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I am wondering if you can get all the necessary nutrients a plant needs my mixing wood ash with forest soil or leaf compost and growing legumes on top of it (and on top of that your actual plant you want to harvest)?

Based on my poor understanding of soil, you should get enough potassium and phosphorus and other trace elements the plant needs from the ash. Forest soil or leaf compost make sure there is lots of organic matter in the soil and cancel out the pH issues. Legumes, the ones that actually fixate more than nitrogen than they need it, make sure that the nitrogen levels are sufficient.

Am I missing something here or is this the perfect soil combo?

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There is no perfect soil, and so your recipe is not perfect. Plants have different requirements, also considering that crops (and vegetables) originates in different part of the world.

Some vegetables doesn't like over-fertilized soil, or better: vegetables like it, but not you. [you want to harvest fruits and vegetables, not to create the most vigorous plant, additionally too much nitrogen will increase diseases].

Over-fertilization would reduce paradoxically some element absorbing (e.g. iron), or it could be toxic. Additionally it could sensibly change the soil so all the living being in the soil (from worms to microbes decomposing and creating nutrients), which is bad in short term.

So, your recipe could work, but don't overdo it. It could create a good base, especially if you start from a poor soil. And every year check the outcome and adapt. Take into account also that different plants have different needs, and crop rotation is good for soil, but also to reduce diseases.

Note: if you had grass before, you could have a lack of Boron, which could be solved adding hay in compost or as mulch.

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It could work, but it's not perfect.

Wood ash has no nitrogen and sulphur and is rather low in phosphorus because much of it is lost during combustion. It is also extremely alkaline so you have to do some maths to avoid over-applying it, though dilution with lots of water and using the mixture to irrigate your garden after adding lots of organic matter should work.

Legumes can add nitrogen to soil, but it's not guaranteed unless you inoculate the seeds with the right bacteria before planting. And even in that case, I feel it takes too long because a legume plant's fixed nitrogen isn't added to the soil until it is killed and starts to decompose. Urea is more cost-effective as a nitrogen source.

Leaves would add organic matter, but organic matter in a fertile and moist soil gets eaten up pretty quickly by worms, microbes etc, so you would need to keep adding the leaves regularly to keep it up. Living and dead roots in the soil are the best and most sustainable source of OM.

My preferred soil building approach is to cover it with plants (highly productive cover crops, or what you actually want to grow) and fertilize them with a balanced commercial liquid plant food or soybean meal, but the fastest shortcut to good soil is to buy lots of compost and mix it into your soil. It provides organic matter and, if added in large quantities, nutrients, too.

Questions to improve my answer: What kind of soil do you have? Why do you feel it needs improvement? What do you want to grow on the soil when it's ready?

  • Thanks. I have zero soil and this is an attempt to produce something out of nothing. A fresh start so to say. I mean, I have soil, but this is more of an experiment for cases where nearby soil pollution is to high. I would be growing tomatoes, peppers, onions and garlic. – sanjihan May 5 '17 at 17:59
  • Unless you have some clay component for water to bind to, presumably it will had very low moisture retention properties. There's one area in my garden where I used leaf mould and shop bought general purpose compost to build up a bed, and it's very dry compared to the areas with soil. – David Liam Clayton May 11 '17 at 9:29

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