'When I was a boy' (I get great amusement saying that of late) cheap potting mix and a bag of wood chips were different things. Now, though, there doesn’t seem to be much of a difference…
The problem with buying the cheapest potting mix (I’ll not name the brand pictured, it's a common thing) is that it’s mostly wood chips. When you plant into it, the microbes that find themselves there take easily available nitrogen out of mix in order to break down the wood. That means that there’s less for the plants and their leaves go yellow.
Nitrogen is a crucial nutrient – it is used to make proteins, including the enzymes that bacteria use to break down nutrients into plant accessible forms. It’s most important role, though, is that it is part of DNA and RNA, without which, growth and life itself is impossible.
Lack of nitrogen shows itself as yellowing in the older leaves at first, as the plant reclaims it and moves it to new growth. Eventually though, a long term deficiency will yellow the whole plant. Because of this, nitrogen is called a 'mobile' nutrient. Iron is another mobile element and deficiency of it causes similar symptoms.
The key when using this kind of cheap potting mix is the same as when you’re using woodchips as mulch. You need to add a nitrogen rich fertilizer such as dilute chook poo, Blood and Bone or even dilute urine before you use the mix. That’ll add the nitrogen needed to break down the wood and leave enough for the needs of the plants that you plant into it.
