On Soil Volume and Pot Size and Type

How much soil to use and in what size and kind of pot is a oft-debated topic in the professional growers’ world. Obviously costs are a major consideration in an operation where thousands of plants are being grown, so the easy answer is “as little as we can get away with without making the plant suffer”. But how much is that? And do cloth pots help? The following is excerpted from Plant Physiology by Lincoln Taiz and Eduardo Zeiger”:

“In the late 1930’s, H. J. Dittmer examined the root system of a single winter rye plant after 16 weeks of growth and estimated that the plant had 13 x 106 primary and lateral root axes, extending more than 500 km in length and providing 200 m2 of surface area (Dittmer 1937). This plant also had more than 1010 root hairs, providing another 300 m2 of surface area. In total, the surface area of roots from a single rye plant equaled that of a professional basketball court” [emphasis added].

That’s a lot of surface area to fit into a pot of any size–and we are growing plants that are much bigger than rye! In my medicinal grow, I used #5 nursery pots for plants that were sped through flowering as quickly as possible. The incredible root area in such a small pot sometimes created a severe decrease in pH later in the plant’s life (explained here), which resulted in some leaf spotting and nutrient lock-out, and created labor. An increase in pot size for strains where such problems had occurred resolved the problem. More soil, not an adjustment of soil characteristics, was the better cure.

Which leads me to cloth pots. The hypothesis behind cloth pots is that the roots’ exposure to air sends the root in a reverse direction, thus directing the root inward and therefore utilizing more of the soil surface area whereas roots in plastic pots simply grow downward along the edge of the pot. Now, I have heard good things from my colleagues who use these pots and I don’t mean to doubt them, but I am not aware of any special mechanism in the plant that would create such a difference between the two materials. Roots have a gravitropic response, and auxin-induced “turn” response to hard objects, and air “prunes” the roots, but I am not aware of any plant mechanism induced by exposure to air that would cause a plant to abandon its gravitropic response. I think that the difference must be in the “air pruning” effect of dryer soil at the margins of a cloth pot. The root, in other words, tends toward a medium of greater moisture content–at the center of the pot.

What do you think? Are cloth pots a clever way to squeeze more out of the same volume of soil? What has been your experience? Comment below if you have an opinion you’d like to share.

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