Cad­mium – it's com­pli­cated

Cad­mium in agri­cul­tural soils is an emotive topic about which a lot of con­fus­ing state­ments are made, Greg Sneath, ex­ec­u­tive man­ager of the New Zealand Fer­tiliser As­so­ci­a­tion told Onions NZ’s Con­fer­ence.

NZ Grower - - NEWS - By Glenys Chris­tian

He is also par­tic­i­pates in the Cad­mium Man­age­ment Group where Hor­ti­cul­ture New Zealand is rep­re­sented through the Veg­etable Re­search and In­no­va­tion Board.

Cad­mium is an el­e­ment which be­haves like zinc and oc­curs nat­u­rally at trace lev­els in soils. It is also found at a range of dif­fer­ent con­cen­tra­tions in phos­phate rock, from which phos­phate fer­tiliser is man­u­fac­tured. So, over many years of re­peat ap­pli­ca­tion, it can very grad­u­ally ac­cu­mu­late in the top layer of soil, he said.

“What’s in the par­ent rock is what you get in phos­phate fer­tiliser made from it.”

There is no fea­si­ble mech­a­nism for re­mov­ing it from su­per-phos­phate but in prod­ucts like di­ammo­nium sul­phate (DAP) with a man­u­fac­tur­ing process us­ing phos­phoric acid, again de­pend­ing on the cad­mium con­cen­tra­tion in par­ent ma­te­rial, there is gen­er­ally a slightly lower level.

For many years be­fore the 1990s, Nauru phos­phate rock was New Zealand’s pri­mary source of rock for man­u­fac­tur­ing phos­phate fer­tiliser. It was rel­a­tively high in cad­mium. How­ever, lev­els in fer­tiliser are now re­duced through sourc­ing and blend­ing phos­phate rock from a range of other sources, which con­tain less cad­mium.

Cad­mium in soil can be taken up by plants and thereby en­ter the food chain at trace lev­els. Ab­sorbed cad­mium can ac­cu­mu­late in the kid­neys and liver and over a life­time, can cre­ate a risk of ad­verse health ef­fects.

“But food isn’t the only source of cad­mium,” he said.

“Ex­po­sure is dou­bled through smok­ing; in­dus­trial ex­po­sure is also pos­si­ble.”

For ex­am­ple weld­ing rods can emit cad­mium.

The Min­istry for Pri­mary In­dus­tries (MPI) Food Safety Au­thor­ity mon­i­tors cad­mium lev­els in the typ­i­cal New Zealand diet, us­ing the To­tal Diet Study. The re­sults are well be­low the World Health Or­gan­i­sa­tion’s (WHO) rec­om­mended monthly tol­er­a­ble in­take lev­els. The Euro­pean Union lim­its are lower than those of the WHO, even though they are based on the same in­for­ma­tion. They make much more con­ser­va­tive as­sump­tions, but typ­i­cal di­etary in­take of New Zealan­ders is also well be­low the Euro­pean rec­om­mend in­take value. These val­ues rep­re­sent a safe level of in­take ev­ery day over a life­time, so an

oc­ca­sional mild ex­cedance in a food stan­dard is of no con­se­quence for an in­di­vid­ual.

The Aus­tralia and New Zealand Food Stan­dards pro­vide lim­its for cad­mium in veg­eta­bles, as con­sumed. Root and tu­ber veg­eta­bles have a limit of 0.1 mil­ligrams of cad­mium per kilo­gram fresh weight, but this does not ap­ply to onions, which are classed as a bulb veg­etable. This means the prin­ci­ple of “as low as rea­son­ably achiev­able” ap­plies.

When it comes to ex­ports, onions have to meet in­di­vid­ual coun­try stan­dards. For ex­am­ple, the Euro­pean Union stan­dards for veg­eta­bles, ex­clud­ing leafy veg­eta­bles, fruit, herbs, stem and root veg­eta­bles and pota­toes, are 0.05 mil­ligrams of cad­mium per kilo­gram of fresh weight.

Sneath said plant va­ri­ety and species had quite an in­flu­ence on cad­mium lev­els, with leafy green veg­eta­bles typ­i­cally show­ing greater up­take of soil cad­mium, com­pared with other veg­eta­bles. Fruit shows the low­est. But dif­fer­ent va­ri­eties for each crop can show con­sid­er­able dif­fer­ences in up­take.

Soil cad­mium ac­cu­mu­la­tion is in­flu­enced by the length of time phos­phate fer­tilis­ers have been ap­plied as well as the cad­mium con­tent of fer­tiliser ap­plied and the rate of ap­pli­ca­tion. Soils in the Waikato and Taranaki re­gion had been found to have typ­i­cally higher lev­els, due to their long his­tory of phos­phate fer­tiliser ap­pli­ca­tion and soils with a high phos­pho­rus re­quire­ment.

Sneath said the Fer­tiliser As­so­ci­a­tion worked with MPI, the Min­istry for the En­vi­ron­ment, re­gional coun­cils, and pri­mary sec­tor groups in the Cad­mium Man­age­ment Group, which meets twice a year. It has im­ple­mented a Cad­mium Man­age­ment Strat­egy and >

pro­vides a gover­nance role, as well as mon­i­tor­ing progress and supporting re­search and ed­u­ca­tion.

The fer­tiliser in­dus­try main­tains a vol­un­tary limit of no more than 280 mil­ligrams cad­mium per kilo­gram of phos­pho­rus. It had de­vel­oped a Tiered Fer­tiliser Man­age­ment Sys­tem, en­dorsed by the Cad­mium Man­age­ment Group to con­trol ac­cu­mu­la­tion based on soil cad­mium lev­els. In Tier 0 where soil cad­mium is within the range, which oc­curs for back­ground lev­els, no re­stric­tions ap­ply, but at Tier 1 where lev­els are slightly el­e­vated, from 0.6 to 1.0 mil­ligram per kilo­gram of soil, some re­stric­tions on the rate and type of fer­tiliser ap­ply. At Tier 2, from 1.0-1.4mg/kg soil, and Tier 3, from 1.4-1.8 mg/kg soil, choice of fer­tiliser be­comes more re­stricted. Then at Tier 4 with lev­els of 1.8 mg /kg soil or more, lim­its are im­posed to stop any fur­ther ac­cu­mu­la­tion un­less a site spe­cific study is car­ried out to iden­tify the risks and path­ways for po­ten­tial harm.

The pro­gramme is dis­cussed with farm­ers at field days, sem­i­nars, small groups and mostly a one-to-one ap­proach, due to the com­plex­ity and sen­si­tiv­i­ties of the topic.

Mit­i­ga­tion op­tions to re­duce plant up­take are im­por­tant. Onions NZ has con­trib­uted to re­cent re­search on plant up­take of cad­mium with sam­pling car­ried out in Pukekohe, Mata­mata, Can­ter­bury and Hawke’s Bay. Soil sam­ples were taken where one va­ri­ety of onions was grown, se­lect­ing five sam­ples from each plot, from 10 dif­fer­ent grow­ers on 19 dif­fer­ent sites.

Soil cad­mium does not nec­es­sar­ily re­flect the plant up­take. For ex­am­ple, Can­ter­bury soils had lower cad­mium with lit­tle vari­a­tion, but there was one site at which onions showed in­creased up­take. This demon­strates that the re­la­tion­ship be­tween soil cad­mium and plant up­take is a com­plex one.

The same plant va­ri­ety might show dif­fer­ent up­take re­sults in dif­fer­ent ar­eas. For ex­am­ple, Sneath said the same va­ri­ety of onion grown in Pukekohe and Pukekawa showed dif­fer­ent up­take lev­els, show­ing there were dif­fer­ent fac­tors at play. This was also ob­served with spinach grown in dif­fer­ent re­gions.

“In these tri­als we’ve looked at soil car­bon, and soil pH but the re­la­tion­ship isn’t al­ways clear,” he said.

“We can’t fully ex­plain dif­fer­ences in up­take with these char­ac­ter­is­tics.”

Sneath said there were a num­ber of prac­tices which grow­ers could use to limit plant up­take of cad­mium. These are pre­sented in the Tiered Fer­tiliser Man­age­ment Sys­tem, such as mon­i­tor­ing their soils, us­ing fer­tiliser with lower cad­mium lev­els, avoid­ing fer­tiliser with a high level of chlo­ride and grow­ing va­ri­eties which take up lower lev­els of cad­mium, if these are known.

“Soil pH is a key is­sue be­cause cad­mium is more avail­able in low pH soils,” he said.

The be­hav­ior of cad­mium was also mod­i­fied by the level of zinc in the soil, as chem­i­cally they are sim­i­lar in na­ture. Cad­mium is gen­er­ally more avail­able in sandy soils and there is ev­i­dence that in­creas­ing soil or­ganic mat­ter might mit­i­gate plant up­take.

“On their own each of these prac­tices may not be as ef­fec­tive in re­duc­ing cad­mium up­take as they are in com­bi­na­tion.”

Grow­ers should also make sure their soils were well up in the rec­om­mended pH range, main­tain or­ganic mat­ter in the soil as high as pos­si­ble, keep zinc at the high end of re­quire­ments to over­come any de­fi­ciency and buy fer­tiliser from a rep­utable source, es­pe­cially any zinc sup­ple­ments.

▴ Greg Sneath – lots more to learn about cad­mium.

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