David L. Rowell

Soil acidification: comparison of acid deposition from the atmosphere with inputs from the litter/soil organic layer

Abstract At three relatively unpolluted sites in beech ( Fagus sylvatica ) woodlands on the Chiltern Hills of southern England, measurements were made over one year of the amount and composition of precipitation, throughfall, stemflow, and drainage below the soil organic layer. At two sites the soils were acidic and at the third the soil contained calcium carbonate. Comparison was made between the soil acidification potential (A.P.) of throughfall calculated as (H + + 2NH 4 + ) and the A.P. of the drainage water from the soil organic layer calculated as the difference between the sum of cations and the sum of Cl − , SO 4 2− and NO 3 − , that is, as HCO 3 − + organic anions. In the two woodlands with acidic soil the A.P. of throughfall was 198 mmol c m −2 a −1 and that of the drainage water was 224 mmol c m −2 a −1 ; the corresponding figures for the woodland with calcareous soil were 176 and 511 mmol c m −2 a −1 . The increases in the drainage water are attributed mainly to organic anions in the acidic soils and bicarbonate in the calcareous soil. The relative importance of the components in throughfall and the organic anions in drainage from the soil organic layer is discussed in relation to soil acidification.

Effect of ammonium and nitrate on indigenous mycorrhizal infection, rhizosphere pH change, and phosphorus uptake by sorghum

Abstract Sorghum (Sorghum bicolor L.) plants were grown in split pots in three Rothamsted soils with different soil pH values and phosphorus (P) contents. Ammonium addition resulted in higher plant dry weight and P content than comparable nitrate treatments. The pH of soils in the rhizosphere (0.51-mm average thickness) differed from the bulk soil depending on nitrogen (N) form and level. Ammonium application resulted in a pH decrease, but nitrate application slightly increased pH. To examine the effect of rhizosphere acidification on mobilization of phosphate, 0.5 M NaHCO3 extractable phosphate was measured. The lowering rhizosphere pH enhanced the solubility of P in the soil and maybe availability of P to plants. Rhizosphere-P depletion increased with increasing ammonium supply, but when N was supplied as nitrate, P depletion was not related to increasing nitrate supply. Low P status Hoosfield soils developed mycorrhizal infection, and as a result, P inflow was increased. Geescroft soil, which initially had a high P status, did not develop mycorrhizal infection, and P inflow was much smaller and was unaffected by N treatments. Therefore, plant growth and P uptake were influenced by both rhizosphere pH and indigenous mycorrhizal infection.

Effect of Mycorrhizae and pH Change at the Root-Soil Interface on Phosphorus Uptake by Sorghum Using a Rhizocylinder Technique

Abstract Sorghum (Sorghum bicolor) was grown for 40 days in a rhizocylinder (a growth container which permitted access to rhizosphere and nonrhizosphere soil), in two soils of low P status. Soils were fertilized with different rates of ammonium and nitrate and supplemented with 40 mg phosphorus (P) kg−1 and inoculated with either Glomus mosseae (Nicol. and Gerd.) or nonmycorrhizal root inoculum. N-serve (2 mg kg−1) was added to prevent nitrification. At harvest, soil from around the roots was collected at distances of 0–5, 5–10, and 10–20 mm from the root core which was 35 mm diameter. Sorghum plants, with and without mycorrhiza, grew larger with than with application. After measuring soil pH, suspensions of the same sample were titrated against 0.01 M HCl or 0.01 M NaOH until soil pH reached the nonplanted pH level. The acid or base requirement for each sample was calculated as mmol H+ or OH− kg−1 soil. The magnitude of liberated acid or base depended on the form and rate of nitrogen and soil type. When the plant root was either uninfected or infected with mycorrhiza, soil pH changes extended up to 5 mm from the root core surface. In both soils, ammonium as an N source resulted in lower soil pH than nitrate. Mycorrhizal (VAM) inoculation did not enhance this difference. In mycorrhizal inoculated soil, P depletion extended up to 20 mm from the root surface. In non-VAM inoculated soil P depletion extended up to 10 mm from the root surface and remained unchanged at greater distances. In the mycorrhizal inoculated soils, the contribution of the 0–5 mm soil zone to P uptake was greater than the core soil, which reflects the hyphal contribution to P supply. Nitrogen (N) applications that caused acidification increased P uptake because of increased demand; there is no direct evidence that the increased uptake was due to acidity increasing the solubility of P although this may have been a minor effect.

Potassium leaching potential and fertilizer recommendations for smallholder coffee gardens of Papua New Guinea

The need to assess, as part of a fertilizer recommendation scheme, a soils tendency to lose potassium (K) by leaching has been examined by determining the mineralogy, K–calcium (Ca) exchange isotherms and K status of eight surface soils which fall into two groups, (a) four with curvilinear isotherms and (b) four with straight line isotherms. Group (b) soils have smaller buffer powers, smaller equilibrium soil solution concentrations and generally lower exchangeable K. All the soils contain kaolinite, sesquioxides and high organic matter contents. Group (a) soils contain mica in the whole soil and 2:1 clay minerals in the clay fraction except for one containing cristobalite. Group (b) soils have larger amounts of sesquioxides and any 2:1 minerals are in intimate association with the oxides. These mineralogical differences appear to be the reason for the differences in the isotherms. The lower K status of the Group (b) soils may be the result of greater leaching losses due to (i) their lower buffer powers especially at low potassium concentrations, and (ii) dilution of soil solution causing decrease in the amount of variable charge. Site-specific K fertilizer recommendations should take account of amounts of exchangeable K and of the tendency of soils to lose K by leaching, based on either mineralogy, exchange isotherms or related soil-mapping units.

Cation exchange, hydrolysis and clay movement during the displacement of saline solutions from soils by water

Deterioration of soil physical conditions occurs when rain or irrigation water displaces soluble salts during reclamation and subsequent management of salinesodic soils. Damage, which depends primarily on the presence of exchangeable Na+, appears to be ameliorated during leaching by exchange of Ca2+ and Mg2+ for Na+ and loss of exchangeable Na+ by hydrolysis. The extent of these processes has been measured by leaching columns of repacked soil with water after preparation with Na+ and Ca2+ or Na+ and Mg2+ as the exchangeable cations and high or low (1 or 0.1 molcl−1) initial salinities. Structural deterioration was monitored by changes in flow rate, and soil properties were measured both initially and after cutting the leached columns into layers. Preliminary studies established reliable methods for measuring exchangeable cations and cation exchange capacity in the saline soils. In a sandy loam (Na-Ca system), clay dispersion and movement occurred particularly in the upper layers as measured both by decreases in CEC and by the amount of clay in the leachate. Cation exchange and hydrolysis of exchangeable Na+ during leaching reduced the exchangeable Na+ percentage, although cation exchange was restricted to columns with high initial salinity. In a clay textured soil (Na-Ca system) there was negligible clay movement, and cation exchange and hydrolysis occurred in columns with both high and low initial salinities: cation exchange may have been encouraged by diffusion limited preferential release of Na+ from aggregates during by-pass flow. In the sandy loam (Na-Mg system) Mg2+ increased the preference of the soil for exchangeable Na+ compared to the Na-Ca system. There was no cation exchange even in columns with high initial salinity. The amounts of clay movement and hydrolysis were similar in the two systems. Conditions conductive to cation exchange are a high initial salinity, a Na-Ca rather than a Na-Mg system and, possibly, restricted release of the divalent cation from within soil aggregates. Attempts to model these changes are complicated by difficulties in predicting the effects of hydrolysis and by-pass flow.

Effect of ph on amount of phosphorus extracted by 10 mm calcium chloride from three Rothamsted soils

Abstract Three silty clay loams from the classical field experiments at Rothamsted Experimental Station (UK) with low phosphorus (P) status were treated with phosphate fertilizer and incubated for 15 days at field capacity with added acid [hydrochloric acid (HCl)] or base [sodium hydroxide (NaOH)] to give pH ranges measured in 1:5 suspensions of soil in 10 mM calcium chloride (CaCl2) of 6.3–8.7 (Hoosfield A, pH 8.2), 3.8–8.2 (Hoosfield B, pH 6.3), and 2.8–7.1 (Geescroft, pH 5.3). Extractable ? was measured by the 10 mM CaCl2 extraction procedure. For the Hoosfield ? and Geescroft soils without added phosphate fertilizer, extractable ? was decreased at both ‘low’ and ‘high’ pH values, the maximum being at about 5.4 in both soils. In the calcareous Hoosfield soil, extractable ? decreased with increasing pH over the range studied. These changes in extractable ? were magnified in soils treated with phosphate fertilizer but the maxima and trends were unaltered. The results indicate that native soil ? appears to be changed by pH in the same way as added ? to the soil. In the calcareous Hoosfield A soil, added acid did not reduce soil pH to less than 6.4 (because of its high buffer capacity) and so the pH level for maximum ? solubility was not found.

Influence of Aluminum Hydroxide on Potassium Release from Two Colombian Soils

Abstract The effect of sesquioxides on the mechanisms of chemical reactions that govern the transformation between exchangeable potassium (Kex) and non‐exchangeable K (Knex) was studied on acid tropical soils from Colombia: Caribia with predominantly 2∶1 clay minerals and High Terrace with predominantly 1∶1 clay minerals and sesquioxides. Illite and vermiculite are the main clay minerals in Caribia followed by kaolinite, gibbsite, and plagioclase, and kaolinite is the major clay mineral in High Terrace followed by hydroxyl‐Al interlayered vermiculite, quartz, and pyrophyllite. The soils have 1.8 and 0.5% of K2O, respectively. They were used either untreated or prepared by adding AlCl3 and NaOH, which produced aluminum hydroxide. The soils were percolated continuously with 10 mM NH4OAc at pH 7.0 and 10 mM CaCl2 at pH 5.8 for 120 h at 6 mL h−1 to examine the release of Kex and Knex. In the untreated soils, NH4 + and Ca2+ released the same amounts of Kex from Caribia, whereas NH4 + released about twice as much Kex as Ca2+ from High Terrace. This study proposes that the small ionic size of NH4 + (0.54 nm) enables it to enter more easily into the K sites at the broken edges of the kaolinite where Ca2+ (0.96 nm) cannot have access. As expected for a soil dominated by 2∶1 clay minerals, Ca2+ caused Knex to be released from Caribia with no release by NH4 +. No Knex was released by either ion from High Terrace. After treatment with aluminum hydroxide, K release from the exchangeable fraction was reduced in Caribia due to the blocking of the exchange sites but release of Knex was not affected. The treatment increased the amount of Kex released from the High Terrace soil and the release of Knex remained negligible although with Ca2+ the distinction between Kex and Knex was unclear. The increase in Kex was attributed to the initially acidic conditions produced by adding AlCl3 which may have dissolved interlayered aluminum hydroxide from the vermiculite present, thus exposing trapped K as exchangeable K. The subsequent precipitation of aluminum hydroxide when NaOH was added did not interfere with the release of this K, and so was probably formed mostly on the surface of the dominant kaolinite. Measurement of availability of K by standard methods using NH4 salts could result in overestimates in High Terrace and this may be a more general shortcoming of the methods in kaolinitic soils.

Cation Flux in Incubated Plant Residues and Its Effect on pH and Plant Residue Alkalinity

Plant residues offer a viable alternative to the costly and non-readily available commercial lime in addressing the constraint of soil acidity among the rural smallholders. The liming potential of the residues, attributable to excess cations over inorganic anions, exists in either available or non-available forms. This study investigates cation flux and its effect on pH and plant residue alkalinity of four plant residues, maize (Zea mays), soya beans (Glycine max), Leucaena (Leucaena leucocephala) and Gliricidia (Gliricidia sepium), upon incubation for 100 days with and without application of lime in an acidic Zambian Ferralsol. Initial characterization of base cation content ranged from 239 to 879 (Ca2+), 188 to 458 (Mg2+) and 298 to 477 mmolc kg–1 plant material (K+). Of these, 26–60, 62–92 and 76–96% in that order were water soluble. On incubation, up to 70% Ca2+ and at least 80% Mg2+ and K+ added in the residues were initially present eventually increasing to 84 and 95%, respectively. Potential alkalinity values were 373 (maize), 1264 (soya beans), 794 (Leucaena) and 1024 (Gliricidia) mmolc kg–1. Of these, between 42 (Gliricidia) and 52% (Leucaena) constituted available alkalinity. Exchangeable aluminium was absent or appeared in insignificantly very low amounts towards the end of the incubation, while base cations were fixed. There was initial dependence of pH on both total cation concentration and residue alkalinity, but this relationship was later lost, suggesting incomplete activation of the non-available fraction of the potential alkalinity. Nitrogen mineralization affected both cation flux and residue alkalinity. This study highlights the importance of residues on amelioration of major cation deficiencies and aluminium phytotoxicity.

Partikeloberflächen und Bodenlösung

Viele wichtige chemische Bodeneigenschaften werden von Reaktionen gesteuert, die sich zwischen Bodenwasser und Partikeloberflachen abspielen. Das Bodenwasser ist eine Losung, die eine Vielzahl von Kationen, Anionen und organischen Molekulen, meist in geringen Konzentrationen, enthalt.

Salz- und natriumhaltige Böden

Versalzung zahlt zu den altesten Problemen der „Bodenverschmutzung“. Einige Historiker sehen darin eine Ursache fur den Niedergang des Babylonischen Reichs: Die zu starke Salzakkumulation brachte den Bewasserungsfeldbau zum Erliegen (Hillel 1992). Trotz unseres Wissens um die zugrundeliegenden Probleme und ihr Management unterliegt auch heute noch etwa ein Drittel des weltweit bewasserten Landes der Degradation und geht fur die Nutzpflanzenproduktion verloren.