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We've just put all new soil in our flower bed and due to it being slightly moist it was a lovely deep rich black colour.

Is it possible to keep it that black colour even when dry?

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Well soil is just the color it is. Some soils are dark brown, some are red, some are yellow. After several years of conditioning with good compost all soil will darken with organic material.

If you really want it to look dark right now, you will probably best be served by top dressing your soil with a dark colored mulch. Mulch however, unless it's dyed is also going to get lighter as it dries and gets sun faded eventually into a silver color.

Is there a reason you need dark?

  • 1
    The only reason i like it dark is for the aesthetics. I notice how nice it looked when i put fresh moist top soil on. – Terry Apr 10 '16 at 16:45
  • Yeah, I think we all have that feeling of how nice it looks when it's fresh. See if you can find some dark, composted mulch. If you get the right kind it will look shredded rather than like wood chips. This will probably be the closest you can get without using dyed mulch. – Escoce Apr 10 '16 at 17:27
  • As long as your flowers like moist soil, you'll likely be keeping it moist so it should retain the colour as long as you dig in some compost and turn it over once in a while. If your soil lightens it also gives you a handy indication of when it's dried out and needs watering again, – Ben Cannon Apr 10 '16 at 19:07
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From my own experience I've found that if your plants can tolerate a little acidity, you can blend in dark hemlock bark fines. They can be had around my area for about 20 dollars a cubic yard. Dark when dry, JET BLACK when wet!

As they break down offer a good food source to the microbiology. I believe the mycology (fungus) in the soil particularly utilize it, breaking it down further in to inorganic matter making it bio available to the plant life. Having raised beds that have been amended with dark hemlock fines you can really see the difference. The soil in this beds is VERY dark. Even when dry, it is considerably darker than surrounding soils.

The other interesting thing worth mentioning is that after just the first year, you cannot tell that its tree bark. It has the texture, consistency, and look of dirt! I personally love it as a cheap way to get fungus friendly organic matter into my garden beds. I also don't worry much about pH issues. I do everything in small increments and do my best to foster soil life / microbiology. When the microbes are happy, issues like pH work themselves out without intervention.

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A few ideas:

Keep it moist by putting in some hydrogel near the surface. Mind that it becomes more difficult to tell when watering is needed. You could also make it so there is less evaporation from the surface, but then you don't get to see it as much.

A light-colored crust forms at the top? Filter chlorine out of your water, and use less fertilizer. Also replace the top layer.

The dark mulch washes out? Replace once in a while. The darkest natural I've found, kinda like black clay, had been collected from drying puddles in a forest. Does brighten when dry, but still darker than many soils. Just check the pH, and mix with something larger-grained. mr-fackler's idea of dark hemlock fines is great, too.

Alternative: Iron(II) oxide (black paint pigment) mixed into the mulch.

Or: a layer of black stones.

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