do damage with it :). Your potting mix could be deficient in nutrients, but I really don't know. How you made yours sounds sensible to me. The lab I use makes it dead simple- they tell me the nutrient content for all the nutrients I paid them to test for, the target for 'normal' fertile soil, and the target for the intended crop I listed on the form. Then I just find a fertilizer with the nutrients I need to add, and add however much the label says.
    – MackM
    Commented Jun 30, 2023 at 22:26
  • @MichaelSherpa Organic potting mix is definitely possible, I think I was just unclear on what the NPK means- '2-2-2' means 2% 2% 2% by weight in the fertilizer, but you're aiming to adjust the soil by parts per million. It's to compare different fertilizers easily- I know I need to apply 2 pounds of 10-5-5, but I have 5-2-2, so I need 4ish pounds. Natural fertilizers are available that are totally adequate. I suggest you get a soil test on your potting mix and google "Natural fertilizer _____" where ____ is the nutrient(s) you need. Big you're welcome, enjoy the strawberries!
    – MackM
    Commented Jun 30, 2023 at 22:34

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