Ero­sion chal­lenge to new sea­son sow­ing

South Waikato News - - RURAL DELIVERY - By BALA TIKKISETTY

The post-win­ter pe­riod is the time for cul­ti­vat­ing pad­docks so they are at their best to pro­vide fresh pas­ture or crops.

In par­tic­u­lar, sac­ri­fice pad­docks are nor­mally cul­ti­vated about now and sown with a sum­mer crop to re­store the soil. Any plough­ing can lead to sed­i­ment and as­so­ci­ated nu­tri­ents get­ting into wa­ter­ways, af­fect­ing wa­ter qual­ity. Also, soil ero­sion gen­er­ally can be a ma­jor con­trib­u­tor to sed­i­ment and nu­tri­ents get­ting into wa­ter.

Farm­ers can min­imise wind ero­sion­caused losses to wa­ter by pro­vid­ing shel­ter with well-de­signed wind­breaks. An­other way to pre­vent ero­sion is to main­tain as much veg­e­ta­tive cover as pos­si­ble to pro­tect bare soil.

Given the amount of prepa­ra­tion of pad­docks go­ing on about now, this is one of the times of great­est ero­sion risk be­cause pro­tec­tive plant cover is lost through cul­ti­va­tion.

Even with well-es­tab­lished shel­ter­belts, wind ero­sion can still oc­cur be­cause of in­ap­pro­pri­ate cul­ti­va­tion tech­niques.

I rec­om­mend that farm­ers in wind prone ar­eas and with lighter soils con­sider us­ing ‘‘con­ser­va­tion cul­ti­va­tion’’ mea­sures. These are cul­ti­va­tion prac­tices aimed at main­tain­ing max­i­mum veg­e­ta­tive cover on the soil sur­face and which en­cour­age mois­ture re­ten­tion in soils. The aim is to pro­duce an un­even soil sur­face in as ‘‘rough’’ a con­di­tion as prac­ti­ca­ble and keep­ing the cul­ti­va­tion pe­riod to a min­i­mum.

The use of the chisel plough or grub­bers gives a fine deep seedbed while still re­tain­ing a cloddy sur­face. Top-work­ing im­ple­ments, disc­ing and rolling can cre­ate a fine seedbed prone to wind ero­sion even in well-shel­tered sit­u­a­tions.

Cul­ti­vat­ing and sow­ing at right an­gles to the pre­vail­ing wind and ridg­ing is also good for min­imis­ing soil blow.

Sat­is­fac­tory re­sults are achieved when cul­ti­va­tion is car­ried out at a suit­able soil mois­ture con­tent and at a suit­able depth. If good pre­cau­tions are ob­served, two-pass cul­ti­va­tion is of­ten all that is needed to pre­pare a seedbed. Re­peated passes to get a good tilth are avoided and the risk pe­riod for sur­face ero­sion be­tween ini­tial veg­e­ta­tion clear­ance and ground cover by a grow­ing crop is short­ened.

Win­ter-feed pad­docks can be dif­fer­ent as the top few cen­time­tres of soil has been ‘‘pud­dled’’ by stock in wet con­di­tions and is very prone to wind ero­sion once it has dried out. This is be­cause this top layer of soil has lost all its struc­ture and can lift from the pad­dock very eas­ily. Turn­ing this type of ground over as soon as soil con­di­tions per­mit in the early spring will min­imise the risk of los­ing this fine layer to the north­west­erly wind.

Other con­ser­va­tion cul­ti­va­tion tech­niques in­clude the suite of prac­tices known as min­i­mum tillage or no tillage. If soil has been con­tin­u­ously cul­ti­vated for many years, the struc­ture is likely to be poor be­cause cul­ti­va­tion re­duces soil or­ganic mat­ter lev­els. No tillage will not re­pair the dam­age overnight but, with residue re­ten­tion, it will even­tu­ally. Chem­i­cal spray­ing fol­lowed by di­rect drilling is an op­tion on light erodi­ble soils.

Runoff oc­curs when the rate of wa­ter

in­fil­tra­tion into the soil is slower than the ap­pli­ca­tion rate ( rain or ir­ri­ga­tion). Be­cause of cer­tain tex­tu­ral dif­fer­ences, in some soils the nat­u­ral rate of wa­ter in­fil­tra­tion is low.

But the in­fil­tra­tion rate can be low due to fre­quent tillage or other man­age­ment re­lated con­straints like com­paction.

Run-off will move into low-ly­ing ar­eas or to the edge of the field where it can pond for longer pe­ri­ods or move into a nearby sur­face wa­ter course.

A range of ma­te­rial from ad­ja­cent land can con­tam­i­nate wa­ter cour­ses.

This can in­clude sed­i­ment, nu­tri­ents such as ni­tro­gen and phos­pho­rous, other chem­i­cals and mi­crobes. Sed­i­ment and some nu­tri­ents, par­tic­u­larly phos­pho­rus, are car­ried to streams pri­mar­ily in the over­land flow of wa­ter.

Dis­solved nu­tri­ents such as ni­tro­gen and other ma­te­ri­als (in­clud­ing dis­solved or­ganic car­bon) can also move through the soil in un­der­ground flows and con­tam­i­nate wa­ter cour­ses.

The ri­par­ian mar­gin be­tween land and wa­ter is a cru­cial buf­fer that helps stop sed­i­ment, pathogen and other ma­te­rial get­ting into wa­ter.

An ef­fec­tive ri­par­ian fil­ter strip is valu­able where over­land wa­ter en­ters wa­ter bod­ies.

Healthy ri­par­ian veg­e­ta­tion in these ar­eas im­proves bank sta­bil­ity, in­creases wa­ter qual­ity, re­duces stock losses, fil­ters sur­face run-off and pro­vides habi­tat for wild life.

Stud­ies show that up to 90 per cent of sed­i­ment can be caught in an ef­fec­tively con­structed fil­ter strip.

Any fae­cal bac­te­ria trapped in long grass fil­ter strips will die off in sun­light.

Ri­par­ian veg­e­ta­tion also has an im­por­tant ad­di­tional ben­e­fit in pro­vid­ing shade to the stream, thereby re­duc­ing wa­ter tem­per­a­tures and growth of nui­sance plants and al­gae.

It is im­por­tant to main­tain fil­ter strips so that there is al­most com­plete ground cover and a good height of veg­e­ta­tion, which max­imises the po­ten­tial to trap sed­i­ment and nu­tri­ents.

De­sign­ing plant­ings to pro­vide shade to streams while al­low­ing enough light on the banks to main­tain grass cover is the se­cret to suc­cess.

The Clean Streams Guide on the web­site www.ew.govt.nz can help with this.

In the fil­ter strips, gen­er­ally, grasses should be kept to a height of at least 10-15cm with a high den­sity of stems and leaves at ground level for max­i­mum trap­ping ef­fect.

Also, un­der the Waikato Re­gional Plan, farm­ers should not cul­ti­vate pad­docks within two me­tres of a river, stream or lake bed.

Bala Tikkisetty is a sus­tain­able agri­cul­ture co-or­di­na­tor at En­vi­ron­ment Waikato.

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