I know this is heresy, but hear me out. We’ve been told by northern hemisphere garden and permaculture articles for so long that soil health is everything (it is). But we’ve also been told that healthy soil should look like the kind of nutrient-rich loams that almost never occur in nature on the ancient, weathered geology that makes up our landscape.
The most easily visible and understood problem is that wellfertilised or composted soils result in rapid soft growth, dependent on supplementary water to survive. Even if they are adapted, rapidly grown leaves don’t develop all the features that make local indigenous plants able to breeze through a normal dry season without extra water. These features include leathery structures that withstand wilting, the silvery hairs and felting that shelter the pores leaves breathe through, and even the waxy coatings that prevent water loss. You should be selecting plants bearing in mind such traits, and promoting their development by providing the bare minimum of water and nutrients your plants need.
The more important reason minimal or no compost is better is entirely invisible, unless you have a powerful microscope. Nearly every plant has symbiotic relationships with specialised fungi called mycorrhizae. This vast network of invisible fungal threads extends the effective length of plant roots to many times what they have by themselves. It is well known that this symbiosis helps the plant to gather nutrients. Less well known is that mycorrhizae usually greatly increase a plant’s water uptake ability and drought tolerance.
So what’s the problem with compost? Well, if you raise soil phosphorous and nitrogen to the point where they are plentiful, plant roots separate themselves from the fungi, reducing their ability to survive dry periods. Compost produced by commercial or home hot-composting contains a lot of highly soluble and available phosphorous and nitrogen, more than enough to disrupt mycorrhizal development and drought tolerance.
In addition, in many situations roots stay within the rich, well-watered composted hole, not extending laterally into the relatively poor and dry surrounding soil. Where this happens, initial rapid growth stops and the plant sulks. Some sensible horticultural scientists have recommended against composting tree planting holes for this reason alone.
Mulch, however, can actually increase soil nutrient status to the same degree as regular surface compost application, as the soil surface becomes colonised by nitrogen-fixing bacteria but without the destructive flush of nutrients associated with compost. So avoid the compost, or at least go easy, and use plenty of mulch.
Often, plant deficiencies and yellow leaves are a result of a pH or nutrient imbalance, and heavy feeding does not fix the underlying problem. Of course, highly soluble artificial fertilisers should be used with extreme restraint for all the same reasons.