The effectiveness of Langebaan rock phosphate and superphosphate in two acid, phosphate-deficient soils

Abstract

Acid, P-deficient soils with a large capacity to immobilize P favour the rapid decomposition of apatite. Such soils are widespread in the east of South Africa and the possibility exists that Langebaan rock phosphate (Langfos) may be a more cost-effective source of P than superphosphate on these soils. This study compared the effectiveness of granular Langfos to that of double superphosphate in a pot experiment using a strongly P-fixing Salmoral clay (Typic Haplorthox) and a weakly P-fixing Avalon sandy loam (Plinthic Paleudult). Five rates of double superphosp,hate (0,100,250,500 and 1 000 mg Pkg-1 ) and six of Langfos (0,100, 250, SOD, 1 000 and 2000 mg P kg-) were compared at three lime rates. Three consecutive crops of maize (lea mays L.) were grown for periods of four weeks. The average of the initial response slopes from the relationship between cumulative yield and P applied for the different lime applications showed that the effectiveness of Langfos relative to superphosphate was 10% and 18% for the Salmoral and Avalon soils, respectively. Lime had relatively little effect on plant response to superphosphate on the Salmoral, but had a marked positive effect on the Avalon. The response to Langfos was negatively affected by lime on the Salmoral, but only slightly so on the Avalon. These results suggest that granular Langfos will be more effective in acid sandy soils than in acid clay soils but that it is not an effective substitute in either for water-soluble P fertilizers.Suur, P-gebrekkige gronde met 'n hoe P-vasleggingsvermoe bevorder die vinnige ontbinding van appetiet. Sulke gronde kom wydverspreid in die ooste van Suid-Afrika voor, en die moontlikheid bestaan dat Langebaanrotsfosfaat (Langfos) op hierdie gronde dalk 'n meer koste-effektiewe bron van P as superfosfaat mag wees. Hierdie studie het die effektiwiteit van gekorrelde Langfos met dubbelsuperfosfaat vergelyk in 'n potproef op 'n hoe P-vasleggende Salmoral-klei (Typic Haplorthox) en 'n lae P-vasleggende Avalon sanderige leem (Plinthic paleudult). Vyf peile van dubbelsuperfosfaat (0, 100, 250, 500 en 1 000 mg P kg-1) en ses peile van Langfos (0, 1 ~O, 250, SOD, 1 000 en 2000 mg P kg-1) is vergelyk by drie verskillende kalk peile. Drie agtereenvolgende oeste van mielies (lea mays L.) is verbou vir periodes van vier weke elk. Die gemiddelde van die aanvangsgedeelte van die reaksie-kurwe van die verwantskap tussen kumulatiewe opbrengs en P toegedien vir die verskillende peile van kalk toon dat die effektiwiteit van Langfos relatief tot superfosfaat, respektiewelik 10% en 18% vir die Salmoral en die Avalon gronde was. Kalk het plantreak~ie op superfosfaat op die Salmoral grond relatief min geaffekteer, maar 'n merkbare positiewe effek op die Avalon grond gehad. Die reaksie op Langfos was deur kalk negatief op die Salmoral grond geaffekteer, maar slegs gering negatief opdie Avalon grond geaffekteer. Hierdie resultate dui aan dat gekorrelde Langfos nie 'n effektiewe plaasvervanger vir wateroplosbare bemestingsstowwe is nie en dat gekorrelde Langfos se...

Maize response to Langebaan rock phosphate—superphosphate mixtures under glasshouse conditions

“…1992), are given in Table 1. The properties of Langfos and double superphosphate have also been reported previously (Thibaud et al .• 1992).…”

“…The two soils used in this investigation were the same as those used in an earlier investigation (Thibaud et al .. 1992) except that bulk soil samples were taken from P plots that differed in acidity from those used previously. The bulk soil samples were air-dried and ground to pass through a 2-mm sieve.…”

“…The bulk soil samples were air-dried and ground to pass through a 2-mm sieve. Chemical and physical properties of the two soils, determined according to the same procedures used previously (Thibaud et al. 1992), are given in Table 1.…”

“…Subsamples of the Avalon (7 000 g) and Balmoral soils (3 000 g) were treated with a total of 0, 50, 100, 175, 250 and 350 mg P kg-1 and 0, 200, 400, 800 and 1600 mg P kg-I, respectively, applied as a mixture of double superphosphate (ground to pass through a O.5-mm sieve) and granular Langfos to provide P from the two sources in the ratios of 1:0, 4:1, 2:1, 1:1, 1:2, 1:4 and 0:1. Other nutrients were applied at the same rates as those used previously (Thibaud et al .. 1992). The soils were transferred to plastic pots, watered to field moisture capacity and allowed to incubate for four weeks before planting them to maize' (Zea mays L. cv.…”

“…For both soils, a randomized complete block design with three replications was used. The cropping procedure and growing conditions were the same as those used previously (Thibaud et al .. 1992), except that plants were harvested after six weeks and only a single crop was grown. After harvesting, the plant material was dried at 70°C and weighed prior to milling to pass through a 0.84-mm screen.…”