Get­ting value for fer­tiliser dollars

Matamata Chronicle - - Rural Delivery - By BALA TIKKISETTY Bala Tikkisetty is a sus­tain­able agri­cul­ture co­or­di­na­tor at Waikato Re­gional Coun­cil. Con­tact him on bala.tikkisetty@waika­tore­gion.govt.nz or 0800 800 401.

Get­ting the best bang for buck out of fer­tiliser while pro­tect­ing eco­nomic and en­vi­ron­men­tal bot­tom lines is a key goal for farm­ers.

Find­ing that bal­ance can be tricky for farm­ers and re­quires ad­vice from their fer­tiliser reps and con­sul­tants as it’s a pretty tech­ni­cal area.

That’s be­cause soils are a very dy­namic mix­ture of min­er­als, or­ganic residues and liv­ing mi­cro and macro or­gan­isms – all of which sup­port farm pro­duc­tion.

So a clear as­sess­ment of fer­tiliser re­quire­ments will both im­prove eco­nomic re­turns from pas­ture and help avoid con­tam­i­na­tion of ground and sur­face wa­ter with nu­tri­ents, par­tic­u­larly ni­tro­gen and phos­pho­rus.

There are a range of tools for help­ing make this as­sess­ment.

Nu­tri­ent bud­get­ing is widely ac­cepted as the ap­pro­pri­ate first step in man­ag­ing nu­tri­ent use and it’s also the pre­ferred tool for eval­u­at­ing the en­vi­ron­men­tal im­pact of farm man­age­ment prac­tices.

Over­seer, a com­puter de­ci­sion sup­port model, is used to ad­vise on nu­tri­ent man­age­ment and green­house gas emis­sions. It pre­dicts what hap­pens to the nu­tri­ents that are brought on to the farm in the form of fer­tilis­ers and sup­ple­men­tary feed in the same way that a fi­nan­cial bud­get can track money.

A nu­tri­ent man­age­ment plan builds on the bud­get and iden­ti­fies what the farmer will do to im­prove the ef­fi­ciency of the farm nu­tri­ent sys­tem and min­imise losses to the en­vi­ron­ment. There’s a wide range of op­tions for re­duc­ing losses, de­pend­ing on in­di­vid­ual sit­u­a­tions, and many of them can be found in the ‘‘menus of prac­tices to im­prove wa­ter qual­ity’’ on the Waikato Re­gional Coun­cil web­site.

An un­der­stand­ing of the be­hav­iour and fate of nu­tri­ents in the soil-plan­tan­i­mal sys­tem helps in pre­par­ing a nu­tri­ent man­age­ment plan, as does a plain English knowl­edge of some im­por­tant ter­mi­nol­ogy when us­ing the Over­seer model.

Min­er­al­i­sa­tion in­volves con­ver­sion of soil or­ganic ni­tro­gen into plant avail­able forms.

Min­eral N stands for those ni­tro­gen fer­tilis­ers used to di­rectly sup­ple­ment the ni­trate and am­mo­nium pools in soil.

Am­moni­fi­ca­tion is the break­down of or­ganic ni­tro­gen (from dung, urine and dead plant ma­te­rial) into am­mo­nium ions. Th­ese ions are sub­se­quently ni­tri­fied in the pres­ence of a good oxy­gen sup­ply to ni­trate ions. Am­moni­fi­ca­tion is car­ried out by a va­ri­ety of soil mi­cro-or­gan­isms.

Nitri­fi­ca­tion in­volves the bi­o­log­i­cal con­ver­sion of am­mo­nium to ni­trate. Ni­trate con­cen­tra­tions in aer­o­bic, warm, and moist cul­ti­vated soils are nor­mally higher than am­mo­nium con­cen­tra­tions, so in such soils the main source of ni­tro­gen taken up by plants is ni­trate. This is be­cause in warm aer­o­bic soils the rate of nitri­fi­ca­tion is rapid. In acidic soils nitri­fi­ca­tion is slow and am­mo­nium is prob­a­bly the main plant ni­tro­gen source.

Im­mo­bil­i­sa­tion is the con­ver­sion of plant avail­able ni­tro­gen into or­ganic form. If the min­er­al­i­sa­tion rate is less than the im­mo­bil­i­sa­tion rate then net-im­mo­bil­i­sa­tion oc­curs and pas­ture and crops grow­ing in such a soil will be­come ni­tro­gen de­fi­cient. Whether net-min­er­al­i­sa­tion or net-im­mo­bil­i­sa­tion oc­curs de­pends mostly on the car­bon to ni­tro­gen (C:N) ra­tio in the or­ganic sub­strate added to the soil.

De-nitri­fi­ca­tion in­volves mi­cro­bial re­duc­tion of ni­trate to ni­trous ox­ide and ni­tro­gen gas. This process re­quires, in ad­di­tion to soil mi­cro-or­gan­isms, a source of ni­trate and de­com­pos­able or­ganic mat­ter, and anaer­o­bic con­di­tions such as wa­ter logged soils.

Vo­latil­i­sa­tion is con­ver­sion of am­mo­nium to am­mo­nia gas. This com­monly oc­curs in an­i­mal urine spots and af­ter urea ap­pli­ca­tion dur­ing high tem­per­a­tures.

In New Zealand, the com­mon ni­troge­nous fer­tilis­ers are urea (46 per cent N), am­mo­nium sul­phate (21 per cent N), DAP (18 per cent N) and cal­cium am­mo­nium ni­trate (27 per cent N).

There is in­creas­ing pres­sure for farm­ers to im­prove their nu­tri­ent man­age­ment be­cause of the ef­fects that ni­tro­gen and phos­pho­rus can have on wa­ter, and be­cause im­prov­ing nu­tri­ent use ef­fi­ciency is im­por­tant for farm prof­itabil­ity. So it’s im­por­tant for farm­ers to work closely with their qual­i­fied nu­tri­ent man­age­ment ad­vi­sor and their farm con­sul­tant to en­sure that, as much as pos­si­ble, the nu­tri­ents in their farm sys­tem are used pro­duc­tively.

The re­sult will be op­ti­mum pro­duc­tion and cleaner wa­ter.

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