I have lost my Cereus peruvianus mostruosus

[ianc]

I have lost my Cereus peruvianus mostruosus

I repotted the plant as it had outgrown its pot last August into Lorbex cactus compost, (a mix Of Loam And Peat, With Added High Grade Horticultural Grit), however I didn't remove the old soil from the roots. I repotted three others at the same time.
All the cacti are on a south facing window sill so get some sun all year round when it shines.

I noticed a few weeks ago the peruvianus was starting to fade to a yellow colour, then a week ago saw it was starting to shrivel so I gave it extra water. Now it has shrivelled a lot and looks withered and dead. It seemed to go this way quite rapid and is too far gone to save.

Reading up on care has suggested that it should be watered sparingly and allowed to dry out completely during colder months, usually October through March and that it tolerates intense heat and drought.
Since October I have been watering them all about once a month. just enough to make the soil damp then pouring away the drainage.

I just want to understand what has gone wrong. It certainly looks like desiccation.
All the other plants seem to be fine.

Being indoors, it is difficult to keep them below 50°f / 10°c but I am wondering if they should be moved from a window during the dormant period so they do not get direct sun when it shines, then move them back to the sunny window in the spring.

[ElieEstephane]

Old soil should be removed. The peat will form a tight ball and the roots cannot escape it in the new soil. In addition, dry peat will not rewet easily so the roots starve in a dry ball of peat they can't escape.
Again the original soil, which is peat, is terrible for cacti. Once you buy a cactus you need to remove the old soil if you plan to keep it. New soil should have a high mineral content and no peat.
The 3rd strike is watering during winter. Winter should be almost completely dry or you risk rot and etiolation

[esp_imaging]

Being indoors, it is difficult to keep them below 50°f / 10°c but I am wondering if they should be moved from a window during the dormant period so they do not get direct sun when it shines, then move them back to the sunny window in the spring.

If they are in strong light and above 10deg C, they will probably appreciate some water water.

As Elie says, plants are often sold in a peat compost, which can harden into an impenetrable, root-trapping block when allowed to dry completely. This is unlikely to re-wet when only given sporadice watering, hence the plant can dessicate even if watered. Obviously this will happen quicker if it is kept in a warm place.

[ianc]

Being indoors, it is difficult to keep them below 50°f / 10°c but I am wondering if they should be moved from a window during the dormant period so they do not get direct sun when it shines, then move them back to the sunny window in the spring.

If they are in strong light and above 10deg C, they will probably appreciate some water water.

As Elie says, plants are often sold in a peat compost, which can harden into an impenetrable, root-trapping block when allowed to dry completely. This is unlikely to re-wet when only given sporadice watering, hence the plant can dessicate even if watered. Obviously this will happen quicker if it is kept in a warm place.

That is most probably the reason. It certainly looks like it has 'dried up'.
The other cacti seem to be doing ok - green and healthy. Once spring arrives I will repot them again, this time releasing the roots from the old soil.

I have read Daiv's article on making your own soil, however I'm not sure what is meant by 'standard' potting soil as they all contain some level of peat. The only way to avoid peat would be to use one of the peat-free varieties.

Rather than waste it, I could add some extra horticultural sand or potting grit to the Commercial cactus compost that I bought, presumably that will make it more suitable to re-wetting?
Also, with reference to Daiv's article, I could add a sprinkle of bone meal.

[ElieEstephane]

Yes add lots and lots of grit to the commercial cactus mix and you'll probably be fine.