Surface Soil pH Research Articles

Impacts of urban storm-water drainage channels on a northern Australian mangrove forest

A common practice in tropical mangrove forests is to widen, deepen or extend the natural channels, or create new channels, in order to increase drainage to limit mosquito breeding habitats. Such excavations were made around 1984 in the upper shore regions of a forest near Darwin in northern Australia. To determine whether changes near the channels appeared to be affecting the mangroves, we measured the survival of experimentally planted seedlings, vegetation structure and a set of environmental variables at several distances from the channels. We focused primarily on Ceriops australis, the most common species. Surface and sub-surface (30 cm) soil salinity, pH and moisture showed no consistent pattern with distance from channels but measurements before and after spring tides showed significantly greater changes in these soil parameters in the surface than in the sub-surface. The survival of planted C. australis seedlings was lower in the forests immediately adjacent to the channels than in forests further away. The growth of the seedlings, however, did not depend on location. Total basal area increased with distance from channels. The forests adjacent to some channels had a greater proportion of Lumnitzera racemosa and Avicennia marina. Although no consistent patterns were evident in soil variables, some patterns in the vegetation, and poorer survival of the seedlings, suggest that the channels might be negatively impacting C. australis. If this was the case, the effects on the forest were, however, relatively minor and apparently similar to those of natural channels.

Long‐Term Liming Effects on Coastal Plain Soils and Crops

Most soils in the southern Coastal Plain need liming; however, long‐term data are needed on its value and changes in soil profile pH, Ca, or Mg. Field studies with lime rates have been maintained on a Tifton soil (Plinthic Kandiudult) for 31 yr and on Pelham soil (Arenic Paleaquult) for 24 yr to determine soil chemical changes, rates needed for high yields, and if yield of cotton (Gossypium hirsutum L.) on low‐pH soils can be increased by Ca or Mg fertilization. Total amounts [calcium carbonate (CaCO3) equivalent] of dolomitic lime were 0, 7.5, 15.0, and 30.0 Mg ha−1 for Tifton and 0, 11.7, and 34.0 Mg ha−1 for the Pelham. Liming increased pH, Ca, and Mg to a depth of 90 cm. Decreases in pH, Ca, and Mg were closely related to the amount of ammoniacal N (NH4–N) applied. Application of 15 Mg ha−1 dolomitic lime on the Tifton soil for 31 yr and 34 Mg ha−1 on the Pelham soil for 24 yr maintained surface soil (0–15 cm) pH near 6.0 and provided greatest crop yield. In no‐lime plots, in 2000, exchangeable Al (0–45 cm depth) was >0.3 cmolc kg−1, and Al saturation was >15% of the effective cation exchange capacity on Tifton soil and >20% on Pelham soil. In those plots, low cotton yields were not increased by fertilization with Ca or Mg salts without liming. Providing 5.25 and 14 kg lime kg−1 N for the Tifton and Pelham soils, respectively, decreased soil Al saturation to <7%.

Biotic Soil Crusts of Oregon's Shrub Steppe: Community Composition in Relation to Soil Chemistry, Climate, and Livestock Activity

Abstract We examined biotic soil crust cover and composition at nine shrub-steppe sites in central and eastern Oregon, U.S.A. One pair of livestock-grazed and excluded transects was established at each site. Data were collected on the cover of biotic soil crust and vascular plant species, soil surface pH and electrical conductivity, and other environmental variables. Using gradient analysis, we found that differences in community composition among sites were most strongly related to soil pH, electrical conductivity (EC), and Calcareous Index Value (CIV; a scale representing the relative calcium carbonate content of soils). Other important variables included precipitation, elevation, aspect, and temperature. We found total crust cover to be highest at sites with lower pH, EC, and CIV. Dominant species differed markedly between the more calcareous sites with higher pH, and the less calcareous, lower pH sites. Livestock exclusion was not an important gradient in the ordination of these data, being overshadow...

Effect of flue gas desulfurization residue on plant establishment and soil and leachate quality.

Effects on soil quality and crop establishment after incorporation of flue gas desulfurization by-product (FGD) into soil as an amendment was assessed in a mesocosm study. Mesocosm units received applications equivalent to 0, 2.5, 5.0, 7.5, and 10% FGD residue [0, 25, 50, 75, and 100 tons acre(-1)]. Germination, biomass production, and elemental composition of corn (Zea mays L. var. Dekalb DK-683), soybean [Glycine max (L.) Merr. var. Haskell Pupa 94], radish (Raphanus sativus L. var. Sparkler), and cotton (Gossypius hirsutus L. var. Deltapine 51) were determined. The quality of leachates and soil were also determined periodically. Flue gas desulfurization residue did not affect germination and all application rates stimulated aboveground biomass. Plants grown in FGD-amended soil contained significantly elevated tissue concentrations of As, B, Se, and Mo. The FGD residue elevated surface soil pH from 5.5 (Control) to 8.1 (at 10% FGD). Leachate pH was unaffected by FGD, but salinity rose sharply with increasing application rates of FGD. Leachates contained higher concentrations of B, with small increases in Se and As. Flue gas desulfurization residue application caused an increase in total B, As, Mo, Se, and extractable Ca in the soil, but decreased Mn and Zn. Using FGD residues could have beneficial effects on crop establishment without detrimental effects on soil or leachate quality, at an optimum rate of approximately 2.5%. This material could alleviate surface acidity, and B and Mo deficiencies in plants.

Effect of plant residue return on the development of surface soil pH gradients

In the field, surface soil pH gradients were observed under senescing plants over late spring and summer. A soil incubation experiment was conducted (119 days, 20 °C) to provide direct evidence of the influence of plant residue incorporation on soil pH. This was investigated in terms of plant residue type (wheat and subterranean clover) and dry matter addition rate (0, 6.25, 12.5 and 25.0 g kg–1), as well as the soil layer of incorporation (0–2.5 and 7.5–10 cm) and moisture regime (continuously moist and moist-dry cycles). During incubation, moist unamended soils slowly acidified. In contrast, the addition of plant residue resulted in a rapid (day 0–7) increase of soil pH due to the association, and particularly oxidation, of added organic anions. This was followed by a gradual (day 7–119) pH decline attributed to the mineralization and subsequent nitrification of added organic N. The addition of 12.5–25.0 g kg–1 of cereal crop residues, and 6.25–25.0 g kg–1 of legume-based pasture residues, resulted in a net alkalization of the surface 2.5 cm of soil. It was therefore concluded that surface soil pH gradients observed in the field were largely attributable to an increase of pH at the surface 2.5 cm in response to plant residue return. The magnitude of such gradients will be particularly large with the return of large quantities of plant residues of high ash alkalinity in soils of relatively low initial pH and biological activity, and when the surface of the soil is exposed to moist-dry cycles.

Development of nitrogen mineralisation gradients through surface soil depth and their influence on surface soil pH

Following mixing of the surface soil to about 7.5 cm depth in the field, soil layers (0–2.5, 2.5–5, 5–10 and 10–15 cm) were separately incubated in the laboratory to determine the rate of development of net N mineralisation gradients through surface soil depth under fallow, wheat and subterranean clover plots. Gradients in net N mineralisation were compared with those observed in the field, and their contribution to the observed pH changes was investigated. Heterotrophic activity, and thus net N mineralisation, decreased only slightly with depth immediately after soil mixing. This pattern persisted over time in soil layers sampled from fallow plots. In contrast, within 1 growing season after soil mixing, heterotrophic activity and net N mineralisation decreased significantly with depth in soil sampled from wheat and clover plots. In 0–15 cm soil sampled from under senescing plants, 32–38% of CO2-C produced and net N mineralised originated from the surface 2.5 cm, while 52–56% originated from the surface 5 cm of soil. This resulted from an increase of pH and organic substrate concentration within the surface 2.5 cm of soil following plant residue return. Limitations of the in situ measurement of net N mineralisation in fallow soil was identified. Laboratory incubation studies showed that since most net N mineralisation occurred within the surface 2.5 cm of soil under senescing plants, nitrification and acidification were also concentrated at this depth. Despite this, compared to fallow soil, high potential acidification rates of 0–2.5 cm soil under senescing plants were not realised in the field due to the exposure to prolonged dry periods and moist-dry cycles. As a consequence, in the field the large magnitude of surface soil pH gradient which resulted from the return of alkaline plant residues was maintained over summer and autumn.

Soil N dynamics associated with deforestation, biomass burning, and pasture conversion in a Mexican tropical dry forest

Deforestation and land conversion in deciduous dry forests are occurring throughout the Neotropics, yet few studies have described soil N dynamics associated with this land-use/land-cover change. Soil nitrogen dynamics were measured before, and after, slash burning in two different fire-severity treatments (a high-severity fire with 80% of the aboveground biomass consumed, and a low-severity fire where 63% was consumed) in a neotropical dry forest ecosystem near Chamela, Jalisco, Mexico. We repeated these measurements three months after burning (corn field), the first year after slash-and-burn (pasture), and before, and after, a fire ≈2 years after the initial slash fires. Soil pH, total and inorganic soil nitrogen pools, and potential net mineralization and nitrification rates were measured. Slash burning resulted in dramatic but short-term increases in inorganic nitrogen and soil pH. Mineral N pools in surface soils (0–10 cm) increased from 9 to 44 kg ha −1 in the low-severity treatment and from 18 to 57 kg ha −1 in the high-fire severity treatment. This was due to an increase in soil NH 4-N; NO 3-N concentrations were lowered by fire. Surface soil pH increased 1.1 units following the low-severity fires and 2.3 units following the high-severity fires. Soil pH remained elevated in the perturbed sites for the duration of the study. Immediately after the initial slash fires, N mineralization was higher in the burned plots compared to the adjacent undisturbed forest. Potential nitrification was >9.5 μg NO 3-N g −1 soil day −1 in the slash-and-burn plots, and 3.9 μg NO 3-N g −1 soil day −1 in an adjacent reference forest. Potential nitrification was significantly higher in the high-severity treatment compared to the low-severity treatment. The increases in mineralization were likely a response to the large pulse of NH 4-N created via pyromineralization during the slash fires. The pattern of increased mineral N pools and potential rates of N mineralization in perturbed sites was short-lived. Compared to the reference forest, N mineralization rates in burned plots were similar 3 months after the fire, and lower 2 years following conversion. In contrast to the initial slash fire, we did not find a significant increase in available mineral N (NH 4-N) pools, or potential mineralization immediately following the pasture fires in 1995. Soil surface total N concentrations decreased from 4.3 and 5.4 mg g −1 at the start of the study, to 2.5 and 2.4 mg g −1 at the end of the study in the low- and high-fire severity treatments, respectively.

Liming effects of beef cattle feedlot manure or compost

Soil pH can be increased by manure or compost application because feed rations usually contain calcium carbonate. This study was conducted from 1992 to 1996 to evaluate effects of phosphorus (P) and nitrogen (N)‐based manure and compost application management strategies on soil pH level. Composted and uncomposted beef cattle (Bos taunts) feedlot manure was applied to supply N or P needs of corn (Zea mays L.) for either a one‐ or two‐year period. Phosphorus‐based manure or compost treatments also received additional fertilizer N (ammonium nitrate) to provide for a total of 151‐kg available N ha‐1 yr‐1. Fertilized and unfertilized checks also were included. Manure and composted manure contained about 9 g CaCO3kg‐1 resulting in application rates of up to 1730 kg CaCO3 ha‐1 in 4 years. The surface soil (0–150 mm) pH was significantly decreased with ammonium nitrate application compared to soil in the unfertilized check or to soil receiving manure or compost. Soil pH was significantly increased with the N‐based management strategy compared with the soil original level. In contrast, 4 yr of P‐based manure and compost application strategy maintained soil pH at the original level. Nitrogen‐based applications resulted in higher soil pH than P‐based. Beef cattle feedlot manure and compost can be good sources of CaCO3 for soils requiring lime addition.

Regression Analysis of Soil Chemical Composition for Two Shrimp Farms in Texas

Abstract.Several nutrients, pH, organic matter, and total soluble salts of surface soil from 81 ponds on two Texas shrimp farms were measured. The relationship between shrimp yield and soil characteristics was analyzed statistically. Shrimp yield was negatively correlated with soil concentrations of S, P, NO3‐, B, Ca, Mg, K, Na, Cu, and total water‐soluble salts. Shrimp yield was directly correlated with soil concentrations of Fe, Mn, and organic matter. Stepwise multiple regression analysis indicated that Ca and S concentrations explained 54% of the variation in shrimp yield.

Acidic Soil Amendment with a Magnesium‐Containing Fluidized Bed Combustion By‐Product

AbstractRemoval of SO2 from the emissions of coal‐fired boilers produces by‐products that often consist of CaSO4, residual alkalinity, and coalash. These by‐products could be beneficial to acidic soils because of their alkalinity and the ability of gypsum (CaSO4·2H2O) to reduce AI toxicity in acidic subsoils. A 3‐yr field experiment was conducted to determine the liming efficacy of a fluidized bed combustion boiler by‐product (FBC) that contained 129 g Mg kg−1 as CaMg(CO3)2 and MgO and its effects on surface and subsurface soil chemistry. The FBC was mixed in the surface 10 cm of two acidic soils (Wooster silt loam, an Oxyaquic Fragiudalf, and Coshocton silt loam, an Aquultic Hapludalf) at rates of 0, 0.5, 1, and 2 times each soil's lime requirement (LR). Soils were sampled in 10‐cm increments to depths ranging from 20 to 110 cm, and corn (Zea mays L.) and alfalfa (Medicago sativa L.) were grown. Application of Mg‐FBC increased alfalfa yields in all six site‐years, whereas it had no effect on corn grain yield in five site‐years and decreased grain yield in one site‐year. Plant tissue concentrations of Mg, S, and Mo were increased by Mg‐FBC, while most trace elements were either unaffected or decreased. Application of Mg‐FBC at one or two times LR increased surface soil pH to near 7 within 1 wk. Although surface soil pH remained near 7 for 2 yr, there was minimal effect on subjacent soil pH. Application of Mg‐FBC increased surface soil concentrations of Ca, Mg, and S, which promoted downward movement of Mg and SO4. This had different effects on subsoil chemistry in the two soils: in the high‐Ca‐status Wooster subsoil, exchangeable Ca was decreased and exchangeable Al was increased, whereas in the high‐Al‐status Coshocton subsoil, exchangeable Al was decreased and exchangeable Mg was increased. The Mg‐FBC was an effective liming material and, because of the presence of both Mg and SO4, may be more effective than gypsum in ameliorating subsoil Al phytotoxicity.

SOIL CHEMICAL CHANGES AFTER NINE YEARS OF DIFFERENTIAL N FERTILIZATION IN A NO-TILL DRYLAND WHEAT-CORN-FALLOW ROTATION

Intensively cropped dryland systems in the central Great Plains require adequate N fertilization for optimum residue and grain production. However, this N fertilization could be slowly changing the chemistry of the surface soil because of a decrease in soil pH and an increase in soil organic matter (SOM) and basic cations, even in previously well buffered calcareous soil systems. We investigated the effects of five increasing ammonium-N fertilizer rates in a Platner loam, on physical and chemical changes at the 0 to 5, and 0 to 15-cm depths after three cycles of no-till wheat (Triticum aestivum L.)-corn (Zea mays L.)-fallow rotation. We measured soil pH, texture, bulk density, cation exchange capacity (CEC), total P, soluble and total soil organic carbon (SOC), nitrate-N to a depth of 60 cm, and grain yields. No significant changes were found with soil texture, bulk densities, CEC, and total P. The data showed a significant reduction in surface (0-5 cm) soil pH (6.5 to 5.1) with the highest N rate (112 kg/ha), but this was accompanied by a 40% increase in SOC. Although there were significant increases in Al and Mn and decreases in Ca concentrations in the surface 0 to 5 cm at the highest N rate, no reduction in grain yields occurred relative to lower N levels with near neutral pHs. Because only a shallow depth of the soil was affected, residue, SOM, and rapid root growth could be compensating for surface acidity. Over the longer term, we need to monitor the effects of ammoniacal-N on downward soil acidity and yield trends under these new intensive cropping systems.

The independent effects of heat, smoke and ash on emergence of seedlings from the soil seed bank of a heathy Eucalyptus woodland in Grampians (Gariwerd) National Park, western Victoria

Abstract The independent effects of smoke, ash, and wet and dry heat treatments on seedling emergence from the soil seed bank were tested for soils from fire‐prone heathy woodlands in western Victoria. A total of 763 individuals from 56 species were recorded from the surface soil samples (which covered a total area of 1 m2). Both species richness and density of seedlings was greater for smoke‐ and heat‐treated samples than for controls and ash‐treated samples. However, only the density differences were significant. Mean seed bank densities for the smoke and heat treatments ranged from 855 ± 70m−2 to 1080 ± 58 m−2 and are similar to estimates obtained elsewhere in Australia for heat‐treated soils from dry sclerophyll communities. Of the 56 species recorded, 46 occurred in the smoke and heat treatments but only 33 in the control and ash treatments. The sudden increase in surface soil pH, exchangeable cations and extractable phosphorus which was associated with the ash treatment did not act as a trigger for germination in any of the species recorded here. Chemical constituents from smoke do appear to provide a stimulus separate from the effects of heat, but were not identified with any particular taxa.

Effect of sewage sludge and ammonium nitrate on wheat yield and soil profile inorganic nitrogen accumulation1

The beneficial effect of sewage sludge in crop production has been demonstrated, but there is concern regarding its contribution to nitrate (NO3) leaching. The objectives of this study were to compare nitrogen (N) rates of sewage sludge and ammonium nitrate (NH4NO3) on soil profile (0–180 cm), inorganic N [ammonium nitrate (NH4‐N) and nitrate nitrogen (NO3‐N)] accumulation, yield, and N uptake in winter wheat (Triticum aestivum L.). One field experiment was established in 1993 that evaluated six N rates (0 to 540 kg·ha‐1·yr‐1) as dry anaerobically digested sewage sludge and ammonium nitrate. Lime application in 1993 (4.48 Mg ha‐1) with 540 kg N ha‐1·yr‐1 was also evaluated. A laboratory incubation study was included to simulate N mineralization from sewage sludge applied at rates of 45, 180, and 540 kg N ha‐1·yr‐1. Treatments did not affect surface soil (0–30 cm) pH, organic carbon (C), and total N following the first (1994) and second (1995) harvest. Soil profile inorganic N accumulation increased when ≥270 kg N ha‐1 was applied as ammonium nitrate. Less soil profile inorganic N accumulation was detected when lime was applied. In general, wheat yields and N uptake increased linearly with applied N as sewage sludge, while wheat yields and N uptake peaked at 270 kg N ha‐1 when N was applied as ammonium nitrate. Lime did not affect yields or N uptake. Fertilizer N immobilization was expected to be high at this site where wheat was produced for the first time in over 10 years (previously in native bermudagrass). Estimated N use efficiency using sewage sludge in grain production was 20% (average of two harvests) compared to ammonium nitrate. Estimated plant N recovery was 17% for sewage sludge and 27% for ammonium nitrate.

Field Scale Spatial Analysis of pH on Sodium Soils in Northern Burkina Faso

Geografisk Tidsskrift, Danish Journal of Geography 97: 66–75, 1997. Microscale variations of soil properties may have a pronounced impact on crop yields in the low external input agriculture in the Sahel. Using classical and geostatistical methods, spatial patterns of soil pH at 0.0–0.10 m, 0.30–0.50 m and 0.70–0.90 m depth have been analysed and mapped based on two hundred and eighty-one samples collected at 10 m x 10 m grid nodes on a one hectare test plot on a clayey, sodium affected soil used for millet cultivation in northern Burkina Faso. The mean surface soil pH is 7.39 and semi-variance analysis shows that the nugget effect accounts for nearly 100% of the sample variance, and therefore surface pH exhibits no spatial dependency at the separating distance. The mean soil pH increases to 7.83 and 7.90 at 0.30–0.50 m and 0.70–0.90 m depth, respectively. However, the coefficient of variation also increases, and at 0.70–0.90 m the field has areas of both alkaline and acid soil. The range of influence for soil pH at 0.30–0.50 m was 60 m, increasing to 80 m at 0.70–0.90 m as a result of stronger dependency on geology. On application of the semivariograms, pH values between the grid points were interpolated by point kriging. The study illustrates that pH of sodium soils varies considerably and that millet roots may grow in a diversity of pH conditions. Furthermore, soil characterization depends very much upon the sampling strategies, but the determination of the range allows for choosing the minimum distance required for spacing of non spatially correlated samples. The application of chemical fertilizers are likely to produce very different effects given the chemically variable soil environment.

SOIL SPATIAL VARIABILITY RELATIONSHIPS IN A STEEPLY SLOPING ACID SOIL ENVIRONMENT

Management of pastures in steep terrain can be expensive in terms of time, chemical inputs, and equipment requirements. Management of those portions of pasture that will yield the most for minimum inputs requires a knowledge of the spatial patterns of the soil and atmospheric environments. A steep 2.4-ha unimproved pasture in southern West Virginia was studied to determine the spatial dependency of aboveground biomass production on soil characteristics. The predominant soil of the study site was Gilpin silt loam (fine-loamy, mixed, mesic, Typic Hapludults) with an average A-horizon pH (1:1 H2O) of 4.5 (range 3.8 to 6.1). The study site was broadcast seeded with red fescue (Festuca rubra L.) in 1982, making it the major plant species. However, several other plant species, including orchardgrass (Dactylis glomerata L.), red clover (Trifolium pratense L.), white clover (Trifolium repens L.), broomsedge (Adropogon virginicus L.), velvet grass (Holcus lanatus L.), yarrow (Achillea millegolium L.), and wild strawberry (Fragaria virginiana L.), and others were common. Transects were laid out up and down the north-facing, south-facing, and west-facing slopes and one transect was established on the contour for measurement of soil characteristics at a 1-m spacing. Plant aboveground biomass was measured at a 2-m spacing on three of the slope transects and on the contour transect. Although correlation analysis failed to find many statistically significant relationships between biomass production and soil characteristics, cross-semivariogram analyses of biomass production and surface soil pH and extractable Al, Mn, and P indicated that a strong spatial dependency exists. Those relationships were used for identifying homogeneous areas of biomass production and associated soil characteristics by ordinary kriging.

Chemical Conditions Conducive to the Release of Mobile Colloids from Ultisol Profiles

AbstractDispersed colloidal clay particles may enhance the transport of environmental contaminants adsorbed to the colloid surfaces. The objective of this study was to determine how soil pH (4.3–6.9), low total electrolyte concentration (TEC, 0.4–2.8 molc m‐3), and low Na adsorption ratios (SAR, 0.3–1.8 [molc m‐3]0.5) affected the concentration of mobile colloids released from 13.5‐m3 reconstructed Ultisol profiles. Critical flocculation concentration (CFC, the minimum TEC to induce flocculation of a clay suspension) tests were conducted using the water‐dispersible clay fraction of the Ap horizon of these profiles, which is the primary source for the mobile colloids. Surface soil pH, TEC, and SAR levels of the profiles that released high concentrations of colloids fell within the dispersion domain identified by the laboratory CFC tests. The effect of SAR on dispersion appears to be a continuum that approaches zero, and this effect is significantly (P ≤ 0.01) dependent on TEC. The dispersible nature of these soils is in part attributed to their very low TEC, generally <2 molc m‐3, which results from low concentrations of weatherable minerals. The TEC of 10 of the 13 profiles were <2 molc m‐3, the CFC for SAR = 1 (molc m‐3)0.5, indicating that if the pH is not limiting (pH greater than ≈6.0), colloidal dispersion will occur. Thus, these weathered soils can be highly dispersive within limited chemical conditions, and only small increases in SAR or pH may induce dispersion. By identifying these limits, the risk of enhancing subsurface contaminant transport by soil‐derived mobile colloids can be reduced.

Ammonia Impacts on Atrazine Leaching through Undisturbed Soil Columns

AbstractAmmonia‐based fertilizers, such as anhydrous ammonia, aqua ammonia, and urea, initially increase soil pH, reducing atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐1,3,5‐triazine) sorption and increasing atrazine desorption. Increased amounts of atrazine in soil solution may increase atrazine's leaching potential. This laboratory study investigated atrazine leaching behavior when ammonia and atrazine applications overlap. Nondisturbed 15‐cm diam. by 15‐cm depth soil columns were excavated from a Brandt silty clay loam (fine silty, Pachic Udic Haploboroll) and a Ves clay loam (fine silty, mixed mesic Typic Hapludalf). Concentrated NH4OH was applied to the soil surface at 0 or 220 kg N ha−1. Immediately after fertilizer application, 1.9 kg atrazine (spiked with ring‐labeled 14C‐atrazine) ha−1 was applied. One day after chemical application, soil columns were leached with 5.4 L of water. The ammonia application increased leachate and surface soil pH by about 2.5 and 3.5 pH units, respectively. The amount of 14C collected in leachate from ammonia‐treated columns was 60 and 30% greater for the Brandt and Ves soils, respectively, compared with untreated columns. Less 14C remained in the surface of the ammonia‐treated columns than in the surface of the untreated columns. These data indicate that the interaction between ammonia‐based fertilizers and atrazine must be considered when evaluating atrazine movement through soil. Applications of atrazine and ammonia‐based fertilizers that increase pH should be physically separated to limit the leaching potential of atrazine.

Effects of surface application of lime and superphosphate to acid soils on growth and N 2 fixation by subterranean clover in mixed pasture swards

Forage production in south-eastern Australia is well below potential. Poor productivity in the higher rainfall zones has often been attributed to effects of soil acidity and phosphorus deficiency on N 2 fixation by legumes in pasture swards. Difficult terrain and lack of suitable equipment commonly prevents the incorporation of lime or phosphatic fertilisers in permanently grazed pastures, and amelioration of soils in these situations must rely upon surface applications. A study was initiated in May 1990 at field sites at Bungendore and Braidwood in N.S.W. and at Beechworth in Victoria to investigate the effect of surface applied lime (0 or 2500 kg ha −1) and superphosphate (0, 10 or 20 kg P ha −1) on N 2 fixation and growth of subterranean clover ( Trifolium subterraneum). By November 1991 (18 months after treatment application) liming had increased surface soil pH (from 4.2–4.5 to 4.9) and significantly decreased the concentrations of CaCl 2 extractable aluminium and manganese at all sites. However, pasture growth and subterranean clover content was increased by lime or superphosphate applications only at Bungendore. Plant analyses indicated that phosphorus availability may have limited pasture response, despite the inclusion of two rates of superphosphate. Additional superphosphate (50 kg P ha −1) was applied to the original 10 kg P ha −1 plots at all sites in the autumn of 1992. Amounts of N accumulated in pastures at the end of spring were increased by 2–3-fold with the additional superphosphate at Braidwood and by 24–61% at Beechworth and Bungendore. These effects on productivity were principally determined by changes in the subterranean clover component. Estimates of the proportion of subterranean clover N derived from symbiotic N 2 fixation (P NFix) using the 15N natural abundance procedure, indicated that clover reliance on N 2 fixation for growth was high at Bungendore and Beechworth (87–96%), regardless of season, rate of superphosphate application, or lime treatment. At Braidwood, P NFix ranged from 50–73% in 1991 to 81–97% in 1992. Amounts of N 2 fixed at the three experimental sites ranged from <2 kg N ha −1 to 119 kg N ha −1, and were determined by pasture composition, or by seasonal and treatment-induced differences in growth of subterranean clover. The implications of the findings are discussed in terms of management to alleviate nutritional stress and improve the contribution of subterranean clover in permanent pastures on acid and phosphorus deficient soils.

A field study on the influence of soil ph on trace element levels in spring wheat (Triticum aestivum), potatoes (Solanum tuberosum) and carrots (Daucus carota)

Soil pH is one of the main factors influencing the solubility and availability of trace elements in arable soils. Thus pH can affect the trace element contents of agricultural crops and thereby indirectly influence human health. The aim of this study was to determine Cd, Ni, Zn, Cu, Mn, Cr, Al and Se contents in spring wheat, potatoes and carrots (Cd, Ni and Zn) and estimate their correlations with certain soil factors (surface and subsurface soil pH and organic matter content) governing the plant availability of these elements. Commercial fields were sampled in Sweden in order to cover a wide range of soil types with respect to pH, soil texture and organic matter content. Concentrations of Zn, Mn, Ni (grain) and Cd (straw) in spring wheat (n=43); Cd, Ni, Zn, Mn, Cu and Al in potatoes (n=69); and Cd, Ni and Zn in carrots (n=36) showed significant negative correlations with surface soil pH (0–25 cm). The Se content of potatoes and Cr content of spring wheat straw were positively correlated with soil pH. Stepwise multiple regressions including a combination of soil pHs (0–25 and 25–50 cm) and organic matter contents (0–25 cm) showed that the organic matter content as well as the surface and subsurface soil pH significantly influenced concentrations of several trace elements in one or more of the studied crops. It was concluded that, if acid deposition together with other acidifying processes (fertilisation, harvest of biomass, etc.) are not balanced by a sufficient amount of liming there might be a decrease in the pH of arable soils, which, in turn will lead to decreased levels of Se in edible crops but an overall increase concentrations of other trace elements.

Constraints of Sahelian Sandy Soils and Management Implications for Sustainable Rainfed Crop Production

Constraints to sustainable production of the main food crops, sorghum (Soragum bicolor (L.) Moench) and millet (Pennicetum thyphoides (M.) Schulm), are diverse but site specific in the Sahel. Contraints were assessed at three Sahelian sites (N'Dounga, Chikal and Kala Pate). Results suggest that constraints other than water may limit sustaihable cereal crop production. Rapid hydraulic conductivity (> 0.25 m h-I), low soil fertility (less than 13 mg total N kg-1. 8 mp Bray-1 available P kg-1. 30 mg exchangeable K kg-1), soil mineralogy dominated by quartz/kaolinite/iron-oxide low available water holding capacity (< 12 Mg m-3), and low nutrient holding capacitv (CEC < 4 mole(+) 100 g-1 were common to all sites. At Kala Palte where soil pH was <5, crop roots and leaves were affected by l1 and Mn toxicities. Soil liming or acid tolerant cultivars are needed more than fertilizer or water management. At N'Dounga where surface soil pH was 6.5, constraints were the fewest. Constraints at Chikal were the late onset of the rains and high surface soil temperatures, therefore, production strategy should insure water availability early and/or late in the growing season. Due to the socio-economical environment at these sites, biological nitrogen fixation, acid and/or drought tolerant cultivars, dust mulching, domestic garbage, animal traction, and agroforestry may offer sustainable alternatives.