Evaluation of chemical methods for estimating available zinc and response of green gram (Phaseolus aureus roxb) to applied zinc in non-calcareous soils

Summary Studies were conducted in 22 non-calcareous soils (India) to evaluate various extractants,viz. (6N HCl, 0.1N HCl, EDTA (NH₄)₂CO₃, EDTA NH₄OAc, DTPA+CaCl₂ and 1M MgCl₂) to find critical levels of soil and plant Zn for green gram (Phaseolus aureus Roxb.). The order of extractability by the different extractants was 6N HCl>0.1N HCl>EDTA (NH₄)₂CO₃<EDTA NH₄OAc DTPA+CaCl₂>1M MgCl₂. Critical levels of 0.48 ppm DTPA × CaCl₂ extractable Zn, 0.80 ppm EDTA NH₄OAc extractable Zn, 0.70 ppm EDTA (NH₄)₂CO₃ extractable Zn, and 2.2 ppm 0.1N HCl extractable Zn were estimated for the soils tested. The critical Zn concentration in 6 weeks old plants was found to be 19 ppm. The 0.1N HCl method gave the best correlation (r=0.588^**) between extractable Zn and Bray's per cent yield, while with DTPA+CaCl₂, it was slightly low (r=0.542^**). The DTPA + CaCl₂ method gave significant (r=0.73^**) correlation with plant Zn concentration. The 0.1N HCl gave the higher correlation with Zn uptake (r=0.661^**) than DTPA (r=0.634^**) 6N HCl and 1M MgCl₂ method gave nonsignificant positive relationship with Bray's per cent yield. For noncalcareous soils apart from the common use of DTPA+CaCl₂, 0.1N HCl can also be used for predicting soil available Zn. The use of 0.1N HCl would be much cheaper than DTPA and other extractants used in the study.     

Evaluation of a soil test method and plant analysis for determing the sulphur status of alluvial soils

Summary In a pot experiment with soils of Alfisol, Entisol, and Inceptisol orders, the relative yield of Egyptian clover (Trifolium alexandrinum L.) was significantly correlated with Morgan's reagent (N NaOAc+HOAc, pH 4.8)—extractable soil S (r=0.88), plant S (r=0.82), and plant N/S ratio (r=−0.77) suggesting suitability of these tests for diagnosing S deficiency. Total plant S lower than 0.21 per cent, plant N/S ratio wider than 17, and extractable soil S lower than 10 ppm were indicative of S deficiency, and were suggested therefore to be critical limits for these tests. Nitrogen and S in plant proteins were in near constant ratio of 16 and were significantly correlated (r=0.99). Sixty one per cent of 250 surface soil samples had less than 10 ppm extractable S and hence were deficient in S, suggesting a widespread S deficiency in soils under study. Extractable soil S in all soil series was significantly correlated with electrical conductivity and alkaline KMnO₄-extractable N, but not with pH, organic C, and CaCO₃.     

The critical level of copper in sierozem soils of haryana as evaluated with different extractants

Summary The relative efficiency of seven extractants for estimating available Cu in sierozem soils of Haryana was studied. Bulk samples of 15 soils ranging in neutral normal amm. acetate extractable Cu from 0.12 to 0.20 ppm were subjected to screen-house study. The quantities of Cu extracted with neutral normal amm. acetate, Morgan’s reagent (pH 4.8)N ammonium acetate (pH 4.8), 0.1N HCl, 0.02M EDTA, critrate-EDTA and DTPA from soils were examined for their correlation with responses of maize in terms of Bray’s percentage yield and percentage Cu uptake. The highest coefficient of correlation was obtained between Cu extractable with neutral 1N NH₄OAc and Bray’s per cent yield and per cent Cu uptake. All other methods showed lower values of correlation. The critical level of available Cu estimated with use of neutral normal NH₄OAc was 0.16 ppm. Below this value, responses to applied Cu can be expected.     

Alder and lupine enhance nitrogen cycling in a degraded forest soil in Northern Sweden

Positive effects of legumes and actinorhizal plants on N-poor soils have been observed in many studies but few have been done at high latitudes, which was the location of our study. We measured N₂ fixation and several indices of soil N at a site near the Arctic Circle in northern Sweden. More than 20 years ago lupine (Lupinus nootkatensis Donn) and gray alder (Alnus incana L. Moench) were planted on this degraded forest site. We measured total soil N, net N mineralization and nitrification with a buried bag technique, and fluxes of NH⁺ ₄ and NO^– ₃ as collected on ion exchange membranes. We also estimated N₂ fixation activity of the N₂-fixing plants by the natural abundance of ¹⁵N of leaves with Betula pendula Roth. as reference species. Foliar nitrogen in the N₂-fixing plants was almost totally derived from N₂ fixation. Plots containing N₂-fixing species generally had significantly higher soil N and N availability than a control plot without N₂-fixing plants. Taken together, all measurements indicated that N₂-fixing plants can be used to effectively improve soil fertility at high latitudes in northern Sweden.     

Phytoalexin production in lucerne roots inoculated withVerticillium albo-atrum

Summary Out of various soil and plant test methods tested for predicting response of rice to K application in soils of a rice growing valley region the Hanway and Heidal extractant neutralN NH₄ OAc turned out to be the best. The critical limit of extractable K was 160 ppm by the Hanway and Heidal extractant, and by the Bray's reagent 175 ppm. Critical K level in the rice plant is 0.4%. Correlations between the extractable K and K uptake were highly positive for various extractants: Hanway and Heidal, Morgan, Hunter and Pratt No. 2, Blanchet and Perigand and MacLean. Although majority of the soils of the region was Inceptisols followed by Alfisols and Vertisols, all soil types had a similar available nutrient status and a similar pattern in relative grain yields. K response was noticeable in Alfisols with respect to grain and straw yields. The grain P concentration in Vertisols, and straw K in Alfisols indicated the contribution of K towards the productivity of two soil groups.     

Effects of Shading Soybean Plants on N2O Emission from Soil

The effect of photosynthesis on N₂O emission from soil was investigated by shading soybean (Gycline max. L) plant at flowering, pod-setting and grain-filling stages. The results showed that by stopping photosynthesis through shading the plants stimulated N₂O emission significantly at flowering stage and pod-setting stage, and suppressed N₂O emission dramatically at grain-filling stage. At flowering stage, soybean species seem to rely mainly on fertilizer N and shaded plants decreased the N uptake. Interaction between the relative increase in available N for N₂O production by shading and the presence of root exudates promoted N transformation (nitrification/denitrification) and N₂O emission. At pod-setting stage, the available soil nitrogen seems to be a critical limiting factor and without substantial release of symbiotically fixed N through plant roots, resulted in a weak effect of shading on N₂O emission. At grain-filling stage, available N for N₂O production was derived from symbiotically fixed N and was greatly affected by photosynthesis. These results indicated that the effect of soybean growth on N₂O emission from soil varies with plant growth stages as available N for N₂O production is mainly from fertilizer N and organic mineralization during the early growth of soybean plants, while N₂O emission is controlled by the quantity and perhaps also the quality of root exudates, which is closely related with plant photosynthesis in the late season of soybean growth.     

Transformations of fertilizer nitrogen in soil

Summary Five crops of oats were grown over a 14-month period on a Chester silt loam soil fertilized with N¹⁵-labelled (NH₄)₂SO₄. All plant material from the first four crops was returned to the soil. Following a fifth crop, oat tops and roots were harvested, and the soil was subjected to repeated extractions by autoclaving in 0.01M CaCl₂. The distribution of N¹⁵ and of indigenous soil N among chemical fractions of the extracts, and in the acid-soluble and acid-soluble and acid-insoluble portions of the soil residues following 0.01M CaCl₂ extraction, was remarkably similar. Since appreciable equilibrations between added N¹⁵ and the more resistant forms of soil organic N is unlikely, it is postulated that fertilizer N became incorporated in newly-formed complexes, similar to those already present in the soil. This view is in harmony with the finding that percentage removals of total and N¹⁵-labelled N remained almost the same, even with recovery of approximately 55 per cent of the amounts originally present. N mineralization capacity of the soil was reduced appreciably as a result of extraction.     

EUF-analysis of tea soils of Southern India and tea productivity

Summary The relationship between EUF extractable nutrients and conventional soil test extractable nutrients in the acid soils of Southern India on one hand and that between EUF values and tea productivity on the other are described. Close correlation exists between EUF-NO₃–N at 20°C and CuSO₄–Ag₂SO₄-extractable NO₃–N (r=0.98^***), EUF-N_org and Morgan's reagent extractable NH₄–N (r=0.97^***), total EUF-N and CuSO₄–Ag₂SO₄-extractable NO₃–N plus Morgan's reagent NH₄–N (r=0.96^***), EUF-P at 20°C and modified Bray II-P (r=0.93^***) and EUF-P at 20°C plus that at 80°C and modified Bray II-P (r=0.91^***). The EUF-K at 20°C shows close correlation with NH₄OAc–K (r=0.80^***), Ag-thiourea-K (r=0.86^***) and Morgan's reagent-K (r=0.84^***) whereas the EUF-K at 80°C shows close correlation with the difference in K contents of NH₄OAc–K and Ag-thiourea-K (r=0.92^***) or of NH₄OAc–K and Morgan's reagent-K (r=0.93^***) and fixed NH₄–N (r=0.89^***). EUF-Ca, EUF-Mg and EUF-Mn do not show any relationship with conventional soil test values. Tea productivity is strongly associated with EUF-N and EUF-P extracted at 20°C.     

Evaluation of phosphorus in forest soils: Comparison of phosphorus uptake,extraction method and soil properties

Summary Phosphorus in soils from plantation of red pine (Pinus resinosa Ait.) was determined using six extractants: 0.002N H₂SO₄ (pH 3.0); 0.025N HCl+ +0.03N NH₄F; 0.5N NaHCO₃ (pH 8.5);N NH₄OAc (pH 4.8); anion exchange resin (Dower –2, Cl-form); H₂O. Correlations of extractable P with Al- and Al-+Fe-P indicated that these fractions are the dominant forms of inorganic P in most of the soils.Uptake of P by corn and Monterey pine seedlings grown in greenhouse culture was correlated with soil P extracted by the different methods. The most successful of the extractants for predicting P uptake was resin extractable P; the simple correlation coefficients were 0.811 and 0.609 for pine and corn respectively. P uptake by pine correlated significantly with 0.002N H₂SO₄ P (r=0.679),N NH₄OAc P (r=0.443), H₂O P (r=0.549) and Al-+Fe-P (r=0.532) while P uptake by corn correlated with 0.002N H₂SO₄ P (r=0.579), H₂O P (r=0.477) and organic P (r=0.460). Per cent P in pine seedling tops correlated significantly with 0.002N H₂SO₄, resin andN NH₄ OAc extractable P. Multiple regressions which included silt+clay and organic P improved correlations of some soil tests with P uptake in corn and pine seedlings respectively.Research supported by the School of Natural Resources, College of Agricultural and Life Sciences, Univ. of Wisconsin, Madison, the Wisconsin Department of Natural Resources and FAO Fellowship to the senior author.     

Induction of somatic embryos by macrosalt stress from mature zygotic embryos of Panax ginseng

Mature zygotic embryos of ginseng (Panax ginseng C. A. Meyer) were germinated on a Murashige and Skoog medium lacking growth regulators. However, when the zygotic embryos were cultured on MS medium containing increased levels of macrosalts (NH₄NO₃, KNO₃, KH₂PO₄, MgSO₄, or CaCl₂) to result is a mild salt stress, growth of zygotic embryos was strongly suppressed and eventually browning occurred. Somatic embryos or embryogenic calli were formed directly from these abnormal stressed zygotic embryos. Cotyledons were the most competent tissue for somatic embryo production. The highest frequency of somatic embryo formation (56.3%) was observed on medium containing 61.8 mM of NH₄NO₃. The highest frequency of somatic embryo formation by five different macrosalt treatments occurred in the following order: NH₄NO₃> KNO₃> KH₂PO₄> MgSO₄> CaCl₂. Somatic embryos were regenerated into plants with a shoot and root, and the plants survived on soil in the greenhouse. This revised version was published online in June 2006 with corrections to the Cover Date.     

Amino acid content in xylem sap of regrowing alfalfa (Medicago sativa L.): Relations with N uptake,N2 fixation and N remobilization

During vegetative regrowth of Medicago sativa L., soil N, symbiotically fixed N₂ and N reserves meet the nitrogen requirements for shoot regrowth. Experiments with nodulated or non-nodulated plants were carried out to investigate the changes in N flows originating from the different N sources and in xylem transport of amino acids during regrowth. Exogenous N uptake, N₂ fixation and endogenous N remobilization were estimated by ¹⁵N labelling and amino acids in xylem sap were analysed. Removal of shoots resulted in great declines of exogenous N flows derived either from N₂ or from NH₄NO₃ during the first week of regrowth, thereafter recovery increased linearly. Mineral N uptake as well as N₂ fixation occurred mainly between the 10th and 18th day after removal of shoots while exogenous N assimilation in intact plants remained at a steady level. Nitrogen remobilization rates in defoliated plants increased by at least three to five-fold, especially during the first 10 days following shoot removal. Compared to control plants, contents of amino acids in xylem sap, during the first 10 days of regrowth, were reduced by about 72% and 82% in NH₄NO₃ grown and in N₂ fixing plants, respectively. Asparagine was the main amino acid transported in xylem sap of both treated plants. Its relative contents during this period significantly decreased from 75% to 59% and from 67% to 36% respectively in non-nodulated plants and in nodulated ones. This decline was accompanied by compensatory increase in the relative contents of aspartate and glutamine.     

Assessment of potential sources of error in nitrogen fixation measurements by the nitrogen-15 isotope dilution technique

Although the use of ¹⁵N fertilizers to measure nitrogen (N₂) fixed in crops has increased substantially in recent years, some methodological uncertainties still remain unresolved. The results obtained from a greenhouse study of soybean [Glycine max. (L.) Merrill] inoculated by six different methods have been examined for potential errors arising from incorporating ¹⁵N labelled fertilizer into soil to estimate N₂ fixed in pods or shoots or the whole plant at three growth stages (50% flowering, pod-initiation and physiological maturity) using as reference crops, an uninoculated soybean cultivar and a non-nodulating soybean isoline. At the first harvest when N₂ fixed was very low, the estimates of N₂ fixed by the two reference crops did not match. At this stage the uninoculated soybean estimated about four times as much N₂ fixed in the symbiotic soybean as that measured using the non-nodulating soybean. For the second and third harvests, there were substantial increases in N₂ fixed, and both the non-nodulating and uninoculated soybean were equally suitable as reference crops for assessing N₂ fixed in the symbiotic soybean. These results indicate how critical and difficult the choice of the reference crop could be at early harvests, or when N₂ fixed is low. Even though there were significant differences in ¹⁵N enrichments in different organs (generally nodules < pods < roots < shoots), the estimates of N₂ fixed in soybean plants obtained by excluding roots and nodules did not differ much from those based on the whole plant. Of the above-ground organs, % N₂ fixed in pods (containing seeds) was closest to that of the whole plant (similar at P<0.05 at physiological maturity). However, the total N₂ fixed in pods or shoots was substantially lower than that fixed by the whole plant (P<0.05), although that for the pods and enclosed seeds once again was closer to N₂ fixed in the whole plant than that in the shoots.     

Influence of pH,exchangeable aluminium and 0.02M CaCl2-extractable aluminium on the growth and nitrogen-fixing activity of white clover (Trifolium repens) in some New Zealand soils

The effects of soil acidity on the growth and N₂-fixing activity of white clover in seven acid topsoils and subsoils of New Zealand were investigated using a glasshouse experiment.The application of phosphate (Ca(H₂PO₄)₂) to the soils resulted in very large increases in white clover growth on all soils. The application of phosphate, as well as increasing P supply, also decreased 0.02M CaCl₂-extractable Al levels, but had little effect on exchangeable Al levels.Where adequate phosphate was applied, increasing rates of lime (CaCO₃) resulted in increased plant growth on most soils. N₂[C₂H₂]-fixing activity was increased by the first level of lime for one soil, but generally remained approximately constant or declined slightly at higher rates of lime. Up to the point of maximum yield, white clover top weight was more highly correlated with 0.02M CaCl₂-extractable soil Al than with exchangeable Al or pH. At pH values greater than 5.5, plant yield declined on some soils, apparently because of Zn deficiency. The data suggest that white clover is unlikely to be affected by Al toxicity at 0.02M CaCl₂-extractable Al levels of less than about 3.3 g g^–1. However, there were differences between soils in apparent plant tolerance to 0.02M CaCl₂-extractable Al, which appeared to be caused by differing C levels in the 0.02M CaCl₂ extracts.     

Effects of continuous nitrogen application and nitrogen preconditioning on nodulation and growth ofCeanothus griseus varhorizontalis

Rooted cuttings ofCeanothus griseus varhorizontalis were irrigated with 0, 10, 20, 50, 75 or 100ppm nitrogen as NH₄NO₃ for eight weeks prior to inoculation with infectiveFrankia. After inoculation, half of the plants for each treatment nitrogen level continued to be irrigated with the preconditioning nitrogen level and half were given no more supplemental nitrogen. For plants continuously receiving nitrogen, nodule initiation (nodule number) was inversely correlated with increasing supplemental nitrogen levels, and suppressed above 50 ppm N. Leaf nitrogen above 2% in continuous-N plants correlated with greatly reduced or suppressed nodulation. Plants maintained after inoculation without supplemental nitrogen showed influence of the prior nitrogen treatment on nodulation. Preconditioning at 50 ppm and above greatly reduced the number of nodules formed. The evidence suggests that stored internal nitrogen can regulate nodulation.Plant biomass accumulated maximally when nodulation was suppressed, at 75 and 100 ppm supplemental N applied continuously. Internode elongation during the nodulation period occurred only on nodulated plants, or in the presence of supplemental N (10 ppm and above).     

Effect of solarization of soil within plastic bags on root rot of gerbera (Gerbera jamesonii L.)

Solarization of soil, (potting mix = coarse sand:Eucalyptus marginata fines = 1∶1) infested with 3 fungi pathogenic to gerbera (Phytophthora cryptogea, Fusarium oxysporum andRhizoctonia solani), for 3 to 4 weeks within transparent polyethylene bags controlled root rot of gerbera. Solarization for 2 weeks however, was less effective. All plants grown in the infested potting mix which had been kept in shade for 2, 3 or 4 weeks were severely attacked. Solarization of soil within plastic bags for 4 weeks also increased availability of nutrients such as NH₄ ⁺-N, PO₄ ⁻ and K⁺ in comparison to bagged soil kept in the shade for the same period.     

Evaluation of suitable extractant for available lead in soils

Summary Four extractants were used to extract Soil-Pb from alluvial soils and the amounts of Pb extracted was correlated with Pb concentration in wheat crop. Of all the reagents, the Grigg's reagent was found to extract maximum amount of Pb from soils and the N NH₄OAc the least. A highly significant correlation (r=0.295) was observed between Grigg's reagent extractable Pb and Pb concentration in wheat plants followed by ammonium acetate (r=0.238). While 0.02M EDTA did not show any significant correlation, a negative significant correlation was observed with 0.1N HCl. re]19750512     

Toxicity of nitrapyrin to sunflower under different N nutrition regimes

Summary The purpose of this study was to investigate the phytotoxicity of nitrapyrin 2-chloro-6-(trichloromethyl)pyridine to sunflower (Helianthus annuus L.) under different N regimes and to see if N forms affect the phytotoxicity of nitrapyrin. Sunflower was grown in pot culture for 21 days and was fertilized with (NH₄)₂SO₄, NH₄NO₃ and NaNO₃ to provide 0, 100 and 200 ppm N and with nitrapyrin application of 0 and 20 ppm. All N-treated sunflower plants in all N regimes and regardless of titrapyrin treatment produced more root and shoot dry weights and contained a significantly higher N than untreated check. Nitrapyrin toxicity appeared as a curling of leaf margin and a tendril type of stem growth, the visible toxicity symptoms decreased in the order: (NH₄)₂SO₄>NH₄NO₃>NaNO₃. Furthermore nitrapyrin addition suppressed sunflower growth in each N regime, the suppressing effect being greater with (NH₄)₂SO₄ and NH₄NO₃ than as with NaNO₃. Although, shoot growth from plants receiving nitrapyrin was not significantly affected by any N regime, root growth of nitrapyrin-treated plants was somewhat restricted by NH₄ ⁺−N nutrition relative to NO₃ ⁻−N nutrition.     

Sulphur fractionation in calcareous soils and bioavailability to plants

Sulphur fractionation and availability to plants are poorly understood in calcareous soils. Sixty-four calcareous soils containing varying amounts of CaCO₃ were collected from ten provinces in China and their S fractions determined. Organic S was the predominant fraction of S, accounting for on average 77% of the soil total S. The amounts of adsorbed sulphate were found to be negligible. 1 M HCl extracted substantially more sulphate than either 0.01 M CaCl₂ or 0.016 M KH₂PO₄, indicating the existence of water-insoluble but acid-soluble sulphate, probably in the form of sulphate co-precipitated with CaCO₃. The concentrations of water-insoluble sulphate correlated positively with the contents of CaCO₃ and accounted for 0.03–40.3% (mean 11.7%) of soil total S. To test the bioavailability of water-insoluble sulphate, a sulphate-CaCO₃ co-precipitate labelled with ³⁵S was prepared and added to a calcareous soil in a pot experiment with either NH₄⁺ or NO₃^– as the N source. In 29 days, wheat plants took up 10.6% and 3.0% of the ³⁵S added to the soil in the NH₄⁺ and NO₃^– treatments, respectively. At the end of the pot experiment, the decrease of water-insoluble, acid-soluble, sulphate was more apparent in the NH₄⁺ than in the NO₃^– treatment. The results indicate that sulphate co-precipitated with CaCO₃ in calcareous soils may become partly available for plant uptake, depending on rhizosphere pH, if the field precipitate is similar to the laboratory prepared sample studied.     

氮磷添加与不同栽植密度交互对樟树幼苗土壤化学性质的短期影响

研究氮磷添加对不同密度樟树(Cinnamomum camphora)幼苗土壤化学性质的影响,以期为全球化背景下樟树人工林生态系统的土壤养分管理提供依据。以1年生樟树幼苗为试验材料,选择氯化铵(NH4Cl)作为氮肥模拟大气氮沉降,以二水合磷酸二氢钠(NaH_2PO_4·2H_2O)模拟磷添加。氮磷处理设置CK、施N、施P和施N+P 4个水平,其中N、P和N+P施肥量分别为40 g m~(-2)a~(-1)(NH_4Cl)、20 g m-2a-1(NaH_2PO_4·2H_2O)和40g m~(-2)a~(-1)(NH_4Cl)+20 g m~(-2)a~(-1)(NaH_2PO_4·2H_2O)。种植密度设置4个水平:10、20、40和80株/m~2,试验时间为2017年6月至9月。研究结果表明,在各密度幼苗土壤中,N和N+P处理引起pH值的显著下降,N、P和N+P处理的土壤有机质和碱解N含量的变化规律不明显,P处理的幼苗土壤全P含量上升,P和N+P处理的土壤有效P含量增加,N+P处理的土壤全K含量以及N、P和N+P处理的土壤速效K含量均下降。在10、20和40株/m~2幼苗的土壤中,P处理的土壤全N含量高于N和N+P处理的,而80株/m~2幼苗的土壤全N含量低于其他密度幼苗。随着种植密度的增加,各施肥处理的土壤pH、全P、有效P、全K和速效K含量均呈现上升趋势,而施N和施P处理的土壤有机质呈现下降趋势,各施肥处理的土壤碱解N含量变化规律不明显。施肥和密度处理对樟树幼苗土壤有机质、碱解氮和速效钾含量有显著的交互作用。     

Effects of N source on proton excretion,ionic balance and growth ofAlnus glutinosa (L.) Gaertner: comparison of N2 fixation with single and mixed sources of NO3 and NH4

Summary Non-nodulatedalnus glutinosa plants were grown for 6 weeks in nutrient solutions using 3 combined-N treatments (NO₃; NO₃/NH₄; and NH₄) at a total N level of 4 meq.l^–1, and growth was ccmpared with nodulated plants at zero N (N₂ fixation). Of the combined-N sources, 100 per cent NH₄ resulted in the highest dry matter yields when the solution pH was adjusted daily atc. 6. The dry matter yield was lowest with NO₃.During the first 3 weeks, the yield of the N₂-fixing plants was as high as that of the NH₄ plants, but fell relatively behind during the second 3-week period. These effects could be attributed to higher initial N contents and higher shoot:root ratios, respectively, in the N₂-fixing plants. Specific rates of N acquisition in the root were of a comparable order of magnitude for the combined-N and zero-N treatments.When NO₃ was taken up, it was almost completely reduced in the roots. Regardless of N source there was a large excess of cations (C) relative to inorganic anions (A) in the plants, which was presumed to be balanced by an equivalent amount of organic anions (C-A). The relatively small differences in generation of organic anions for the various modes of N supply indicated the relative importance of the proton pump when NH₄ or N₂ was the N source. Proton or hydroxyl-ion effluxes, calculated on the basis of plant analyses, were generally in good agreement with measured excretion values. The acidity generation with N₂ fixation amounted toc. 0.5 meq H⁺.mmol^–1 N_org, which was distinctly higher than the range of 0.1–0.2 mentioned by Raven and Smith⁴³ for dinitrogen-fixing plants.Without pH adjustment, specific rates of cation uptake and carboxylate generation were strongly depressed as the acidity increased, when NO₃/NH₄, NH₄ and N₂ were the N sources. Growth ofAlnus glutinosa appeared to be still normal at a pH ofc. 2.8. During the final 3 weeks, only the NH₄ plants ceased growing at a pH of 2.6.