Keywords 16Organic matter, microbial phosphorus, mineralisation, phosphate, soil phosphorus pools 17 18 19 1 2 to enable accurate predictions of the required external P inputs to achieve optimum growth of 46 subsequent crops. While our understanding of soil inorganic phosphate (Pi) pools is relatively 47 comprehensive, the value of P returned to the soil in crop residues has not been fully resolved. 48 Agronomically significant amounts of P can be present in crop residues and the microbial biomass 49 associated with their decomposition, and the potential contribution of this pool to the P nutrition of 50 cropping systems is significant (eg. Chauhan et al., 1979;Dalal, 1979;White and Ayoub, 1983; Thibaud 51 et al., 1988;Umrit and Friesen, 1994;Kwabiah et al., 2003a;Nachimuthu et al., 2009). The main factors 52 influencing the amount of crop residue P, its rate of mineralisation and subsequent availability to crops 53 have been identified (Stockdale and Brookes, 2006;Guppy and McLaughlin, 2009;; 54 but their interactions remain poorly elucidated and largely unquantified. By reviewing the published 55 literature in which quantitative measurements of P transformations from plant residues applied to soil 56 have been reported, we will evaluate the contribution of crop residue-derived P to the P nutrition of 57 subsequent crops, assess the key factors involved and summarise the knowledge as an empirical model. 58 59The dynamics of organically-derived nitrogen (N) and carbon (C) in agricultural soils has been extensively 60 described, and a wide range of predictive tools have been developed. These have proved a valuable 61 asset for landholders, agronomists and policy makers by providing good estimates of the impacts of 62 agronomic management options on the dynamics of both C (eg. Parton et al., 1988; Coleman and 63 Jenkinson, 1999;Grace et al., 2006) and N (see Herridge et al., 2008) in agricultural soils. Considering 64 our extensive knowledge of the N cycle in agricultural systems, and the benefits (economic, social and 65 environmental) that have been obtained by our ability to predict and manipulate it, similar knowledge 66 of the organic P cycle could also yield significant benefits. Yet, although the principal driving factors of 67 organic P cycling have long been recognised and modelled (Cole et al., 1977), models have not proven 68 to be universally applicable (Gijsman et al., 1996;Schnepf et al., 2011). Several models have 69 demonstrated a capacity to incorporate P release from crop residues and manures into projected crop 70 growth and yield, notably The Agricultural Production Systems Simulation (APSIM) (Keating et al., 2003), 71 3 Century (Parton et al., 1988) and CERES-Wheat (Ritchie et al., 1988;Godwin et al., 1989; Singh et al., 72 1991;Daroub et al., 2003) modelling frameworks. However, these models require detailed climate and 73 site information that may not be available, and are specialised tools that cannot be operated by the 74 layperson. The contribution of crop residue P to the nutriti...

Summary. This project was designed to study various aspects of wild oat competition in spring wheat and flax. From ten to forty wild oat plants/yd2 were sufficient to cause significant yield reductions in wheat when grown on summerfallow land or when ammonium phosphate fertilizer was added to stubble land. However, when wheat was grown on stubble land without the benefit of a fertilizer treatment, seventy to one hundred wild oat plants/yd2 were needed to suppress wheat yields significantly. This would suggest that on stubble land, soil fertility was a more important factor than moderate densities of wild oats in determining eventual crop yields. In these experiments, wild oats reduced the number of tillers per plant, but did not significantly affect the protein content of the harvested grain. Only ten wild oat plants/yd2 were sufficient to reduce flax yields significantly on both summerfallow and stubble land. The only exception was in 1966, when flax grown on summerfallow land was not significantly affected until the density of wild oats reached forty plants/yd2. This confirms the general observation that flax is a poor competitor with wild oats. The results suggest that wild oat competition had already commenced prior to emergence of wheat, particularly with the higher densities of wild oats. In general, competitive effects increased with time and with wild oat density. In flax, severe competition had already taken place prior to the 2–3‐Ieaf stage of the weed in 1964, but did not become severe until after the 2–3‐leaf stage of wild oat growth in 1965 and 1966. Again, competitive effects increased with time and with wild oat density. Results of a final series of experiments, suggested that the optimum seeding date for flax in Manitoba is the latter part of May or the first week of June. Yield reductions due to wild oat competition became very severe as seeding dates were delayed. La compétition de la folle avoine (A vena fatua L.) avec le blé et le lin

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Heterogeneous distribution of mineral nutrients in soil proWles is a norm in agricultural lands, but its inXuence on nutrient uptake and crop growth is poorly documented. In this study, we examined the eVects of varying phosphorus (P) and potassium (K) distribution on plant growth and nutrient uptake by wheat (Triticum aestivum L.) grown in a layered or split soil culture in glasshouse conditions. In the layered pot system the upper soil was supplied with P and either kept watered or allowed to dry or left P-deWcient but watered, whereas the lower soil was watered and fertilised with K. Greater reductions in shoot growth, root length and dry weight in the upper soil layer occurred in -P/wet than in +P/dry upper soil treatment. Shoot P concentration and total P content were reduced by P deWciency but not by upper soil drying. Genotypic responses showed that K-eYcient cv. Nyabing grew better and took up more P and K than K-ineYcient cv. Gutha in well-watered condition, but the diVerences decreased when the upper soil layer was dry. In the split-root system, shoot dry weight and shoot P and K contents were similar when P and K were applied together in one compartment or separated into two compartments. In comparison, root growth was stimulated and plants took up more P and K in the treatment with the two nutrients supplied together compared with the treatment in which the two nutrients were separated. Roots proliferated in the compartment applied with either P or K at the expense of root growth in the adjoining compartment with neither P nor K. Heterogeneous nutrient distribution has a direct decreasing eVect on root growth in deWcient patches, and nutrient redistribution within the plant is unlikely to meet the demand of roots grown in such patches.