This review highlights future needs for research on potassium (K) in agriculture. Current basic knowledge of K in soils and plant physiology and nutrition is discussed which is followed by sections dealing specifically with future needs for basic and applied research on K in soils, plants, crop nutrition and human and animal nutrition. The section on soils is devoted mainly to the concept of K availability. The current almost universal use of exchangeable K measurements obtained by chemical extraction of dried soil for making fertilizer recommendations is questioned in view of other dominant controlling factors which influence K acquisition from soils by plants. The need to take account of the living root which determines spatial K availability is emphasized. Modelling of K acquisition by field crops is discussed. The part played by K in most plant physiological processes is now well understood including the important role of K in retranslocation of photoassimilates needed for good crop quality. However, basic research is still needed to establish the role of K from molecular level to field management in plant stress situations in which K either acts alone or in combination with specific micronutrients. The emerging role of K in a number of biotic and abiotic stress situations is discussed including those of diseases and pests, frost, heat/drought, and salinity. Breeding crops which are highly efficient in uptake and internal use of K can be counterproductive because of the high demand for K needed to mitigate stress situations in farmers’ fields. The same is true for the need of high K contents in human and animal diets where a high K/Na ratio is desirable. The application of these research findings to practical agriculture is of great importance. The very rapid progress which is being made in elucidating the role of K particularly in relation to stress signalling by use of modern molecular biological approaches is indicative of the need for more interaction between molecular biologists and agronomists for the benefit of agricultural practice. The huge existing body of scientific knowledge of practical value of K in soils and plants presents a major challenge to improving the dissemination of this information on a global scale for use of farmers. To meet this challenge closer cooperation between scientists, the agrochemical industry, extension services and farmers is essential.

Research on potassium in agriculture: needs and prospects

Soils are the product of the activities of plants, which supply organic matter and play a pivotal role in weathering rocks and minerals. Many plant species have a distinct ecological amplitude that shows restriction to specific soil types. In the numerous interactions between plants and soil, microorganisms also play a key role. Here we review the existing literature on interactions between plants, microorganisms and soils, and include considerations of evolutionary time scales, where possible. Some of these interactions involve intricate systems of communication, which in the case of symbioses such as the arbuscular mycorrhizal symbiosis are several hundreds of millions years old; others involve the release of exudates from roots, and other products of rhizodeposition that are used as substrates for soil microorganisms. The possible reasons for the survival value of this loss of carbon over tens or hundreds of millions of years of evolution of higher plants are discussed, taking a cost-benefit approach. Co-evolution of plants and rhizosphere microorganisms is discussed, in the light of known ecological interactions between various partners in terrestrial ecosystems. Finally, the role of higher plants, especially deep-rooted plants and associated microorganisms in the weathering of rocks and minerals, ultimately contributing to pedogenesis, is addressed. We show that rhizosphere processes in the long run are central to biogeochemical cycles, soil formation and Earth history. Major anticipated discoveries will enhance our basic understanding and allow applications of new knowledge to deal with nutrient deficiencies, pests and diseases, and the challenges of increasing global food production and agroecosystem productivity in an environmentally responsible manner.

Plant-microbe-soil interactions in the rhizosphere: an evolutionary perspective

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Alteration, formation, and occurrence of minerals in soils

Laboratory experiments are performed to understand the controlling parameters of the electrical field associated with the seepage of water through a porous material. We use seven glass bead packs with varying mean grain size in an effort to obtain a standard material for the investigation of these electrical potentials. The mean grain size of these samples is in the range 56–3000 μm. We use pure NaCl electrolytes with conductivity in the range 10−4 to 10−1 S m−1 at 25°C. The flow conditions cover viscous and inertial laminar flow conditions but not turbulent flow. In the relationship between the streaming potential coupling coefficient and the grain size, three distinct domains are defined by the values of two dimensionless numbers, the Dukhin and the Reynolds numbers. The Dukhin number represents the ratio between the surface conductivity of the grains (due to conduction in the electrical double layer coating the surface of the grains) and the pore water electrical conductivity. At high Dukhin numbers (≫1) and low Reynolds numbers (≪1), the magnitude of the streaming potential coupling coefficient decreases with the increase of the Dukhin number and depends on the mean grain diameter (and therefore permeability) of the medium. At low Dukhin and Reynolds numbers (≪1), the streaming potential coupling coefficient becomes independent of the microstructure and is given by the well-known Helmholtz-Smoluchowski equation widely used in the literature. At high Reynolds numbers, the magnitude of the streaming potential coupling coefficient decreases with the increase of the Reynolds number in agreement with a new model developed in this paper. A numerical application is made illustrating the relation between the self-potential signal and the intensity of seepage through a leakage in an embankment.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006JB004673

Streaming potentials of granular media: Influence of the Dukhin and Reynolds numbers

Fertilizer K and P requirements for rice (Oryza sativa L.) can be determined with site-specific nutrient management (SSNM) using estimated target yield, nutrient balances, and yield gains from added nutrient. We used the QUEFTS (QUantitative Evaluation of the Fertility of Tropical Soils) model with >8000 plot-level observations to estimate the relationship between grain yield and nutrient accumulation in above-ground dry matter of irrigated rice with harvest index ≥ 0.4. Predicted reciprocal internal efficiencies (RIEs) at 60–70% of yield potential corresponded to plant accumulation of 14.6 kg N, 2.7 kg P, and 15.9 kg K per tonne of grain yield. These RIEs enable determination of plant requirements for K and P and net output of K and P in harvested grain and removed crop residues at a target yield. Yield gains for nutrient applied to irrigated rice averaged 12% for K and 9% for P for 525 to 531 observations. For fields without certain yield gain, fertilizer K and P requirements can be determined by a partial maintenance approach (i.e., fertilizer input < output in nutrient balance), which considers nutrient supply mediated through soil processes and balances trade-offs between financial loss with full maintenance rates and risk of excessive nutrient depletion without nutrient application. When yield gains to an added nutrient are certain, partial maintenance plus yield gain can be used to determine fertilizer requirements. The SSNM-based approach and algorithms enable rapid development of field-specific K and P management.

Field-specific potassium and phosphorus balances and fertilizer requirements for irrigated rice-based cropping systems

Which 2 : 1 clay minerals are involved in the soil potassium reservoir? Insights from potassium addition or removal experiments on three temperate grassland soil clay assemblages

Spontaneous Potentials associated with volcanic activity are often interpreted by means of the electrokinetic potential, which is usually positive in the flow direction (i.e. Zeta potential of the rock is negative). The water-rock interactions in hydrothermal zones alter the primary minerals leading to the formation of secondary minerals. This work addresses the study of calcite precipitation in a sand composed of 98% quartz and 2% calcite using streaming potential measurements. The precipitation of calcite as a secondary mineral phase, inferred by high calcite saturation indices and by a fall in permeability, has a significant effect on the electrokinetic behaviour, leading to a significant reduction in the Zeta potential (in absolute value) and even a change in sign. The measured decrease in Zeta potential from -16 mV to -27±4 mV takes place as the pH rises from 4 to 7, while it remains constant at -25±1 mV as the pH increases from 8 to 10.5. For pH higher than 10.5, calcite precipitates and is expected to coat the quartz surface. The measured Zeta potential vary from -17 to +8 mV for pH ranging from 10.6 to 11.7 depending on the amount of precipitated calcite indicated by the decrease in permeability. The observed change in sign of the electrical surface potential rules out the usual qualitative interpretation of SP anomalies in order to determine fluid circulations, even at pH lower than 9 if calcite is widely present as a secondary mineral phase, since the electrical surface potential of calcite depends also on CO2 partial pressure and [Ca2+]. Therefore SP anomalies as measured in hydrothermal field, without mineralogical analyses of hydrothermal deposits, and without geochemical fluid survey, should be interpreted with caution.

https://hal.archives-ouvertes.fr/hal-00108351/document

Modification of streaming potential by precipitation of calcite in a sand–water system: laboratory measurements in the pH range from 4 to 12

Potassium (K) availability influences many processes in cultivated and natural ecosystems. Several studies suggest that “non-exchangeable” K+ ions fixed in 2:1 clay mineral interlayers contribute to plant nutrition. Although depletion of these K+ ions could be observed by X-ray diffractometry, this technique has never been considered for the observation of short-term changes in illitic 2:1 clay minerals. We established in this study that new treatments of X-ray diffraction patterns allow quantification of short-term 2:1 clay mineral changes through K addition in solution and removal of interlayer K by Lolium multiflorum. Moreover, we obtained a significant relationship (r
 2 = 0.95, P < 0.0001) between an indicator calculated from X-ray diffraction patterns and analyzed clay K content. X-ray diffraction should therefore be considered as an appropriate tool to follow qualitatively and quantitatively clay mineral modifications induced by soil K balance. Our results suggest that 2:1 clay minerals behave as a huge, renewable K reservoir whose theoretical capacity in fertile soils could exceed 3 t/ha. Beyond obvious agronomical implications, this new vision of soil K cycle raises ecological questions about plant inter specific competition and soil fertility. Finally, our study clearly shows that soil 2:1 clay minerals could react as quickly as a biological system.

Nicole Catel

Papers

Which 2 : 1 clay minerals are involved in the soil potassium reservoir? Insights from potassium addition or removal experiments on three temperate grassland soil clay assemblages

Modification of streaming potential by precipitation of calcite in a sand–water system: laboratory measurements in the pH range from 4 to 12

Soil-plant potassium transfer: impact of plant activity on clay minerals as seen from X-ray diffraction

Modification of streaming potential by precipitation of calcite in a sand-water system: laboratory measurements pH range from 4 to 12