Odd color on side of top soil

[BigBallerBonsai]

There is some light brown colored formation on the side of my top soil. Does anyone know what this is?

 

[HorseloverFat]

Hmm.. never seen that...

I hope you figure it out!

 

[Leo in N E Illinois]

Those are accumulating minerals. Most often calcium carbonate. As soil dries out, moisture wicks to the surface of the soil, as the soil surface gets dry, it leaves behind any minerals that were dissolved in the water. Your tap water probably has only a few hundred parts per million concentration of dissolved minerals, but over time, many cycles of watering the pot, then letting it approach dry, these minerals accumulate as a crust on the surface of the soil. And just like coffee stains on a tablecloth or a piece of paper. The mineral accumulation tends to be thickest at the edges of the soil, right were the soil meets the pot, and water wicks onto the pot and carries the minerals there. Generally the minerals are mostly calcium carbonate, and possibly calcium sulfate, perhaps a little iron sulfate to give the brownish color. None of these are very soluble in water, so once deposited they do not re-dissolve easily. Fertilizer in your water can contribute to the accumulation too. Some of the mineral crust will be phosphates from your fertilizer that you have been using.

The summary is, this is normal accumulation of minerals from your water & fertilizer. Repotting, and cleaning the pot with a "safe for Teflon coating" scrub pad will remove most of the "gunk" from the pot. Take the tree out of the pot, clean the pot, repot the tree with fresh bonsai potting media.

The "crud" is not harmful, it is primarily unsightly. So there is no "emergency", the repotting can wait until the appropriate season to repot the particular tree in the pot. Do the repotting sometime within the next 2 years.

 

[LittleDingus]

Those are accumulating minerals. Most often calcium carbonate. As soil dries out, moisture wicks to the surface of the soil, as the soil surface gets dry, it leaves behind any minerals that were dissolved in the water. Your tap water probably has only a few hundred parts per million concentration of dissolved minerals, but over time, many cycles of watering the pot, then letting it approach dry, these minerals accumulate as a crust on the surface of the soil. And just like coffee stains on a tablecloth or a piece of paper. The mineral accumulation tends to be thickest at the edges of the soil, right were the soil meets the pot, and water wicks onto the pot and carries the minerals there. Generally the minerals are mostly calcium carbonate, and possibly calcium sulfate, perhaps a little iron sulfate to give the brownish color. None of these are very soluble in water, so once deposited they do not re-dissolve easily. Fertilizer in your water can contribute to the accumulation too. Some of the mineral crust will be phosphates from your fertilizer that you have been using.

The summary is, this is normal accumulation of minerals from your water & fertilizer. Repotting, and cleaning the pot with a "safe for Teflon coating" scrub pad will remove most of the "gunk" from the pot. Take the tree out of the pot, clean the pot, repot the tree with fresh bonsai potting media.

The "crud" is not harmful, it is primarily unsightly. So there is no "emergency", the repotting can wait until the appropriate season to repot the particular tree in the pot. Do the repotting sometime within the next 2 years.

Exactly what Leo said

This tends to be a more noticeable problem when plants are indoors for the winter. When indoors, we tend to water just enough to wet the pot while outdoors we tend to water until water flows out the bottom. That, plus lower indoor humidities tend to make this more of a winter problem though it certainly does happen outdoors as well.

The only thing I'll add is that you can be a little proactive about it by flushing the pot once in a while. This is something I learned from orchid growing. The salts can damage orchid roots so accumulation can be harmful. Most tree roots are sturdy enough there's no negative impact.

The way I flush the pot is to water normally to get the soil wet. As Leo mentioned, the salts to not re-dissolve readily. I let the pot sit for a while with the soil damp so anything that might re-dissolve has a little time to do so. Later on, I come back and flush copious amounts of water through the pot to wash out some of the salt accumulation.

The pots still accumulate salts, but less obviously. Doing this once a month usually gets me through the winter without a lot of buildup and once back outside rains and a more copious watering regime tend to keep buildup at bay.

You mileage will vary with the amount of dissolved salts in your water supply.

 

[BigBallerBonsai]

Thanks so much guys! This is super helpful!

 

[leatherback]

My tropicals sit on top of a heated wnidowsill most of the winter. To avoid this crusting, I reduce water evaporation from the pot: I have pieces of plastic that I place on top of the substrate, greatly reducing the areas that can loose water. Also, I water with water from the rain barrel as much as winter allows. This also reduced mineral accumulation.