Su­per soil Se­crets from the ex­perts at Kew. Plus, grow­ing flow­ers and fam­ily

The Observer Magazine - - CONTENTS -

This month I was lucky enough to sneak in on a be­hind-the-scenes visit to the newly re­stored Tem­per­ate House at Kew Gar­dens, a build­ing that blew my mind as a kid with its sheer size, its won­der­ful plants and its fas­ci­nat­ing his­tory. But step­ping into this cathe­dral of glass this time, still smelling of wet paint, I couldn’t help but be en­tranced by, well, the soil. Yes, weird. I know. But here’s why…

Any­one who has grown in con­tain­ers will quickly learn that pot­ting com­post has a ten­dency to col­lapse af­ter a sea­son or two, as the plant-based fi­bres it is made up of start to break down. This means the costly, and frankly back­break­ing, ex­er­cise of re­plac­ing it can be al­most an an­nual chore. Yet no mat­ter how mas­sive the ex­panses of glass at Kew, the plants grown in these great green­houses are es­sen­tially still con­tainer plants. With the beds as lit­tle as 40cm deep, even for tow­er­ing trees, the hor­ti­cul­tur­ists at the Tem­per­ate House face sim­i­lar is­sues to most con­tainer gar­den­ers, just on an enor­mous scale.

So back in the early 2000s the plant geeks at Kew be­gan ex­per­i­ment­ing with a pot­ting mix that would sur­vive the test of time, fill­ing dif­fer­ent beds at the Tem­per­ate House with var­i­ous for­mu­la­tions be­fore plant­ing them up. A decade later, as they be­gan to re­move the plant­ing from the house to pre­pare for its planned restora­tion, one sim­ple mix was found to have ba­si­cally zero break­down. This blend should work just as well as a gen­eral mix in the con­tain­ers of my tiny out­door space as it does in the houses at Kew, and the best bit is it’s pretty fool­proof to put to­gether.

It’s sim­ply equal parts loam (ie reg­u­lar gar­den soil), grit, sand and com­posted bark. The rel­a­tively low con­tent of or­ganic mat­ter (just 25%) means this for­mula breaks down far more slowly than tra­di­tional mixes which can verge on 100% plant-based ma­te­rial. Yet it’s still rich enough in these nat­u­ral fi­bres to re­tain wa­ter and sup­port a healthy bac­te­rial com­mu­nity. This might seem ex­tremely low, if you are used to tra­di­tional peat or com­post-based mixes, but is ac­tu­ally much more sim­i­lar to most fer­tile soils found in na­ture than those mixes are.

Also, note Kew’s use of com­posted bark as the or­ganic ma­te­rial here, not peat: it’s a far more sus­tain­able al­ter­na­tive. The ad­di­tion of loam helps mimic nat­u­ral soils fur­ther, en­sur­ing a far broader spec­trum of the es­sen­tial min­er­als nec­es­sary for healthy plant growth, which greatly re­duces our re­liance on fer­tilis­ers, ei­ther nat­u­ral or syn­thetic. Sand and grit, at a re­ally high level (I was sur­prised), help re­tain an open, free-drain­ing struc­ture and pro­vide enough weight to an­chor the roots of large trees. All in all, it’s pretty per­fect for raised beds, tubs and troughs.

How­ever, if you, like me, are grow­ing things on bal­conies or roof ter­races, this mix can be pro­hib­i­tively heavy, both for you to lug up flights of stairs, and po­ten­tially even from a struc­tural point of view. So I have made a lightweight tweak for us sky­wardliv­ing gar­den­ers. Just re­place the grit with ex­panded clay pel­lets (also sold as hy­droleca) and the sand with per­lite and you are good to go. Two per­fect grow­ing mixes to last you years at a time, based on the best the plant geeks at Kew could con­jure up.

@Botanygeek

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