I have used burnt soil successfully the very first time to grow northiana partly it was cheap ( a bag cost only RM2.00 or 0.58USD in today's rate ). Farmers here burn the soil and spread around vegetable as fertilisers. Artificial ferts are costly hence the alternative. Burning probably (?) bumped up the nitrogen and other chemical contents present like nitrates, manganese and iron which gave rise to the reddish appearance,
Natural clay pellets worked wonderful for most neps as it's well drain and retain moisture. It was readily available esp around disturbed hills or where there are minor landslides. weathering breaks them into smaller particles and washed down the hills during heavy rain.
There are various neps growing around this patch of the hill-ampullaria, reinwardtiana, albo,mirabilis, and hybrids -ferrugineomarginata, trichocarpa, neglecta and kuchingensis.
Reddish clay and clump of reinwardtiana ( denoted with red )
some examples neps grow in open ground
Instaed of using burnt clay, i collect the natural clay pellets, rinse off the fine particles, transfered a northiana and had since growing without negative effects. Most neps can be grown using clay pellets and they show no bad effects.