Soil mois­ture mon­i­tor­ing and wa­ter use

The Orchardist - - Orchard Management - Jack Hughes & Bren­dan Hamil­ton

Wa­ter qual­ity and avail­abil­ity is a top­i­cal is­sue and likely to be­come more con­tro­ver­sial as de­mand in­creases and our fresh wa­ter qual­ity de­te­ri­o­rates.

In this ar­ti­cle, we dis­cuss soil mois­ture mon­i­tor­ing and man­age­ment. We con­sider some of the fac­tors in­flu­enc­ing the qual­ity of in­for­ma­tion and com­pare the ir­ri­ga­tion prac­tices of two dif­fer­ent hor­ti­cul­tural sec­tors. Our tech­ni­cal dis­cus­sion be­comes slightly po­lit­i­cal as we con­sider as­pects of pro­posed changes to the cost of wa­ter.


Fruition pro­vides soil mois­ture mon­i­tor­ing ser­vices to hor­ti­cul­ture. Dur­ing the win­ter of 2017 we took time to try and an­swer some nag­ging ques­tions. We were cu­ri­ous to com­pare the per­for­mance of the two types of mon­i­tor­ing equipment we use – neu­tron probe and di­viner (elec­tri­cal ca­pac­i­tance). We wanted to test the man­u­fac­turer’s rec­om­men­da­tion that 3 tubes be used for each soil mois­ture mon­i­tor­ing site. How much soil, plant and ir­ri­ga­tion sys­tem per­for­mance vari­a­tion is there and how much does that af­fect the ac­cu­racy of read­ings. Why mon­i­tor 3 tubes if one would do? To do this we sep­a­rated the 3 in­di­vid­ual tube data read­ings at 50cm soil depth taken from 134 neu­tron probe and di­viner sites. The amount of vari­a­tion be­tween the 3 tubes for each site was cal­cu­lated us­ing Minitab sta­tis­ti­cal soft­ware (Fig­ure 1). Re­sults showed that the neu­tron probe read­ings were a lot less vari­able than the di­viner. This is prob­a­bly ex­plained by the neu­tron probe’s abil­ity to ‘read’ a much larger (16x) soil vol­ume than the elec­tri­cal ca­pac­i­tance sig­nal from the di­viner.

In ad­di­tion, the amount of vari­a­tion be­tween the 3 tubes at each site was con­sid­er­able and con­firmed the need for 3 tubes to ac­count for nat­u­ral site vari­a­tion. In a third of the mon­i­tor­ing sites, the co­ef­fi­cient of vari­a­tion was tol­er­a­ble at 10% or less, but a sig­nif­i­cant pro­por­tion of sites had vari­a­tion of 30, 60 even 80%. Clearly a sin­gle read­ing in these sit­u­a­tions would give very mis­lead­ing in­for­ma­tion. As a re­sult of this in-house study, we are stick­ing with 3 tube sites and will pro­gres­sively con­vert our sites in vari­able soils to neu­tron probe tech­nol­ogy.


Can we ex­plain the data vari­a­tion within sites? It turns out that soil type vari­a­tion is a big part of the ex­pla­na­tion. On closer ex­am­i­na­tion we found that sites in ar­eas of fre­quent changes in soil types had greater vari­a­tion (Fig­ure 2) while those in blocks of sin­gle soil types had more uni­for­mity with their tube read­ings.

The Here­taunga and Ru­atani­wha Plains were formed from al­lu­vial and vol­canic de­po­si­tion over mil­len­nia and are a ver­i­ta­ble patch­work quilt ever chang­ing soil types, wa­ter hold­ing ca­pac­i­ties and drainage classes. Cor­rect place­ment of mon­i­tor­ing tubes is im­por­tant and some­thing we pay par­tic­u­lar at­ten­tion to.


We also re­viewed sea­sonal ir­ri­ga­tion ap­plied by our ap­ple and grape grower clients. Over the past 5 years, grape grow­ers have ap­plied an av­er­age of 52 mm of ir­ri­ga­tion over the whole sea­son while ap­ple grow­ers have ap­plied 191 mm (3.7 times more) (Fig­ure 4).

Most ap­ple grow­ers ap­ply fre­quent ir­ri­ga­tion and main­tain soil mois­ture at rel­a­tively high lev­els com­pared to grape grow­ers who re­strict ir­ri­ga­tion and man­age their soil mois­ture lev­els down to pre­cise, pre-de­ter­mined lev­els (Fig­ure 5).

Yes, grape and ap­ple grow­ers face dif­fer­ent com­mer­cial driv­ers. Grape grow­ers are in­cen­tivized by fruit qual­ity (brix and free­dom from dis­ease) while ap­ple grow­ers are re­warded more by fruit vol­ume and ap­pear­ance (size and colour). At the risk of over sim­pli­fy­ing, grape grow­ers ir­ri­gate spar­ingly be­cause they know that ex­cess ir­ri­ga­tion may re­duce fruit value and in­crease costs of canopy man­age­ment. Ap­ple grow­ers, on the other hand, err on the side of ‘putting more on’ be­cause it might in­crease yield.

There is a sub­jec­tive as­pect to ir­ri­ga­tion. Grow­ers want to be ‘kind’ to their trees and it feels good to put wa­ter on when its hot, dry and windy. Nev­er­the­less, Fruition has gath­ered ev­i­dence from two sea­son’s of repli­cated tri­als that show that ‘reg­u­lated’ ir­ri­ga­tion pays. When soil mois­ture lev­els were man­aged down to ‘re­fill’ lev­els (at the on­set of plant stress) fruit colour was im­proved while fruit size

and yield was un­af­fected.The eco­nomic case for in­creased fruit rev­enue with lower elec­tric­ity pump­ing cost­sis a ‘no brainer’.


Wa­ter was a hot is­sue in the 2017 elec­tion as New Zealan­ders have be­come more con­cerned about the de­grad­ing state of our fresh­wa­ter. (this ar­ti­cle is be­ing writ­ten while post elec­tion ne­go­ti­a­tions to form a gov­ern­ment are tak­ing place). The Labour Party’s pro­posed pol­icy to charge for wa­ter po­lar­ized ur­ban and ru­ral sec­tors dur­ing the elec­tion. Hor­ti­cul­tureNZ re­sponded the day af­ter Labour’s wa­ter tax an­nounce­ment say­ing that the 1-2 cents/cube blan­ket tax would make “healthy food more ex­pen­sive” and “would not de­liver long term benefits to New Zealan­ders”.

This rapid re­sponse is nor­mal for the ‘tooth and nail’ lob­by­ing game but is there room for a broader and longer view? Us­ing HB 5 year av­er­age data, we es­ti­mate the pro­posed wa­ter tax would cost grape grow­ers $5-10/ha pa and ap­ple grow­ers $19-38 (Table 1.)

At these lev­els, the ex­tra cost is neg­li­gi­ble. If grow­ers had to pay a nom­i­nal amount for wa­ter, they might use it more ef­fi­ciently and even im­prove their eco­nomic per­for­mance by im­prov­ing fruit qual­ity and value. Grape grow­ers are lead­ing the way here, but imag­ine if all hor­ti­cul­ture could de­velop an ‘IFP’ of wa­ter use (In­te­grated Fruit Pro­duc­tion, syn­ony­mous with care­ful and jus­ti­fied use of in­puts).

Imag­ine if this ex­am­ple could pos­i­tively in­flu­ence those who se­ri­ously over-use wa­ter and pol­lute our rivers and streams with nu­tri­ents and sed­i­ment. Could that lead the way to a long term ben­e­fit of en­vi­ron­men­tal restora­tion that our grand­chil­dren would ap­pre­ci­ate?

“Wa­ter was a hot is­sue in the 2017 elec­tion as New Zealan­ders have be­come more con­cerned about the de­grad­ing state of our fresh­wa­ter.”

Fig­ure 1.

Fig­ure 5. Typ­i­cal Soil Mois­ture & Ir­ri­ga­tion for HB Grapes & Ap­ples

Fig­ure 4. To­tal av­er­age sea­sonal ir­ri­ga­tion by HB ap­ple and grape grow­ers over the past five years

Fig­ure 2. High vari­a­tion Makoha site show­ing changes in soil type.

Fig­ure 3. Low vari­a­tion Scot­land Farms site with a sin­gle soil type.

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