Modeling the uptake of sulfur by crops on two alluvial soils of Louisiana: Cotton

Abstract

The Cushman and Barber mechanistic model was used to help elucidate the mechanisms that govern the uptake of sulfur (S) by plants. Sulfur uptake predicted by the model was compared to measured S uptake by cotton, (Gossypium hirsutum [L]) grown on Mhoon silty clay loam (Typic Fluvaquent) and a Norwood silt loam (Typic Udifluvent) under glasshouse conditions. Predicted S uptake was significantly correlated with observed uptake r²=0.71. However, the model overpredicted S uptake by a factor of 3.5. The assumption that the maximun ion Influx rate (I _max) for roots growing in soils is the same as the I_max measured in solution cultures experiments may not be appropriate. Better agreement between observed and predicted S uptake values was achieved by using an estimated I_max obtained from soil studies (I_s) with the method of Warncke and Barber. Using I, predicted vs observed S uptake had a slope of 1.00 and r²=0.93. The model predicted that S concentration in soil solution (C_lo) at the root surface (r_o) increased with time for soils with initial S concentration in solution of lmM or higher. This high C_lo, will trigger a higher I_s of about 4 nmol/m²sec, while plants grown on soils with low S content will show a lower I_s, suggesting that S uptake by cotton plants is biphasic and depends on C_lo at r_o.     

Modeling the uptake of sulfur by crops on three alluvial soils of Louisiana: Wheat

Abstract

The Cushman and Barber mechanistic model was used to help elucidate the mechanisms that govern the uptake of sulfur (S) by plants. Sulfur uptake predicted by the model was compared to measured S uptake by wheat (Triticum aestivum [L]) grown on Gallion very fine sandy loam (Typic Hapludalf), Mhoon silty clay loam (Typic Fluvaquenf), and a Norwood silt loam (Typic Udifluvent) under glasshouse conditions. Predicted S uptake was significantly correlated with observed uptake r²=0.85. However, the model over predicted S uptake by a factor of 10.4. The assumption that the maximum ion Influx rate (I_max ) for roots growing in soils is the same as the I_max measured in solution cultures experiments may not be appropriate. Better agreement between observed and predicted S uptake values was achieved by using an estimated I_max obtained from soil studies (I_s ). Using I_s , predicted vs observed S uptake had a slope of 1.5 and r²=0.93. The model predicted that when S concentration in soil solution (C_lo ) at the root surface (r_o ) was about 2mM or higher, this high C_lo will trigger a higher ion Influx rate of about 2.6 nmol/m²sec. Plants grown on soils with C_lo less than 1 mM at r_o will show a lower ion Influx rate of about 0.8 nmol/m²sec, suggesting that S uptake by wheat plants is biphasic and depends on C_lo at r_o .     

Effect of the fraction of the root system supplied with p on p uptake and growth characteristics of wheat roots

An understanding of the phosphorus, P, uptake characteristics of plant roots is important for developing practices that improve P fertilizer efficiency. Phosphorus uptake by plant roots is influenced by plant root properties and solution P level. Since little information about the nutrient uptake characteristics of spring wheat (Triticum vulgare L.) roots is available, this research was undertaken with wheat to determine the relation between the proportion of the roots supplied with P on P influx and root growth characteristics. An experiment was conducted with wheat plants grown in solution culture in a controlled climate chamber.

Phosphorus uptake kinetics were measured on 30‐day‐old wheat using split‐root experiments. Supplying P to only part of the root system resulted in lower plant P concentration and higher I_max(maximum influx) by the roots. The I_max value of wheat roots was much lower than corn (Zea mays L.) and soybeans (Glycine max L.), but the values of Km (the solution P concentration where influx, I_n is 1/2 I_max) and C_min (the solution P concentration where influx, I_n is 1/2 I_max) were greater than those of both corn and soybean crops grown in similar experiments. Phosphorus concentrations in wheat plant's shoots and roots were higher than those for corn and soybean with the same proportions of roots in P solution. Decreasing the proportion of the roots supplied with P had no statistically significant (p = 0.05) effect on shoot dry weight. This differs from the results for corn and soybeans where it decreased significantly as the proportion of the roots exposed to P decreased. These results indicate that the effect of P placement on P uptake and on plant root growth varied among species.     

Potassium efficiency mechanisms of wheat,barley, and sugar beet grown on a K fixing soil under controlled conditions

Plant species differ in their potassium (K) efficiency, but the mechanisms are not clearly documented and understood. Therefore, K efficiency of spring wheat, spring barley, and sugar beet was studied under controlled conditions on a K fixing sandy clay loam. The effect of four K concentrations in soil solution ranging from low (5 and 20 μM K) to high (2.65 and 10 mM K) on plant growth and K uptake was investigated at 3 harvest dates (14, 21, and 31 days after sowing). The following parameters were determined: shoot dry matter (DM), K concentration in shoot dry matter, root length (RL), root length/shoot weight ratio (RSR), shoot growth rate/average root length ratio (GR_s/aRL), K influx, and soil solution K concentrations. Wheat proved to have a higher agronomic K efficiency than barley and sugar beet, indicated by a greater relative yield under K‐deficient conditions. As compared to both cereals, sugar beet was characterized by higher K concentrations in the shoot dry matter, only 30—50 % of the root length, 15—30 % of the RSR and a 3 to 6 times higher GR_s/aRL. This means that the shoot of sugar beet had a 3 to 6 times higher K demand per unit root length. Even at low K concentrations in the soil solution, sugar beet had a 7 to 10 times higher K influx than the cereals, indicating that sugar beet was more effective in removing low available soil K. Wheat and barley were characterized by slow shoot growth, low internal K requirement, i.e. high K utilization efficiency, and high RSR, resulting in a low K demand per unit root length. At low soil K concentrations, both cereals increased K influx with age, an indication of adaptation to K deficiency. The mechanism of this adaptation merits closer investigation. Model calculations were performed to estimate the K concentration difference between the bulk soil and the root surface (ΔC_L) needed to drive the measured K influx. For the two cereals, the calculated ΔC_L was smaller than the K concentration in the soil solution, but for sugar beet, ΔC_L was up to seven times higher. This indicates that sugar beet was able to mobilize K in the rhizosphere, but the mechanisms responsible for this mobilization remain to be studied.     

Growth and nutrient influx patterns during early pumpkin development

Pumpkin specie Cucurbita moschata cv ‘Libby‐Select’ was grown in nutrient solution in the greenhouse to characterize growth and ion uptake for the period up to 56 days. Pumpkin relative growth rate was highest between 26 and 32 days, just after vines began to run. Dry matter accumulation was highest between 38 and 50 days. Root growth in terms of dry weight and total length generally kept pace with shoot growth up until 26 days. Thereafter, root growth increased linearly but at a slower rate than shoot growth. Significant differences in influx (uptake rate per length of root) of P, K, Ca, and Mg occurred during the growth period. Influx rates were generally highest between 26 and 32 days. For each nutrient, the relative absorption rate exceeded plant growth rate. Ion influx parameters (I_max, K_m, C_O) were determined at 18, 28, 40, and 48 days from depletion measurements. For each nutrient, I_max and max C_O tended to decrease as plant age increased. K values were generally variable.     

Soil phosphorus availability to millet roots

Abstract

Soil solution P level is believed to be important in determining P uptake rates from soil. The objective of this research was to investigate the relation between initial P concentration in the soil solution and P flux into the root. Millet (Panicum milaceum) was grown on five soils each of which was adjusted to six C_li levels by addition of P. Millet was also grown in solution culture and P influx vs. P concentration in solution measured. There was a curvilinear relation between P influx and relative yield of the C_li levels on each soil (R²=0.74). A P influx of at least 16 pmoles cm^‐1 sec^‐1 was needed to obtain 90% of maximum yield. However, yield response was not correlated with C_li, indicating C_li was not a suitable indicator of P availability on these soils. Influx of P on soils with C_li less than 6 μM was greater than occurred at similar P concentrations in solution culture indicating P influx was increased by the effect of the root on the soil.     

Availability of phosphate and potassium as the result of interactions between root and soil in the rhizosphere

A number of findings are summarized in order to show the significance of individual plant properties and soil factors on the availability of phosphate and potassium to plants growing in soil. The flux of a nutrient into a given plant root depends directly on the concentration of the nutrient in the adjacent solution. In nutrient solution, P and K influx follows Michaelis-Menten kinetics. Almost maximum rates of influx have been observed in the range of soil solution concentrations usually found in German arable soils. Roots exhaust P and K from solutions to about 0.2 μmol P and 1 μmol K 1^?1 if not replenished. At the root surface P and K concentrations in soil decrease rapidly within one day; small changes occur after this period. Initially, the extent of the depletion zone is very small but it extends radially with time. After the initial phase therefore, P and K supply to the plant depends on transport from more remote parts of the soil and also on release from undissolved sources. The degree of depletion and the extent of the depletion zone are related to the diffusion coefficient; they decrease with increasing clay content of soil. Root hairs penetrate the soil and extend the volume of soil supplying nutrients to a unit of root. P and K influx therefore increase with the length of root hairs. Proton release of roots mobilize P and K in soil. This is clearly detected by the HCl-soluble P and K fractions within 2 mm of the root surface. The activity of acid and alkaline phosphatases strongly increase in the soil in the vicinity of the root surface of several plant species. It is supposed that organic P compounds can therefore be utilized by plants. P and K influx per unit of root length and root length per unit of shoot weight differed widely between species. The product of these two parameters however was closely related to the P and K concentration of the shoots. Calculations from a mathematical model were in good agreement with measured K depletion profiles and K uptake by plants. It is therefore concluded that the main factors influencing the P and K availability of plants growing in soil have been accounted for in the mathematical model and that the parameters have been accurately measured.     

Manganese uptake and Mn efficiency of wheat cultivars are related to Mn‐uptake kinetics and root growth

Wheat cultivars differ widely in manganese (Mn) efficiency. To investigate the reasons for different Mn efficiencies, a pot experiment with soil, a solution‐culture experiment, and model calculations were carried out. The pot experiment was conducted with wheat (Triticum aestivum L. cvs. PBW 373, PBW 154, PBW 343, PBW 138, and Triticum durum L. cvs. PBW 34 and PDW 233) grown in a screen house in India. The soil was a loamy sand with pH 8.1, DTPA‐extractable Mn 1.62 mg (kg soil)^–1, and initial soil solution Mn concentration (C_Li) of 0.19 μM. When fertilized with 50 mg Mn (kg soil)^–1, C_Li increased to 0.32 μM. At C_Li 0.19 μM, wheat cv. PBW 373 produced 74% of its maximum shoot dry weight (SDW) with 64% of its maximum root length (RL), while cv. PDW 233 produced only 25% of its maximum SDW with 11% of its maximum RL. The other wheat cultivars were between these extremes. Manganese deficiency caused a reduction in shoot growth, but more strongly reduced root growth. The low Mn efficiency of T. durum cv. PDW 233 was related to a strong depression of its root growth. Manganese influx was similar for all cultivars. In solution culture below 1 μM Mn, under controlled climate‐chamber conditions, Mn influx was linearly related to Mn concentration. Both the efficient cv. PBW 343 and the inefficient cv. PDW 233 had a similar influx. Uptake kinetic parameters from the solution experiment together with soil and plant parameters from the pot experiment were used in a mechanistic nutrient‐uptake model. Calculated values of Mn influx for wheat grown in soil were 55% to 74% of measured values. A sensitivity analysis showed that increasing C_Li or the slope of the uptake isotherm by about 30% would be enough to reach the observed influx. The results of this research indicate that an increase of Mn solubility by microbial or chemical mobilization would increase Mn uptake. But on the other hand, no chemical mobilization would be required to increase Mn uptake if the plant improved its uptake kinetics. Low Mn efficiency of some wheat cultivars was related to their reduced root growth at low soil Mn supply.     

Growth and uptake rates of P,K, Ca and Mg in wheat 1

The relation between plant age and nutrient absorption properties of red winter wheat (Triticum aestivum L.) roots were investigated. Understanding the change in ion uptake parameters with increasing plant age is helpful in devising efficient fertilization systems. Such information can be used to determine the nutrient levels needed in the soil to supply nutrients rapidly enough to the root surface to minimize deficiencies. Wheat was grown for periods up to 40 days in solution culture in a controlled climate chamber. Sequential harvest and nutrient influx measurements were made. Shoot growth was exponential with time to 30 days and linear thereafter. Root dry weight increased linearly with time at a slower rate than shoot dry weight. Root length increased linearily with time. With increasing plant age there was a reduction in average P and K uptake rate while average uptake rates for Ca and Mg remained relatively unchanged. With increasing plant age, the maximum influx, I_max. for P and Mg remained constant, but for K and Ca, there was a decrease. For the Michael is constant, Km, no change was observed for P, an increase occurred for K, and a decrease for Ca and Mg, as the wheat plant grew from 5 to 40 days.     

Differences in manganese efficiency of wheat (Triticum aestivum L.) and raya (Brassica juncea L.) as related to root‐shoot relations and manganese influx

Manganese efficiency is a term used to describe the ability of plants to obtain higher relative yields at low Mn supply compared to other species. To evaluate Mn efficiency of wheat (Triticum aestivum L.) and raya (Brassica juncea L.), a greenhouse pot experiment was conducted using Mn deficient Typic Ustochrept loamy sand soil, treated with 0, 50, and 100 mg Mn (kg soil)^–1. In the no‐Mn treatment, wheat had produced only 30 % of its maximum dry matter yield (DMY) with a shoot concentration of 10.8 mg Mn (kg DM)^–1 after 51 days of growth, while raya had produced 65 % of its maximum DMY with 13.0 mg Mn (kg DM)^–1. Taking relative shoot yield as a measure of Mn efficiency, raya was more efficient than wheat. Both crops produced the maximum DMY with 50 mg Mn (kg soil)^–1. Even though raya had a lower root length : DMY ratio and a higher shoot growth rate, it acquired higher Mn concentrations in the shoot than wheat under similar soil conditions, because of a 2.5 times higher Mn influx. Model calculations were used to calculate the difference of Mn solution concentration (ΔC_L) between the bulk soil (C_Li) and the root surface (C_L0) that is needed to drive the flux by diffusion equal to the measured influx. The results showed that ΔC_L was smaller than C_Li, which indicates that chemical mobilization of Mn was not needed to explain the observed Mn uptake even for raya. According to these calculations, the higher Mn influx of raya was caused by more efficient uptake kinetics, allowing for a 4.5 times higher Mn influx at the same Mn concentration at the root surface.     

Evaluation of differences in potassium availability in soils of the same exchangeable potassium level

Abstract

Potassium availability was determined by growing corn (Zea mays L.) in pots of soil in a controlled environment chamber. Comparison of K uptake rate per unit root area indicated differing degrees of K availability. The differences in availability among soils were not predicted by differences in cation exchange capacities. The K concentration in solution and the average rate of diffusion more nearly reflected differences in K availability, particularly when the kinetic rate factors of diffusion and mass flow were integrated with root growth and uptake factors in the Cushman mathematical model of K uptake. Utilization of this model demonstrated the importance of using rate factors for ion movement in soil along with plant factors of root growth in predicting K availability.     

Potassium and magnesium uptake by wheat and soybean roots as influenced by fertilizer rate

Abstract

Greenhouse‐pot experiments were conducted to compare wheat (Triticum aestivum L.) and soybean [Glycine max (L.) Merrill] in terms of their potassium (K) and magnesium (Mg) uptake. Previously, a field study indicated that various rates of K and Mg fertilization did not produce a significant wheat‐yield response. However, a yield increase with residual K and Mg was measured for the subsequent soybean crop. The 0 to 15 cm layer of Norfolk loamy fine sand (fine loamy, siliceous, thermic Typic Kandiudult) from two different sites was used for the pot experiments. Soil from both sites had a pH of 5.1. Potassium as potassium sulfate (K₂SO₄) was mixed into the soil from the K‐deficient site and Mg as magnesium sulfate (MgSO₄) was mixed into the soil from the Mg‐deficient site. ‘Florida 301’ wheat and ‘Cobb’ soybean were grown in winter and summer, respectively.

Soybean and wheat were similar in K uptake/g of roots on the first and second sampling dates. However, by the third sampling date, K uptake/g of wheat roots was about twice as high as for soybean. Potassium uptake/cm of soybean roots was two to five times that of wheat at each sampling date. Magnesium uptake/g of soybean roots was about four to five times as high as wheat on each sampling date. Similarly, Mg uptake/cm of soybean roots was 10 to 30 times higher than for wheat. Soybean showed higher total K and Mg content than wheat, suggesting that soybean has a higher demand for both K and Mg. The higher demand for K and Mg by soybeans than by wheat suggests that wheat could meet its demand for K and Mg at much lower soil levels than that for soybean. This would also explain a grain‐yield response to K and Mg by soybeans in the previously reported field study, despite a lack of yield response by wheat grown on the same site.     

Shoot cadmium concentration of soil‐grown plants as related to their root properties

Cadmium (Cd) is toxic to plants, animals, and humans. However, different plant species growing on the same soil may have very different shoot Cd concentrations depending on properties such as size of the root system, Cd net influx, shoot‐growth rate, Cd translocation from root to shoot, and the ability to affect Cd availability in the soil. To investigate possible reasons for different shoot Cd concentrations maize, sunflower, flax, and spinach were grown on an acid sandy soil (pH<$>_{{\rm{(CaCl}}_{\rm{2}} {\rm)}<$> 4.5, and C_org 2.8%) in a growth chamber with Cd additions as Cd(NO₃)₂ of none, 14, and 40 μmol (kg soil)^–1 resulting in Cd soil‐solution concentrations of 0.04, 0.68, and 2.5 μM. Only the high Cd addition caused a significant growth reduction of flax and spinach. The shoot Cd concentration was up to 30 times higher in spinach than in maize; the other species were intermediate. Of the plant properties studied only the variation of the Cd net influx explained the differences in shoot Cd concentrations. This was due to a decreased (maize, sunflower) or increased (flax) Cd concentration in soil solution or more effective uptake kinetics (spinach).     

Simulation of effects of soil bulk density and p addition on k uptake by soybeans

Abstract

Increasing soil bulk density has been shown to reduce root growth and decrease K uptake by soybeans (Glycine max L. Merrill). Changing soil bulk density also affects soil buffer power, b, and effective diffusion coefficient, De, which affect K influx. The relative decrease in K uptake due to reduced root growth as compared to reduced K influx is not known. Addition of P may affect root growth and P influx properties of plant roots. The objectives of this paper were (1) to use the Cushman mechanistic model to simulate the effect of changing soil bulk density and soil P on K uptake by soybeans, and (2) to determine the parameters that are changed by changes in bulk density and added P and their effect on K uptake. Plant and soil data of an experiment where Williams soybeans were grown for 21 days in pots of Raub (Aquic Argiudoll) silt loam with factorial treatments of two rates of K (0 and 100 mg K kg^‐1 soil), two rates of P (0 and 100 mg P kg^‐1 soil), and two bulk densities (1.25 and 1.45 g cm^‐3 ) were used to verify the model. Plant and soil parameters for the model were measured independently of the verification experiment. Predicted K (y) uptake agreed with observed uptake (x) (y = 1.09x‐0.19; r = 0.97) for the P x K factorial and (y = 1.19X‐0.22; r = 0.90) for the K x soil bulk density factorial treatments. In a sensitivity analysis, the model predicted a maximal K influx at a soil bulk density of 1.38 g cm^‐3. The greatest effect of soil bulk density on K uptake was due to reduction of root growth. Increased K uptake as a result of P addition was because of the effect on root growth.     

Kaliumdynamik im wurzelnahen Boden in Beziehung zur Kaliumaufnahme von Maispflanzen

Potassium dynamics at the soil-root interface in relation to the uptake of potassium by maize plants Young maize plants were grown in flat containers on a sandy and a silt loam soil after addition of ⁴³K as tracer. Changes of the K concentration in soil in the vicinity of the roots were determined by scanning the film density of autoradiographs. A distinct zone of K depletion in the soil adjacent to the root surface was observed, similar to those found earlier with phosphate and rubidium. The highest degree of depletion occured within a distance of 0.7 mm from the surface of the root cylinder which corresponds to the average length of root hairs of the cultivar used. The quantity of K released within 2.5 days per unit of this part of the soil exceeded the exchangeable K by a factor of two. In a radial direction the zone of maximum depletion was followed by a depletion profile which extended over 5 mm in the sandy and over 3 mm in the silt loam soil. The K concentration of the soil solution decreased to 2–3 μmoles K/l at the root surface. In order to determine the effect of depleting the K concentration by plant roots on the release of soil K, desorption studies were carried out in parallel. For this purpose the soil was successively extracted by solutions with cation concentrations corresponding to the soil solution, except for K. With this procedure a massive release of K from the soil was observed after the equilibrium concentration decreased to 2–3 μmoles K/l. It is concluded that

• – in one growing season only part of the soil volume of the rooted layer contributes potassium to the plant and, on the other hand
• – substantial part of the potassium absorbed by plants is derived from nonexchangeable soil K, even in short periods of time.

     

Does transport of water to roots limit water uptake of field crops?

The question was examined under which conditions the water extraction rate of plant roots in the field can be limited by water transport to the roots. For this purpose we used a numerical solution of the single root model. Scenario calculations were carried out in order to investigate the general model behaviour. A sensitivity analysis showed that initial volumetric water content and root length density are of greater importance than root diameter in determining the maximum water transport rate to the roots. Data from a field experiment were taken, describing root length density, volumetric water content and water uptake rates under oats (Avena saliva L.) and faba beans (Vicia faba L.) as model input parameters. With this data the model calculated the water content difference between the bulk soil and the root surface which is necessary to induce a water flow to the roots matching the observed water uptake rate. Root length densities below the grain legume crop faba beans are one order of magnitude lower compared to that of the cereal crop oats. The therefore higher specific water influx rates of faba beans roots resulted in a higher decrease in water content near the root surface. However, water uptake by faba beans was controlled by the water flow towards the roots probably only in deeper soil layers with very low root length density. For the given conditions no transport limitation of water uptake was calculated, when rooting densities were higher than about 0.1 cm.cm^?3.     

Phosphorus Influx and Root‐Shoot Relations as Indicators of Phosphorus Efficiency of Different Crops

Abstract

The large variation in phosphorus acquisition efficiency of different crops provides opportunities for screening crop species that perform well on low phosphorus (P) soil. To explain the differences in P efficiency of winter maize (Zea mays L.), wheat (Triticum aestivum L.), and chickpea (Cicer arietinum L.), a green house pot experiment was conducted by using P‐deficient Typic ustochrept loamy sand soil (0.5 M NaHCO₃‐extractable P 4.9 mg kg^?1, pH 7.5, and organic carbon 2.7 g kg^?1) treated with 0, 30, and 60 mg P kg^?1 soil. Under P deficiency conditions, winter maize produced 76% of its maximum shoot dry weight (SDW) with 0.2% P in shoot, whereas chickpea and wheat produced about 30% of their maximum SDW with more than 0.25% P in shoot. Root length (RL) of winter maize, wheat, and chickpea were 83, 48, and 19% of their maximum RL, respectively. Considering relative shoot yield as a measure of efficiency, winter maize was more P efficient than wheat and chickpea. Winter maize had lower RL/SDW ratio than that of wheat, but it was more P efficient because it could maintain 2.2 times higher P influx even under P deficiency conditions. In addition, winter maize had low internal P requirement and 3.3 times higher shoot demand (i.e., higher amount of shoot produced per cm of root per second). Even though chickpea had 1.2 times higher P influx than winter maize, it was less P efficient because of few roots (i.e., less RL per unit SDW). Nutrient uptake model (NST 3.0) calculations satisfactorily predicted P influxes by all the three crops under sufficient P supply conditions (C_Li 48 µM), and the calculated values of P influx were 81–99% of the measured values. However, in no‐P treatment (C_Li 3.9 µM), under prediction of measured P influx indicated the importance of root exudates and/or mycorrhizae that increase P solubility in the rhizosphere. Sensitivity analysis showed that in low P soils, the initial soil solution P concentration (C_Li) was the most sensitive factor controlling P influx in all the three crops.     

A sensitivity analysis for assessing the relevance of fine‐root turnover for P and K uptake

Plant fine roots are subject to continual turnover, i.e., old roots die during the plant life cycle and are quickly replaced by new roots. New roots grow partly into undepleted soil areas and can take up nutrients at a higher rate than old roots. This is one possible advantage of root turnover. It has been shown that root turnover of several plant species increases when P and/or K supply is limited, indicating an efficiency mechanism. The objective of this study was to assess the maximum benefit for nutrient uptake by root turnover and to determine which soil or plant properties influence this process. Based on a data set of field‐grown faba beans, a sensitivity analysis with a transport and uptake model was performed, i.e., several input parameters were systematically varied to assess their importance for nutrient uptake of a root system with and without fine‐root turnover. The calculations were based on the assumptions that all new roots grow into undepleted soil areas and that no inter‐root competition occurs. Model calculations indicated that a root system with a high but realistic turnover rate can take up twice the amount of P or K compared to a stable root system without any turnover. This benefit on uptake is higher at low concentrations of these nutrients in soil solution, low soil water content, or high maximum inflow. However, measured uptake under poor conditions of nutrient supply is often higher than calculated uptake, even when root turnover is taken into account. This indicates that root turnover might be an efficiency mechanism, but not the only one.     

Interrelationships between growth and nutrient uptake in alfalfa and corn

Nutrient requirements o£ plants during their various phases of growth are affected by several internal and external factors. The changes in rate of uptake by root with age are an important factor to meet the increasing plant demand for nutrients. Nutrient culture experiments were carried out under controlled greenhouse conditions with corn (Zea Mays L.) and alfalfa (Medicago sativa L.) to investigate the relationship of stage of growth to changes in plant parameters and nutrient uptake properties. With advancement of age. both plant species increased their ambient growth medium pH towards neutrality. With increasing age in alfalfa there was very little change in observed S:R ratio and root growth rate. On the other hand in corn plants the S:R ratio increased and growth rate for root and shoot decreased with age. Alfalfa contained higher concentrations of N, K, Na, and Ca than corn; while ion concentrations in both crops decreased with plant age. At all stages of growth, alfalfa absorbed less nutrients than corn. The rates of nutrient influx, I_n in both the crops showed various degrees of correlation with age and rate of shoot growth. In corn. I_n for ions reached a maximum at 25 days growth; whereas, in alfalfa, I_n reached maximum at 30 days of growth. The differences in influx rates for different ions in the two species are probably due to the difference in development of shoot and root parameters and shoot demand for the ions.     

Mechanistic Model for Predicting NO3-N Uptake by Plants and Its Verification

Some mechanistic models have been proposed to predict the No3^- concentrations in the soil solution at root surface and the NO3-N uptake by plants,but all these relatively effective non-steady state models have not yet been verified by and soil culture experiment.In the present study,a mathematical model based on the nutrient transport to the roots,root length and root uptake kinetics as well as taking account of the inter-root competition was used for calculation,and soil culture experiments with rice,wheat and rape plants grown on alkali,neutral and acid soils in rhizoboxes with nylon screen as a isolator were carried out to evaluate the prediction ability of the model through comparing the measured NO3-concentrations at root surface and N uptake with the calculated values.Whether the inter-root competition for nutrients was accounted for in the model was of less importance to the calculated N uptake but could induce significant changes in the relative concentrations of NO3^- at root surface.For the three soils and crops,the measured NO3-N uptake agreed well with the calculated one,and the calculated relative concentrations at root surface were approximate to the measured values.But an appropriate rectification for some conditions is necessary when the plant uptake parameter obtained in solution culture experiment is applied to soil culture.In contrast with the present non-steady state model,the predicted relative concentrations,which show an accumulation,by the Phillips‘ steady-state model were distinct from the measured values which show a depletion,indicating that the present model has a better prediction ability than the steady-state model.