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Enhanced ammonium supply, soil pH and electrical conductivity effects on spring wheat growth
Journal of Plant Nutrition
Abstract
Spring wheat (Triticum aestivum L.) dry matter (DM), N content and tillering are increased by increasing the proportion of N available to the plant as NH4 (enhanced ammonium supply‐EAS) at soil pH ≥ 7.0. Using different N sources to provide different levels of EAS effects soil pH and electrical conductivity (EC) as well as soil NH4. Both pH and EC may affect plant growth and response to EAS.Two greenhouse experiments were conducted to determine the effects of EAS, pH, and EC on the DM, N content, and tillering of spring wheat. The collinearity between pH and NH4 was eliminated over a pH range of 5.8 to 7.2 by adjusting lime rates to compensate for the effect of each N source on pH. Even though EC was somewhat correlated with soil NH4 in both experiments, there were a sufficient number of comparisons to separate the effects of EC and NH4 on plant growth. Differences in plant growth resulted from differences in soil NH4 levels. Soil pH and EC did not affect plant growth. Plant responses to soil NH4 levels were quadratic. Maximum plant growth occurred at approximately 100–200 mg/kg KCl‐extractable NH4.
... An acid environment is characterized by an abundance of H + ions, while an alkaline environment is dominated by OHions. Different pH levels typically regulate the leaching or precipitation of basic cations such as NH 4 + , Ca 2+ , Mg 2+ , Fe 2+ , and PO 4 3− (Camberato et al. 1992;Zhang et al. 2016), thereby directly impacting the availability and utilization of essential elements by plants (Hipps et al. 2005;Neina 2019). ...
Interactions between plant species from different families have been extensively studied and practiced globally, particularly in agroecosystems. However, the effects of pH environments on plant growth regulation remains an area that warrants further investigations. To explore the effects of pH on plant interactions and growth, a hydroponic microcosm experiment was conducted with two crop species: rice (Oryza sativa L.) and pea (Pisum sativum L.). These two crop species were grown by mono- and intercropped under three pH conditions of acid (pH = 3.0), neutral (pH = 6.0), and alkaline (pH = 9.0) and were harvested at the tillering, flowering, and fruiting stages of pea, respectively. Results revealed that rice grew better in acid and neutral conditions than in alkaline environments. In alkaline environments, intercropping restrained the growth of both crops at tillering stage but promoted growth at flowering and fruiting stages. The alkaline environments largely decreased the P (phosphorus) and Fe (iron) content in the nutrient solution promoting rice to allocate more carbon (C) towards root development. A strong correlation was observed between dissolved organic carbon (DOC) levels and plant C investment in roots. In the intercropping regime, the DOC concentration was significantly higher in the alkaline than neutral environment during the flowering stage. These findings indicate that under pH stress, rice invested more C belowground and released more root exudate C to support its growth, in contrast to pea. While interactions between rice and pea did not consistently result in positive growth effects, they did exhibit a competitive effect. This study provides valuable insights into how interspecies interactions can regulate plant growth under pH stress conditions.
Calcium chloride combined with an enhanced ammonium supply (EAS) (0.5:1 Ca/NH4 molar ratio) has increased yields of horticultural crops. Greenhouse experiments were conducted to determine if Ca or Cl promote wheat (Triticum aestivum L., cv. Len) responses to an EAS. Wheat was grown in a Shano silt loam soil (coarse-silty, mixed, mesic Andic Mollic Camborthid) with 0 to 400 mg N kg-1 applied as (i) 100% NO3-N supplied by Ca(NO3)2; (ii) 50% NO3-N supplied by Ca(NO3)2, 50% NH4-N supplied by urea with a nitrification inhibitor (NI) (50:50 NO3/NH4, 0.5:1 Ca/NH4 molar ratio); (iii) 50:50 NO3/NH4 + CaCl2 (1:1 Ca/NH4); (iv) 100% NH4-N supplied by urea with a NI; and (v) NH4 + CaCl2 (0.5:1 Ca/NH4). Grain yield averaged 9 to 30% less for the NH4 and 50:50 NO3/NH4, but 15 to 37% more for the NH4 + CaCl2, than for the NO3, indicating that CaCl2 promoted the response to an EAS. A response to CaCl2 added to the 50:50 NO3/NH4 treatment, in which the NO3-N was supplied as Ca(NO3)2, suggested that the response to CaCl2 was due to the Cl rather than the added Ca. The individual roles of Ca and Cl in EAS responses were evaluated in a second experiment by combining Ca, Cl, and Ca + Cl with NO3 and NH4 (300 mg N kg-1 soil) in a factorial design. There was no effect of added Ca on grain yield with either N form. Furthermore, grain yield was higher for the NH4 + Cl than for the NH4, NH4 + Ca, or NO3 treatments, indicating that supplemental Cl elicited wheat responses to an EAS. Chloride also stimulated Ca uptake, which may have played a secondary role in the EAS response.
Four soils, ranging in texture from loamy sand to clay, were fertilized differently and equilibrated moist for several days. Soil solutions were then separated by column‐displacement, by simple centrifugation, and by immiscible displacement with CCl 4 via centrifugation. The ionic compositions of soil solutions were unaffected by the method used to obtain the solutions.
Effects of varying calcium (Ca) concentration at constant or varying solution ionic strength on root elongation of soybean (Glycine max (L.) Merr.) and subterranean clover (Trifolium subterraneum L.) were determined in aluminium (Al) free nutrient solution or solutions containing monomeric Al activities (+aAl mono) of 8-10 8M for subterranean clover and 20-22 8M for soybean. In Al-free solutions, the root length for subterranean clover was not significantly influenced by Ca or ionic strength. However, soybean root length was greater at 500 8M Ca than at higher Ca concentrations. Raising the ionic strength at 500 and 5000 8M Ca significantly decreased root length. In the presence of Al, maximum root length of both species occurred at 15 000 8M Ca. Soybean root length at 500 and 5000 8M Ca was 35% and 87% respectively of that at 15000 8M Ca. The corresponding values for subterranean clover were 53% and 81%. The positive effect of Ca concentration on root length, despite a nearly constant +aAL mono, confirms the existence of a protective action of Ca against Al toxicity. Raising the solution ionic strength at 500 8M Ca in the presence of Al improved the root growth of soybean by 86% and that of subterranean clover by 45%. At 5000 8M Ca, a small beneficial effect of increased ionic strength (14%) was found only in subterranean clover. Increasing Ca concentration in solution decreased water extractable and 0-1 M HNO3 extractable Al in roots of both plant species. Transfer of soybean seedlings to Al-free nutrient solutions containing 500, 1500 or 5000 pM Ca after 24, 48 or 96 h in a solution containing +aAL mono of 22 8M resulted in a substantial recovery in primary root growth. Relative root lengths were in each case significantly higher at 5000 8M Ca than at 1500 or 500 pM Ca. Roots transferred to 500 8M Ca after exposure to Al for 6 or 18 h underwent a period of accelerated elongation after a lag period of 30-40 h. By 138 h there were no significant differences in root length between the unstressed control plants and those subjected to 6 or 18 h Al-stress.
The rhizosphere pH (pH r ) of field and container‐grown wheat plants was measured and compared to the nonrhizosphere soil pH (pH b ). The predominant form of N supplied to the roots was a major factor influencing the pH r . The pH r was generally lower than pH b where NH 4 was supplied, higher where NO 3 was used, and relatively unchanged where both forms were added. Differences in pH r of up to 2.2 units were recorded for NH 4 ‐ vs NO 3 ‐fed wheat plants ( Triticum aestivum L.) in pots. Differences of up to 1.2 units occurred in the field. The pH b and pH r also differed by as much as 1.2 units. Changes in pH r were correlated with the concentrations of NH 4 in potted soils, although the acidifying influence of NH 4 decreased with time, as nitrification proceeded. Reductions in pH r values persisted longer in NH 4 treated, fumigated soils, and in non‐fumigated soils where NH 4 fertilizers had been treated with a nitrification suppressant. The pH r differences in NH 4 ‐ vs NO 3 ‐N treated soils were reduced by organic amendments. Difference with respect to pH r were found among wheat varieties and plant genera. The ability of plants to alter the pH r was related to assimilation of NO 3 primarily in the roots (monocots) or shoots (dicots).
Roots of intact plants (Helianthus annuus; Calathea warscewizii; Zea mays) were embedded in an agar medium (0.75%) containing nutrient solution and either bromocresol purple (pH indicator) or BPDS (chelator for FeII). Around the apical root zones of Fe deficient sunflower plants, enhanced reduction of FeIII (formation of red coloured FeII(BPDS)3) and increased proton efflux (i.e. acidification of the medium) could be observed within a few hours. Under Fe deficiency, enhanced reduction of manganese (MnFeIV) could also be demonstrated in the same root zones when the roots were embedded in agar with MnCO2.
Excised tomato roots supplied their nitrogen as nitrate in media of pH 4–5 or pH 7 have an essential requirement for molybdenum. Their growth is markedly stimulated and their continued culture made possible by the addition of 0.000005 p.p.m. Mo. The minimum addition which fully satisfies the molybdenum requirement is 0.001 p.p.m. The depression of growth resulting from a deficiency of molybdenum is not due to the toxicity of accumulating nitrate.
Excised roots supplied their nitrogen as nitrite, ammonium, urea, casein hydrolysate or as a mixture of L(+)— arginine and glycine are capable of growth in molybdenum-omitted medium. Their growth is stimulated by addition of molybdenum and a similar addition is required for optimum growth whether nitrogen is supplied as nitrate or as ammonium.
The tolerance of excised tomato roots to excess manganese at pH 7.0 is greater in ammonium than in nitrate media. The optimum manganese addition is increased by supraoptimal concentrations of ammonium. Optimum growth is only achieved when the medium has a satisfactory magnesium: manganese ratio. The deleterious effects of excess manganese can be counteracted by increasing the concentration of magnesium. At low manganese concentration, growth is depressed by enhancing the magnesium concentration.
Using a medium buffered with calcium carbonate (pH 7) and with a manganese addition of 0.01 p.p.m. the continuous culture of excised tomato roots supplied their nitrogen solely as ammonium has been achieved. No improvement of the very poor growth occurring in ammonium media at pH 4–5 has been achieved by altering the absolute and relative concentrations of a number of macronutrient elements.
Growth of bean plants was limited when nitrogen was supplied as (NH 4 ) 2 SO 4 in sand culture in comparison to the growth of plants receiving nitrate nitrogen. Mixing relatively insoluble carbonates with the sand, using (NH 4 ) 2 CO 3 instead of (NH 4 ) 2 SO 4 or maintaining the pH of the nutrient solution near neutrality with NaOH all prolonged the life and increased the growth of plants. Experiments in which C ¹³ ‐labeled carbonate was used revealed no absorption or incorporation of C ¹³ into the plant tissues. The changes in mineral element contents of plant tissue induced by the treatments did not account for the increased growth.
In the absence of pH control, ammonium, amide, and amino acid accumulation was very high in the leaves. Maintaining the pH of the nutrient solution near neutrality stimulated N assimilation in roots, especially the synthesis of amino acids and amides. Acidity control thus prevented the accumulation of ammonium in the tops and hence lessened the possibility of toxic effects from ammonium. The increased conversion of ammonium to nontoxic metabolites in roots is considered to be thedominant process by which acidity control improved the growth of plants on ammonium nutrition.
An N source experiment was conducted on infertile Mountview sil (Typic Paleudult) to evaluate four N sources at multiple rates of applied N and P. Granular ammonium nitrate (AN), sulfur‐coated urea (SCU), oxamide (Ox), and isobutylidene diurea (IBDU) were evaluated for corn ( Zea mays L.) at N rates of 0, 400, 800, and 1,200 mg/pot (5 kg of soil), each at P rates of 0, 60, 120, 480, and 960 mg/pot. Yield response to applied N was in the order AN >> SCU > Ox > IBDU at the higher rates of applied P. At 60 and 120 mg of applied P/pot, however, P was too deficient for satisfactory evaluation of the N sources. In an experiment with flooded rice ( Oryza sativa L.), P limited yields less than did N. In other N‐P and P‐K factorial experiments, rates of other nutrients and length of growth period also greatly affected crop response to applied N and K. These results show that satisfactory evaluations of N or K sources are possible only at adequate rates of nontest nutrients. Nutrient rates adequate for small greenhouse pots are much higher than rates equivalent to normal rates recommended for crops grown under field conditions. Length of growth period and other growth‐limiting factors are equally important.
Corn ( Zea mays L.) seedlings were grown for 11 days on 3 soils of pH 7.1, 5.5, and 4.3, and fertilized with monocalcium phosphate, and either (NH 4 ) 2 SO 4 , Ca(NO 3 ) 2 , or CaCl 2 .
Rhizocylinder (roots plus adhering soil) and bulk soil solutions were isolated by centrifugal filtration and analyzed for pH and phosphate.
The pH of the rhizocylinder solution was lowered by the absorption of NH 4 ⁺ and increased slightly by NO 3 ‐ absorption. Reduced rhizocylinder pH was associated with increased rhizocylinder solution phosphate concentration and P uptake by corn seedlings. The difference in P absorption between NH 4 ⁺ ‐fertilized and NO 3 ‐ ‐fertilized corn can be explained by induced changes in the H 2 PO 4 ‐concentration associated with rhizocylinder pH. But, in addition, NH 4 ⁺ and NO 3 ‐ enhanced P absorption probably as a result of increased physiological capacity to absorb P and/or increased root growth.
Soybeans ( Glycine max L.) fertilized with either NH 4 ‐N or NO 3 ‐N were grown in a growth chamber using soil with four different initial pH levels. Liming the soil used in this research to increase pH decreased the P level in solution. Fertilization of soybeans with NH 4 ‐N decreased the pH of the rhizocylinder (root plus strongly adhering soil); fertilization with NO 3 ‐ increased rhizocylinder pH. The difference between the rhizocylinder pH of the NH 4 ⁺ and NO 3 ‐ treatments was as large as 1.9 pH units with an initial soil pH of 5.2 and as small as 0.2 units when soil pH prior to N application was 7.8.
Ammonium‐fertilized soybeans absorbed more P and had a higher P concentration than NO 3 ‐fertilized soybeans. The results for soybeans grown with NH 4 ⁺ and NO 3 ‐ treatments at four initial soil pH levels showed that the P content of the shoots and roots was closely correlated with the pH of the rhizocylinder, but not the pH of the bulk soil. This suggests that the increased availability of P from the soil where NH 4 ⁺ was used was mainly due to the effect of the nitrogen source on the pH of the rhizosphere soil.
Soybean root length decreased from 180 to 120 m/gram of dry roots as the pH of the rhizocylinder increased from 4.7 to 7.5.
Spring wheat (Triticum aestivum L.) grain dry matter (DM) has been increased in previous greenhouse experiments through the use of a nitrification inhibitor (NI), which provides an enhanced ammonium supply (EAS). The objective of this research was to determine, under greenhouse conditions, the effect of EAS on the accumulation and partitioning of DM and N at several stages of plant development and on the yield components (heads plant⁻¹, kernels head⁻¹, and kernel weight). Enhanced NH4 supply increased plant DM at 18 d after plant emergence, boot stage, 7 d after anthesis, and maturity. Enhanced NH4 supply did not alter the relative partitioning of DM among leaf, stem, and grain at any stage of development. Effects of EAS on plant and grain DM at maturity increased with increasing N rate. At the highest N rates, grain DM was increased 18 and 6% in two experiments. Nitrogen and moisture stress appeared to alter grain DM response to EAS. Increased grain DM was associated with an increased number of heads plant⁻¹. Kernels head⁻¹ were decreased with EAS, but kernel weight was not affected. Consequently, EAS increased heads plant⁻¹ more than grain DM. At growth stages prior to maturity, EAS increased plant N uptake. At maturity, plant N uptake was slightly and inconsistently affected by EAS; however, the proportion of plant N in the grain was increased by EAS. This indicated a greater translocation of N from the stem to the grain with EAS. In general, increased spring wheat grain DM is associated with proportional increases in plant DM throughout plant development, but not necessarily with proportional increases in plant N uptake. Heads plant⁻¹ is the only yield component increased positively by EAS and thus warrants further research.
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Spring wheat ( Triticum aestivum L.) grain dry matter (DM) is increased by increasing the proportion of fertilizer N as NH 4 in the presence of a nitrification inhibitor (enhanced ammonium supply, EAS). This yield increase is manifest in increased heads plant ⁻¹ . The objective of this study was to determine the effects of EAS on the number and origin of vegetative and headed tillers and on the number of fertile spikelets and kernels of each tiller. Knowledge of these effects will aid in determining when and how EAS increases grain DM. Increased grain DM with EAS in this study resulted from an increase in the number of headed tillers plant ⁻¹ , which, for the most part, resulted from an increase in the number of vegetative tillers plant ⁻¹ . Increases in vegetative tillers plant ⁻¹ were detectable within several days after the onset of tillering and were similar for a low‐tillering and a high‐tillering cultivar. The rate of mainstem development was not affected by EAS, but several tillers appeared earlier with EAS. For some tillers, EAS also increased the proportion of vegetative tillers forming heads. Increased grain DM was not proportional to increases in heads plant ⁻¹ , because the additional headed tillers produced with EAS treatments were primarily T0 (coleoptilar mainstem) tillers and subtillers that had fewer spikelets head ⁻¹ and kernels head ⁻¹ . The number of fertile spikelets head ⁻¹ of several tillers was increased by EAS treatments, but the number of kernels head ⁻¹ was increased for the T1 and T20 tillers, only. Negative effects of EAS treatments on late‐appearing tillers (T4, T5, and T22) were thought to result from increased self‐shading, increased plant wilting, and reduced soil‐N supply occurring at anthesis and resulting from the much greater plant DM prior to anthesis in EAS treatments. However, in general, increased spring wheat grain DM with EAS can be ascribed to the positive effects of EAS on tiller development.
Nitrogen uptake by rooted cuttings of sweet potato [Ipomoea batatas (L.) Lam.] has not been studied extensively although “slips” (rooted sprouts) are the normal plant part used in commercial propagation. Experiments with rooted cuttings of sweet potato, cultivar ‘Jewell’, were conducted in solution culture to investigate the effects of ambient acidity and the presence and concentration effects of ammonium and nitrate on rates of nitrate and ammonium uptake. Plant stem cuttings were rooted in Hoagland's solution (plus or minus nitrate) then transferred to 980 ml containers of experimental solutions. In experiment I rates of N uptake from 0.1 mM NH4NO3 were measured at pH of 4.5, 5.5, and 6.5. In experiment II rates of N uptake were measured from 0.1 mM solutions with and without the presence of the ammonium in the case of nitrate uptake and vice versa. In experiment III nitrate uptake was measured from 0.1 mM nitrate without ammonium and in the presence of 0.1 mM and 1 mM ammonium. Solutions were sampled periodically and analyzed for nitrate, ammonium, and K. Nitrate uptake rate was decreased by increasing ambient pH while ammonium uptake rate was increased. Presence of ambient ammonium at equimolar concentration decreased rate of nitrate uptake ,∼30% at pH 5.5. However, a tenfold increase in ammonium concentration did not further reduce rate of nitrate uptake although rate of ammonium uptake was doubled. Presence of equimolar nitrate had no effect on rate of ammonium uptake at 0.1 mM ammonium. Efflux of K to initially K-free solutions was increased by ambient ammonium, but a transition from K efflux to influx occurred without a consistent change in rates of ammonium or nitrate uptake. A suggestion is made that two components of nitrate uptake by sweet potato roots exist, one of which is assumed to be associated with nitrate reductase and inhibited by ammonium.
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In a greenhouse experiment with wheat, sandy loam or clay soils were salinized by additions of 0, 3. or 8 g NaCl/3L pot. Ammonium and nitrate nitrogen mixtures in ratios of 0/100, 25/75 and 50/50 together with DCD, a nitrification inhibitor, were applied with irrigation water.Salinity significantly reduced dry matter yields, and N and P content in grain and stover. In accordance with previous reports, a mixed ammonium and nitrate N source produced larger dry matter and protein yields than nitrate alone, particularly in grains. The relative increases in yields and N and P accumulation, due to mixed N nutrition, were significantly higher in salinized soils and increased with increasing proportions of ammonium. Grain dry matter and N yields at medium salinity with a 50/50 N mixture, were equal to, or higher than those in non‐salinized soil fertilized with nitrate only.Chloride concentrations in stover increased with salinity and proportion of ammonium in the mixed source indicating that the advantage of mixed N nutrition is not due to nitrate‐chloride antagonism. Sodium concentration in plant tissues increased with salinity, but was reduced by increasing proportions of ammonium in the medium. This effect may be attributed to competition between ammonium and sodium for root uptake sites.The results obtained emphasize the effectiveness of mixed N sources to increase salt tolerance of crops such as wheat.
Three greenhouse pot experiments were conducted with corn ( Zea mays L.) grown on infertile soils to evaluate four fertilizers as sources of P. The granular sources were concentrated superphosphate (CSP, 90% of P water‐soluble), monoammonium phosphate (MAP, 100% of P water‐soluble), a mixture of 30% of the P as CSP and 70% as dicalcium phosphate (DCP, water‐insoluble), and a 10‐90% mixture of CSP and DCP. Rates of 0, 50, 200, 400, and 800 mg of P/pot from each source were compared in Exp. 1; and 0, 60, 120, and 480 mg rates of P in Exp. 2 and 3. These P sources and rates were compared at various levels of N and K. The order of effectiveness was the same at all levels of applied N and K: MAP ≥ CSP > 30% CSP + 70% DCP > 10% CSP + 90% DCP. This indicates that on these soils, P was the chief limiting nutrient at all levels of N and K. However, yield levels and the precision of determining differences in effectiveness among sources relative to experimental error increased greatly at higher rates of applied N and K.
These results show that effectiveness of P sources and also source differences relative to experimental error are greater at adequate than at deficient levels of nontest nutrients or other growth‐limiting factors. Adequate levels of nutrients for greenhouse pot experiments are much higher per pot equivalent than the normal recommended application rates for crops grown under field conditions.
Semi-dwarf bread wheat (Triticum aestivum L.) and durum wheat (Triticum turgidum L., Durum Group) are often grown on saline soils in the western United States. Because of the lack of information on salinity effects on vegetative growth and seed yield of these two species, a 2-yr field plot study was conducted. Six salinity treatments were imposed on a Holtville silty clay [clayey over loamy, montmorillonitic (calcareous), hyperthermic Typic Torrifluvent] by irrigating with waters salinized with NaCl and CaCl2 (1:l by wt). Electrical conductivities of the irrigation waters were 1.5, 2.5, 5.0, 7.4, 9.9, and 12.4 dS/m the first year, and 1.5, 4.0, 8.0, 12.0, 16.1, and 20.5 dS/m the second year. Grain yield, vegetative growth, and germination were measured. Relative grain yields of one semi-dwarf wheat cultivar and two durum cultivars were unaffected by soil salinity up to 8.6 and 5.9 dS/m (electrical conductivity of the saturated-soil extract), respectively. Each unit increase in salinity above the thresholds reduced yield of the semi-dwarf cultivar by 3.0% and the two durum cultivars by 3.8%. These results place both species in the salt-tolerant category. Salinity increased the protein content of both grains but only the quality of the durum grain was improved. Vegetative growth of both species was decreased more by soil salinity than was grain yield. Both species were less salt tolerant at germination than they were after the three-leaf stage of growth.
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Water culture, growth chamber, greenhouse and field experiments were conducted to compare the effect of NH4−N and NO3−N on yield and N uptake of rapeseed (Brassica campestris L.). In water culture, the yields of 28-day old rapeseed plants grown at 14 μg N ml−1 were double with NO3 compared to NH4, but N uptake was little affected. There was no such effect when concentration was reduced to 3.5 or 7 μg N ml−1. The yield and N uptake of 26-day old rapeseed grown on six soils (pH 4.6 to 6.5) in pots in a growth chamber were much greater
with NO3 than with NH4, although N concentration was more in the NH4- than the NO3-grown plants. In a greenhouse experiment with rapeseed grown on 12 potted soils, the N uptake of applied N was greater with
NO3 than with NH4 on all soils. Averages were 63% with NH4 and 78% with NO3. However, NH4-fixation capacities of the soils were only weakly correlated with yield from the two sources of N (r=0.48) and the relation
was similar with N uptake. In contrast to the behavior of water culture, growth chamber and greenhouse experiments, the 33
field experiments did not show consistent difference in seed yield with NH4 and NO3 applied at time of seeding. In nine field experiments where band application was used for Ca(NO3)2, (NH4)2 SO4, NH4 NO3, yield tended to be greatest for (NH4)2SO4. However, in 19 experiments on acid soils with and without lime, yields in most cases were similar with (NH4)2SO4 and NH4 NO3. Nitrification inhibitors were added to spring banded NH4-based fertilizers in five experiments, but the yields were not influenced.
Dry matter accumulation of plants utilizing NH4+ as the sole nitrogen source generally is less than that of plants receiving NO3- unless acidity of the root-zone is controlled at a pH of about 6.0. To test the hypothesis that the reduction in growth is a consequence of nitrogen stress within the plant in response to effects of increased acidity during uptake of NH4+ by roots, nonnodulated soybean plants (Glycine max [L.] Merr. cv Ransom) were grown for 24 days in flowing nutrient culture containing 1.0 millimolar NH4+ as the nitrogen source. Acidities of the culture solutions were controlled at pH 6.1, 5.1, and 4.1 +/- 0.1 by automatic additions of 0.01 N H2SO4 or Ca(OH)2. Plants were sampled at intervals of 3 to 4 days for determination of dry matter and nitrogen accumulation. Rates of NH4+ uptake per gram root dry weight were calculated from these data. Net CO2 exchange rates per unit leaf area were measured on attached leaves by infrared gas analysis. When acidity of the culture solution was increased from pH 6.1 to 5.1, dry matter and nitrogen accumulation were reduced by about 40% within 14 days. Net CO2 exchange rates per unit leaf area, however, were not affected, and the decreased growth was associated with a reduction in rates of appearance and expansion of new leaves. The uptake rates of NH4+ per gram root were about 25% lower throughout the 24 days at pH 5.1 than at 6.1. A further increase in solution acidity from pH 5.1 to 4.1 resulted in cessation of net dry matter production and appearance of new leaves within 10 days. Net CO2 exchange rates per unit leaf area declined rapidly until all viable leaves had abscised by 18 days. Uptake rates of NH4+, which were initially about 50% lower at pH 4.1 than at 6.1 continued to decline with time of exposure until net uptake ceased at 10 days. Since these responses also are characteristic of the sequence of responses that occur during onset and progression of a nitrogen stress, they corroborate our hypothesis.
The effect of Ca2+ on NO3- assimilation in young barley (Hordeum vulgare L. var CM 72) seedlings in the presence and absence of NaCl was studied. Calcium increased the activity of the NO3- transporter under saline conditions, but had little effect under nonsaline conditions. Calcium decreased the induction period for the NO3- transporter under both saline and nonsaline conditions but had little effect on its apparent Km for NO3- both in the presence and absence of NaCl. The enhancement of NO3- transport by Ca2+ under saline conditions was dependent on the presence of Ca2+ in the uptake solution along with the salt, since Ca2+ had no effect when supplied before or after salinity stress. Although Mn2+ and Mg2+ enhanced NO3- uptake under saline conditions, neither was as effective as Ca2+. In longer studies, increasing the Ca2+ concentration in saline nutrient solutions resulted in increases in NO3- assimilation and seedling growth.