Effects of phosphate carrier, phosphate placement, and level of lime application on plant growth on two oxisols in natal*

Part of a thesis by the senior author for the M.Sc. Agric. degree, University of Natal.

Abstract

A glass‐house study on two Oxisols revealed a marked response by trudan (Sorghum sudanense) to phosphate (P) placement. Plant response to water‐soluble forms of P (superphosphate and superphosphate lime mixture) was greater when this phosphale was granulated or when it was banded than when the powdered form was broadcast. Granulation or banding did not, however, affect plant responses to rock phosphate or thermophos. This response to granulation of P was strong in the Farningham soil, but weak in the Clovelly soil which had a lower P fixing capacity. Although both the soils studied can fix large proportions of added P, the considerable difference in their ability to fix P explains why, for Oxisols, fertilizer recommendations based only on estimates of available P in the soil could be unsatisfactory.

Growth responses to amelioration by lime occurred until exchangeable Al was eliminated (except on the Clovelly soil when treated with rock phosphate). Additional lime significantly reduced yields where P was banded even when a water soluble P carrier was used. High levels of lime drastically reduced yield where rock phosphate was used, even when P was broadcast. With insufficient lime, root growth and proliferation was severely restricted.

Uittreksel

‘n Aanmerklike reaksie op P plasing is deur “trudan” (Sorghum sudanense) in ‘n glashnisstudie op twee Oksisols geloon. Bandgeplaasde en gekorrelde P het opmerkbaar hoir opbrengste gegee as die by gestrooide P, maar slegs met superfosfaat en super‐met‐kalk as bronne van P, en nie met rotsfosfaat of thermophos as bron nie. Die reaksie op gekorrelde P was sterk in die Farningham‐grond dog swak in die Clovelly‐grond wat ‘n laer P‐bindende kapasiteil besit. Alhoewel nlbei gronde in staat is om groot hoeveelhede P te kan vaslé is die aansienlike verskil in die P‐bindende hoedanighede van die twee gronde die rede waarom, vir sover dit Oksisols betref, bemestingsaanbevelings, gebaseer slegs op beramings van die beskikbare P in die grond, in ‘n ernstige mate misteidend mag wees.

‘n Groeiverbetering as gevolg van kalktoediening is verkry (behalwe op die Clovelly grond wanneer die met rotsfosfaat behandel is) totdat uitniilbure Al uitgeskakel is. Daarna het ‘n betekenisvolle afname in apbrengs plaasgevind waar bandplasing van P toegepas is, selfs wanneer ‘n wateroplosbare P‐draer gebruik is. Hoë toedienings van kalk het opbrengste drasties veriaag waar rotsfosfaat gebruik is, selfs waar die P gestrooi is. Met ontoereikende kalk is wortelomtwikkeling en wortelverspreiding aansienlik beperk.

Notes

Part of a thesis by the senior author for the M.Sc. Agric. degree, University of Natal.