What limits nitrate uptake from soil?

Abstract. An accepted view, that unless nitrate concentrations in the soil solution are very low (e.g. below 0.1–0.2 mol m^?3) the growth of high-yielding crops is not limited by the availability of nitrogen, is challenged. Conventional analyses of nutrient supply and demand, based on calculations of apparent inflow rates (uptake rates per unit total root length) are invalid. Apparent inflow rates are inversely proportional to root length. The convention of using total root length grossly overestimates the fraction of the root system active in nutrient uptake. Consequently, inflow rates based on total root lengths underestimate the true values, indicating unrealistically low nutrient concentration differentials between bulk soil and root surfaces required to drive uptake. An alternative method of analysis is suggested. This is based on total nutrient uptake rather than on inflow rate. Measurements of the former do not depend on estimates of active root length and can be made directly and reliably. The method was applied to data obtained from a pot experiment using spring wheat (Triticum aestivum L., cv. Wembley) grown in soil without nitrogen fertilizer (N₀) or with nitrogen fertilizer equivalent to 200kg N ha^?1 (N⁺). Soil nitrate concentrations calculated using the conventional method based on total root length, suggested that any increases in concentration above those measured in the N⁰ treatment should not have resulted in increased uptake and growth. However, the N⁺ plants were always bigger than those in the No treatment, refuting this suggestion. Theoretical uptakes of nitrogen (calculated initially on the basis of a fully active root system) were adjusted, by reducing the effective root length incrementally, until the theoretical uptake matched the measured net uptake of nitrogen. The mean fractions of the root systems likely to have been involved in nitrate uptake were 11% and 3.5% of the total lengths of root in the N⁰ and N⁺ treatments, respectively.     

Modelling zinc uptake by rice crop using a Barber-Cushman approach

The Barber-Cushman mechanistic nutrient uptake model which has been utilized extensively to describe and predict nutrient uptake by crop plants at different stages of crop growth was evaluated for its ability to predict the Zn uptake by rice seedlings. Uptake of the nutrient is, therefore, determined by the rate of nutrient supply to the root surface by mass flow and diffusion. Inter root competition and time dependent root density are accounted for by soil volume that delivers nutrients. The radii of these cylinders decline with increasing density. Since mass flow and diffusion each supply zinc to the root, the process can be described mathematically using the model of Barber-Cushman (1984). The 11 parameters of the model for the uptake by rice cultivars were measured by established experimental techniques. Zinc uptake at different growth stages predicted by the model was compared to measured zinc uptake by rice cultivars grown on sandy loam soil in a green house. Predicted zinc uptake was significantly correlated with observed uptake r ²=0.99^**. Sensitivity analysis was also used to investigate the impact of changes in soil nutrient supply, root morphological and root uptake kinetic parameters on simulated nutrient uptake. Overall results of sensitivity analysis indicate that the half distance between root axes, rate of root growth and water flux affect the uptake of zinc particularly at their higher values rather than at lower values and DaZn is the most sensitive parameter for zinc uptake at its lower values.     

长期施肥对水稻生长和抗虫性的影响: 解析土壤生物的贡献

合理施肥对保障土壤质量和粮食安全具有重要作用。有机肥促进土壤生物群落的发展已被认为是其优于化肥的重要方面, 然而有机肥影响下的土壤生物群落对作物生长的贡献却了解甚少。了解土壤生物因素对作物抗虫性的贡献不仅可以揭示施肥影响土壤功能的生物调控机制, 而且有助于制定土壤-作物的综合管理措施。本研究采集长期施用有机肥和化肥的水稻土, 通过制备灭活与否的土壤悬液, 在砂培条件下探究土壤生物群落对水稻生长及其抗虫性的影响。结果显示, 土壤生物群落和施肥措施均极显著地影响了土壤养分含量(P < 0.01)。土壤生物的存在降低了土壤铵态氮含量、水稻生物量、茎叶全氮含量以及褐飞虱(Nilaparvata lugens)生物量; 增加了土壤硝态氮含量、水稻的根冠比及水稻根系全氮、可溶性糖以及酚类含量(P < 0.05); 同时, 有机肥处理的土壤生物群落还能够促进水稻茎叶可溶性糖和酚类的合成。接入褐飞虱后, 土壤生物群落的存在显著降低了水稻整体的全氮含量, 促进了酚类的合成(P < 0.05)。研究结果表明, 土壤生物群落, 尤其是有机肥处理的土壤生物群落, 主要通过改变水稻养分向地下部的分配格局、增加根冠比、促进防御性代谢物质(如酚类)的合成来提高水稻地上部对害虫的 抗性。     

Complementarity in root architecture for nutrient uptake in ancient maize/bean and maize/bean/squash polycultures

Background and Aims

During their domestication, maize, bean and squash evolved in polycultures grown by small-scale farmers in the Americas. Polycultures often overyield on low-fertility soils, which are a primary production constraint in low-input agriculture. We hypothesized that root architectural differences among these crops causes niche complementarity and thereby greater nutrient acquisition than corresponding monocultures.

Methods

A functional–structural plant model, SimRoot, was used to simulate the first 40 d of growth of these crops in monoculture and polyculture and to determine the effects of root competition on nutrient uptake and biomass production of each plant on low-nitrogen, -phosphorus and -potassium soils.

Key Results

Squash, the earliest domesticated crop, was most sensitive to low soil fertility, while bean, the most recently domesticated crop, was least sensitive to low soil fertility. Nitrate uptake and biomass production were up to 7 % greater in the polycultures than in the monocultures, but only when root architecture was taken into account. Enhanced nitrogen capture in polycultures was independent of nitrogen fixation by bean. Root competition had negligible effects on phosphorus or potassium uptake or biomass production.

Conclusions

We conclude that spatial niche differentiation caused by differences in root architecture allows polycultures to overyield when plants are competing for mobile soil resources. However, direct competition for immobile resources might be negligible in agricultural systems. Interspecies root spacing may also be too large to allow maize to benefit from root exudates of bean or squash. Above-ground competition for light, however, may have strong feedbacks on root foraging for immobile nutrients, which may increase cereal growth more than it will decrease the growth of the other crops. We note that the order of domestication of crops correlates with increasing nutrient efficiency, rather than production potential.     

Modelling potassium uptake by wheat (Triticum aestivum) crops

Spring wheat was grown in the field under deficient and sufficient levels of soil K and with high and low supplies of fertiliser nitrogen. Measurements were made of K uptake, soil nutrient supply parameters, root growth and, in solution culture, root influx parameters. Mechanistic models predicted uptake reasonably well under K-deficient conditions, but over-predicted uptake, by as much as 4 times, under K-sufficient conditions. The over-prediction was apparently due to poor characterisation of plant demand.     

Simulation of nutrient uptake by a growing root system considering increasing root density and inter-root competition

A simulation model is presented which describes uptake of a growth limiting nutrient from soil by a growing root system. The root surface is supposed to behave like a zero-sink. Uptake of the nutrient is therefore determined by the rate of nutrient supply to the root surface by mass flow and diffusion. Inter-root competition and time dependent root density are accounted for by assigning to each root a finite cylindrical soil volume that delivers nutrients. The radius of these cylinders declines with increasing root density. Experiments with rape plants grown on quartz sand were used to evaluate the model. Simulated nitrogen uptake agreed well with observed uptake under nitrogen limiting conditions. In case no nitrogen limitation occurred nitrogen uptake was overestimated by the model, probably because the roots did not behave like a zero-sink any more.     

On the assessment of root and soil respiration for soils of different textures: interactions with soil moisture contents and soil CO2 concentrations

Estimates of root and soil respiration are becoming increasingly important in agricultural and ecological research, but there is little understanding how soil texture and water content may affect these estimates. We examined the effects of soil texture on (i) estimated rates of root and soil respiration and (ii) soil CO₂ concentrations, during cycles of soil wetting and drying in the citrus rootstock, Volkamer lemon (Citrus volkameriana Tan. and Pasq.). Plants were grown in soil columns filled with three different soil mixtures varying in their sand, silt and clay content. Root and soil respiration rates, soil water content, plant water uptake and soil CO₂ concentrations were measured and dynamic relationships among these variables were developed for each soil texture treatment. We found that although the different soil textures differed in their plant-soil water relations characteristics, plant growth was only slightly affected. Root and soil respiration rates were similar under most soil moisture conditions for soils varying widely in percentages of sand, silt and clay. Only following irrigation did CO₂ efflux from the soil surface vary among soils. That is, efflux of CO₂ from the soil surface was much more restricted after watering (therefore rendering any respiration measurements inaccurate) in finer textured soils than in sandy soils because of reduced porosity in the finer textured soils. Accordingly, CO₂ reached and maintained the highest concentrations in finer textured soils (> 40 mmol CO₂ mol⁻¹). This study revealed that changes in soil moisture can affect interpretations of root and soil measurements based on CO₂ efflux, particularly in fine textured soils. The implications of the present findings for field soil CO₂ flux measurements are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Promotion of AM utilization through reduced P fertilization 2. Field studies

The hypothesis of this study was that cumulative P fertilization decreases the contribution of arbuscular mycorrhiza (AM) to crop growth and nutrient uptake in Northern European field conditions. The modes of action of P fertilization were evaluated through effects on mycorrhization, crop dependence on AM, and AM fungal (AMF) community. Field studies were carried out within long-term experiments on soils with low and intermediate initial content of extractable P, where no P fertilization and 45 kg ha^–1 a^–1 P were applied for 20 years. AM effectiveness in terms of growth and nutrient uptake of flax, red clover and barley, percentage root length colonized by AMF, P response of flax, and spore densities and species composition of the AMF communities, were assessed. In the soil with low initial P supply, cumulative P fertilization decreased AM contribution to crop growth and nutrient uptake. The higher AM effectiveness in soil with no added P compensated the cumulative P fertilization (soil P_H2O 2.5 v. 9.5 mg kg^–1) for flax, but not completely for clover. In contrast, barley obtained no benefit from AM at harvest and only a slight benefit from cumulated P. In the soil with intermediate initial P supply, AM reduced growth of flax and barley, especially with no added P, and no response to AM was obtained on clover due to retarded mycorrhization. Cumulative P fertilization reduced yield losses of flax by AM (P_H2O 18.8 v. 5.4 mg kg^–1), because fertilization inhibited mycorrhization. In both soils, root colonization and spore density were decreased by cumulative P fertilization, but no changes in AMF species composition were observed.     

Improving crop nutrient efficiency through root architecture modifications

Improving crop nutrient ef ficiency becomes an essential consideration for environmentally friendly and sustainable agriculture. Plant growth and development is dependent on 17 essential nutrient elements,among them,nitrogen(N) and phosphorus(P) are the two most important mineral nutrients. Hence it is not surprising that low N and/or low P availability in soils severely constrains crop growth and productivity,and thereby have become high priority targets for improving nutrient ef ficiency in crops. Root exploration largely determines the ability of plants to acquire mineral nutrients from soils. Therefore,root architecture,the 3-dimensional con figuration of the plant's root system in the soil,is of great importance for improving crop nutrient ef ficiency. Furthermore,the symbiotic associations between host plants and arbuscular mycorrhiza fungi/rhizobial bacteria,are additional important strategies to enhance nutrient acquisition. In this review,we summarize the recent advances in the current understanding of crop species control of root architecture alterations in response to nutrient availability and root/microbe symbioses,through gene or QTL regulation,which results in enhanced nutrient acquisition.     

Root proliferation,nitrate inflow and their carbon costs during nitrogen capture by competing plants in patchy soil

The responses of roots to nitrogen- and phosphorus-rich patches of soil include proliferation of laterals and stimulation of nutrient inflow (uptake rate per unit root length) within the patch. Nitrate uptake from an N-rich patch is thereby maximised and, perhaps, compensates for an uneven supply of nitrate to the whole root system. Paradoxically, the often weak correlation between root length density and N uptake found in experiments on single plants and crop monocultures suggests that root proliferation in patches has only a minor compensatory influence on N capture. This paradox was resolved when it was realised that localised root proliferation during inter-specific competition for nitrate can lead to a strong association between root length density and nitrate capture. Here, a simple model of inter-specific competition is used to estimate the stimulation in inflow required in one plant to match the N capture of a competitor that responds only by root proliferation, and to estimate associated carbon costs. The model predicts that nitrate inflow must increase proportionally more than root length density to achieve the same N capture. For example, the N capture possible with a 10% increase in root length density can be matched by increasing N inflow by anything from 20% to 20-fold, depending on the initial conditions: the faster the rate of change in root length density, the greater the required relative increase in inflow. In those terms, proliferation would seem the better option, but one that may be more costly in terms of its carbon requirement.     

Limited costs of wrong root placement in Rumex palustris in heterogeneous soils

Nutrient hot spots in the soil have a limited life span, but the costs and benefits for root foraging are still underexposed. We assessed short-term costs that may arise when a nutrient-rich patch induces root proliferation, but then rapidly disappears. Rumex palustris plants were grown with a homogeneous or a heterogeneous nutrient application. After root proliferation in a nutrient-rich patch, nutrient supply was switched from homogeneous to heterogeneous, and vice versa, or the patch location was changed. R. palustris proliferated its roots in the rich patch. After switching, the relative growth rates of the roots were adjusted to the novel pattern of nutrient availability. However, the changes in local root biomass lagged behind the rapid shift in nutrient supply, because the root mass realized in specific sectors could not be rapidly relocated. Despite this, R. palustris did not exhibit costs of switching in terms of biomass or nitrogen uptake. Our data suggest that rapid shifts in uptake rate and redistribution of nitrogen within the plant may have lowered the costs of incorrect root placement.     

Soil fumigation and phosphorus supply affect the formation of Pisolithus-Eucalyptus urophylla ectomycorrhizas in two acid Philippine soils

To examine the effects of microbial populations and external phosphorus supply of two Philippine soils on mycorrhizal formation, Eucalyptus urophylla seedlings were inoculated with two Pisolithus isolates and grown in fumigated, reinfested and unfumigated soil fertilized with four rates of phosphorus. The Pisolithus isolates used were collected from under eucalypts in Australia and in the Philippines. Soils were infertile acid silty loams collected from field sites in Pangasinan, Luzon and Surigao, Mindanao.Significant interaction was observed between inoculation, soil fumigation and phosphorus supply on mycorrhizal formation by the Australian isolate in Surigao soil but not in Pangasinan soil. Soil fumigation enhanced mycorrhizal formation by the Australian isolate but did not affect root colonization by the Philippine isolate. Root colonization by the Australian isolate was highest in the reinfested soil while for the Philippine isolate it was highest in the unfumigated soil. The Australian isolate was more effective than the Philippine isolate in promoting growth and P uptake of E. urophylla seedlings in both soils. Total dry weight and P uptake of E. urophylla seedlings inoculated with the Australian isolate were maximum in fumigated and in the reinfested Pangasinan and Surigao soils supplied with 8 mg P kg^-1 soil. In the unfumigated soil, growth of seedlings inoculated with the Australian isolate was significantly reduced. Seedlings inoculated with the Philippine isolate had the largest dry weights and P contents in unfumigated Pangasinan and Surigao soils supplied with 8 mg P kg^-1 soil.These results indicate that the performance of the Australian Pisolithus isolate was markedly affected by biological factors in unfumigated soil. Thus, its potential use in the Philippines needs to be thoroughly tested in a variety of unfumigated soils before its widespread use in any inoculation programme.     

Root length dynamics in agroforestry with Gliricidia sepium as compared to sole cropping in the semi-deciduous rainforest zone of West Africa

Tree root systems may improve soil fertility through carbon inputs, uptake of leachable nutrients and maintenance of soil biomass, but can at the same time reduce crop yields by competition for water and nutrients. Quantitative information about the positive and negative effects of tree roots and their changes in space and time are necessary for the optimization of agroforestry associations. An alley cropping experiment was layed out as a randomized complete block design on a Plinthic Lixisol/Ferralic Cambisol with Gliricidia sepium hedgerows at 5 m distance, including a sole cropping control. The development of root systems was monitored by sequential soil coring (eight samplings) during one year, with maize and groundnut as crops. Additional information is presented from a single sampling for rice during the foregoing year. Pronounced fluctuations of live root length density indicated an important variability in the nutrient and water uptake capacity of the vegetation. At low total root length density, the hedgerows affected the root development in the agroforestry plots directly by the presence of their root systems. At high root length density, they affected root development mainly by improving crop root growth and influencing the composition of the spontaneous vegetation. The root length density of the hedgerows was too low to compete with the crops for soil resources. The hedgerows tended to increase root length densities in the subsoil when few roots were present, thus possibly reducing the risk of nutrient leaching. However, the length density of the perennial root systems decreased during the cropping season, presumably as an effect of repeated pruning, and attained minimum values almost at the same time as the crops. Trees with denser root systems which are less frequently pruned may be more efficient in achieving closer nutrient cycles, though at the cost of higher root competition with crops.     

Effect of sub-optimal root zone temperatures at varied nutrient supply and shoot meristem temperature on growth and nutrient concentrations in maize seedlings (Zea mays L.)

Maize seedlings were grown for 10 to 20 days in either nutrient solution or in soils with or without fertilizer supply. Air temperature was kept uniform for all treatments, while root zone temperature (RZT) was varied between 12 and 24°C. In some treatments the basal part of the shoot (with apical shoot meristem and zone of leaf elongation) was lifted up to separate the indirect effects of root zone temperature on shoot growth from the direct effects of temperature on the shoot meristem.Shoot and root growth were decreased by low RZT to a similar extent irrespective of the growth medium (i.e. nutrient solution, fertilized or unfertilized soil). In all culture media Ca concentration was similar or even higher in plants grown at 12 as compared to 24°. At lower RZT concentrations of N, P and K in the shoot dry matter decreased in unfertilized soil, whereas in nutrient solution and fertilized soil only the K concentration decreased.When direct temperature effects on the shoot meristem were reduced by lifting the basal part of the shoot above the temperature-controlled root zone, shoot growth at low RZT was significantly increased in nutrient solution and fertilized soil, but not in unfertilized soil. In fertilized soil and nutrient solution at low RZT the uptake of K increased to a similar extent as plant growth, and thus shoot K concentration was not reduced by increasing shoot growth rates. In contrast, uptake of N and P was not increased, resulting in significantly decreased shoot concentrations.It is concluded that shoot growth at suboptimal RZT was limited both by a direct temperature effect on shoot activity and by a reduced nutrient supply through the roots. Nutrient concentrations in the shoot tissue at low RZT were not only influenced by availability in the substrate and dilution by growth, but also by the internal demand for growth.     

Root growth,macro-nutrient uptake dynamics and soil fertility requirements of a high-yielding winter oilseed rape crop

Shoot growth, root growth and macro-nutrient uptake by a high-yielding (5t/ha grain) winter oilseed rape crop have been measured. Maximum rooting density in the top 20cm of soil was 9.4 cm cm⁻³ and roots reached a depth of at least 1.8 m. Maximum nutrient uptakes were 364 kg ha⁻¹ for N, 43 kg ha⁻¹ for P, 308 kg ha⁻¹ for K, 287 kg ha⁻¹ for Ca and 16 kg ha⁻¹ for Mg. A 30-day drought coincided with the flowering period and root and shoot growth, as well as nutrient uptake rates, were reduced. Nutrient concentrations in the soil solution necessary to sustain the nutrient fluxes into the root system by diffusive supply have been calculated. Peak values were in the range 10 μM for P to 87 μM for N, lower than the observed concentrations, and it was concluded that nutrient transport to roots was not a limitation to uptake by this rape crop.     

A multiple ion-uptake phenotyping platform reveals shared mechanisms affecting nutrient uptake by roots

Nutrient uptake is critical for crop growth and is determined by root foraging in soil. Growth and branching of roots lead to effective root placement to acquire nutrients, but relatively little is known about absorption of nutrients at the root surface from the soil solution. This knowledge gap could be alleviated by understanding sources of genetic variation for short-term nutrient uptake on a root length basis. A modular platform called RhizoFlux was developed for high-throughput phenotyping of multiple ion-uptake rates in maize (Zea mays L.). Using this system, uptake rates were characterized for the crop macronutrients nitrate, ammonium, potassium, phosphate, and sulfate among the Nested Association Mapping (NAM) population founder lines. The data revealed substantial genetic variation for multiple ion-uptake rates in maize. Interestingly, specific nutrient uptake rates (nutrient uptake rate per length of root) were found to be both heritable and distinct from total uptake and plant size. The specific uptake rates of each nutrient were positively correlated with one another and with specific root respiration (root respiration rate per length of root), indicating that uptake is governed by shared mechanisms. We selected maize lines with high and low specific uptake rates and performed an RNA-seq analysis, which identified key regulatory components involved in nutrient uptake. The high-throughput multiple ion-uptake kinetics pipeline will help further our understanding of nutrient uptake, parameterize holistic plant models, and identify breeding targets for crops with more efficient nutrient acquisition.

A platform for quantifying root uptake rates of multiple, simultaneous nutrients reveals these rates are correlated among nutrients, are heritable, and may have a common genetic basis.     

Role of Arbuscular Mycorrhizal Fungi in Uptake of Phosphorus and Nitrogen From Soil

Abstract

Colonization of plant roots by arbuscular mycorrhizal fungi can greatly increase the plant uptake of phosphorus and nitrogen. The most prominent contribution of arbuscular mycorrhizal fungi to plant growth is due to uptake of nutrients by extraradical mycorrhizal hyphae. Quantification of hyphal nutrient uptake has become possible by the use of soil boxes with separated growing zones for roots and hyphae. Many (but not all) tested fungal isolates increased phosphorus and nitrogen uptake of the plant by absorbing phosphate, ammonium, and nitrate from soil. However, compared with the nutrient demand of the plant for growth, the contribution of arbuscular mycorrhizal fungi to plant phosphorus uptake is usually much larger than the contribution to plant nitrogen uptake. The utilization of soil nutrients may depend more on efficient uptake of phosphate, nitrate, and ammonium from the soil solution even at low supply concentrations than on mobilization processes in the hyphosphere. In contrast to ectomycorrhizal fungi, nonsoluble nutrient sources in soil are used only to a limited extent by hyphae of arbuscular mycorrhizal fungi. Side effects of mycorrhizal colonization on, for example, plant health or root activity may also influence plant nutrient uptake.     

Buckwheat (Fagopyrum esculentum Moench) has high capacity to take up phosphorus (P) from a calcium (Ca)-bound Source

Two experiments were carried out in a growth chamber to investigate the phosphorus (P)-uptake efficiency of Fagopyrum esculentum Moench (buckwheat) and Triticum aestivum (spring wheat) from a Ca-bound form. The first experiment was based on a sand-culture system with either rock phosphate (RP) or CaHPO₄ (CaHP) as the P source and nitrate or ammonium nitrate as nitrogen source. A highly calcareous soil was used in the second experiment. Buckwheat was shown to be highly efficient in taking up Ca-bound P compared to spring wheat. When plants were supplied with nitrate, the total P uptake by buckwheat from RP was nearly 10-fold higher than that of spring wheat (20.1 compared with 2.1 mg P pot^–1). Changing nitrogen source from nitrate only to ammonium nitrate increased P uptake by spring wheat substantially, but not buckwheat. High P-uptake efficiency of buckwheat was also demonstrated using the field soil, but to a lesser extent, which may be related to the difference in Zn supply between sand culture and field soil. It is suggested that buckwheat may be included in intercropping or crop rotation systems to activate P sources in calcareous soils. The principal mechanism of P uptake efficiency of buckwheat may be its ability to acidify the rhizosphere; however, further study is needed to unravel the regulation of root excretion of H⁺ and its molecular basis in order to exploit buckwheat's genetic capability to utilise sparingly soluble P from soil.     

Site factors,foliar nutrient levels and growth of Cordia alliodora plantations in the humid lowlands of Northern Costa Rica

Within the perhumid, Atlantic lowlands of northern Costa Rica, Cordia alliodora plantations were studied in order to explain the observed pattern of growth irregularities. The soils, that were partly used as pastures over long periods, could be classified roughly into two units: (i) red, deeply weathered, slightly acidic soils from Mg-enriched parent material and (ii) brown, strongly acidic soils with high saturation of exchangeable Al (up to 80%).Leaf analysis revealed that Cordia is a highly demanding species in respect to macronutrients. Poorly growing trees in slope position suffered from an insufficient supply of N and P. K/Mg ratios of chlorotic leaves are very low. Soil analysis showed that nutrient deficiencies were related to (i) insufficient soil nutrient reserves of the poor, tropical soils or (ii) to an inhibition of nutrient uptake by soil physical or chemical factors.All sites are characterized by very low K reserves and losses of nutrients that are organically bound (N, P) caused by erosion. In the Mg-rich red soils, low amounts of K lead to K/Mg imbalances. Soil compaction caused by cattle grazing occurs on both soil units. In negatively influences the root development of Cordia, and hence nutrient uptake. In the brown soils, high amounts of exchangeable Al hinder a sufficient supply of nutrients (e.g.P) to the assimilation organs.     

Responses of various crop species and cultivars to fertilizer application

Summary Crop response to fertilizer application depends not only on the level of available plant nutrients in the soil but is also related to crop physiology and morphology. For a well balanced nutrition the rate of nutrient supply to the roots must correspond with the rate of nutrient required for growth. Species or cultivars with a high growth rate generally respond more favourably to fertilizer application than those with low growth rates. An analogous relationship holds for the biomass produced per unit soil surface. Thus modern rice and wheat cultivars tolerate a more dense spacing than older ones. Due to the dense stand the yield and particularly the grain yield of the modern varieties may be several times higher than those of older cultivars, and therefore also the nutrient requirement, especially the demand for N and P, is higher for the modern cultivars.Modern cereal cultivars are characterized by a high crop index which means that after flowering a high proportion of grain filling material must be produced by photosynthesis. Assimilation and translocation of photosynthates are favoured by K⁺. Thus in particular modern cultivars require a high K⁺ content for optimum grain filling.Nutrient exploitation of soils by plant roots depends on root morphology and root physiology. Grasses generally have much longer roots than dicots. Thus the rate of K⁺ and phosphate uptake per unit root length is lower for grasses than for dicots. It is for this reason that dicots respond earlier to a K⁺ and phosphate dressing than grasses.Species living symbiotically with Rhizobium may depress the rhizosphere pH considerably and thus promote the dissolution of phosphate rock.