The effect of soil flooding on the transformation of Fe oxides and the adsorption/desorption behavior of phosphate

As repeatedly reported, soil flooding improves the availability of P to rice. This is in contrast with an increased P sorption in paddy soils. The effects of soil flooding on the transformation of Fe oxides and the adsorption/desorption of P of two paddy soils of Zhejiang Province in Southeast‐China were studied in anaerobic incubation experiments (submerging with water in N₂ atmosphere). Soil flooding significantly increased oxalate‐extractable Fe (Fe_ox), mainly at the expense of dithionite‐soluble Fe (Fe_DCB), as well as oxalate‐extractable P (P_ox), but decreased the ratio of P_ox/Fe_ox. Flooding largely increased both, P adsorption and the maximum P adsorption capacity. The majority of newly sorbed P in the soils was P_ox, but also more newly retained P was found to be not extractable by oxalate. Flooding also changed the characteristics of P desorption in the soils. Due to a decrease of the saturation index of the P sorption capacity, P adsorbed by flooded soils was much less desorbable than that from non‐flooded soils. There are obviously significant differences in the nature of both, the Fe_ox and P_ox fractions under non‐flooded and flooded conditions. The degree of the changes in Fe_ox, P_ox, P adsorption and P desorption by flooding depended on the contents of amorphous and total Fe oxides in non‐flooded soils. Our results confirm that the adsorption and desorption behavior of P in paddy soils is largely controlled by the transformation of the Fe oxides. The reasons of the often‐reported improved P availability to rice induced by flooding, in spite of the unfavorable effect on P desorbability, are discussed.     

Soil Properties Influencing Iron Chlorosis in Grapevines Grown in the Montilla‐Moriles Area,Southern Spain

Abstract

Iron (Fe) chlorosis, an Fe deficiency commonly observed in grapevines cultivated on calcareous soils, generally inhibits plant growth and decreases yield. The objective of this research was to relate the incidence of Fe chlorosis in vines of the Montilla‐Moriles area, southern Spain, to indigenous soil properties. Thirty‐five grapevines (V. vinífera L. cv. Pedro Ximenez grafted on V. berlandieri×V. rupestris 110 Ritcher) showing different degree of Fe chlorosis were selected from 13 vineyards. The leaf chlorophyll concentration (estimated by the SPAD value measured with a Minolta meter) was positively correlated with the contents in different soil Fe forms but not with alkalinity‐related soil properties (pH, calcium carbonate equivalent, and active lime). The acid NH₄ oxalate‐extractable Fe (Fe_o) was the most useful simple variable to predict the occurrence of Fe chlorosis. A Fe_o/active lime ratio of 25×10^–4 was found to be useful to class soils into two groups according to the probability of inducing Fe chlorosis.     

Predicting the incidence of iron chlorosis in calcareous soils of southern Spain

Abstract

Iron chlorosis is a serious crop production problem in many calcareous soils of Southern Spain. The objective of this study was to determine which indigenous soil properties (i.e., those which are essentially permanent) were related to Fe chlorosis. Experiments, using two chickpea (Cicer ariethinum L.) cultivars and a sunflower (Helianthus annuuus L.) cultivar, were carried out in a growth chamber with 25 calcareous soils representing widespread Xerofluvents, Xerorthents, Xerochrepts, Haploxeralfs, Rodoxeralfs, Chromoxererts, and Pelloxererts of Southern Spain. The average chlorophyll contents for the three cultivars were significantly correlated with several properties of the carbonate and Fe oxide phases, such as calcium carbonate equivalent (r = 0.69***), “active lime”; (r = 0.58**), acid NH₄‐oxalate extractable Fe (r = 0.68***), Tiron‐extractable Fe (r = 0.61**), and DTPA‐extractable Fe (r = 0.55**). The present and other studies indicate that the soil property most consistently related to Fe chlorosis is acid NH₄‐oxalate extractable Fe (Feo). The Feo critical level separating soils with a high probability from those with a low probability of responding to Fe fertilization was 0.63 g/kg soil, a value similar to those found in other studies. This further supports the use of Feo as a key property to predicting the appearance of Fe chlorosis.     

Resistance of mineral soils to Fe(III) reduction

Gleying and enhancement of hydromorphism in wetland soils due to Fe(III) reduction entail a series of degradation processes. The resistance of wetlands to degradation can be calculated from the content of potentially reducible iron, Fe(III)_pr, which is found from the van Bodegom equation taking into account the contents of oxalate-soluble iron Fe_ox and dithionite-soluble iron Fe_dit in the soil. In addition, this makes it possible to distinguish relict and actual gleysols. The van Bodegom equation is applicable to soils from which the oxalate solution extracts only amorphous and poorly crystallized iron compounds, which are quickly reduced by Fe-reducing bacteria. These soils have a low proportion of Fe(II) (no more that 15% of the total iron), as well as an accumulative profile distribution of Fe_ox. The van Bodegom equation is unsuitable for calculating the Fe(III)_pr content in soils with a high proportion of Fe(II) and a nonaccumulative profile distribution of Fe_ox.     

Assessing alternative approaches to predicting soil phosphorus sorption

Twenty‐five pasture soils were sampled from high‐rainfall zones of southeastern Australia to examine relationships between soil properties, and between soil properties and P buffering capacity (PBC) measures. Correlations between PBC values and soil properties were generally poor, with the exception of oxalate‐extractable Al (Al_ox) (r ≥ 0.97). Predictions of PBC were further improved when clay, as well as Al_ox, was included in a linear regression model (r² ≥ 0.98). When Al_ox and oxalate‐extractable Fe were excluded from the modelling exercise, a more complex three‐term linear regression model, including pH_H2O, exchangeable H and cation exchange capacity, adequately fitted both PBC values of the 25 soils examined in this study (r² ≥ 0.76). However, the Al_ox, Al_ox plus clay and the three‐term models gave poor predictions of the PBC values when the models were validated using 28 independent soils. These results emphasize the importance of model validation, because predictive models based on soil properties were not robust when tested across a broader range of soil types. In comparison, direct measures of PBC, such as single‐point P sorption measures, are more practical and robust methods of estimating PBC for Australian soils.     

Phosphate sorption by Egyptian,Ethiopian and German soils and P uptake by rye (Secale cereale L.) seedlings

Phosphate sorption was studied in samples (0 - 20 cm depth) of five soils from Egypt (pH 7.4 - 8.7), four soils from Ethiopia (pH 3.9 - 5.3) and six soils from Germany (pH 3.3 - 7.2). Sorption parameters were calculated according to Pagel and Van Huay (1976) and according to Langmuir (Syers et al., 1973). Phosphate sorption parameters and oxalate extractable Fe and Al (Fe_ox, Al_ox) were related to the phosphate uptake by young rye plants in Neubauer pot experiments. P sorption parameter after Pagel and Van Huay (A) correlated significantly positively with the Fe_ox and Al_ox content in acid (r = 0.73) as well as in calcareous soils (r = 0.89) if the whole equilibrium concentration range (0 - 14 mg P/L) was considered. The relations calculated after Langmuir (B) were similar. P uptake by rye in acid soils was negatively correlated with the affinity constant n (r = ?0.76, (A)). In calcareous soils, a negative correlation between P uptake and affinity constant was calculated in the lower P equilibrium range (0 - 2.8 mg P/L) only for (B). Thus, P uptake decreased with increasing strength of P bonding to soil. From these results it is concluded that phosphate sorbed to Fe/Al oxides is an important P source for plants in acid and calcareous soils.     

Colloidal stability in some tropical soils of southeastern Nigeria as affected by iron and aluminium oxides

The stability of microaggregates in soils as opposed to its dispersion is a very important soil phenomenon that checks degradation arising from unguided tillage and soil erosion. Ten soils from southeastern Nigeria were sampled from their typical A and B horizons for the study. The aim was to identify the extent of colloidal stability of the soils and the forms of Fe and Al oxides in the soils contributing to their stability. The soils are mostly Ultisols and Inceptisols formed on sandstones and shale parent materials. The soils are low in soil basic cations including the soil organic carbon (SOC). The major clay mineral is kaolinite while the soil is acid in reaction. The various forms of soil Fe and Al oxides are high with the total forms of Fe and Al being most dominant and > dithionite extracted Fe and Al > oxalate extracted Fe and Al > pyrophosphate extracted Fe and Al. The water-dispersible clay and silt (WDC) and (WDSi) which are index of dispersion in most soils are low to medium thus reflecting in the low to medium dispersion ratio (DR). The clay flocculation index (CFI) and aggregated silt + clay (ASC) were moderate to high implying the high potential stability of the soils. Soil organic carbon did not seem to be contributing much to the stability of the microaggregates while oxalate and pyrophosphate extractable Fe (Fe_ox, Fe_p) and to some extent total Al (Al_t) were among the different forms of oxides that act as aggregating agents. We propose here that rather than SOC acting as a disaggregating agent in the soils, it might have acted in association with these oxides in a linkage or bridge such as C–P–OM–C to ensure stability of the soils.     

Micronutrient deficiencies in rainfed calcareous soils of Pakistan. I. Iron chlorosis in the peanut plant

Abstract

Peanut (Arachis hypogaea L.) is susceptible to iron (Fe) chlorosis, however, plant analysis diagnostic criteria are lacking for determining the intensity of chlorosis in this crop. As total Fe content is a misleading index of Fe nutritional status of plants, determination of physiologically active Fe fraction (Fe²⁺) is suggested for the purpose. In a nutrient indexing survey of the chlorosis‐affected peanut crop grown in the rainfed Potohar plateau of Pakistan, o‐phenanthroline extractable Fe²⁺ concentration in plants decreased with increasing severity of chlorosis and thus proved an effective technique for determining the intensity of Fe chlorosis. Green plants contained 40.1 to 67.3 mg Fe²⁺/kg, mildly chlorotic 32.1 to 40.0 mg Fe²⁺/kg, moderately chlorotic 28.0 to 32.0 mg Fe²⁺/kg, and severely chlorotic <28.0 mg Fe²⁺/kg. The minimum Fe²⁺ requirement in green plants was estimated to be 40 mg/kg on dry weight basis. In rainfed field experiments on a calcareous Typic Hapludalfs soil, foliar sprays of 1% solution of sequestrene (NaFeEDDHA) proved superior to the foliar sprays of 0.5% FeSO₄.7H₂O in correcting Fe chlorosis in two cultivars of peanut. Maximum increase in pod yield with sequestrene was 42% in cv. BARD‐92 and 27% in cv. BARD‐699 over the respective control yields. Ferrous concentration in plants increased with both the Fe sources, however, a substantial increase was recorded only with sequestrene. As peanut is a low‐input high‐risk rainfed crop, correction of Fe chlorosis by using sequestrene may not be economically feasible. Thus, development and/or screening of peanut varieties tolerant to Fe chlorosis is suggested by employing Fe²⁺ analysis technique.     

Potential Iron and Phosphate Mobilization During Flooding of Soil Material

This study shows that mobilization of phosphate from soils under anaerobic conditions can be intimately coupled with reductive dissolution of iron from iron oxides. Among four soil samples from the reclaimed Skjernå estuary in Denmark incubated anaerobically and amended with glucose, 28–39% of the dithionite-citrate-bicarbonate-extractable iron and 10–25% of the oxalate-extractable phosphorus (P_ox) were released to the soil solution after 31 days. Significant correlation (r = 0.992**) between the molar ratio P_ox/(Fe_ox + Al_ox) for the aerobic samples and (P_{P sol}/Fe_sol) (the molar ratio between phosphate and iron in solution during anaerobic incubation), indicates that the phosphate saturation status of the soil is an important determinant of the amount of phosphate released during flooding of moderately acid soils.     

Estimation of the phosphorus sorption capacity of acidic soils in Ireland

The test for the degree of phosphorus (P) saturation (DPS) of soils is used in northwest Europe to estimate the potential of P loss from soil to water. It expresses the historic sorption of P by soil as a percentage of the soil's P sorption capacity (PSC), which is taken to be α (Al_ox + Fe_ox), where Al_ox and Fe_ox are the amounts of aluminium and iron extracted by a single extraction of oxalate. All quantities are measured as mmol kg soil^?1, and a value of 0.5 is commonly used for the scaling factor α in this equation. Historic or previously sorbed P is taken to be the quantity of P extracted by oxalate (P_ox) so that DPS = P_ox/PSC. The relation between PSC and Al_ox, Fe_ox and P_ox was determined for 37 soil samples from Northern Ireland with relatively large clay and organic matter contents. Sorption of P, measured over 252 days, was strongly correlated with the amounts of Al_ox and Fe_ox extracted, but there was also a negative correlation with P_ox. When PSC was calculated as the sum of the measured sorption after 252 days and P_ox, the multiple regression of PSC on Al_ox and Fe_ox gave the equation PSC = 36.6 + 0.61 Al_ox+ 0.31 Fe_ox with a coefficient of determination (R²) of 0.92. The regression intercept of 36.6 was significantly greater than zero. The 95% confidence limits for the regression coefficients of Al_ox and Fe_ox did not overlap, indicating a significantly larger regression coefficient of P sorption on Al_ox than on Fe_ox. When loss on ignition was employed as an additional variable in the multiple regression of PSC on Al_ox and Fe_ox, it was positively correlated with PSC. Although the regression coefficient for loss on ignition was statistically significant (P < 0.001), the impact of this variable was small as its inclusion in the multiple regression increased R² by only 0.028. Values of P sorption measured over 252 days were on average 2.75 (range 2.0–3.8) times greater than an overnight index of P sorption. Measures of DPS were less well correlated with water‐soluble P than either the Olsen or Morgan tests for P in soil.     

Genetic Differences in Resistance to Iron Deficiency Chlorosis in Peanut

Iron (Fe) deficiency has been a widespread problem in peanut (Arachis hypogaea L.) grown on calcareous soils of northern China and has resulted in significant yield losses. Field observations showed considerable variability in visual chlorosis symptoms among peanut cultivars in the same soil. The objective of this study was to confirm the genetic differences in resistance to Fe-deficiency chlorosis in peanut and to identify feasible indicators for screening Fe-efficient genotypes. Resistance to Fe chlorosis of sixteen peanut cultivars grown on calcareous soil was evaluated in the field and physiological responses to Fe-deficiency stress were studied in nutrient solution. There were significant differences in resistance to Fe-deficiency chlorosis among the sixteen peanut cultivars tested, which was identified with SPAD readings, active Fe concentrations in young leaves in the early growth stages, and the pod yield. For Fe-resistant peanut cultivars, Fe-reduction capacity and quality of releasing hydrogen ions from roots increased under Fe-deficiency stress. Highly correlated relationships were observed between the summation of root Fe reduction and field chlorosis scores for sixteen cultivars (r² = 0.79). It was concluded that Fe-reduction capacity was a better physiological indicator for screening Fe-efficient peanut genotypes of the mechanisms measured.     

Iron treatment of lime‐induced chlorosis: Implications for chlorophyll,Fe2 +, Fe3 + and K+ in leaves 1

This study addressed some complementary aspects related to plant Fe nutrition. A field and a greenhouse experiment were conducted to monitor changes in chlorophyll, Fe³⁺, Fe²⁺, Ca²⁺ and K⁺ along with the progressive evolution of lime‐induced chlorosis, and following soil (Fe‐EDDHA, Fe‐EDTA, Fe‐DTPA, DTPA) and foliar (Fe‐EDDHA, FeSO₄, “Fe‐Metalosate") treatments, in a chlorosis‐susceptible ornamental plant, Hydrangea macrophylla, over a year's growing period. Though soil Fe‐EDDHA was the most effective compound in alleviating chlorosis symptoms, it became less so with time and was only partly effective as a foliar spray. Leaf analysis showed that as chlorosis intensified and chlorophyll content decreased, phenanthroline ‐ Fe (Fe²⁺) decreased with corresponding increases in total iron (Fe³⁺) and K⁺ concentrations. The reliability of these chlorosis‐indicators was confirmed as the reverse changes occurred upon chlorosis plant recovery.     

Volcanic ash additions control soil carbon accumulation in brown forest soils in Japan

Abstract

The objective of the present study was to clarify the influence of volcanic ash addition on soil carbon stocks and the carbon accumulation process in brown forest soils (BFS) in Japan. The degree of volcanic ash addition to the soil was estimated according to the acid ammonium oxalate extractable aluminum (Al_ox) and lithic fragment contents, and their vertical distribution patterns. The BFS was classified in order of increasing volcanic ash influence on the soil into the following types: high Al_ox content with no gravel (H-Al_ox-NGv), high Al_ox with a high gravel content (H-Al_ox-Gv), moderate Al_ox (M-Al_ox), and low Al_ox (L-Al_ox), and then analyzed for carbon content, carbon amount, carbon stock, Al_ox amount and pyrophosphate extractable aluminum (Al_py) amount. The correlation between the carbon and Al_py amounts and the relationship between the Al_py and Al_ox amounts in the BFS samples indicated that the amount of carbon is determined by Al—humus complex formation, which is defined by the active Al generated from additional volcanic ash in BFS soil samples of BFS. Therefore, soils with thicker horizons and greater amounts of Al_ox had higher carbon levels in deeper horizons. For this reason, soil carbon stocks at depths of 0–30 cm and 0–100 cm, and in the effective soil depth of BFS, were larger and followed the order H-Al_ox-NGv = H-Al_ox-Gv > M-Al_ox > L-Al_ox. Furthermore, successive accumulations of volcanic ash on the soil surface promoted soil carbon accumulation as a result of the development of the surface horizon in H-Al_ox-NGv BFS. Our results suggest that volcanic ash additions control the soil carbon accumulation of forest soil in Japan.     

Soil organic matter stabilization in acidic forest soils is preferential and soil type-specific

Minerals with large specific surface areas promote the stabilization of soil organic matter (SOM). We analysed three acidic soils (dystric, skeletic Leptic Cambisol; dystric, laxic Leptic Cambisol; skeletic Leptic Entic Podzol) under Norway spruce (Picea abies) forest with different mineral compositions to determine the effects of soil type on carbon (C) stabilization in soil. The relationship between the amount and chemical composition of soil organic matter (SOM), clay content, oxalate‐extractable Fe and Al (Fe_o; Al_o), and dithionite‐extractable Fe (Fe_d) before and after treatment with 10% hydrofluoric acid (HF) in topsoil and subsoil horizons was analysed. Radiocarbon age, ¹³C CPMAS NMR spectra, lignin phenol content and neutral sugar content in the soils before and after HF‐treatment were determined and compared for bulk soil samples and particle size separates. Changes in the chemical composition of SOM after HF‐treatment were small for the A‐horizons. In contrast, for B‐horizons, HF‐soluble (mineral‐associated) and HF‐resistant (non‐mineral‐associated) SOM showed systematic differences in functional C groups. The non‐mineral associated SOM in the B‐horizons was significantly depleted in microbially‐derived sugars, and the contribution of O/N‐alkyl C to total organic C was less after HF‐treatment. The radiocarbon age of the mineral‐associated SOM was younger than that of the HF‐resistant SOM in subsoil horizons with small amounts of oxalate‐extractable Al and Fe. However, in horizons with large amounts of oxalate‐extractable Al and Fe the HF‐soluble SOM was considerably older than the HF‐resistant SOM. In acid subsoils a specific fraction of the organic C pool (O/N‐alkyl C; microbially‐derived sugars) is preferentially stabilized by association with Fe and Al minerals. Stabilization of SOM with the mineral matrix in soils with large amounts of oxalate‐extractable Al_o and Fe_o results in a particularly stable and relatively old C pool, which is potentially stable for thousands of years.     

Effects of soil flooding on P transformations in soils of the Mesopotamia region,Argentina

In the Mesopotamia region (Argentina), rice is cropped on a wide range of soil types, and the response of rice to fertilizer application has been inconsistent even in soils with very low levels of available phosphorus. Phosphorus transformations in flooded soils depend on soil characteristics that may affect phosphorus availability. This study was conducted to determine which soil characteristics were related to the changes in P fractions during soil flooding. Soils were chosen from ten sites within the Mesopotamia region that are included in five different soil orders: Oxisols, Ultisols, Alfisols, Mollisols, and Vertisols. Soil phosphorus (P) was fractionated by a modified Hedley method before and after a 45 d anaerobic‐incubation period. Changes in the inorganic P extracted with resin depended on soil pH and were related to the exchangeable‐Fe concentration of soils (extracted with EDTA). Inorganic P extracted with alkaline extractants (NaHCO₃ and NaOH) increased due to soil flooding. This increase was related to the organic‐C (OC) percentage of soils (r² = 0.62, p < 0.01), and ranged from 13 to 55 mg kg^–1. Even though previous studies showed that P associated with poorly crystalline Fe played an important role in the P nutrition of flooded rice, in this study, there was no relationship between ammonium oxalate–extractable Fe and P changes in soils due to flooding. Our results suggest that in the Mesopotamia region, changes in P fractions due to soil flooding are related to soil OC, soil pH, and soluble and weakly adsorbed Fe.     

Soil flooding and rice straw addition can increase isotopic exchangeable phosphorus in P‐deficient tropical soils

Soil flooding increases phosphorus (P) availability due to reductive dissolution of P‐bearing Fe(III) minerals. It is, however, unclear whether such processes also act in P‐deficient soils of the tropics that have large Fe/P ratios (dithionite‐ and oxalate‐extractable P and Fe). The objective was to identify the extent of P release induced by flooding in such soils and the soil characteristics involved. Six topsoils (0.4–5% Fe) from rice fields in Madagascar were incubated aerobically and anaerobically for 66 days amended with factorial combinations of (0, 50 mg P/kg); half of the flooded soils were also amended with 1 g rice straw/kg prior to flooding to stimulate soil oxygen depletion. The release of P after flooding was measured at day 40 with ³³P isotopic exchange, which detects both changes of labile P (exchangeable P) and changes in P solubility. Flooding increased labile P concentration in soil compared with aerobic soils by 1.4–60 mg P/kg, effects being significant in 6 of the 12 soil samples. Rice straw addition further increased the labile P in 5 of the 12 flooded soil samples by 2–27 mg P/kg. The release of labile P by flooding increased with soil oxalate‐extractable P concentration. Flooding combined with rice straw addition can increase the labile P in soil, even in soils with large amount of Fe; however, this release in unfertilized soils is likely insufficient for optimal nutrition of rice plants when evaluated against critical values for P solubility.     

CHLOROSIS IN WILD PLANTS: IS IT A SIGN OF IRON DEFICIENCY?

ABSTRACT

Chlorosis in crops grown on calcareous soil is mainly due to iron (Fe) deficiency and can be alleviated by leaf application of soluble Fe²⁺ or diluted acids. Whether chlorosis in indigenous plants forced to grow on a calcareous soil is also caused by Fe deficiency has, however, not been demonstrated. Veronica officinalis, a widespread calcifuge plant in Central and Northern Europe, was cultivated in two experiments on acid and calcareous soils. As phosphorus (P) deficiency is one of the major causes of the inability of many calcifuges to grow on calcareous soil we added phosphate to half of the soils. Leaves in pots with the unfertilized and the P-fertilized soil, respectively, were either sprayed with FeSO₄ solution or left unsprayed. Total Fe, P, and manganese (Mn) in leaves and roots and N remaining in the soil after the experiment were determined. In a second experiment, no P was added. Leaves were either sprayed with FeSO₄ or with H₂SO₄ of the same pH as the FeSO₄ solution. Degree of chlorosis and Fe content in leaves were determined. Calcareous soil grown plants suffered from chlorosis, which was even more pronounced in the soils supplied with P. Newly produced leaves were green with Fe spray but leaves that were chlorotic before the onset of spraying did not totally recover. H₂SO₄ spray even increased chlorosis. This demonstrated that chlorosis was due to Fe deficiency. As total leaf Fe was similar on acid and calcareous soil, it was a physiological Fe deficiency, caused by leaf tissue immobilization in a form that was not metabolically “active”. Iron in the leaves was also extracted by 1,10-phenanthroline, an Fe chelator. In both experiments, significant differences between leaves from acid and calcareous soil were found in 1,10-phenanthroline extractable Fe but not in total leaf Fe, when calculated on a dry weight basis. Differences in 1,10-phenanthroline extractable Fe were more pronounced when calculated per unit dry weight than calculated per leaf area, whereas the opposite condition was valid for total leaf Fe.     

Investigation of relationships between iron status of peach leaves and soil properties

This study investigated those soil factors related to iron (Fe) chlorosis between Fe status of peach leaves and some soil properties in the Antalya region of Turkey. The total Fe content of leaves was negatively correlated with soil pH and the organic matter content of the soils. Extractable Fe (by 1N HCl) was negatively correlated with the calcium carbonate (CaCO₃) and bicarbonate (HCO₃‐) content of the soils. In addition, both total‐ and extractable‐Fe contents of leaves were also negatively correlated with the copper (Cu) content of the soils. On the other hand, significant correlations were found among the Fe index, P/Fe ratio of leaves, and soil pH, phosphorus (P), zinc (Zn), and Cu content of the soils. It appears from these studies that high pH, and the CaCO₃, HCO₃‐, and Cu contents are effective soil factors affecting the availability of Fe and its uptake by the peach trees, and these soil factors were associated with severity of Fe chlorosis in the studied area.     

INHERITANCE OF RESISTANCE TO IRON DEFICIENCY CHLOROSIS IN CHICKPEA (CICER ARIETINUM L.)

Iron (Fe)-deficiency chlorosis causes considerable yield losses in chickpea (Cicer arietinum L.) when susceptible genotypes are grown in calcareous soils with high pH. The most feasible method for alleviating Fe deficiency is the selection of suitable cultivars resistant to Fe deficiency chlorosis. ICC 6119 (desi type), which is Fe-deficient chlorosis, was crossed with CA 2969 and Sierra (kabuli types), resistant to Fe deficiency chlorosis. Inheritance of resistance to Fe deficiency in chickpea revealed that the resistance was controlled by a single dominant gene in these genotypes crossed. A negative selection for resistance to Fe deficiency chlorosis will be effective after segregating generations.     

Oxalate-soluble compounds of Al, Fe, and Si in soils of cold humid regions as a function of weathering

Original and published data on the contents of X-ray amorphous oxalate-soluble compounds of Al, Fe, and Si in mesomorphic eluvial soils of cold, moderately cold, and moderately warm continental humid and semihumid regions are generalized. The groups of soils developed from mafic igneous, metamorphic, and pyroclastic rocks are considered. It is shown that the content of oxalate-soluble oxides (OSOx) in the horizons of their maximal accumulation varies from less than 1% to 20–30%; the Al_ox/Fe_ox ratio varies from 1 to 6.5. The leading factor dictating the amount and quality of the OSOx in the soils is the presence or absence of volcanic glass in the parent materials. The boundary between the soils with and without volcanic glass corresponds to the OSOx content of 5% and the Al₂O_3ox/Fe₂O_3ox ratio equal to 2. These criteria are more reliable than the Al_ox/Fe_ox ratio used by foreign soil scientists to specify Andosols (Al_ox/Fe_ox > 2). The contents of oxalate-soluble oxides of Al and Fe do not depend on the total contents of these oxides in the parent material and seem to be related to the presence of these elements in minerals with different resistance to weathering. Under the natural conditions described in this paper, the content of OSOx shows a very weak response to zonal (temperature-controlled) climatic changes and/or to changes in the degree of humidity and the continentality of the climate.