Showing papers in "Plant and Soil in 1986"

Effects of vegetation on the emission of methane from submerged paddy soil

Abstract: Methane emission rates from rice-vegetated paddy fields followed a seasonal pattern different to that of weed-covered or unvegetated fields. Presence of rice plants stimulated the emission of CH4 both in the laboratory and in the field. In unvegetated paddy fields CH4 was emitted almost exclusively by ebullition. By contrast, in rice-vegetated fields more than 90% of the CH4 emission was due to plant-mediated transport. Rice plants stimulated methanogenesis in the submerged soil, but also enhanced the CH4 oxidation rates within the rhizosphere so that only 23% of the produced CH4 was emitted. Gas bubbles in vegetated paddy soils contained lower CH4 mixing ratios than in unvegetated fiels. Weed plants were also efficient in mediating gas exchnage between submerged soil and atmosphere, but did not stimulate methanogenesis. Weed plants caused a relatively high redox potential in the submerged soil so that 95% of the produced CH4 was oxidized and did not reach the atmosphere. The emission of CH4 was stimulated, however, when the cultures were incubated under gas atmospheres containing acetylene or consisting of O2-free nitrogen.

Development and function of Azospirillum -inoculated roots

Abstract: The surface distribution of Azospirillum on inoculated roots of maize and wheat is generally similar to that of other members of the rhizoplane microflora. During the first three days, colonization takes place mainly on the root elongation zone, on the base of root hairs and, to a lesser extent, on the surface of young root hairs. Azospirillum has been found in cortical tissues, in regions of lateral root emergence, along the inner cortex, inside xylem vessels and between pith cells. Inoculation of several cultivars of wheat, corn, sorghum and setaria with several strains of Azospirillum caused morphological changes in root starting immediately after germination. Root length and surface area were differentially affected according to bacterial age and inoculum level. During the first three weeks after germination, the number of root hairs, root hair branches and lateral roots was increased by inoculation, but there was no change in root weight. Root biomass increased at later stages. Cross-sections of inoculated corn and wheat root showed an irregular arrangement of cells in the outer layers of the cortex. These effects on plant morphology may be due to the production of plant growth-promoting substances by the colonizing bacteria or by the plant as a reaction to colonization. Pectic enzymes may also be involved. Morphological changes had a physiological effect on inoculated roots. Specific activities of oxidative enzymes, and lipid and suberin content, were lower in extracts of inoculated roots than in uninoculated controls. This suggests that inoculated roots have a larger proportion of younger roots. The rate of NO 3 - , K+ and H2PO 4 - uptake was greater in inoculated seedlinds. In the field, dry matter, N, P and K accumulated at faster rates, and water content was higher in Azospirillum-inoculated corn, sorghum, wheat and setaria. The above improvements in root development and function lead in many cases to higher crop yield.

Nutrient cycling and nitrogen mineralization in eucalypt forests of south-eastern Australia: II. Indices of nitrogen mineralization

Abstract: Nutrient pools in litter and soil and the major nutrient transfers and additions in rainfall, throughfall and litterfall were measured in eight mature, undisturbed eucalypt forests covering a range of species, climate, productivity and soil type. Litterfall is the major pathway for the return of N, P, Ca and usually Mg, to the soil. The forests covered almost the range of litterfall reported for eucalypt forests and, in conjunction with published data, litterfall was strongly related to climatic variables. Extractable P in the soil and P concentrations in litter and litterfall were significantly higher in two sub-alpine forests (Eucalyptus pauciflora andE. delegatensis) than in all other forests. In general, nutrient turnover, particularly N turnover, was related to the rate of organic matter turnover. Rates of organic matter turnover in these forests and in other studies of eucalypts were correlated with climatic conditions using the simple climatic scalar developed by Vitousek. Nitrogen turnover, especially that proportion cycling via leaf litterfall is primarily a function of organic matter turnover, but litter quality appears also to have an influence.

Comparison of soil extractions by EUF and by some conventional extraction procedures.

Abstract: The extractive powers of different extraction procedures (Electro-Ultrafiltration, 0.01M CaCl2 and standard Dutch methods) were compared mutually for a limited number of nutrients in soil samples from 21 locations. The results showed that for almost all parameters under study (Na, K, Mg, Mn, P, N) the methods are interchangeable. Drawbacks of the EUF technique are lower reproducibility of the results, laboriousness and high cost. Moreover, the extraction of exchangeable forms of Mn and Mg with this technique was incomplete. Extraction with 0.01M CaCl2 seems recommendable due to the simplicity of the analytical procedure giving sufficient information for practical soil-analytical purposes.

Development and function ofAzospirillum-inoculated roots

Abstract: The surface distribution ofAzospirillum on inoculated roots of maize and wheat is generally similar to that of other members of the rhizoplane microflora. During the first three days, colonization takes place mainly on the root elongation zone, on the base of root hairs and, to a lesser extent, on the surface of young root hairs.Azospirillum has been found in cortical tissues, in regions of lateral root emergence, along the inner cortex, inside xylem vessels and between pith cells. Inoculation of several cultivars of wheat, corn, sorghum and setaria with several strains ofAzospirillum caused morphological changes in root starting immediately after germination. Root length and surface area were differentially affected according to bacterial age and inoculum level. During the first three weeks after germination, the number of root hairs, root hair branches and lateral roots was increased by inoculation, but there was no change in root weight. Root biomass increased at later stages. Cross-sections of inoculated corn and wheat root showed an irregular arrangement of cells in the outer layers of the cortex. These effects on plant morphology may be due to the production of plant growth-promoting substances by the colonizing bacteria or by the plant as a reaction to colonization. Pectic enzymes may also be involved. Morphological changes had a physiological effect on inoculated roots. Specific activities of oxidative enzymes, and lipid and suberin content, were lower in extracts of inoculated roots than in uninoculated controls. This suggests that inoculated roots have a larger proportion of younger roots. The rate of NO− 3, K+ and H2PO− 4 uptake was greater in inoculated seedlinds. In the field, dry matter, N, P and K accumulated at faster rates, and water content was higher inAzospirillum-inoculated corn, sorghum, wheat and setaria. The above improvements in root development and function lead in many cases to higher crop yield.

Establishment of inoculated Azospirillum spp. in the rhizosphere and in roots of field grown wheat and sorghum

Abstract: Four field experiments were carried out with wheat or sorghum in different regions of Brazil. The aim was to study the establishment of inoculatedAzospirillum strains, marked with resistance to various antibiotics, in the rhizosphere and in roots. The levels of the various antibiotics were chosen according to the resistance of the indigenousAzospirillum population.Azospirillum brasilense strains Sp 107 and Sp 245 could be established in all three wheat experiments and predominated within theAzospirillum population in washed, and especially in surface sterilized, roots. Strains Sp 7 and Cd established poorly in wheat roots.Azospirillum lipoferum Sp S82 represented 72% of the root isolates from sorghum inoculated with this strain. This strain and naturalAzospirillum infection became concentrated in the upper parts of the root system. Improved methods for root surface sterilization in which the absence ofAzospirillum on the root surface was established by pre-incubating roots with paraffin-capped ends in NFb medium confirmed the establishment of inoculatedAzospirillum strains within sorghum roots in the field.

Effect of inoculation of Azospirillum spp. on nitrogen accumulation by field-grown wheat

Abstract: Two experiments were performed to examine the effects of inoculation of field grown wheat with various Azospirillum strains. In the first experiment the soil was sterilized with methyl bromide to reduce the Azospirillum population and15N labelled fertilizer was added to all treatments. Two strains ofAzospirillum brasilense isolated from surface sterilized wheat roots and theA. brasilense type strain Sp7 all produced similar increases in grain yield and N content. From the15N and acetylene reduction data it was apparent that these increases were not due to N2 fixation.

Land use and organic carbon content of some subtropical soils

Abstract: The assumption that the organic matter content of tropical forest soils is oxidized to atmospheric carbon dioxide when these soils are converted to agricultural use was tested using results of soil surveys in Puerto Rico (1940's, 1960's, and 1980's). Results showed that under intensive agricultural use, soil carbon in the top 18 cm of soil was about 30–37 Mg/ha, regardless of climatic conditions. Reduced intensity of agricultural use resulted in an increase of soil carbon in the order of 0.3–0.5 Mg.ha−1. yr−1 over a 40-yr period. Rates of soil carbon accumulation were inversely related to the sand content of soils. The relation between rates of soil carbon accumulation and climate or soil texture were better defined at higher soil carbon content. Soils under pasture accumulated soil carbon and often contained similar or greater amounts than adjacent mature forest soils (60–150 Mg/ha in the top 25 or 50 cm). Soils in moist climates exhibited greater variations in soil carbon content with changes in land use (both in terms of loss and recovery) than did soils in dry climates. However, in all life zones studied, the recovery of soil carbon after abandonment of agriculture was faster than generally assumed. Low carbon-to-nitrogen ratios suggested that intensively used soils may be stable in their nutrient retention capacity. The observed resiliency of these soils suggested that their role as atmospheric carbon sources has been overestimated, while their potential role as atmospheric carbon sinks has been underestimated.

Post-harvest residue input to cropland

Abstract: Total annual input of carbon with crop residues to the soils in Missouri (North Central Region of the USA) was found to be about 3.7 t ha−1 for wheat, 3.4 t ha−1 for soybeans and 9.2 t ha−1 for corn. Root biomass represented 40–50% of total residues and its accumulation appears to be influenced by climatic conditions. Estimates of carbon outflow from the ecosystem in the form of CO2 were in good agreement with the biomass input.

Family-dependent cadmium accumulation characteristics in higher plants

Abstract: Thirty four plant species belonging to 9 families were grown on a sand soil with various levels of added Cd. The Cd content of aerial parts of the plants (tc) was correlated to the soil Cd level (sc) in a double-logarithmic relationship: log (tc)=α+β log (sc). The regression coefficients α and β statistically differed among plant families. Plant families were classified into 3 groups based on the α values; (1) low accumulation, Leguminosae; (2) moderate accumulation, Gramineae, Liliaceae, Cucurbitaceae and Umbelliferae; and (3) high accumulation, Chenopodiaceae, Cruciferae, Solanaceae and Compositae. Values of β also enabled to discriminate the families into (1) Umbelliferae, Cucurbitaceae and Cruciferae in which Cd accumulation was promoted at higher soil Cd levels, and. (2) the other families which were of the excluder type.

Phosphorus deficiency induced by nitrogen input in Douglas fir in the Netherlands

Abstract: A re-examination of earlier NPK fertilization experiments in Douglas fir stands on sandy soils shows the effects of high nitrogen input by air pollution during the last 10–15 years on plant nutrition at these sites. In 1960, experimental plots showed a positive growth reaction to nitrogen, phosphorus, and potassium fertilization. All suffered from severe phosphorus deficiency in 1984, low phosphorus in the needles was invariably accompanied by a high nitrogen content, with all N/P ratios between 20 and 30. The same conclusion emerges from an independent investigation of nutrient status of a selection of Douglas fir stands. Hence, if stand productivity and a balanced nutrient status of the trees is to be maintained, the increase in atmospheric input of nitrogen calls for supplementary fertilization. Given the current N/P ratios in the needles, a positive growth response to phosphorus fertilization is to be expected.

Microclimatic controls of nitrogen mineralization and nitrification in shortgrass steppe soils

Abstract: The depth distributions of rates of net nitrogen mineralization and nitrification were measured in a series of field and laboratory incubations. Field studies suggested that the highest rates of mineralization and nitrification occurred in the surface 2.5 cm such that forty to sixty percent of the N mineralization in 20-cm soil column occurred in the surface 2.5cm. Some upward nitrate movement occurred but laboratory studies suggested that surface rates were not an artifact of nitrate mobility alone. Microclimatic data indicate that either dew or upward movement and condensation of soil water vapor may drive biological activity at the soil surface. High rates of N mineralization even in dry horizons were sustained as long as water was stored within the 0-to 20-cm depth. High rates of nitrification were found in all incubations, and field measurements showed NO 3 − to be the predominant form of inorganic N, despite previous characterization of the shortgrass steppe as an NH 4 + -dominated system.

Maximum axial and radial growth pressures of plant roots

Abstract: The axial root growth force exerted by seedlings of pea (Pisum sativum cv. Greenfeast), cotton (Gossypium hirsutum cv. Sicot 3) and sunflower (Helianthus annuus cv. Hysun) was measured. Effects of different seedling age and different batches of seeds on axial root growth pressure were investigated.

Effects of differentiated applications of fertilizer N on leaching losses and distribution of inorganic N in the soil

Abstract: Nitrogen fertilizer was applied to field plots at rates of 0, 50, 100, 150 and 200 N kg/ha yr, in order to determine the effects of differentiated N applications on drainage water and groundwater quality. Water samples, collected monthly or bimonthly from 1974 to 1983, were analysed for inorganic and total N content. In order to see the impact of residual N on leaching losses, soil samples were collected to a depth of 2 m in the N0, N100 and N200 plots, usually in September and April. Leaching of nitrate was moderate to the N100 level but increased substantially with increasing fertilization, up to 91 N kg/(ha-yr) for the highest application rate (N200), during the wet year of 1980/81. The losses were greatest during the fall, mainly due to high levels of N remaining in the soil after harvest combined with high precipitation. The N content of the groundwater did not show any significant correlation to the fertilization intensity. A buildup of inorganic N in the soil occurred only when excessive amounts of fertilizer were applied (N200), while the contents of the N0 and N100 treatments fluctuated around states of balance, approximately 45 and 70 N kg/ha respectively. Spring rape followed by winter wheat showed a great ability to reduce N contents in the tile effluent from highly fertilized plots (N150 and N200), even though the plots had received excessive amounts of fertilizer for several years. Results of this experiment in central Sweden demonstrate the importance of applying nitrogen fertilizer in balance with crop needs and of maintaining a growing crop cover as much of the time as possible in order to minimize water pollution.

Micronutrient uptake and distribution in mycorrhizal or phosphorus-fertilized soybeans

Abstract: Soybean plants were grown in a soil very low in available P. Seedlings were inoculated with two vesicular-arbuscular mycorrhizal (VAM) fungi or were left non-inoculated and fertilized with P. Assimilation and allocation of micronutrients (Fe, Mn, Zn, and Cu) were determined during host development, and the uptake of trace elements in VAM plants was compared to P-fertilized, non-VAM plants of similar weight, growth stage, and P status. Copper and zinc concentrations were always higher in VAM plants, while iron and manganese concentrations were lower than in the equivalent P-fertilized soybeans. Differences in the micronutrient content of fully-mature soybean pods reflected differences in the leaves and roots. Thus, for trace elements, seed quality can be altered by VAM colonization in a fashion not duplicated by P fertilizer.

A method for determining the cation-exchange capacity of organic materials

Abstract: A method for determining the cation-exchange capacity (CEC) of organic matter was developed by modifying the BaCl2 triethanolamine, pH 8.1, method used in calcareous soils. Problems arising from the presence of sulphates and losses of water soluble organic matter were solved. The proposed method for organic matter is comparable to that of soil in terms of sensitivity, reproducibility and in the time necessary for each analysis. In the second part of the work the CEC's of diverse organic samples were determined and found to be reproducible with an average coefficient of variation of 3.56%.

Estimation of biological nitrogen fixation associated with Brachiaria and Paspalum grasses using 15N labelled organic matter and fertilizer

Abstract: Six pasture grasses, Paspalum notatum cv batatais, P. notatum cv pensacola, Brachiaria radicans, B. ruziziensis, B. decumbens and B. humidicola, were grown in concrete cylinders (60 cm diameter) in the field for 31 months. The soil was amended with either a single addition of 15N labelled organic matter or frequent small (2 kg N. ha-1) additions of 15N enriched (NH4)2SO4. In the labelled fertilizer treatment soil analysis revealed that there was a very drastic change in 15N enrichment in plant-available nitrogen ( 3 - + NH 4 - with depth. The different grass cultivars recovered different quantities of applied labelled N, and evidence was obtained to suggest that the roots exploited the soil to different depths thus obtaining different 15N enrichments in soil derived N. This invalidated the application of the isotope dilution technique to estimate the contribution of nitrogen fixation to the grass cultivars in this treatment. In the labelled organic matter treatment the 15N label in the plant-available N declined at a decreasing rate during the experiment until in the last 12 months the decrease was only from 0.274 to 0.222 atom % excess. There was little change in 15N enrichment of available N with depth, hence it was concluded that although the grasses recovered different quantities of labelled N, they all obtained virtually the same 15N enrichment in soil derived N. Data from the final harvests of this treatment indicated that B. humidicola and B. decumbens obtained 30 and 40% respectively of their nitrogen from N2 fixation amounting to an input of 30 and 45 kg N.ha-1 year-1 respectively.

Verification of a mathematical model by simulating potassium uptake from soil

Abstract: This work develops the mathematical models suggested by various authors to simulate nutrient uptake of plants from soil. The simulation is based on ion transport from the soil to the roots by mass flow and diffusion and on Michaelis-Menten kinetics of nutrient uptake from soil solution by plant roots. For this purpose a differential equation is numerically integrated. Inter-root competition is allowed for by the choice of the boundary conditions. The integration procedure used makes it possible to take into account a variable buffer power which depends on soil solution concentration. The model calculates the change of nutrient concentrations in soil as a function of distance from the root surface for preestablished periods of time. Furthermore, the rate of uptake and the quantity of nutrients taken up per cm of root length is obtained. If the growth function of the root is known, nutrient uptake of a growing root system can be calculated. In order to verify the model two experiments were made: It is therefore concluded that the theoretical conception of the model is realistic and that the parameters have correctly been measured. The model thus appears to be useful to simulate such aspects of nutrient uptake of plants from soil which cannot be measured.

Phosphorus requirements of the wheat plant in various stages of its life cycle

Abstract: In pot experiments root growth and P uptake were found to precede shoot growth. The high rate of P uptake in the early stages of the life cycle is not an expression of luxury consumption but reflects a high P requirement in plants. Plants cultivated in nutrient solutions with different P concentrations during various stages of development showed that a high P supply (1 ppm) between Feekes stages 6 and 9 (30 days) caused a higher grain yield than the same P concentration between Feekes stages 11 and 17 (30 days). The early applied P caused a high number of fertile ears per area, a high number of grains per ear, and a high P pool in vegetative parts. The latter could be mobilized during the grain-filling period. Therefore, for high grain yields soil and fertilizers have to meet the high P requirement (about 20 μg P/m root · day) in an early stage of plant growth. During the grain filling period the P supply can be much lower.

Model experiments on the behaviour of roots at the interface between a tilled seed-bed and a compacted sub-soil. II: Entry of pea and wheat roots into sub-soil cracks

Abstract: SummaryRoots which grow down through a seed-bed and encounter a strong, untilled sub-soil beneath may be deflected horizontally. They will continue to grow horizontally along the top of the sub-soil either until the seed-bed dries out and the roots wilt and cease elongating, or until they find some path of low resistance down through the sub-soil. A major source of such paths is vertical cracks in the sub-soil.Model experiments were done with artificial impenetrable sub-soils containing parallel cracks with widths in the range 0.5–3 mm. Roots of pea and wheat were grown down through beds of aggregates to encounter the artificial sub-soil at random positions. The roots were deflected horizontally until they encountered the vertical cracks. The proportions of roots which entered the cracks were found to decrease strongly with decreasing crack width and increasing (more perpendicular) angle of incidence between the root and the crack.The experimental results were combined with the results from computer simulation studies which gave the proportions of roots encountering cracks at various angles with both hexagonal and parallel crack patterns. This showed that parallel crack patterns should enable a greater proportion of roots to enter the cracks than hexagonal crack patterns. Monocotyledonous plants which have several seminal root axes per plant have a statistical advantage over dicotyledonous plants which have only one seminal axis per plant with regard to crack entry.

role of trichomes in sunflower tolerance to manganese toxicity

Abstract: In comparison with other crop species, sunflower (Helianthus annuus L.) has been found to be very tolerant of high manganese (Mn) concentrations in nutrient solution. Furthermore, sunflower was able to accumulate high Mn concentrations in plant tops without apparent detrimental effect on growth. The first symptom of excess Mn supply (c. 30μM Mn in solution) was the appearance of small, dark-brown to black spots ( 2 mm in size) developed on the lower leaves, especially along the veins. A concentration of 2205 μg Mn g−1 in the tops was associated with a 10% reduction in plant dry matter yield.

Nutrient cycling and nitrogen mineralization in eucalypt forests of south-eastern Australia. I: Nutrient cycling and nitrogen turnover

Abstract: Summary Nutrient pools in litter and soil and the major nutrient transfers and additions in rainfall, throughfall and litterfali were measured in eight mature, undisturbed eucalypt forests covering a range of species, climate, productivity and soil type. Litterfall is the major pathway for the return of N, P, Ca and usually Mg, to the soil. The forests covered almost the range of litterfall reported for eucalypt forests and, in conjunction with published data, litterfall was strongly related to climatic variables. Extractable P in the soil and P concentrations in litter and litterfall were significantly higher in two sub-alpine forests (Eucalyptus pauciflora and E. delegatensis) than in all other forests. In general, nutrient turnover, particularly N turnover, was related to the rate of organic matter turnover. Rates of organic matter turnover in these forests and in other studies of eucalypts were correlated with climatic conditions using the simple climatic scalar developed by Vitousek. Nitrogen turnover, especially that proportion cycling via leaf lJtterfaU is primarily a function of organic matter turnover, but litter quality appears also to have an influence.

Plant-induced changes in the rhizosphere of maize and wheat

Abstract: The release of organic materials by roots of maize and wheat was studied using a growth chamber with a14CO2 atmosphere at constant total CO2 concentration and constant specific activity. The distribution of14C within shoots, roots and soil was determined for both plants after 4 and 6 weeks. After 6 weeks, 1.5% of the total amount of14C fixed by maize was found as a residue in the soil, while for wheat this figure was 2.0%.

Model experiments on the behaviour of roots at the interface between a tilled seed-bed and a compacted sub-soil

Abstract: When elongating seminal roots of developing plants reach the base of a tilled seedbed, they often encounter a layer of dense, strong untilled soil. At this interface, they may be deflected horizontally and instead of penetrating the sub-soil, they may form a horizontal mat of roots at the base of the seed-bed. If this occurs, the plants are unable to absorb the reserves of water in the sub-soil, and are very sensitive to short periods of drought.

Quantitative relationships for the dependence of growth rate of arable crops on their nitrogen content, dry weight and aerial environment

Abstract: Quantitative relationships for growth rate and its dependence on plant nitrogen concentration are developed from the results of experiments on potatoes, cereals and vegetables. The relationships appear to be of general applicability and most coefficients in them are similar for widely different crops.

Effects of NaCl stress on proline and cation accumulation in salt sensitive and tolerant turfgrasses

Abstract: Concentrations of proline, sodium and potassium in shoot tissues of five turfgrass species were measured following exposure to 170 mM NaCl salinity stress. Salt tolerant ‘Fults’ alkaligrass and ‘Dawson’ red fescue restricted the accumulation of Na-ions to significatnly low levels compared to the salt sensitive Kentucky bluegrasses (‘Adelphi’ and ‘Ram I’) and ‘Jamestown’ red fescue. Accumulation of proline began in all species within 24 h of initiation of salt stress but at a more rapid rate and higher overall concentration for ‘Fults’ alkaligrass. Proline levels were variable and too low in relation to sodium accumulations to have any significant osmoregulatory role in salt tolerance among all cultivars tested with the possible exception of alkaligrass.

Drought and shade interact to cause fine-root mortality in Douglas-fir seedlings

Abstract: Both desiccation and depleted carbohydrate reserves have been suggested as causes of fine-root (≤2 mm in diameter) mortality in trees. In this study, Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings were subjected to four combinations of shading and watering to determine whether shading increases drough-induced root mortality and, if so, whether this effect is due to reduced levels of carbohydrate reserves or increased susceptibility to desiccation. Two correlated measures of root mortality (counting root tips and weighing roots) showed that significantly more fine roots died only when seedlings were both shaded and unwatered. Concentrations of suberin, a compound synthesized by plant roots to control desiccation, were unaffected by any combination of shading and watering; however, carbohydrate reserves were nearly exhausted in the shaded and unwatered treatment — the treatment with highest root mortality. Water stress may have increased root mortality indirectly by increasing root temperature and maintenance respiration and by inhibiting photosynthate transport to the root system, but massive die-off in response to drought was apparent only when starch and sugar reserves were nearly depleted. Drought cannot be considered directly responsible for death of fine roots. Instead, a root's ability to continue to respire, which in turn depends on the status of its starch and sugar reserves, seems to be the primary physiological control of fine-root mortality.

Potassium availability in relation to soil moisture I. Effect of soil moisture on potassium diffusion, root growth and potassium uptake of onion plants

Abstract: SummaryThe objective of this research is to evaluate the influence of soil water content on- the mobility of potassium in soil,- plant growth and- K uptake of plants. The mobility of K increased with soil moisture. Increasing the volumetric water content (θ) from the 0.1 to 0.4 resulted in a rise of the effective diffusion coefficient (De) by a factor of about 10. This is mainly due to the increase of the tortuosity or impedance factor with higher soil moisture.In order to relate K mobility in soil to the availability of K for plant uptake, onion plants were grown in special containers under constant water content in the range of 0.1 to 0.4 cm3 H2O cm−3 of soil. Results are- both K content and growth of the plants increased with soil moisture,- water content below θ=0.1 reduced root growth- K inflow per unit of root surface increased with soil moisture. Maximum rate of inflow occurred with θ=0.25 in the soil used. It is therefore concluded that soil moisture affected K availability by affecting both K mobility and root growth.ZusammenfassungDie Arbeit hat das Ziel, den Einfluss des Wassergehaltes des Bodens auf- die Mobilität der Kaliumionen im Boden,- das Pflanzenwachstum und- die K-Aufnahme Zu bestimmen. Hierzu wurden einerseits Messungen der Mobilität von Kalium im Boden durch-geführt. Sie ergaben eine Erhöhung des effektiven Diffusionskoeffizienten (De) mit ansteigendem volumetrischen Wassergehalt (θ). De nahm um mehr als das Zehnfache zu während θ von 0,1 auf 0,4 anstieg. Dies ist der Erhöhung des Tortuositäts-oder Widerstands-faktors mit steigendem Wassergehalt zuzuschreiben. Um zu prüfen, in welchem Masse die Diffusionsbedingungen im Boden die Pflanzenverfügbarkeit von Kalium beeinflussen, wurde ein Vegetationsversuch durchgeführt. Hierzu wurden Zwiebelpflanzen in speziellen Versuchsgefässen bei konstanten Wassergehalten zwischen 0,1 und 0,4 cm3 H2O/cm3 Boden kultiviert. Die Ergebnisse sind:- K-Konzentration und Ertrag der Pflanzen wurden mit zunehmedem Bodenwassergehalt erhöht.- Der Wassergehalt des Bodens beeinflusste das Wurzelwachstum; unter θ=0,1 nahm die Wurzellänge stark ab.- Die K-Aufnahmerate eines Wurzelabschnitts stieg mit dem Wassergehalt an; bei θ=0,25 war die maximale Aufnahmerate in diesem Boden erreicht. Bie niedrigem Wassergehalt des Bodens wird die Kalium-Verfügbarkeit demnach beeinträchtigt sowohl durch den Rückgang der Mobilität von Kalium im Boden als auch die Verringerung des Wurzelwachstums.

Potassium availability in relation to soil moisture II. Calculations by means of a mathematical simulation model

Abstract: In order to study the influence of soil moisture on the availability of potassium a simulation model was used. The model is designed to describe the transport of a nutrient from the soil to plant roots and its distribution around a root. From a pot experiment, the measured K uptake of onion plants, grown in soil under different moisture levels, agreed satisfactorily with the calculated K uptake. The model is therefore regarded as a valid means of quantifying the dynamics of K in the soil around plant roots.