Browsing by Author "Kotze, Theunis Gerhardus"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Acidification of sands in citrus orchards fertilized by drip irrigation
    (Stellenbosch : Stellenbosch University, 2002-12) Kotze, Theunis Gerhardus; Fey, M. V.; Hoffman, J. E.; Stellenbosch University. Faculty of AgriScience. Dept. of Soil Science.
    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.