App to guide ef­flu­ent ap­pli­ca­tion

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

Tak­ing dif­fer­ent soil types into ac­count is a key as­pect of the dairy farm­ing sec­tor’s on­go­ing ef­forts to im­prove ef­flu­ent man­age­ment and as­so­ci­ated is­sues.

Farm dairy ef­flu­ent can be a sig­nif­i­cant source of wa­ter con­tam­i­nants, ei­ther by di­rect run-off or through ni­tro­gen leach­ing.

The amount of wa­ter (or ‘‘hy­draulic load­ing’’) in soil and var­i­ous soil prop­er­ties in­flu­ence th­ese types of ef­fects.

For ex­am­ple, soil tex­ture and struc­ture de­ter­mine the amount of wa­ter that can en­ter and be re­tained in a par­tic­u­lar soil, as well as the trans­mis­sion rate of ex­cess wa­ter through soil.

So th­ese var­i­ous soil prop­er­ties should be weighed up when de­cid­ing on ef­flu­ent ir­ri­ga­tion sys­tems, and when ap­ply­ing ef­flu­ent to pas­ture, to max­imise nu­tri­ent ben­e­fits and water­way pro­tec­tion.

When dis­cussing such de­ci­sions with ef­flu­ent ad­vis­ers, it’s worth bear­ing in mind a num­ber of fac­tors.

Leach­ing oc­curs in re­sponse to move­ment of ex­cess wa­ter from the soil, mean­ing soils with lower wa­ter-hold­ing ca­pac­ity are more sus­cep­ti­ble to leach­ing.

Con­versely, soils with high wa­ter-hold­ing ca­pac­ity (deep silt loams) can store sig­nif­i­cant quan­ti­ties of ef­flu­ent.

Soils with low avail­able wa­ter-hold­ing ca­pac­ity are the shal­low to mod­er­ately deep soils, as well as sandy or stony soils.

Gen­er­ally speak­ing, lower wa­ter-hold­ing ca­pac­ity is the re­sult of re­stricted root­ing depths be­cause of the shal­low na­ture of the soils and high wa­ter ta­bles.

Ef­flu­ent ir­ri­ga­tion on shal­low soils with high wa­ter ta­bles is likely to re­sult in leach­ing.

Per­me­able soils with a deep wa­ter ta­ble and no drainage lim­i­ta­tions are best for putting ef­flu­ent on.

How­ever, on stony soils the risk of ef­flu­ent drain­ing di­rectly to ground wa­ter must be con­sid­ered. In such sit­u­a­tions, ap­pli­ca­tion depths and rates should be ad­justed to ac­count for this risk.

Mean­while, it’s im­por­tant to take into ac­count the rate at which ef­flu­ent can in­fil­trate soil. If too much is put on too quickly, this can lead to runoff into wa­ter­ways.

The na­ture of the ef­flu­ent and cat­tle tread­ing on soils can af­fect the in­fil­tra­tion rate.

Tread­ing dam­age by stock, which oc­curs most when the soils are wet, sig­nif­i­cantly re­duces in­fil­tra­tion rate. For some soils this can re­sult in ef­flu­ent ac­cu­mu­lat­ing be­low slopes and hol­lows. It can then en­ter sur­face wa­ter­ways.

Wa­ter trans­mis­sion through soil pores is gen­er­ally de­scribed as hy­draulic con­duc­tiv­ity. When soil hy­draulic con­duc­tiv­ity is low, ef­flu­ent ir­ri­ga­tion will re­sult in pond­ing and run-off once the to­tal wa­ter ca­pac­ity of the soil is ex­ceeded.

Low rates of hy­draulic con­duc­tiv­ity are found in soils that are poorly drained, and pond­ing and runoff of­ten oc­cur with high rain­fall. Many of th­ese soils need to be ar­ti­fi­cially drained to re­duce the in­ci­dence of pond­ing and wa­ter-log­ging.

For pugged soils af­fected by tread­ing, ef­flu­ent ap­pli­ca­tion rates which are above 10mm an hour are likely to re­sult in pond­ing if soil wa­ter con­tent at the time of ir­ri­ga­tion is high.

When ef­flu­ent ap­pli­ca­tion rates are higher than in­fil­tra­tion rates, wa­ter can en­ter macro-pores open at the soil sur­face, and then move very rapidly through so­called ‘‘by­pass flow’’ through a rel­a­tively dry soil ma­trix.

This gives lit­tle op­por­tu­nity for the wa­ter to be re­tained in the root zone and high leach­ing of ni­trate is likely to oc­cur.

By­pass flow of farm dairy ef­flu­ent can oc­cur in soils that un­dergo shrink­age and fissuring dur­ing dry­ing, es­pe­cially when th­ese soils have been pre­vi­ously com­pacted by tread­ing.

One of the keys to avoid­ing over­ap­pli­ca­tion can be hav­ing ad­e­quate ef­flu­ent stor­age so farm­ers can de­fer ir­ri­ga­tion if con­di­tions aren’t right.

DairyNZ has re­leased a new smart-phone app to help farm­ers ap­ply ef­flu­ent more ef­fi­ciently.

The Dairy Ef­flu­ent Spread­ing Cal­cu­la­tor app pro­vides dairy farm­ers and ef­flu­ent spread­ing con­trac­tors with guid­ance around nu­tri­ent ap­pli­ca­tion rates based on the depth and type of ef­flu­ent they ap­ply.

The app al­lows for nu­tri­ent ap­pli­ca­tion rates for dairy ef­flu­ent to be eas­ily cal­cu­lated, based on a num­ber of cus­tomis­able in­puts so ef­flu­ent nu­tri­ents can be ap­plied with greater pre­ci­sion.

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