Exchangeable cations and CEC determinations of some highly weathered soils

Abstract

Eight methods to determine exchangeable cations and cation exchange capacity (CEC) were compared for some highly weathered benchmark soils of Alabama. The methods were: (1) 1N NH₄OAc at pH 7.0 by replacement (for CEC only), (2) 1N NH₄OAc at pH 7.0 (summation of basic cations plus 1N KCl extractable Al), (3) 1N NH₄OAc at pH 7.0 (summation of basic cations plus exchangeable H⁺), (4) 0.1M BaCl₂ (summation of basic cations plus exchangeable Mn, Fe and Al), (5) Mehlich 1 (summation of basic cations plus 1N KCl extractable Al), (6) Mehlich 1 (summation of basic cations plus exchangeable H⁺), (7) Mehlich 3 (summation of basic cations plus 1N KCl extractable Al), and (8) Mehlich 3 (summation of basic cations plus exchangeable H⁺). The 0.1M BaCl₂ was chosen as the standard method for the highly weathered soils and the other methods compared to it. The results indicated that the 1N NH₄OAc replacement method gave significantly higher CEC values compared to the summation methods. This was probably due to the overestimation of the field CEC caused by measurement of pH dependent cation exchange sites in these soils. There was, however, close agreement between the BaCl₂ method and the summation methods that included extractable Al. The generally good agreement between these summation methods suggests that the Mehlich 1 and Mehlich 3 extractants, commonly used to determine available nutrients in the southeastem USA, may also be used to measure effective CEC of some acid‐rich sesquioxide benchmark soils of Alabama. However, 1N KCl extractable Al as opposed to exchangeable H⁺ should be included in the computation.     

Effect of long‐term fertilization and manuring on potassium release properties in a Typic Ustochrept

A long‐term fertilizer experiment, over 27 years, studied the effect of mineral fertilizers and organic manures on potassium (K) balances and K release properties in maize‐wheat‐cowpea (fodder) cropping system on a Typic Ustochrept. The treatments consisted of control, 100% nitrogen (100% N), 100% nitrogen and phosphorus (100% NP), 50% nitrogen, phosphorus, and potassium (50% NPK), 100% nitrogen, phosphorus, and potassium (100% NPK), 150% nitrogen, phosphorus, and potassium (150% NPK), and 100% NPK+farmyard manure (100% NPK+FYM). Nutrients N, P, and K in 100% NPK treatment were applied at N: 120 kg ha^—1, P: 26 kg ha^—1, and K: 33 kg ha^—1 each to maize and wheat crops and N: 20 kg ha^—1, P: 17 kg ha^—1, and K: 17 kg ha^—1 to cowpea (fodder). In all the fertilizer and manure treatments removal of K in the crop exceeded K additions and the total soil K balance was negative. The neutral 1 N ammonium acetate‐extractable K in the surface soil (0—15 cm) ranged from 0.19 to 0.39 cmol kg^—1 in various treatments after 27 crop cycles. The highest and lowest values were obtained in 100% NPK+FYM and 100% NP treatments, respectively. Non‐exchangeable K was also depleted more in the treatments without K fertilization (control, 100% N, and 100% NP). Parabolic diffusion equation could describe the reaction rates in CaCl₂ solutions. Release rate constants (b) of non‐exchangeable K for different depth of soil profile showed the variations among the treatments indicating that long‐term cropping with different rates of fertilizers and manures influenced the rate of K release from non‐exchangeable fraction of soil. The b values were lowest in 100% NP and highest in 100% NPK+FYM treatment in the surface soil. In the sub‐surface soil layers (15—30 and 30—45 cm) also the higher release rates were obtained in the treatments supplied with K than without K fertilization indicating that the sub‐soils were also stressed for K in these treatments.     

Added nitrogen interaction as affected by soil nitrogen pool size and fertilization – significance of displacement of fixed ammonium

Displacement of NH₄⁺ fixed in clay minerals by fertilizer ¹⁵NH₄⁺ is seen as one mechanism of apparent added nitrogen interactions (ANI), which may cause errors in ¹⁵N tracer studies. Pot and incubation experiments were carried out for a study of displacement of fixed NH₄⁺ by ¹⁵N‐labeled fertilizer (ammonium sulfate and urea). A typical ANI was observed when ¹⁵N‐labeled urea was applied to wheat grown on soils with different N reserves that resulted from their long‐term fertilization history: Plants took up more soil N when receiving fertilizer. Furthermore, an increased uptake of ¹⁵N‐labeled fertilizer, induced by increasing unlabeled soil nitrogen supply, was found. This ANI‐like effect was in the same order of magnitude as the observed ANI. All causes of apparent or real ANI can be excluded as explanation for this effect. Plant N uptake‐related processes beyond current concepts of ANI may be responsible. NH₄⁺ fixation of fertilizer ¹⁵NH₄⁺ in sterilized or non‐sterile, moist soil was immediate and strongly dependent on the rate of fertilizer added. But for the tested range of 20 to 160 mg ¹⁵NH₄⁺‐N kg^–1, the NH₄⁺ fixation rate was low, accounting for only up to 1.3 % of fertilizer N added. For sterilized soil, no re‐mobilization of fixed ¹⁵NH₄⁺ was observed, while in non‐sterile, biologically active soil, 50 % of the initially fixed ¹⁵NH₄⁺ was released up to day 35. Re‐mobilization of ¹⁵NH₄⁺ from the pool of fixed NH₄⁺ started after complete nitrification of all extractable NH₄⁺. Our results indicate that in most cases, experimental error from apparent ANI caused by displacement of fixed NH₄⁺ in clay is unlikely. In addition to the low percentage of only 1.3 % of applied ¹⁵N, present in the pool of fixed NH₄⁺ after 35 days, there were no indications for a real exchange (displacement) of fixed NH₄⁺ by ¹⁵N.     

Leaching Methods Can Underestimate Mineralization Potential of Soils

Aerobic incubations to estimate net nitrogen (N) mineralization typically involve periodic leaching of soil with 0.01 M calcium chloride (CaCl₂), so as to remove mineral N that would otherwise be subject to immobilization. A study was conducted to evaluate the accuracy of leaching for analysis of exchangeable ammonium (NH₄⁺)-N and nitrate + nitrite (NO₃^?+ NO₂^–)-N, relative to conventional extractions using 2 M potassium chloride (KCl). Ten air-dried soils were used, five each from Illinois and Brazil, that had been amended with NH₄⁺-N (1 g kg^?1) and NO₃^–-N (0.6 g kg^?1). Both methods were in good agreement for inorganic N analysis of the Brazilian Oxisols, whereas leaching was significantly lower by 12–48% in recovering exchangeable NH₄⁺-N from Illinois Alfisols, Mollisols, and Histosols. The potential for underestimating net N mineralization was confirmed by a 12-wk incubation experiment showing 9–86% of mineral N recoveries from three temperate soils as exchangeable NH₄⁺.     

Sensitivity of Nitrogen Mineralization Indicators to Crop and Soil Management

Abstract

Soil nitrogen (N) mineralization indicators are useful only if they are sensitive to management practices. The precision of measurement and the sensitivity of the following indicators to crop sequence, tillage, and liming effects were compared: (i) mineral N production during a 24‐day incubation under aerobic conditions, (ii) ammonium (NH₄)‐N production under waterlogged conditions, (iii and iv) hot potassium chloride (KCl)–extractable and hydrolyzable NH₄‐N (the latter obtained by subtracting initial NH₄‐N from extracted NH₄‐N), and (v) protease activity. The coefficients of variation decreased in this order: protease activity>KCl‐hydrolyzable NH₄‐N>aerobic incubation>KCl‐extractable NH₄‐N=anaerobic incubation. Most of the test results obtained using the indicators were correlated with each other. Mineralizable N measured by aerobic and anaerobic incubation was sensitive to tillage, liming, and crop sequences, especially when using soil 5‐cm deep. Hot KCl‐extractable NH₄‐N was influenced by tillage but not liming, and less sensitive than the incubation procedures to crop sequence. The protease assay produced no significant test. It was concluded that anaerobic incubation can provide a relatively sensitive assessment of management effects on soil mineralizable N.     

Correlation of soil test nitrogen with potato yields

Abstract

Yield response of Idaho Russet Burbank potatoes to nitrogen fertilizer was related to soil test for inorganic N (NO₃ ^‐ and NH₄ ⁺) in a total of 27 field experiments over a 3‐year period using polynomial correlation and regression analysis. Nitrate plus ammonium nitrogen content in the surface foot of soil was found to be useful in predicting yield response of potatoes to applied nitrogen.

Correlations between yield and extractable N were considerably better when the data from each cropping system were analyzed separately than when all locations were analyzed as one group. Additional improvement was obtained by including extractable ammonium nitrogen and nitrogen in the second foot of soil respectively. The best correlation with yield was found using (NO₃ ^‐ + NH₄ ⁺)‐N in the surface foot following grain and the top two feet following non‐grain crops with R² values of 0.875 and 0.821 respectively.     

Different Fractions of Soil Potassium,Olsen Phosphorus,and Available Sulfur Status of Intensively Cultivated Berpura Soil Series of India and Nutrient Indexing of Rice Crop

Berpura alluvial soil series of the Indo‐Gangetic Plains is situated in the Ambala District of the Haryana State of India. Soils of this series had medium concentrations of both potassium (K) and phosphorus (P) and large concentrations of sulfur (S) before 1970. To study different fractions of K, Olsen P, and 0.15% calcium chloride (CaCl₂)–extractable (available) S of soils of the Berpura series and to create nutrient indexing of rice crops growing on this series, surface soil samples were collected from 100 farmers' fields after the harvest of the wheat crop in 2005. During kharif season of same year, samples of upper two leaves at anthesis growth stage of rice crop were also collected from the same 100 farmers' fields that had earlier been sampled for soil analysis. Analysis of soil samples showed more K depletion in soils of this series, of which 86% of farmers' fields were deficient in ammonium acetate (NH₄OAc) K (available K). Thirty and 62% of leaf samples of the rice crop growing on the 100 fields of this series were extremely and moderately deficient in K, respectively. The mean values of water‐soluble, exchangeable, nonexchangeable, lattice, and total K were 10.6, 30.3, 390.0, 8204, and 8635 mg kg^?1, respectively. In soils of this series, 0.123, 0.351, 4.517, and 95.009% of total K were found in water‐soluble, exchangeable, nonexchangeable, and lattice K forms, respectively. On the other hand, long‐term farmers' practice of more application of P fertilizer in wheat crop has resulted in P buildup in the soils of the Berpura series. Olsen P in soils of farmers' fields of this series ranged from 9.0 to 153.0 mg kg^?1, with the mean value of 41.8 mg kg^?1. Eighty‐two percent of leaf samples of rice crops grown on this series without application of P fertilizer were sufficient in P. The analysis of soil and rice crops for P and K proved the suitability of 0.5 M sodium bicarbonate (NaHCO₃) and 1 N NH₄OAc for extracting available P and K, respectively, in alluvial soils of the Indo‐Gangetic Plains. The 0.15% CaCl₂–extractable S in this soil ranged from 9.6 to 307 mg kg^?1 with a mean value of 34.6 mg kg^?1. Four and 26% of soil samples had low and medium, respectively, in 0.15% CaCl₂–extractable S. S deficiency was recorded in rice crops, as 29% of the leaf samples were extremely deficient in S and 58% were moderately deficient in S. This indicated the unsuitability of the 0.15% CaCl₂ to extract available S from the Udic ustochrept utilized for cultivation of rice crops.     

Adsorption isotherms of ammonium on coir

Abstract

Coir (coconut mesocarp pith) is the fibrous material that constitutes the thick mesocarp (middle layer) of the coconut fruit (Cocos nucifera L.). It has ion‐exchange and gas adsorption properties that can be utilized to adsorb the important plant nutrient N in its NH₄ ⁺ and/or NH₃ form, protecting it from losses such as during composting of N‐rich wastes. The NH₄ ⁺ adsorption process and the pH buffering capacity of the coir were studied to quantify its effectiveness as an NH₄ ⁺ adsorbent. The one‐ and two‐surface Langmuir, van Bemmelen‐Freundlich, Temkin, and first‐order models were fitted to the NH₄ ⁺ sorption data at an equilibrium pH range of 4.6 to 5.1. Each model was found to describe the NH₄ ⁺ sorption data with comparable success (r²≥0.88), with the Langmuir and Temkin being superior (r²≥0.99). Although the equilibrium pH did not change much after adding buffers of pH 4 to 7 to the coir, there was a consistent increase in the amounts of NH₄ ⁺‐N sorbed with increasing pH, as estimated using the two‐surface Langmuir model. The estimated amounts of NH₄ ⁺‐N sorbed were 516 mmol kg^‐1 at pH 4.6, 521 mmol kg^‐1 at pH 4.63, 571 mmol kg^‐1 at pH 4.88, and 573 mmol kg^‐1 at pH 5.1. The coir showed a good buffering ability at pH range 3.7 to 10.4. The coir buffer strength, defined as the amount of OH” required to increase pH by one unit, was 11.3 cmol kg^‐1 for the pH range 3.7 to 8.4, and 22.1 cmol kg^‐1 for the pH range 8.4 to 10.4. The high CEC, surface area, and buffer strength of the coir indicated its high ability to retain cations and buffer against pH change.     

Soil carbon and nitrogen response to 25 annual cattle manure applications

Improving manure management to benefit both agricultural production and the environment requires a thorough understanding of the long‐term effects of applied manure on soil properties. This paper examines the effect of 25 annual solid cattle manure applications on soil organic carbon (OC), total N (TN), and KCl‐extractable NO₃‐N and NH₄‐N under both non‐irrigated and irrigated conditions. After 25 annual manure applications, OC and TN contents increased significantly with the rate of manure application at the top two sampling depths (0–15 cm and 15–30 cm), and the increases were not affected by the irrigation treatment. The NO₃ content increased at all sampling depths with greater increases observed under non‐irrigated conditions, while NH₄ content was not affected by manure application rates or the irrigation treatment. The changes in OC and TN at the surface (0–15 cm) and 15–30 cm depth were dependent on the cumulative weight of manure added over the years. The relationships between cumulative manure OC added and soil OC content and between cumulative manure TN added and soil TN content were linear and not affected by the irrigation treatment. For every ton of manure OC added, soil OC increased by 0.181 g kg^–1 in the topsoil (0–15 cm). Similarly, for every ton of manure TN added, surface soil TN increased by 0.192 g kg^–1. The linear relationship between manure C added and soil C content suggests that the soil had a high capacity for short‐term C sequestration. However, the total amount of NO₃‐N in the soil profile (0–150 cm) was affected by both the manure application rates and the irrigation treatment. A large amount of NO₃ accumulated in the soil, especially under non‐irrigated conditions. The extremely high level of NO₃ in the soil increases the potential risk of surface and groundwater pollution and losses to atmosphere as N₂O.     

Zinc in wheat grain as affected by nitrogen fertilization and available soil zinc

Greenhouse and field experiments were conducted to determine the influence of nitrogen (N) fertilization and DTPA‐extractable soil zinc (Zn) on Zn concentration in wheat (Triticum aestivum L., cv. Pioneer 2375) grain. Application of zinc sulfate (ZnSO₄) in the range of 0 to 8 mg Zn kg^‐1 increased linearly DTPA‐extractable Zn in an incubated calcareous soil from 0.3 to 5.0 mg kg^‐1. Application of these rates of ZnSO₄ to the same soil under greenhouse conditions increased Zn concentration of wheat grain from 26 to 101 mg kg^‐1. The influence of 134 kg urea‐N ha^‐1 on Zn concentration in wheat grain at eight field sites, with DTPA‐extractable soil Zn levels ranging from 0.3 to 4.9 mg kg^‐1, was studied. Nitrogen fertilizer increased wheat‐grain yields in four of the eight experiments but had little effect on grain‐Zn concentration. Grain‐Zn concentration ranged from 31 to 45 mg kg^‐1 in N‐fertilized plots at the various sites and was related (r=0.74*) to DTPA‐extractable soil Zn.     

Effect of soil amendments on fractionation of Selenium‐enriched soil

Abstract

This study was to determine the effect of soil amendments on the fractionation of selenium (Se) using incubation experiments under simulated upland and flooded conditions. The treatments were as follows: 1) control [soil + sodium selenite (Na₂SeO₃) (1 mg Se kg^‐1)]; 2) control + calcium carbonate (CaCO₃) (5 g kg^‐1); 3) control + alfalfa (40 g kg^‐1); and 4) control + CaCO₃ (5 g kg^‐1) + alfalfa (40 g kg^‐1). After a 90‐day incubation, soil was sampled and fractionated into five fractions: 1) potassium sulfate (K₂SO₄)‐soluble fraction (available to plants); 2) potassium dihydrogen phosphate (KH₂PO₄)‐exchangeable fraction (potentially available); 3) ammonium hydroxide (NH₃H₂O)‐soluble fraction (potentially available); 4) hydrochloric acid (HCl)‐extractable fraction (unavailable); and 5) residual fraction (unavailable). Compared with the control, CaCO₃ increased the K₂SO₄ fraction at the expense of the NH₃H₂O fraction. Alfalfa increased both the K₂SO₄ and residual fractions but reduced the KH₂PO₄ and NH₃H₂O fractions. The CaCO₃‐alfalfa treatment had a similar effect to the alfalfa treatment alone. The comparison between the upland and flooded conditions showed that the flooded condition generally increased the residual fraction and decreased the potentially‐available fractions. In general, CaCO₃ was a better amendment because it not only increased the available fraction but also maintained the potentially available fractions at a high level. The application of Na₂SeO₃ and use of appropriate soil amendments can improve Se availability in soil.     

Comparison of mehlich 3, mehlich 1, ammonium bicarbonate‐DTPA, 1.0M ammonium acetate,and 0.2M ammonium chloride for extraction of calcium,magnesium, phosphorus,and potassium for a wide range of soils

Abstract

Extractants employed for routine soil analysis vary from one laboratory to another. Lack of a universal soil extractant is a serious limitation for interpretation of analytical results from various laboratories on nutritional status of a given soil. This limitation can be overcome by developing functional relationships for concentrations of a given nutrient extractable by various extradants. In this study, extractability of Ca, Mg, P, and K in a wide range of soils (0–15 cm) from citrus groves in Florida representing 21 soil series, with varying cultural operations, were compared using Mehlich 3 (M3), Mehlich 1 (M1), ammonium acetate (NH₄AOc), pH = 7.0 (AA), 0.2M ammonium chloride (NH₄Cl), and ammonium bicarbonate‐DTPA (AB‐DTPA) extractants. Soil pH (0.01M CaCl₂) varied from 3.57 to 7.28. The concentrations of Ca or Mg extractable by M3, M1, AA, and NH₄Cl were strongly correlated with soil pH (r² = 0.381–0.482). Weak but significant correlations were also found between AB‐DTPA extractable Ca or Mg and soil pH (r² = 0.235–0.278). Soil pH relationships with extractable K were rather weak (r² = < 0.131) for M1 and NH₄Cl but non‐significant for M3, AB‐DTPA, and AA. Concentrations of Ca, Mg, and K extractable by M3 were significantly correlated with those by either M1, AA, or NH₄Cl extractants. Mehlich 3‐P was significantly correlated with P extractable by M1 extractant only. Mehlich 3 versus AB‐DTPA relationship was strong for K (r² = 0.964), weaker for Mg and P (r² = 0.180–0.319), and non‐significant for Ca. With the increasing emphasis on possible use of M3 as an universal soil extractant, data from this study support the hypothesis that M3 can be adapted as a suitable extractant for routine soil analysis.     

Impact of Cow Dung Manure on the Solubility of Copper,Lead, and Zinc in Urban Garden Soils from Northern Nigeria

Maintaining the quality of groundwater is a major consideration when developing management practices to effectively use cow dung manure (CDM) as a nutrient source and soil conditioner in agricultural production systems. This study examines the effect of CDM on the solubility of copper (Cu), lead (Pb), and zinc (Zn) in urban garden fields from Kaduna under long-term vegetable production. Soil samples were collected from Kakau, Kakuri, Trikaniya, and Romi in Kaduna metropolis in northern Nigeria. Soil–manure mixtures at the rate of 100 g CDM kg^?1 soil were incubated for 2 weeks and analyzed for exchangeable [0.1 M calcium chloride (CaCl₂)–extractable], mobile [1 M ammonium nitrate (NH₄NO₃)–extractable], and potentially labile [0.05 M ethylenediaminetetraacetic acid (EDTA)–extractable] copper (Cu), lead (Pb), and zinc (Zn). Addition of CDM increased exchangeable Cu in Kakuri and Romi and exchangeable Pb in Kakau and Kakuri, but decreased exchangeable Zn across the sites. The addition of CDM increased mobile Cu and Pb in all the sites. Although there was a decrease in mobile Zn in Kakau and Romi after the soil–manure incubation period, mobile Zn increased in Trikaniya and Kakuri. Furthermore, CDM decreased potentially labile Cu, Pb, and Zn in all the sites except for Romi, which increased labile Cu. To sustain the quality of soil and minimize groundwater pollution and food chain contamination, use of CDM in urban garden soils polluted with Cu, Pb, and Zn should be regulated or discouraged entirely to reduce the mobility of these metals.     

Behavior of soil boron and boron uptake by M.26 apple rootstock as affected by application of different forms and nitrogen rates

Abstract

The changes in availability and uptake of boron (B) by M.26 apple rootstocks as affected by applications of different forms and rates of nitrogen (N) were examined. The study was carried out in a greenhouse using soil with low contents of organic matter, clay, calcium carbonate, NH₄‐oxalate soluble aluminum (Al) and iron (Fe), NH₂OH·HCl extractable manganese (Mn), poor cation exchange capacity and low pH. Soil N application was in the form of urea, calcium nitrate, ammonium sulphate, or ammonium nitrate at rates of 0, 17, 34, and 51 mg N kg^?1. After 1, 3, and 5 days of N application, soil B fractions were determined: B in soil solution, B specifically and non‐specifically adsorbed on soil surfaces, B occluded in Mn oxyhydroxides, and B occluded in crystalline Al and Fe oxides. The results showed that N as calcium nitrate and ammonium nitrate increased B both in soil solution and non‐specifically adsorbed on soil surface and decreased B concentration on Al and Fe oxides. This indicates that N‐NO₃ inhibited B sorption on Fe and Al oxides. Maximum B desorption from Fe and Al oxides was obtained within one day after N‐NO₃ was supplied. Nitrogen application as calcium nitrate and ammonium nitrate increased availability and uptake of B by plant roots. Thus, it was concluded that apple trees planted on coarse‐textured soils where risk of B deficiency is high, calcium nitrate or ammonium nitrates would be appropriately to apply to keep B more available.     

Chloride interference in total nitrogen analysis by the Kjeldahl method

Abstract

Very low recovery of NH₄+‐N was observed in total N determination of (NH₄)₂SO₄ in KC1 solutions by a semimicro Kjeldahl method using permanganate and reduced iron to recover NO₃‐ and NO₂‐, whereas complete recovery was obtained in analysis of NH₄+‐N in water, and of NO₃ ^?‐N or NO₂ ^?‐N in either water or KC1 solutions. The loss of NH₄ ⁺‐N observed with KC1 was attributed to the formation of NCl₃ upon reaction of NH₄ ⁺ with Cl₂ generated during oxidation of Cl^? by MnO₄ ^?. This difficulty is avoided by using K₂SO₄ instead of KC1 for extraction of inorganic N from soil. Complete recovery was obtained by adding ¹⁵N‐labeled NH₄+, NO₃‐, or NO₂‐ to 0.5 M K₂SO₄ soil extracts, and total ¹⁵N analyses of the labeled extracts were in good agreement with values calculated from the additions of ¹⁵N and the total N contents of the soil extracts.     

Does the potential ammonium fixation of soils have an impact on the optimum nitrogen fertilizer rate?

Field experiments were conducted to determine the effect of nitrogen (N) fertilizer forms and doses on wheat (Triticum aestivum L.) on three soils differing in their ammonium (NH₄) fixation capacity [high = 161 mg fixed NH₄-N kg^?1 soil, medium = 31.5 mg fixed NH₄-N kg^?1 soil and no = nearly no fixed NH₄-N kg^?1 soil]. On high NH₄⁺ fixing soil, 80 kg N ha^?1 Urea+ ammonium nitrate [NH₄NO₃] or 240 kg N ha^?1 ammonium sulfate [(NH₄)₂SO₄]+(NH₄)₂SO₄, was required to obtain the maximum yield. Urea + NH₄NO₃ generally showed the highest significance in respect to the agronomic efficiency of N fertilizers. In the non NH₄⁺ fixing soil, 80 kg N ha^?1 urea+NH₄NO₃ was enough to obtain high grain yield. The agronomic efficiency of N fertilizers was generally higher in the non NH₄⁺ fixing soil than in the others. Grain protein was highly affected by NH₄⁺ fixation capacities and N doses. Harvest index was affected by the NH₄⁺ fixation capacity at the 1% significance level.     

Inorganic sulphate extraction from SO2-impacted Andosols

Sulphate sorption on to the surface of short‐range ordered minerals and precipitation of Al‐hydroxy sulphate contribute to the acid neutralizing capacity of soils. The correct measurement of total inorganic sulphate is thus essential in soils that are accumulating SO₄^2– anions. We extracted SO₄^2– by various solutions, namely 0.005 m Ca(NO₃)₂, 0.016 m KH₂PO₄, 0.5 m NH₄F and 0.2 m acidic NH₄‐oxalate (pH 3), from Vitric and Eutric Andosols exposed to prolonged deposition of acid and SO₂ from an active volcano (Masaya, Nicaragua). We attributed sulphate extractable by KH₂PO₄ (20–3030 mg kg^?1) to anion‐exchangeable SO₄^2–, which was much smaller than NH₄F‐ and oxalate‐extractable SO₄^2– (400–9680 and 410–10 480 mg kg^?1, respectively). Our results suggest the occurrence of a sparingly soluble Al‐hydroxy‐mineral phase extractable by both NH₄F and oxalate. The formation of Al‐hydroxy minerals would result from the combination of enhanced weathering caused by strong acid loading and simultaneous occurrence of large SO₄^2– concentrations in soil solution. Oxalate extracted slightly more inorganic SO₄^2– than did NH₄F, this additional amount of SO₄^2– correlating strongly with oxalate‐extractable Si and Fe contents. Preferential occlusion of SO₄^2– by short‐range ordered minerals, especially ferrihydrite, explains this behaviour. If we exclude the contribution of occluded sulphate then oxalate and NH₄F mobilize similar amounts of SO₄^2– and are believed to mobilize all of the inorganic SO₄^2– pool.     

Nitrate and ammonium nutrition in chicory and rocket salad plants

To evaluate chicory (Cichorium intybus L.) and rocket salad [Eruca vesicaria (L.) Cav.subsp. sativa (Mill.)] capability to use ammonium‐nitrogen (NH₄‐N) even in the absence of nitrate‐nitrogen (NO₃‐N) in the nutrient solution, and the chances they offer to reduce leaf NO₃ content, cultivated rocket and two cultivars of chicory ('Frastagliata’, whose edible parts are leaves and stems, and ‘Clio’, a leaf hybrid) were hydroponically grown in a growth chamber. Three nutrient solutions with the same nitrogen (N) level (4 mM) but a different NH₄‐N:NO₃‐N (NH₄:NO₃) ratio (100:0, 50:50, and 0:100) were used. Rocket growth was inhibited by NH₄ nutrition, while it reached the highest values with the NH₄:NO₃ ratio 50:50. Water and N‐use efficiencies increased in rocket with the increase of NO₃‐N percentage in the nutrient solution. In the best conditions of N nutrition, however, rocket accumulated NO₃ in leaves in a very high concentration (about 6,300 mg kg^‐1 fresh mass). For all the morphological and yield features analyzed, chicory resulted to be quite unresponsive to N chemical forms, despite it took more NO₃‐N than NH₄‐N when N was administered in mixed form. By increasing NO₃‐N percentage in the nutrient solution, NO₃ leaf content increased (5,466 mg kg^‐1 fresh mass with the ratio NH₄:NO₃ 0:100). On average, both chicory cultivars accumulated 213 mg NO₃ kg^‐1 fresh mass with the ratio NH₄:NO₃ 100:0 and, differently from rocket, they showed that by using NH₄ produce can be obtained very low in NO₃ content.     

Pedotransfer functions for the pool size of slowly mineralizable organic N in sandy arable soils

The major aim of this study was to evaluate how the pool size of slowly mineralizable, ‘old’ soil organic N can be derived from more easily accessible soil and site information via pedotransfer functions (PTF). Besides modeling, this pool size might be of great importance for the identification of soils with high mineralization potential in drinking‐water catchments. From long‐term laboratory incubations (ca. 200 days) at 35 °C, the pool sizes of easily mineralizable organic N (N_fast), mainly in fresh residues, and slowly mineralizable, ‘old’ soil organic N (N_slow) as well as their first‐order rate coefficients were obtained. 90 sandy arable soils from NW Germany served to derive PTFs for N_slow that were evaluated using another 20 soils from the same region. Information on former land‐use and soil type was obtained from topographical, historical, and soil maps (partly from 1780). Pool size N_slow very strongly depends on soil type and former land‐use. Mean pool sizes of N_slow were much lower in old arable lowland (105 mg N kg^–1) than upland soils (175 mg N kg^–1) possibly due to lower clay contents. Within lowlands, mean pool sizes in former grassland soils (245 mg N kg^–1) were 2 to 3 times larger than in old arable soils due to accumulation of mineralizable N. In contrast, mean pool sizes of N_slow were lowest in recently cleared, former heath‐ and woodland (31 mg N kg^–1) as a result of the input of hardly decomposable organic matter. Neither N nor C in the light fraction (density < 1.8 g cm^–3) was adequate to derive pool size N_slow in the studied soils (r² < 0.03). Instead, N_slow can be accurately (r² = 0.55 – 0.83) derived from one or two basic soil characteristics (e.g. organic C, total N, C : N, mineral fraction < 20 μm), provided that sites were grouped by former land‐use. Field mineralization from N_slow during winter (independent data set) can be predicted as well on the basis of N_slow‐values calculated from PTFs that were derived after grouping the soils by former land‐use (r² = 0.51***). In contrast, using the PTF without soil grouping strongly reduced the reliability (r² = 0.16).     

An evaluation of some manual colorimetric methods for the determination of inorganic nitrogen in soil extracts

Abstract

A number of manual colorimetric methods for the determination of inorganic nitrogen in 1 M KCl soil extracts were investigated to find techniques that were inexpensive, rapid, versatile and suitable for laboratories with limited analytical equipment. Three colorimetric methods for No^? ₃‐N determination were evaluated and only the copperised/cadmium reduction technique suffered no significant interference from the Cl^? present in the extracting solution. A phenol‐hypo‐chlorite (Berthelot) procedure for NH⁺ ₄‐N determination and the Griess‐Ilosvay method for NO^? ₂‐N determination were both found suitable for N determination in 1M KC1 soil extracts. The reliability and accuracy obtainable with the manual colorimetric methods described was shown to be comparable with that obtained from colorimetric analyses performed using an AutoAnalyser.