Assessment of the availability of ions to plant roots from the determination of their chemical activity in nutrient solutions

In soilless culture, the chemical composition of the nutrient solutions is usually expressed in terms of ion concentrations which do not reflect their actual availability to plant roots : even in aqueous media, the ions supplied as salts can give rise to complexes ; furthermore, other kinds of ion interactions can be involved.

The VEGACT model, developed from thermodynamic concepts used in geochemistry, affords a more reliable assessment of the energy level of the ions. Using a particular example, at equal concentrations, the roots are shown to take up potassium or nitrate ions more readily than calcium or phosphate ions. This model also affords the prediction of the possible occurrence of phosphate precipitation within nutrient solutions of a given composition.     

Elemental bioavailability in nutrient solutions in relation to precipitation reactions

In hydroponic plant nutritional research, nutrient solutions can be considered as aqueous solutions of inorganic ions. In this aqueous solution, the ions are submitted to the laws of aquatic inorganic chemistry. This means that the ions are involved in the dynamic equilibria between complexation, dissociation, and precipitation reactions. These chemical reactions seriously impact elemental speciation and bioavailability. As a result, plant roots experience a different nutritional composition. Ions withdrawn from the‐nutrient solution due to precipitation reactions, change the nutritional composition and are not available for uptake by plant roots. Like complexes, precipitates can buffer a nutrient solution, exchanging nutrients as these decrease by plant uptake. This research looks into the precipitation reactions that occur in hydroponic nutrient solutions. In the concentration range of nutrient solutions, no precipitates are formed involving potassium (K⁺), nitrate (NO₃ ^‐), ammonium (NH₄ ⁺), or sulphate (SO₄ ^2‐), while calcium (Ca²⁺) and magnesium (Mg²⁺) form mainly at a higher pH precipitates with hydrogen phosphate (HPO₄ ^2‐). Preparing nutrient solutions with tap water, calcium carbonate (CaCO₃) is likely to precipitate. A good knowledge of the chemical reactions occurring in nutrient solutions is the first prerequisite in hydroponic plant nutritional research.     

Ceramic Aggregate Sorption and Desorption Chemistry: Implications for Use as a Component of Soilless Media

Ceramic aggregates (Turface® and Profile®) are common soilless media components, but their sorption/desorption chemistry is poorly understood. We investigated: labile (readily desorb-able or readily plant-available) ion concentrations; the effect of rinsing and soaking pretreatments on labile ions; sorption of applied nutrients; and nutrient uptake from the aggregates by plants. Variability in labile ions was extremely high among bags of aggregates. Manganese, boron, magnesium, calcium, sulfur and potassium were most likely to desorb in excess for plants. Phosphorus, iron, copper and zinc were sorbed by the aggregates; only copper was found nearly deficient in plant tissue. Rinsing and soaking pretreatments adjusted labile ions to more suitable concentrations for plants. However, growth data suggested a worst-case scenario of high levels of labile ions may not be mitigated by these pretreatments. With frequent leaching after planting or where the aggregates are a minor component of media, excessive nutrient uptake would likely be limited.     

A comparison of three methods of determining the cation exchange capacity of calcareous soils

Three widely used methods for cation exchange capacity determination, viz. sodium acetate, Bascomb's and ⁴⁵Ca isotopic dilution methods have been compared. Sodium acetate and Bascomb's methods gave similar values for most of the samples. Though some correction for the organic matter and calcium carbonate content are of concern for the ⁴⁵Ca method, the results do not preclude use of the latter for the cation exchange capacity determination.In the ⁴⁵Ca method organic matter contributed mostly to the cation exchange capacity, whereas for the other two methods the relative contribution of clay was found to be higher. The lowest contribution of calcium carbonate to the cation exchange capacity was observed in Bascomb's method. Simple and multiple regression relationships between cation exchange capacity and organic matter, clay and calcium carbonate are discussed.     

KINETICS OF ION EXCHANGE IN SOIL ORGANIC MATTER

The rate of ion exchange of humic substances (humic acid and peat in the hydrogen form) with lead ions was investigated. As in processes occurring in nature, the lead solution was added in all our experiments slowly and continuously at a constant feed rate to the well-stirred water-suspended humic substances. Control experiments were also performed with a well-defined synthetic ion- exchange resin (carboxylic acid type). The uptake of divalent lead ions by the humic substance and by the ion-exchange resin, which is coupled with the release of an equivalent amount of hydrogen ions, proceeds as a function of time according to a characteristic sigmoidal curve. During the initial phase, the rate of ion exchange increases continuously with time, even though the humic substances become more saturated with lead ions. Theoretical calculations, based on a film diffusion process as the rate-determining step, show that this behaviour should indeed be observed if any solution, containing exchangeable counter ions, is added continuously to an ion exchanger suspended in a solvent. The theory predicts further, in agreement with the experimental results, that in the initial phase of the ion-exchange reaction, the uptake of the counter ions is proportional to the square of the time. If experiments are performed with different rates of addition of the counter ions, the time required to exchange a certain amount of counter ions should be proportional to α^-1/2, where α is the rate at which the ions are added. This prediction is also in accordance with the experimental results.     

Determining the Moisture and Plant Effect on Nutrient Release,and Plant Nutrient Uptake Using Ion Exchange Resin Membrane

Traditional soil testing has a limited predictability about available nutrients for plant uptake. Potential of ion exchange resin membrane (RM) or plant root simulator probe is evaluated to determine the effect of moisture on nutrient availability and uptake by corn (Zea mays L.), under greenhouse condition. Available nutrient concentrations measured by RM in two soil series at three soil moisture levels (40%, 60%, and 80% of field capacity) with (W) and without (W/O) the plant at V3 and V7 stages were compared with plant nutrient content at the V7 stage. Soil moisture did not influence RM-extracted nutrient concentrations (except for N at V3). Concentrations of nitrogen (N), phosphorus (P), potassium (K), sulfur (S), and iron (Fe) from RM at the V3 stage significantly correlated with shoot uptake. The presence of plant (W- vs. W/O-plant) significantly influenced RM-nutrient concentration at both stages. RM can predict crop nutrient requirements.

Abbreviations: Ion exchange resin membrane (RM); nitrogen (N); phosphorus (P); potassium (K), sulfur (S), field capacity (FC)     

Direct determination of potassium-calcium activity ratio in soils with two ion-selective electrodes. II. Interactions of potassium and calcium ions with soils

The interactions of potassium and calcium ions with two variable charge soils and one constant charge soil were examined by measuring the pK-0.5pCa value with two ion-selective electrodes, together with measurements of single ion activities. The constant charge soil showed a much higher preference for calcium relative to potassium than the variable charge soils. The relative affinities of potassium, calcium, sodium and aluminum ions with exchange sites were not constant, but were a function of surface properties of the soil. pH affected the pK-0.5pCa value in both the variable charge soil and the constant charge soil. The strong competition of aluminum ions for exchange sites was the major cause of the higher preference of soils for calcium ions relative to potassium ions at low pH. The pK-0.5pCa value was always smaller in sulfate systems than in chloride systems, with the difference more distinct in variable charge soils. In addition to an initial fast reaction, some potassium ions were immobilized at later times, even for the highly weathered variable charge soil.     

Specific ion effect on the accumulation of abscisic acid in wheat

Studies were carried out in the glass house using sand culture to find out the specific ion effect on the accumulation of abscisic acid in wheat. Seeds were sown in sixteen different solutions of chloride, sulphate, nitrate, carbonate salts of sodium, potassium, calcium and magnesium used each at ‐2 bar osmotic potential. The leaf samples taken after 25 days of sowing indicated that osmotic as well as salt stress increased significantly the accumulation of abscisic acid. The accumulation of abscisic acid was significantly less under potassium and calcium salt ions as compared to that under sodium and magnesium salts ions. Overall effect of anions showed that accumulation of abscisic acid was more under PEG (osmotic stress and chloride salts but it was less under sulphate and nitrate salts and was the least under carbonate salts. The accumulation of abscisic acid was exceptionally high under potassium dihydrogen phosphate. In general, the accumulation of abscisic acid‐a growth inhibitor varied not only with the type of the cation but also depended on the accompanying anion indicating differential growth response of plants under different types of saline conditions having dominance of different salt ion(s).     

Extraction of metal ions sorbed on Mn(IV) oxides and CaCO3 using ion exchange resins

The ability of ion exchange resins to promote release of metal ions (Cu, Ph, Cd or Zn) presorbed on different structural forms of Mn(IV) oxide or CaCO₃ has been investigated using exchangers possessing different types of functional groups and charged in either the H⁺ or Na⁺ form. The exchanger uptake reflected the degree of soluble salt or labile complex displaced from the substrate surface, and its magnitude varied with metal ion involved, exchanger type and initial counter ion (i.e. H⁺ forms retrieved more than Na⁺ forms). With Mn(IV) oxides substrate structure was important, with CaCO₃ a major factor was dissolution of the matrix. The amount of calcite dissolved was controlled by the number of exchange sites introduced, and exchanger type. The `labile metal' levels determined by the exchanger technique did not directly correlate with bonding fraction category values obtained using a series of chemical extractant solutions.     

Selectivity of Ion Exchange as a Sign of Soil Quality

A hypothesis is tested that allows study of not only the quantity but also the quality of ion exchange in soils, composts, peats, and its mixtures, which is common in natural and synthetic ion exchangers, on the basis of the ion-exchange process and its selectivity. For practical application of ion exchange in the soil not only the quantity of base cations and hydroxon ions bound by the solid soil phase is significant but also their readiness to ion exchange or ion exchange elasticity. Selectivity of ion exchange, expressed by selectivity coefficient, reflects the readiness of ion exchange while ion-exchange thermodynamics show the degree of spontaneity of the ion-exchange process. The results document that soil humus is the highest-quality ion exchanger in soil and that its quality may be assessed by the proposed method. The effort to replace humus by any synthetic ion exchangers cannot be successful: Although they have a high value of cation exchange capacity, they also show high selectivity for ions of higher valence and low ion-exchange elasticity, and their desorption of calcium and magnesium is poor.     

Amending Chemical Fertilizers with Rice Straw Compost and Clinoptilolite Zeolite and Their Effects on Nitrogen Use Efficiency and Fresh Cob Yield of Zea mays L.

A field study was carried out from April 2014 to August 2014 for two consecutive planting cycles of Zea mays L. on Nyalau Series (Typic Tualemkuts) to determine the short-term effects of co-application of chemical fertilizers, rice straw compost, and clinoptilolite zeolite on (i) ammonium adsorption and desorption, (ii) nitrogen, phosphorus, and potassium uptake and use efficiency, and (iii) yield of Zea mays L. (cobs). Amending urea with rice straw compost and clinoptilolite zeolite improved nitrogen use efficiency because of temporary adsorption and desorption of ammonium on the exchange sites of compost and clinoptilolite zeolite. Combined use of chemical fertilizers, rice straw compost, and clinoptilolite zeolite enhanced uptake of nitrogen, phosphorus, potassium, calcium, and magnesium. Co-application of chemical fertilizers, rice straw compost, and clinoptilolite zeolite can improve the availability of soil nutrients. This approach can also improve nutrient use efficiency and yield of Zea mays L.     

Effects of Sulfur Dioxide and Ozone on Ion Uptake of Spruce (Picea abies (L.) Karst.) Seedlings

Shoots of spruce (Picea abies (L.) Karst.) seedlings grown in solution culture were fumigated with sulfur dioxide or ozone, or with a combination of both for 60 days. Dry weights of the shoots exposed to O₃ and SO₂/O₃ were increased over the controls. Uptake rates of calcium, magnesium, potassium and nitrate from the nutrient solutions were altered in dependence on fumigation, time of exposure and stage of plant development. Uptake rates of all ions by the seedlings fumigated with SO₂ first increased, but later on decreased as compared to the control. Fumigation with O₃, or O₃ in combination with SO₂ affected uptake of the various nutrients differently. The effects of the pollutants on the concentrations of calcium and magnesium in the various parts of the seedlings indicate that the changes are not only due to altered water and ion uptake but may be related to various effects in ion transport and plant metabolism.     

A KINETIC APPROACH TO THE DESCRIPTION OF SOIL PHOSPHATE STATUS

A kinetic factor is used to describe soil phosphate status. It is supplementary to capacity and intensity factors which are essentially static factors. The rate of release of phosphate ions from the soil solid phase was measured by the use of an anion exchange resin and a numerical parameter assigned to the release rate. A highly significant correlation was found between this parameter and the uptake of phosphate by ryegrass from a wide range of soils.     

离子交换树脂膜原位提取土壤有效性养分的探讨

连续3年在实验室内和利用盆栽及田间进行了利用离子交换树脂膜对土壤的原位测定,探讨影响树脂膜对土壤中养分离子吸收的因素;制定出一套包括树脂膜埋置方法,时间,洗脱土粒和解吸被树脂膜吸附的养分等的标准方法,称为植物根模拟法(Plant root simulator)。 通过400多个土壤样品测定结果表明,用树脂膜提取的土壤有效氮、磷、钾和硫与用常规化学法提出的量有显著相关。盆栽条件下,树脂膜原位埋置法对预测供试作物油菜吸取土壤有效养分的能力与常规化学法相当,而硫的效果更好。树脂膜在田间直接埋置能合理地表征土壤中硝态氮的分布,并具有同时提取土壤各种有效养分的能力。由于离子树脂交换膜不受地域影响,具有广泛的通用性,因而在土壤养分有效性的研究中应用前景广阔。     

Effect of calcium/potassium ratio ammonium supply on nutrition and yields of cucumber plants

A greenhouse experiment was conducted in order to study the influence of two calcium/potassium (Ca/K) ratios (0.75 and 0.33) and ammonium supply (0 and 1 mmol/L) in the nutrient solution on nutrient uptake, mineral composition, and productivity of cucumber plants grown in sand culture. There were not significant differences in nitrate consumption between the four treatments. Calcium and potassium consumptions were directly related with the Ca/K ratio in the nutrient solution. The treatment with Ca/K = 0.75 and ammonium supply, that showed the lowest potassium and nitrogen plant levels and the highest calcium uptake and concentration in plant, offered the highest yields.     

Effects of ionic strength and nature of the cation on the desorption of fluoride from soil

The reaction between soil and added fluoride was accelerated by incubating at a high temperature. Desorption of the fluoride was then studied using solutions of chloride salts of several cations at a range of solution : soil ratios and for periods which ranged from 1 h to 4 days. Fluoride desorbed was related to the experimental variables by a regression equation. When the solution : soil ratio was small and hence only small amounts of fluoride were desorbed, decreasing the concentration of salts increased the concentration of fluoride in the solution. The concentration in the solution was lower for calcium chloride than for sodium or potassium chloride. Amongst the monovalent cations, the concentration of fluoride was highest for salts of the cations of lowest atomic number. Thus the greater the average distance between the charge conveyed to the surface by the adsorbed fluoride and the cation which balanced it, the higher the fluoride concentration in the solution. As the solution : soil ratio was increased, the differences between the cations in their effects on fluoride desorption decreased and seemed to disappear as the solution : soil ratio became very large. This contrasts with previously observed effects on phosphate. It is suggested that the difference may have arisen because appreciable desorption of fluoride occurs by exchange with hydroxyl ions rather than by escape of the fluoride ion together with its counter ion.     

Nutrient solution monitoring in greenhouse cultivation employing a potentiometric electronic tongue

This work investigates the use of electronic tongues for monitoring nutrient solution compositions in closed soilless systems. This is a horticultural technique in which the nutrient solution is continuously recirculated and an automatic recomposition system maintains the concentration of the different ions in the optimum range for the plants. Electronic tongues used in this study comprised an array of potentiometric sensors and complex data processing by artificial neural networks. A first experiment was able to carry out the simultaneous inline monitoring of ammonium, potassium, sodium, chloride, and nitrate ions during the winter. In the second and third applications, done during summer, some changes were introduced in the sensor array to improve its response toward chloride ions and to incorporate phosphate in the model. This electronic tongue was validated with real greenhouse samples and was also able to detect the variations in the ion concentrations caused by an incorrect configuration of the recomposition system.     

秸秆钾替代化肥钾对莴笋的营养效应研究

在重庆地区多年种植蔬菜的两个基地进行田间试验,研究了秸秆钾替代化肥钾对莴笋的产量、品质、养分吸收量和钾素利用率的影响。秸秆钾替代化肥钾较无钾肥处理极显著提高两个试验点莴笋产量,增产11.4%~13.7%和11.4%~15.3%,各处理产量大小为:MH+KL(有机肥钾50%+化肥钾50%)MM+KM(有机肥钾30%+化肥钾70%)ML+KH(有机肥钾10%+化肥钾90%)CK2(常规施钾处理)CK1(无钾肥处理),以高量(50%)秸秆钾处理(MH+KL)增产作用最大。秸秆钾替代化肥钾较常规施钾提高莴笋产量,但增产作用不显著。两个试验点秸秆钾替代化肥钾显著降低莴笋叶维生素C和可溶性糖含量,降幅分别为4.7%~16.8%和3.2%~30.5%,但对莴笋茎该两个品质指标略有提高;莴笋叶硝酸盐含量增加8.0%~8.2%和11.4%~17.2%,莴笋茎硝酸盐含量在试验点1降低2.5%~11.5%,而试验点2则增加8.0%~13.5%。试验点1莴笋叶钙、镁、铁和锰含量为提高,试验点2降低。两个试验点秸秆钾替代化肥钾均显著提高莴笋对氮、磷和钾吸收量,以MH+KL处理莴笋钾素利用率最高。综合莴笋产量、品质、养分吸收和钾素利用效率,以MH+KL处理效果最佳。     

Chemical properties of upland peats influencing the retention of phosphate and potassium ions

Peat samples representing depths of 0–15 cm and 15–30 cm were obtained from 11 upland sites in southern Scotland. Their pH, ash content, cation exchange capacity, phosphate sorption index and contents of 0.1 m hydrochloric acid-extractable iron and aluminium were determined as indicators of the extent to which phosphate and potassium ions would be retained following afforestation and application of fertilizers to these sites. In all soils cation exchange capacities were considerably larger than the quantity of potassium normally applied as fertilizer. Phosphate sorption indices were closely related to the content of extractable iron and aluminium in the peat samples. When expressed on a volume basis, the indices were low compared with those published for mineral soils, and some peats appeared to have almost no capacity to sorb phosphate ions from solution.     

A QUANTITATIVE ANALYSIS OF THE FACTORS AFFECTING PLANT NUTRIENT UPTAKE FROM SOME SOILS

A model of solute uptake by a growing root system is discussed in relation to the ability of a soil to support the nutritional requirements of plants. It is quantitative, and should apply to the absorption of nitrate, phosphate, and potassium by grasses and arable crops. The principles, if not the detail, are relevant to all soils. The model was tested in an experiment in which the nitrogen and potassium taken up by a rape plant were measured. The results suggest that the model has identified the significant variables in the absorption process. The principal factors affecting the supply of nutrient to a given plant are the total quantity of diffusible nutrient, the rate at which the nutrient can move, and the distance it has to travel to a root surface. The exact relevance of each factor for the different nutrients is readily determined. The diffusion coefficients of potassium and phosphate are often low. Mass flow contributes little to their supply, and an adequate exploitation of the soil reserves depends on a well-developed root system. Nitrate, on the other hand, can usually move easily to roots, either by diffusion or mass flow. The whole rooting volume is depleted fairly quickly, and the amount absorbed depends solely on the quantity present in the rooting volume. These ideas have practical significance. In particular, they suggest that the soil properties which influence root growth may deserve as much attention as soil chemical composition in the drawing up of fertilizer recommendations.