The play­ers in our hid­den bio­sphere

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grubs, ter­mites and wee­vils, for ex­am­ple – but oth­ers are beloved or at least be­guil­ing.

Dung bee­tles con­vert an­i­mal waste into hu­mus, a ser­vice we take for granted. Ants are the most abun­dant soil in­sect. Although some species are pests or nur­ture pests such as aphids, ants with their highly or­gan­ised colonies are es­sen­tial mem­bers of the soil bio­sphere. They as­sist in the con­ver­sion of lit­ter to hu­mus, move and mix large quan­ti­ties of soil, and spread the seed of bulbs and other de­sir­able plants. crus­taceans that like soft plant de­bris and make quick work of green plant ma­te­rial and newly fallen leaves.

One of the most abun­dant, but barely vis­i­ble, arthro­pods in the soil are spring­tails. They are named for a tail-like struc­ture that al­lows them to jump when threat­ened. As many as a bil­lion can live in an acre of soil. De­pend­ing on species, they cy­cle plant de­bris or feed on fungi, al­gae or other spring­tails.

Mites are gen­er­ally re­garded by gar­den­ers as pests, and some are – suck­ing sap from plants and spread­ing dis­ease. But the soil houses an im­mense com­mu­nity of non-pest species that are es­sen­tial to the cy­cle of life. Half the known species of mites live in the soil, where they feed on de­cay­ing plant lit­ter. Nardi writes that they “set the stage for smaller de­com­posers like bac­te­ria and fungi to free most of the en­ergy and nu­tri­ents stored in those leaves”.

Some mites are preda­tory and at­tack ne­ma­todes and other small crea­tures.

Ne­ma­todes are tiny worm­like crea­tures tra­di­tion­ally viewed in agri­cul­ture as se­ri­ous pests that harm plants by feed­ing on their roots. More re­cently, the view of ne­ma­todes has be­come more nu­anced be­cause some species are now com­monly used (and pur­chased) as preda­tors of gar­den pests. Ex­perts be­lieve there may be close to a mil­lion species, of which only a frac­tion have been de­scribed sci­en­tif­i­cally.

Some ne­ma­todes eat soil bac­te­ria and fungi, while oth­ers pre­fer to con­sume other soil arthro­pods and pro­to­zoa. Their value to the gar­den is in con­vert­ing ni­tro­gen into a form that plants can use.

Pro­to­zoa are mi­cro­scopic crea­tures that live in vast num­bers in the film of wa­ter be­tween soil par­ti­cles. The most well-known is the amoeba, but these mi­crobes come in sev­eral forms, in­clud­ing species that move with a sin­gle fla­gel­lum or with hair­like cilia.

They are the ma­jor preda­tor of bac­te­ria, and in con­sum­ing them they re­lease ni­tro­gen and other nu­tri­ents to plants.

Pro­to­zoa, in turn, are eaten by ne­ma­todes and other small arthro­pods.

His­tor­i­cally, bac­te­ria have been as­so­ci­ated with germs. Some of the nas­ti­est human dis­eases – an­thrax, ty­phoid, tu­ber­cu­lo­sis and syphilis, for ex­am­ple – are the re­sult of bac­te­rial in­fec­tions. But we have come to know too that our guts are full of ben­e­fi­cial bac­te­ria and es­sen­tial to our health.

The soil is the same way. The bad ac­tors are out­num­bered and usu­ally out­wit­ted by the good ones. Healthy soil is loaded with bac­te­ria, and be­cause they’re not very mo­bile, they tend to hang out in vast num­bers on and around the roots of plants, a zone known as the rhi­zo­sphere. There can be as much as 100 times more bac­te­ria around plant roots than else­where in the soil, and with good rea­son. The plants feed them car­bon sug­ars. The mi­crobes give back ni­tro­gen.

Fungi break down or­ganic mat­ter, which is why you will see mycelium strands in com­post piles and un­der leaf lit­ter. Two ba­sic forms of fungi form a sym­bi­otic re­la­tion­ship with plants. One ex­ists in prox­im­ity to root tips and as­so­ciates with hard­wood trees and conifers, the other pen­e­trates the cell wall of the roots and is found in plants of the do­mes­tic land­scape of flow­ers, shrubs, grasses and veg­eta­bles.

The fungi grow tiny, frag­ile strands called hy­phae. They are a tenth the thick­ness of human hair, but there are so many of them that they form a vast net­work, ef­fec­tively ex­tend­ing the reach and ef­fi­ciency of plant roots. In her book The Soil Will Save Us, science writer Kristin Ohlson says there can be as much as 515km of hy­phae in a cu­bic foot of soil. At least 80% of the plants on Earth con­nect to these fun­gal part­ners. “Gar­den­ers need to know this stuff,” Lowen­fels said. “A think­ing gar­dener is a bet­ter gar­dener.”

• Adrian Hig­gins has been writ­ing about the in­ter­sec­tion of gar­den­ing and life for more than 25 years, and joined the Washington Post in 1994.

He is the au­thor of sev­eral books, in­clud­ing the Washington Post Gar­den Book and Chan­ti­cleer, a Plea­sure Gar­den.

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