I'm running a test on growing tomato plants with about 80% coffee, and another with about 20% coffee in layers. What one should I expect to do better?

Both have silty sand from the bottom of the compost pile, since I've heard that silt is excellent for growing plants in.

  • NO. Even mulch IS NOT A SOIL SUBSTITUTE. Coffee? Do you know what silt is? Sandy loam? Sand versus clay? Why clay is used to make concrete and how it is used to make concrete? Do you know the pH of coffee grounds? How it affects the pH of the compost and then the soil? Plants need soil...little tiny pieces of rock. Do you know what the pH is of the soil in which your tomatoes are growing? Do you know what pH tomatoes need? I'll bet that soil at the bottom of your compost pile is yummy, there is a reason for that. Do not use garden soil or that soil for pots...
    – stormy
    Commented Jul 5, 2016 at 1:05
  • @stormy the silt has some coloring (black) in it, so it's not true silt, but about as thick as it. There's no rocks in the soil, probably just what animals wallowed in thousand of years ago (what I'm calling silt). My tomatoes say they need 6.2-6.8, but they can tolerate different levels of PH outside of it if needed. If I'm putting in plants on a budget I'm going to just dig some dirt out from the compost area outside of what the compost pile has as we don't care about what happens in the tall grass. Commented Jul 5, 2016 at 2:16
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    Probably the former. The constant jittering from the caffeine should make the stems strong and aid in pollination. ^_^
    – Stephie
    Commented Jul 5, 2016 at 7:25
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    The term 'silt' in terms of soil means a type of soil formed by running water, so the sort of thing you'd get possibly from a river bed, and it contains various minerals. You appear to be using this term to describe material from your compost pile which has composted thoroughly and is ready for use - but that is not silt, it's compost, which, whilst valuable in the garden, is no subsitute for proper alluvial silt.
    – Bamboo
    Commented Jul 5, 2016 at 9:01
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    Great comment Fiasco!! You are a naughty girl Miss Stephie!! Grins!! What do you mean black thumb by 'what happens in the grass'....have you ever had a soil test done? With as much as you are doing with gardening, a soil test will teach an awful lot of necessary stuff about your resources!! Coop. Extension services through your nearest University!! A master gardener course through the same services would help you tremendously!! Have you done the mason jar with water test? Put a cup of your soil in water and shake it up. When it settles you'll see the proportions of sand, silt and clay
    – stormy
    Commented Jul 5, 2016 at 19:37

2 Answers 2


I think you're not really asking if this is soil, which requires inorganic materials ( ground rocks ) with some organic matter ( humus ), but whether this mixture is suitable medium to grow your tomatoes. There are many different types of media including those used in hydroponics ( just water alone ), flood and drain systems which use gravel, and expanded clay pellets, to traditional soil. The elements that provide the necessary growing media include support, nutrients, water and oxygen. Your mixture of coffee grounds is mostly organic matter in a form not yet available to the plants since it hasn't been transformed to humus, doesn't offer support, doesn't allow free draining so that oxygen gets to the roots, and will just clog up with stagnant water. The coffee will decompose and rot the roots. If any survive, it will be the one with the lesser amount of coffee.


100% coffee grounds will not make a great soil, you should mix it with at least 50-75% other soil. Also, coffee grounds are fine for plants that like a bit of acid (azaleas, rhododendrons, tomatoes) but will be bad for other plants.

You should also check your soil pH to make sure it's a good range for the plant that will be in the coffee grounds. I got this 3-in-1 tester for $13usd and it works well enough for me. It tests for light, pH, and moisture.

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