Investigation of the adsorption of I : 1'-dimethyl 4 : 4'-dipyridytium (paraquat) dichloride by a range of soils, using a solution-equilibration technique, has shown that the adsorption is characterized by three factors :I. Up to a limiting value, defined as the Strong Adsorption Capacity (SAC), the solution concentration of paraquat is reduced below the level of chemical detection by suspended soil : this strongly adsorbed paraquat is preferentially held against 0.1 to 2.oN solutions of ammonium ion. A range of strengths of adsorption probably exists within the strong adsorption range, but the greater part of the strongly adsorbed material is de-activated herbicidally ; this accounts for the herbicidal de-activation of paraquat in soils.2. Removal of soil organic matter by treatment with hydrogen peroxide usually does not greatly change the SAC. Strong adsorption of paraquat is primarily a property of clay minerals, and the presence of expanding lattice minerals is of particular importance.3. For fortifications above the strong adsorption region, there is a region of weaker adsorption but the total adsorption capacities of soils at saturation are less than their cation exchange capacities for inorganic cations. Taken with the difficulty of displacement, this indicates that the adsorption of paraquat is strongly influenced by factors other than simple electrostatic interaction.

The results of a field experiment on winter wheat are presented in which the effects of the soil physical properties produced by different cultivation systems were studied in relation to root development.The extension rate of the seminal root axes was slower in undisturbed or shallow (5 cm) cultivated soil than in ploughed or deep (20 cm) cultivated soil leading to earlier lateral branching and the production of an effectively shallower seminal root system which persisted throughout the season. During the first four weeks after drilling the soil in the ploughed and the deeper cultivated plots was of lower bulk density and there were more pores of a diameter suitable for unimpeded root extension. It is suggested that seminal root development in uncultivated soils, which are less readily deformed than cultivated ones, is related to the probability of the extending seminal axes contacting pores of suitable dimensions.The implications of shallower rooting are discussed and related to field experience with cereal crops drilled directly into uncultivated soils where success has consistently been achieved on free-draining, open-textured soils.

Summary. Some characteristics of the interaction of paraquat with a dioctahedral montmorillonite have been studied by a combination of adsorption, radioisotope exchange, and X‐ray diffraction techniques. The clay exhibited a high affinity for paraquat, since when less than 50 me/100 g were absorbed no paraquat was detected in the solution (<0·07 ppm), irrespective of the saturating cation, and in the presence or absence of 3·5 M ammonium nitrate solution. Interlamellar adsorption of paraquat was detected by the occurrence, even in an aqueous environment, of the 12·6 A c‐spacing associated with the adsorption of pyridinium ions. When 14C‐labelled paraquat was added to a clay containing less than 50 me/100 g of adsorbed paraquat, complete self‐exchange was observed within 24 hr. Therefore at all levels of adsorption, paraquat held in the adsorbed phase must be in equilibrium with a solution concentration. Interaction entre le paraquat et le sol; adsorption par un mineral argileux gonflant Résumé. Quelques caractères de l'interaction entre le paraquat et une montmorillonite dioctahèrique ont étéétudiés par une combinaison des techniques suivantes: adsorption, échanges de radioisotopes et diffraction des rayons X. L'argile manifesta une grande affinité pour le paraquat: lorsque moins de 50 me/100 g furent absorbés, le paraquat ne fut plus décelé cans la solution (<0.07 ppm), indépendamment du cation saturant, et en présenee ou non d'une solution 3,5 M de nitrate d'ammonium. L'adsorption interlamellaire du paraquat fut détectée par l'occurence, même dans un environnement aqueaux, d'un espacement selon l'axe c de 12,6 A associé avec l'adsorption d'ions pyridinium. Quand du paraquat marqué au 14C fut ajoutéà une argiie contenant moins de 50 me/100 g de paraquat adsorbé, un autoéchange complet fut observé dans les 24 heures. En conséquence, á tous les niveaux d'adsorption, le paraquat retenu dans la phase adsorbée doit être en êquilibre avec une concentration de la solution. Die Wechselwirkung zwischen Paraquat und Boden: Adsorption an ein aufweit‐bares Schichtgitter‐Tonmineral Zusammenfassung. Mit Hilfe einer Kombination von Adsorptions‐ und Radio‐isotopenaustauschtechniken und Röntgenstrukturanalyse wurden einige Charakteristika der Wechseiwirkungen zwischen Paraquat und einem diocatedrischen Montmorillonit untersucht. Der Ton besass eine hohe Affinität fur Paraquat, da in der Lösung kein Paraquat mehr nachzuweisen war (<0,07 ppm), wenn weniger als 50 mval/100 g adsorbiert worden waren. Dies war sowohl von dem entsprechenden Kation als auch von der Anwesenheit oder Abwesenheit einer 3,5 M Ammoniumnitratlosung unabhangig. Interlamellare Adsorption von Paraquat wurde anhand der sogar in wassrigem Milieu stattfindenden C‐Aufweitung (12,6 Å), verbunden mit einer Adsorption von Pyridiniumionen, festgestellt. Wurde 14C‐markiertes Paraquat einem Boden, der weniger als 50 mval/100 g adsorbiertes Paraquat enthielt, zugegeben, so trat innerhalb von 24 Stunden vollkommener Selbstaustausch ein. Das adsorbierte Paraquat muss dah...

A procedure for examining the distribution of ethirimol in soil is described. Soil samples containing ethirimol are initially freeze-dried at -5 to -1O"C, impregnated with Araldite resin and hardeners under vacuum and then maintained for 16 h at room temperature followed by 24 h at lOO"C, to complete curing. The impregnated soils may then be sectioned and the radiolabelled pesticide located by autoradiography. The distribution of ethirimol within the soil was found to be unaffected by either freeze-drying or impregnation. The method enables the distribution to be determined on a small scale directly in undisturbed soil sections from both the laboratory and the field. It was used to investigate the effects of soil type on the movement of ethirimol. Structural and textural factors appear to have an important influence on the redistribution of this fungicide and may override the effects of adsorption.