Acidification of sands in citrus orchards fertilized by drip irrigation

Kotzé, Theunis Gerhardus (2002)

Thesis (MScAgric) -- Stellenbosch University, 2002.

Thesis

ENGLISH ABSTRACT: The use of drip irrigation in citrus orchards is becoming increasingly important in the Citrusdal region of the Western Cape. Drip irrigation provides an opportunity to optimize water and fertilizer use by inducing a smaller root zone that can be managed more effectively. Ammoniacal fertilizers are an integral part of any drip fertilization (fertigation) programme. However, a disadvantage is that they generate soil acidity upon nitrification. If insufficient plant uptake of nitrate occurs during the fertigation season, soil acidification may become an important yield-limiting factor. This study investigated the effect of drip fertigation on four sandy soils with different buffer capacities, near Citrusdal. Spatial variation in soil chemical properties below irrigation emitters was investigated to determine the nature and extent of soil acidification from drip fertigation. The response ofthe four sands to acid and base addition (and laboratory incubation) was also studied in order to assess the magnitude and origin of pH buffering. The soil types in the four orchards included two poorly buffered and two moderately buffered soils. Both the poorly buffered soils, at Brakfontein and Swartvlei, were classified in the Kroonstad form and contain less than 5% clay. The two moderately buffered soils, at BoHexrivier and OnderHexrivier, were classified in the Vilafontes and Constantia forms, respectively, and contained more than 10% clay in the subsoil. X-ray diffractometry revealed that kaolinite and quartz dominate the clay fraction of all four soils. Organic carbon content in the topsoils ranged from 0.2-0.98 percent. Drip fertigation for a period of between 4 and 10 years at all four sites has resulted in large decreases in soil pH to depths of up to 1 m below the emitter with a sharp increase in acid saturation at pHKc1 values below 4.5. The Brakfontein and Swartvlei orchard soils were the least acid saturated with levels of about 50-60 percent. At the BoHexrivier site acid saturation values of 70 percent were recorded for the topsoil immediately below the emitter, while the whole soil profile of the OnderHexrivier site had extreme acid saturation levels, exceeding 90 percent in the deeper parts of the profile. It was also revealed that a large proportion of the 1M KCI-extractable acidity \ consisted of A~ although Al saturation showed a poorer relationship to pHKCIthan did acid saturation. The Al component of exchangeable acidity in the OnderHexrivier subsoil was significantly larger [Al = 0.84(acidity)] than in the other soils [AI = 0.69(acidity)]. It was found that most of the wetted soil volume was deficient in exchangeable Ca, Mg, K and enriched with Al. No accumulation of NH/ or N03- was found in any of the soils directly below the emitter, indicating either a sufficient degree of nitrification or the leaching of both NH/ and N03- to greater depths. The mobile anions cr and sol- also appeared at the periphery of the wetting front. Phosphate generally accumulated in the soil just below the emitter, except in two of the soils where P showed some degree of leaching. Spatial variation in soil chemical properties indicated that nitrification and over-irrigation had resulted in a significant volume (between 0.1 and 1.1 rn') of severely acidified soil (pHKcl <4.5) below the emitter at all four study sites. Buffering in these naturally acidic sandy soils from the Citrusdal area is weak as a result of the low clay and organic matter contents. The low content of clay, dominated by kaolinite and quartz, implied that organic carbon plays an important role in pH buffering, especially in the topsoils. Laboratory incubation with acid or base confirmed the fact that CEC becomes increasingly saturated by acidic cations (Ir and Ae+) once soil pHKCIvalues decrease below 4.5. Again Al was found to be the major acidic cation [AI = 0.69(acidity)], especially in the subsoils. This confirmed that, even in these poorly buffered, quartz-rich sandy soils, toxic amounts of Al could enter the soil solution quite rapidly following acidification. Lime requirement calculated from the slope of titration curves following incubation provided a useful way of assessing the magnitude of the acidification problem, even though liming the acidified subsoil may present practical difficulties under field conditions in drip-fertilized irrigation systems. These lime requirement values, ranging from 0.9-10.3 tonnes of CaC03/ha, can be applied to field conditions with some calibration refinements.

AFRIKAANSE OPSOMMING: Die gebruik van drup besproeiing in die citrus boorde van Citrusdal in die Wes-Kaap is besig om toenemende belangstelling te werf Drup besproeiing bied die geleentheid om water en misstof toediening te optimiseer deur die meer effektiewe benutting van 'n kleiner wortel sone. Mistowwe wat ammonium bevat is 'n integrale deel van enige drup en misstof toedienings (misstofbesproeiing) program. 'n Nadeel hiervan egter is die vorming van grondsuurheid weens nitrifisering. Indien nitraat onvoldoende deur die plant opgeneem word gedurende die misstof besproeiings seisoen, kan grondversuring 'n belangrike opbrengs-bepalende faktor word. Hierdie studie het die invloed van misstofbesproeiing op vier sanderige gronde met vier verskillende buffer vermoëns naby Citrusdalondersoek. Die ruimtelike variasie in die chemiese einskappe van die grond direk onder die drupper is ondersoek om die aard en omvang van grondversuring van drup besproeiing te bepaal. Die reaksie van die vier sande op die byvoeging van suur en basis (en laboratorium inkubasie) is ook bestudeer om die grootte en oorsprong van pH buffereinskappe te bepaal. Die vier boordgronde het bestaan uit twee swak-gebufferde en twee matig-gebufferde gronde. Beide die swak-gebufferde gronde, by Brakfontein en Swartvlei, is geklassifiseer in die Kroonstad vorm en bevat minder as 5% klei. Die twee matig-gebufferde gronde by BoHexrivier en OnderHexrivier, is geklassifiseer in die Vilafontes en Constantia vorms onderskeidelik, en bevat meer as 10% klei in die ondergrond. X-straal diffraksiometrie het getoon dat kaoliniet en kwarts die klei-fraksie oorheers. Organiese koolstofin die bogrond het gewissel van 0.2-0.98 percent. Drupmisstofbesproeiing, vir 'n tydperk van tussen 4 en 10 jaar, het by al vier persele groot afuames in grond pH tot 'n diepte van 1 m, met 'n skerp toename in suur versadiging, tot gevolg gehad. Suurversadiging by die Brakfontein en Swartvlei boordgronde is die minste met vlakke van omtrent 50-60 percent. By die BoHexrivier perseel, het suurversadiging waardes van 70 percent bereik in die bogrond direk onder die drupper. Die hele profiel van die OnderHexrivier perseel het uiters hoë suurversadigings vlakke gehad, met waardes van hoër as 90 percent in die dieper dele van die profiel. Daar is ook getoon dat 'n groot gedeelte van die 1 M KCI-ekstraeerbare suurheid bestaan uit Al, hoewel Al versadiging 'n swakker verwantskap toon met pHKcl as suurversadiging. Die Al komponent van uitruilbare suurheid in die OnderHexrivier ondergrond is betekenisvol groter [Al = 0.84(suurheid)] as die ander gronde [Al = 0.69(suurheid)]. Daar is gevind dat die grootste gedeelte van die benatle grond volume gebrekkig is aan Ca, Mg en K, en verryk is in Al. Geen akkumulasie van ammonium of nitraat is gevind direk onder die drupper van enige van die persele nie, wat aandui dat daar genoegsame nitrifisering, of diep loging van beide ione plaasgevind het. Die mobiele ione, chloor en sulfaat is gevind aan die buiterand van die benattings volume. Fosfaat het oor die algemeen in die omgewing van die drupper gekonsentreer, behalwe in twee gronde waar daar tog 'n mate van P-Ioging waargeneem is. Die ruimtelike variasie in grondchemiese kenmerke het aan die lig gebring dat groot volumes (tussen 0.1 en 1.1 nr') grond versuur het direk onder drupper van al vier persele as gevolg van nitrifisering en oorbesproeiing. Die buffereinskappe van hierdie sanderige, natuurlike suur gronde van Citrusdal is swak weens hul lae klei en organiese materiaal inhoud. Die lae inhoud van klei, wat deur kaoliniet en kwarts oorheers word, impliseer dat organiese materiaal 'n belangrike rol speel in pH buffereienskappe, veral in die bogronde. Laboratorium inkubasie met suur en basis het die feit bevestig dat die KVK toenemend versadig word met suur katione (Ae+ en W) sodra grond pHKcl na waardes laer as 4.5 verminder. Weereens is gevind dat Al die belangrikste suur katioon is [Al = 0.69(suurheid)], veral in die ondergronde. Dit impliseer dat selfs swak-gebufferde, kwartsryke sande ook die vermoë beskik om toksies hoeveelde Al tot die grondoplossing te voorsien as gevolg van grondversuring. Kalkbehoeftes wat bereken is vanaf die helling van die titrasie kurwes na inkubering, bied bruikbare inligting om die graad van die grondsuurheids probleem te bepaal, hoewel bekalking van die versuurde ondergrond praktiese probleme mag voorsien onder toestande van drupmisstofbesproeiing. Die kalkbehoefte waardes wat wissel van 0.9-10.3 ton/ha kan toegepas word in die praktyk met sommige kalibrasie verfynings.

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