Cultivars with increased efficiency of uptake and utilization of soil nutrients are likely to have positive environmental effects through reduced usage of chemicals in agriculture. This review assesses the available literature on differential uptake and utilization efficiency of K in farming systems. Large areas of agricultural land in the world are deficient in K (e.g. 3/4 of paddy soils in China, 2/3 of the wheatbelt in Southern Australia), with export in agricultural produce (especially hay) and leaching (especially in sandy soils) contributing to lowering of K content in the soil. The capacity of a genotype to grow and yield well in soils low in available K is K efficiency. Genotypic differences in efficiency of K uptake and utilization have been reported for all major economically important plants. The K‐efficient phenotype is a complex one comprising a mixture of uptake and utilization efficiency mechanisms. Differential exudation of organic compounds to facilitate release of non‐exchangeable K is one of the mechanisms of differential K uptake efficiency. Genotypes efficient in K uptake may have a larger surface area of contact between roots and soil and increased uptake at the root–soil interface to maintain a larger diffusive gradient towards roots. Better translocation of K into different organs, greater capacity to maintain cytosolic K+ concentration within optimal ranges and increased capacity to substitute Na+ for K+ are the main mechanisms underlying K utilization efficiency. Further breeding for increased K efficiency will be dependent on identification of suitable markers and compounding of efficiency mechanisms into locally adapted germplasm.

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...

A novel approach to the sustainable management of potassium (K) resources in agro-ecosystems is through better exploitation of genetic differences in the K efficiency of crop plants. Potassium efficiency is a measure of genotypic tolerance to soils with low potassium availability and can be quantified as the K efficiency ratio (the ratio of growth at deficient and adequate K supply). This study investigated the magnitude of variation in K efficiency among wheat (Triticum aestivum L.) genotypes grown in a glasshouse and in the field. Genotypes differed significantly in response to low soil K availability in terms of shoot biomass during the vegetative growth phase and grain yield at maturity under glasshouse (144 genotypes) and field (89 genotypes) conditions. K-efficient and K-inefficient genotypes were identified. The main factor determining K efficiency for grain yield was the capacity of genotypes to maintain a high harvest index (grain yield/total shoot weight) at deficient K supply. Genotypes that had reduced harvest index under deficient K supply were K-inefficient. Capacity to tolerate low concentrations of K in shoot tissue where K supply was deficient was also important in determining K efficiency for grain yield. Potassium-efficient genotypes have the potential to enhance the productivity and sustainability of cereal cropping systems.

High arsenic (As) concentrations in the soil, water and plant systems can pose a direct health risk to humans and ecosystems. Phosphate (Pi) ions strongly influence As availability in soil, its uptake and toxicity to plants. Better understanding of As(V)-Pi interactions in soils and plants will facilitate a potential remediation strategy for As contaminated soils, reducing As uptake by crop plants and toxicity to human populations via manipulation of soil Pi content. However, the As(V)-Pi interactions in soil-plant systems are complex, leading to contradictory findings among different studies. Therefore, this review investigates the role of soil type, soil properties, minerals, Pi levels in soil and plant, Pi transporters, mycorrhizal association and microbial activities on As-Pi interactions in soils and hydroponics, and uptake by plants, elucidate the key mechanisms, identify key knowledge gaps and recommend new research directions. Although Pi suppresses As uptake by plants in hydroponic systems, in soils it could either increase or decrease As availability and toxicity to plants depending on the soil types, properties and charge characteristics. In soil, As(V) availability is typically increased by the addition of Pi. At the root surface, the Pi transport system has high affinity for Pi over As(V). However, Pi concentration in plant influences the As transport from roots to shoots. Mycorrhizal association may reduce As uptake via a physiological shift to the mycorrhizal uptake pathway, which has a greater affinity for Pi over As(V) than the root epidermal uptake pathway.

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