Acidification of sands in citrus orchards fertilized by drip irrigation

[摘要] ENGLISH ABSTRACT: The use of drip irrigation in citrus orchards is becoming increasingly important in the Citrusdalregion of the Western Cape. Drip irrigation provides an opportunity to optimize water and fertilizeruse by inducing a smaller root zone that can be managed more effectively. Ammoniacal fertilizersare an integral part of any drip fertilization (fertigation) programme. However, a disadvantage isthat they generate soil acidity upon nitrification. If insufficient plant uptake of nitrate occurs duringthe fertigation season, soil acidification may become an important yield-limiting factor. This studyinvestigated the effect of drip fertigation on four sandy soils with different buffer capacities, nearCitrusdal. Spatial variation in soil chemical properties below irrigation emitters was investigated todetermine the nature and extent of soil acidification from drip fertigation. The response ofthe foursands to acid and base addition (and laboratory incubation) was also studied in order to assess themagnitude 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 formand contain less than 5% clay. The two moderately buffered soils, at BoHexrivier andOnderHexrivier, were classified in the Vilafontes and Constantia forms, respectively, and containedmore than 10% clay in the subsoil. X-ray diffractometry revealed that kaolinite and quartzdominate the clay fraction of all four soils. Organic carbon content in the topsoils ranged from0.2-0.98 percent.Drip fertigation for a period of between 4 and 10 years at all four sites has resulted in largedecreases in soil pH to depths of up to 1 m below the emitter with a sharp increase in acid saturationat pHKc1 values below 4.5. The Brakfontein and Swartvlei orchard soils were the least acidsaturated with levels of about 50-60 percent. At the BoHexrivier site acid saturation values of70 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 deeperparts 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 acidsaturation. The Al component of exchangeable acidity in the OnderHexrivier subsoil wassignificantly larger [Al = 0.84(acidity)] than in the other soils [AI = 0.69(acidity)]. It was foundthat 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 greaterdepths. 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 whereP showed some degree of leaching. Spatial variation in soil chemical properties indicated thatnitrification and over-irrigation had resulted in a significant volume (between 0.1 and 1.1 rn') ofseverely 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 lowclay 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 increasinglysaturated by acidic cations (Ir and Ae+) once soil pHKCIvalues decrease below 4.5. Again Al wasfound to be the major acidic cation [AI = 0.69(acidity)], especially in the subsoils. This confirmedthat, even in these poorly buffered, quartz-rich sandy soils, toxic amounts of Al could enter the soilsolution quite rapidly following acidification. Lime requirement calculated from the slope oftitration curves following incubation provided a useful way of assessing the magnitude of theacidification problem, even though liming the acidified subsoil may present practical difficultiesunder field conditions in drip-fertilized irrigation systems. These lime requirement values, rangingfrom 0.9-10.3 tonnes of CaC03/ha, can be applied to field conditions with some calibrationrefinements.

[发布机构] Stellenbosch University