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Iron oxide (Pigment Black 11) is a intense black non toxic and very cheap pigment. I think I have read somewhere that soil from shops is sometimes pigmented with iron oxide because it sells better. But I could not find a reference any more.

Some plants need different soil (some with more and less humus content) in my garden and the surface looks very blotchy at the moment and I wonder

what are the long term effects of iron oxide pigments in soil?

  • Will plants absorb the pigment and get darker too?
  • Will it react with rain or natural fertiliser?
  • Where can I find studies on the consequences of iron oxide pigments on the nutritiousness of vegetables and fruits?

According to instinct I expect nothing good from pigments in soil, but I could not find a reliable statement. Please support your answer with references.

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    You seem to be suggesting you might add iron oxide pigment (used in mascaras and paints and the like) to your soil in order to make it look better. If you want to improve your soil (both in appearance and health) and anything that grows in it, add as much humus rich material as you can get hold of - things like composted animal manures, leaf mould, good home made compost. Adding dyes is a waste of time and money.... – Bamboo Apr 12 '17 at 22:47
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    Humus or biochar are the only "black things to add to soil" that are going to be of any benefit. – Ecnerwal Apr 12 '17 at 23:14
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    Recently I have bought potting soil that looked like it contained black paint added to the usual soil. I have used this soil for seedlings trays and one day I forgot to water. The soil compacted to half its volume and become hard as a rock. I had to overwater the seedlings to loosen the soil. The plants grow slowly, but this may be the case because they are also on a north windowsill. – Alina Apr 13 '17 at 5:55
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Whilst the Iron (II,III) oxide in pigments is usually manufactured, it does occur naturally as the mineral magnetite. It will gradually be converted into other iron compounds - exactly which will depend on soil chemistry/water availability, although it will tend to hydrate to form rust in healthy soils. These compounds will not cause you or your plants any harm, unless you are dumping large quantities of it (in the order of kg/m²). The amounts encountered in dyed composts or paper inks will be insignificant.

As for absorbing them - small quantities may be used by plants although they are not readily absorbed (by plants or animals) and are necessary for plant growth. Iron deficiency results in chlorosis - yellowing of the leaves and iron compounds (usually sulphates) are found in many fertilisers. It is unlikely to alter their colour (unless they have a deficiency). Similarly, I would not expect any significant effect on the nutrition of produce unless correcting an existing deficiency.

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