Cases of atrazine damage to maize, and occasional reports of excessive residue persistence that caused injury to susceptible following crops, prompted research on factors which influence the bioactivity and persistence of the herbicide. Bioassays with several indicator species were conducted in the field and in glasshouses. Measurement of atrazine by chemical means was done in only three of the experiments. Maize seedlings were not more sensitive to atrazine when stressed by insufficient supplies of N, P , K, Ca or Mg. Neither high P supply nor phytotoxic P concentrations in maize seedlings lowered their tolerance to atrazine. These findings on the roles of certain nutrients in the tolerance of maize to atrazine concluded a series of investigations into damage reported in the field. No satisfactory evidence for the factors which rendered maize susceptible to atrazine in the field was ever found. Another investigation showed that atrazine threshold concentrations for certain susceptible species varied from soil to soil , indicating that differential availability of the herbicide for uptake by plants in different soils precludes the allocation of fixed atrazine threshold values to different crop species. Atrazine bioactivity and persistence varied considerably in a series of field experiments which were conducted to determine the order of importance of certain soil properties in the prediction of short- and long-term bioactivity. Soil organic matter content (% C) was the best predictor in both instances. Soil pH was a poor predictor of short-term bioactivity, but became more prominent with time. Organic matter, pH and P-reversion accounted for 35%, 19% and 14% of the variation in bioactivity measured six months after atrazine application in the field. In view of the variation in atrazine persistence, and therefore, the potential for variable carry-over from soil to soil, the applicability of the recropping interval recommended for dry beans and sunflower were investigated. It was concluded that the specification could be justified for sunflower only, but then only on certain soils. It is suggested that recropping intervals , which are recommended for certain crops after atrazine use in maize, be refined on the basis of differences in follow-up crop sensitivity to atrazine, and relationships between atrazine persistence and certain soil properties. Chemical analysis showed that soil type and soil water content had greater influences on atrazine persistence than temperature. Irrespective of soil type, the rate of atrazine breakdown was faster in soil at a water content of field capacity, and in water-logged soil (2x FC), than in air-dry soil. A bioassay technique was used to estimate the concentration of atrazine and/or its phytotoxic residues in a soil profile. Basically the same procedure was followed in an incubation study with 25 soils to develop the following regression model for prediction of atrazine half-life in soil: γ = -2.29 + I.77xl + 20.8Ix2, where γ is half-life in days; Xl = [soil pH(H2O)2<f and X2 = % C. The bioassay technique proved useful for estimating total amounts of atrazine and its phytotoxic residues in various soil type AFRIKAANS : Beskadiging van mielies deur atrasien, en sporadiese berigte van oormatige nawerking met gepaardgaande beskadiging van gevoelige opvolggewasse, het aanleiding gegee tot navorsing oor faktore wat die bio-aktiwiteit en nawerking van die onkruiddoder be'invloed, Biotoetse is met verskillende toetsplante in glashuis- en veldproewe uitgevoer. Bepaling van atrasien deur chemiese analise is in slegs drie proewe gedoen , Mieliesaailinge se verdraagsaamheid teenoor atrasien is nie deur tekorte aan N , P, K, Ca en Mg in die plante be'invloed nie, Nog hoe P-voorsiening, nog fitotoksiese Pkonsentrasies in saailinge, het hul weerstand teen atrasien verlaag, Hierdie bevindings oor die rol van sekere voedingselemente by die verdraagsaamheid van mielies teenoOT atrasien het 'n reeks ondersoeke na destydse skade in die veld afgesluit, sonder bevredigende verklarings vir die probleem, In'n ander ondersoek is gevind dat drumpelwaarde-konsentrasies van atrasien vir bepaalde gevoelige gewasplante van grond tot grond varieer. Differensiele beskikbaarheid van atrasien in grond sal dus die toekenning van 'n vaste drumpelwaarde aan 'n bepaalde gewasplant verhoed , Dieorganiese koolstofinhoud van grond was die belangrikste voorspeller van die kort- en langtermyn bio-aktiwiteit van atrasien in 'n reeks veldproewe. Die variasie in bioaktiwiteit wat ses maande na toediening van die doder deur organiese materiaalinhoud, pH en P-reversie verklaar is, was onderskeidelik 35% , 19% en 14%. Derhalwe het grond-pH , wat 'n swak voorspeller (l % van variasie) van die korttermyn-aktiwiteit van atrasien was, belangriker geraak met tyd. Die bio-aktiwiteit en nawerking van atrasien het aansienlik tussen lokaliteite verskil. Weens die varierende nawerking van atrasien in grond , en gevolglike verskille in die potensiele oordraging van atrasien na 'n volgende seisoen , is die toepasLikheid van die wagperiode wat vir droebone en sonneblom gestel word vervolgens ondersoek. Die voorgeskrewe wagperiode kon slegs vir sonneblom as toepaslik bevestig word , en dan slegs op sekere grondsoorte. Dit word voorgestel dat wagperiodes verfyn behoort te word op basis van verski lle tussen grondsoorte en die verdraagsaamheid van gewasplante teenoor atrasien. Met chemiese analise is bepaal dat grondsoort en -waterinhoud 'n belangriker effek op nawerking van atrasien as temperatuur gehad het. Vergeleke met die afbraaktempo in 11Igdroe grond het atrasien vinniger afgebreek in grond waar die waterinholld by veldkapasiteit was, asook wanneer dit twee keer hierdie hoeveelheid water bevat het. 'n Biotoetstegniek is gebruik vir skatting van atrasien- en/of fitotoksiese residu-konsentrasies in 'n grondprofiel. Basies dieselfde prosed lire is in 'n inkubasieproef met 25 gronde toegepas vir ontwikkeling van die volgende model vir voorspelling van die halfleeftyd van atrasien in grond: γ = -2.29 + l.77x1 + 20.8Ix2, waar γ halfleeftyd in dae is; x1 = [grond-pH(H20)]2 en x2 = % C. Die biotoetstegniek was effektief vir skattings van die totale hoeveelheid atrasien en fitotoksiese atrasienresidue in grond. / Thesis (PhD)--University of Pretoria, 2011. / Plant Production and Soil Science / unrestricted