I have been practicing gardening since 2001, but this observation dates back to 1987 or 1988. Maybe I've struck some new stuff. In these grey days, the garden was in a bad shape, due to soil problems, mainly salt accumulation. There are two remote spots in the garden. One harbored a potos, which was climbing a wall, and the other had a philodendron in a large container.

The potos did not grow much beyond it's normal proportions, like those plants you see at the nursery. However, once it's air-roots got hold of the wall (which was mortared), the part of the stem above these roots was thicker, and it's leaves have reached monstrous proportions -- almost two feet (60 cm) long.

The philodendron never flowered before I did one unusual thing: I bagged the air-roots, and filled the bag with sawdust. Fast growth, and flowering, did not come late.

At that time, I did not work yet, and had no funds for gardening, and not a driver's license for me to go to the nursery and stock a pile of manure in the rear trunk. So, I have decided to use whatever materials that seemed adequate, to the best of my understanding at that time. I had some unused mortar which I poured on one plot of the garden. It didn't work. Being close to a carpentry shop, I had access to lots of sawdust. Dazzled by the success of air-layering philodendrons with bags of sawdust, I have poured many bags of sawdust on the soil, and tilled them in. In two weeks, I have witnessed an astounding surprise: The sawdust, once mixed into the soil, did quite the opposite of what I expected it to do, and many plants seemed stressed and wilted. It was like realizing that one plus one equals zero.

Looking back at these days, with today's knowledge, I did understand what happened, but this has also led me to new conclusions.

1- One and one does not necessarily make two (Not mathematically speaking). One of the most significant factors I have learned about, was soil pH. Alkaline soil, has it's own advantages and disadvantages, compared to acidic soil: One has some nutrients which are more available than others, and the other has other nutrients available. The example of the potos, growing on a mildly acidic soil and having air-roots holding onto the alkaline mortar of the wall, was actually getting "the best of each". Which means, one and one makes two, if not mixed. This also happened with the Philodendrons air-layered in bags of sawdust: Sawdust is rich in P and K elements, but poor in N (Nitrogen). However, it does not affect the plant, since it gets all the N it needs from the soil!

2- Air roots absorb nutrients even if they seem dry ? Does anyone has information on this one ? The main roots of the potos are in the ground, where it gets most of the humidity. However, the air roots clinging to the wall are always dry, and they don't appear to draw any nutrients. But, once you see that the stems holding to the wall get thicker, you may reconsider this. Possibly the air-roots secrete water to dissolve nutrients, and then re-absorb the water with the nutrients ? I was looking for a phenomena in nature to support this argument. There are two groups of plants which live in a foggy desert with almost no rainfall, and yet, they manage to grow: The Welwitschia in Namibia, and the Copiapoa cacti in the Atacama desert. They do absorb the fog through their leaves (or stem, in-case of the cacti). However, fog does not contain enough nutrients. They need to draw nutrients from the soil. The only way to do this, is when the roots secrete water into the soil to dissolve the nutrients, then re-absorb it again. But I have found no reference to a plant that does that.