The Effect of Soil Acidity on Sugar Beet Growth Some Natal Soils

During the latter part of 1975 an investigation was conducted in the Natal Midlands to locate sites, representative of the main bioclimatic subregions, that would be suitable for sugar beet experiments. Soil acidity was identified as an important limiting factor in the majority of soils examined. A glasshouse study with sugar beet revealed a marked response to lime where soil pH values were less than 5,3, while deep incorporation was responsible for optimum development of the tap root. This effect is ascribed primarily to the elimination of toxic elements such as Al, with secondary benefits from improved P and N availability. In general the amount of lime required for optimum growth varied between 20 and 40% of the amount needed to raise the pH value to 7,O. Introduction In reviewing the prospects for sugar beet in Natal, Rosel1 considered that it would grow satisfactorily in such areas as Richmond, Howick, Nottingham Road, Rosetta, Mooi River and Seven Oaks despite the fact that conditions were so unlike those found in the traditional beet-growing areas situated in northern temperate regions. On the credit side the physical properties of the soils in the above areas appeared to fulfil four major requirements for successful beet production: (i) good soil depth which is conducive to unimpeded tap rootdevelopment; . (ii) non-plastic consistency of soils which, by minimising soil adherence to roots when wet, promotes more efficient mechanical harvesting and less spoil reaching factories ; (iii) general suitability of texture for good seedbed preparation; . (iv) freedom from stones. With regard to the chemical aspects, however, the factor of greatest concern in the local environment was the strongly acid reaction which characterises these soils and the detrimental effect this would have on root growth. Certainly the wealth of literature on sugar beet nutrition leaves little doubt about the sensitivity of this crop to soil acidity which manifests itself mainly in irregular germination, stunted plant growth with chlorosis of the leaf and fibrous and fangy development of the tap root. In order for the SASA Experiment Station to mount an experimental programme with sugar beet in time for planting in September 1975, and to ensure that soil acidity was corrected, the following priorities were recognized: (i) to locate suitable sites for conducting the preliminary field trials; (ii) to sample top and subsoils from these sites with the main object of determining pH and exchangeable A1 levels in order to apply lime, where necessary, before the first sowing; (iii) to conduct glasshouse experiments at the Station in order to test the effects of top and subsoil acidity on root development and to compare the efficacy of two different procedures for assessing lime requirement. The above have been investigated and this paper reports on the progress achieved; Phase I Siting and sampling of field trials Widely varying soil types and topography and fragmented distribution of land in the areas concerned made it necessary to visit a large number of prospective co-operators in order to locate suitable representative sites. This was accomplished by subdividing the test area into bioclimatic subregions, based on the system of classification developed by Professor Phil l ip~.~ In all, four subregions were identified. Of the twelve growers initially approached, four were located in the Mistbelt Evergreen region, two in the Mistbelt Thicket, two in the Coast Hinterland and four in the extensive Highland Montane region, as shown in Fig. 1. To facilitate further the selection of sites, growers were asked to comment, where possible, on the following: (i) cropping and fertilizer history of the field concerned, particularly with regard to liming; (ii) previous soil analysis ; (iii) type of land preparation equipment available; (iv) rainfall distribution; (v) frequency of hailstorm incidence. Environmental assessment Important bioclimatic features of the identified subregions are given in Table 1. There is a marked difference in altitudes, which is reflected in the mean diurnal temperature ranges as well as in historical rainfall distribution trends. In the Highland Montane region, which lies at nearly twice the altitude of the Coast Hinterland, it might be expected that the significantly milder conditions could exert a marked effect on the production cycle. A comparison of rainfall data shows that distribution is most favourable in the Mistbelt Evergreen area, with no more than two months of the year likely to receive less than 25 mm. This suggests that, in this subregion, sufficient moisture might be available to cover both a Spring sowing and an Autumn drilling, whereas in the Highland Montane area, with 3 to 4 months of the year with less than 25 mm rainfall, the feasibility of an Autumn planting appears less likely. Soil assessment (i) Soil sampling Composite top soil samples (0-20 cm) were taken for analysis, as well as single profile samples (0-60 cm), for identification of the soil series present. A Polymetrohm portable pH meter was used in the field for an assessment of soil acidity to depth. (ii) 'Analytical After extraction with 1N ammonium acetate in the laboratory, the amounts of Ca, Mg, K and Na were determined by atomic absorption spectrophotometry. Exchangeable A1 and 0,02N H,S04 soluble P were determined colorimetrically, using methods based on the reagents pyrocatechol violet4 and molybdenum blue, respectively. Plant available Zn was determined by the Proceedings of The South African Sugar Technologists' Association -June 1976 I. NOODSBERG 2. W I ~ I G 3. TRAUU L. HARDEN HEIGHTS 6. SNAlTH 7. BYRNE 8 . MARWICK 10 ARMSTRONG I I . WOODFORD 12. DAVIES FIGURE I The location of prospective co-operators for sugar beet field experiments. TABLE 1 Selected enyironmental characteristics of the sites sampled * After Phillipss Bioclimatic* Subregion