Cast­ing no doubt on earth­worms’ good work

Matamata Chronicle - - Rural Delivery - By BALA TIKKISETTY

The twists and turns of the hum­ble worm have a ma­jor pos­i­tive ef­fect on the chem­i­cal, phys­i­cal and bi­o­log­i­cal prop­er­ties of farm soils.

This im­pact is a com­bi­na­tion of the worms’ bur­row­ing, feed­ing, di­ges­tion and cast­ings.

Waikato Re­gional Coun­cil’s soil qual­ity re­ports have re­vealed only 34 per cent of sites sam­pled in the re­gion have fully met na­tional soil qual­ity tar­gets.

One key in­di­ca­tor of soil bi­o­log­i­cal health and con­di­tion is the num­ber of earth­worms found in it.

Worm pop­u­la­tion den­sity and species are af­fected by soil prop­er­ties and man­age­ment prac­tices.

Pro­mot­ing good soil health and lim­it­ing soil dam­age in win­ter are im­por­tant fac­tors in sus­tain­ing earth­worm pop­u­la­tion.

Over­all, there are nearly 200 species of na­tive and in­tro­duced earth­worms in New Zealand but only a few in­tro­duced species are ac­tu­ally ben­e­fi­cial to agri­cul­ture. The most com­monly found species ar­rived in New Zealand with early Euro­pean set­tlers.

Earth­worms shed and de­com­pose plant residue con­vert­ing it to or­ganic mat­ter and min­eral nu­tri­ents.

Com­pared with uningested soil, these earth­worm ‘‘casts’’ can con­tain five times as much plant-avail­able ni­tro­gen, three to seven times as much phos­pho­rus, 11 times as much po­tas­sium and three times as much mag­ne­sium.

They may also con­tain more cal­cium and plant avail­able molyb­de­num, and have higher ph, or­ganic mat­ter and mois­ture con­tent.

The break­down of or­ganic mat­ter by worm di­ges­tion also re­sults in in­creased fer­til­ity as it pro­vides for faster nu­tri­ent re­cy­cling and makes soil nu­tri­ents more avail­able for plants. It as­sists in a sig­nif­i­cant in­crease in pas­toral pro­duc­tiv­ity, faster de­com­po­si­tion of or­ganic mat­ter, in­creased plant growth, and bet­ter mix­ing of nu­tri­ents, fer­tiliser and lime through the soil pro­file.

Worm move­ment through, and feed­ing on, soils also re­sults in im­proved soil struc­ture. It pro­vides for bet­ter mix­ing of soil lay­ers, cre­ates aer­a­tion and drainage holes, and al­lows for bet­ter plant root de­vel­op­ment. Less water run-off, less ero­sion and bet­ter drainage and water hold­ing ca­pac­ity are other ben­e­fits.

The dis­posal of or­ganic wastes from do­mes­tic, agri­cul­tural and in­dus­trial sources is caus­ing in­creas­ing en­vi­ron­men­tal and eco­nomic prob­lems. Earth­worms can be used to process these wastes.

Most of these wastes con­tain con­sid­er­able amounts of in­or­ganic and or­ganic con­tam­i­nants such as heavy met­als, pes­ti­cides, aro­matic hy­dro­car­bons and sul­phur com­pounds, as well as hu­man and an­i­mal pathogens.

There are three types of earth­worms, epigeic, en­do­geic and anecic. Epigeic earth­worms (Lum­bri­cus rubel­lus) feed on or­ganic mat­ter on the soil sur­face and do not form per­ma­nent bur­rows. En­do­geic earth­worms (Apor­rec­todea calig­i­nosa) in­gest top­soil and its as­so­ci­ated or­ganic mat­ter, form­ing semi-per­ma­nent bur­rows.

Anecic earth­worms (Apor­rec­todea longa) take or­ganic mat­ter from the soil sur­face into their deep, per­ma­nent bur­rows to feed on. In an ideal soil all three types may be found.

Pre­cise iden­ti­fi­ca­tion of earth­worm species is dif­fi­cult but for most pur­poses it is only nec­es­sary to be able to dis­tin­guish be­tween na­tive and in­tro­duced species. When ma­tured, most in­tro­duced worms have a vis­i­ble ‘‘col­lar’’, called the clitel­lum, about one third of the dis­tance along the body from the head, and of­ten a pair of white glan­du­lar swellings on the un­der­side be­tween the head and the clitel­lum.

In the na­tive worms, this col­lar is close to the head, and there are no glan­du­lar swellings on the un­der­side. Im­ma­ture worms do not have a clitel­lum and are there­fore harder to iden­tify.

The in­tro­duced earth­worms men­tioned are es­sen­tial to the de­vel­op­ment of fer­tile pro­duc­tive soils.

Soils with­out earth­worms are usu­ally poorly struc­tured and tend to de­velop a turf mat or thatch of slowly de­com­pos­ing peat­like ma­te­rial at the sur­face. They are also less pro­duc­tive, hav­ing a harsh un­healthy ap­pear­ance with old dung and dead plant ma­te­rial ly­ing on the sur­face.

Other fea­tures of such soils can in­clude the fact that lower pro­duc­ing grasses are of­ten more ev­i­dent than rye­grass, and pas­ture growth is slow to start in spring and stops early in au­tumn. Plant nu­tri­ents can re­main locked in the or­ganic layer and there is no mix­ing of ap­plied fer­tilis­ers. Plant roots are rel­a­tively shal­low and there­fore pas­tures are sus­cep­ti­ble to drought. Water tends to run off the pas­ture rather than be­ing ab­sorbed into the soil, thereby in­creas­ing water qual­ity prob­lems.

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