The influence of soil chemistry on fine root aluminum concentrations and root dynamics in a subalpine Spodosol,Washington State,USA

The relationship between root Al concentrations and Al fractions in the soil solution was examined in a mature Abies amabilis ecosystem in the Cascade Range of Washington State. The naturally acidic soils in these ecosystems lead to high concentrations of aqueous Al in soil solutions and contribute to the biocycling of Al by the A. amabilis/T. mertensiana stand. Root concentrations of Al were very closely related to aqueous Al³⁺ activities, but poorly correlated with total aqueous Al concentrations. The solution Al/Ca molar ratios followed a seasonal cycle with low values during the fall and high values during the spring. Ratios remained <1 throughout the year in the Oa horizon while they varied between 2 and 14 in the E and Bhs horizons. The vertical distribution of roots and the mortality of fine roots may be linked to the soil solution Al/Ca ratio. Root cation exchange capacity ranged between 180 and 225 mol g^-1 and the exchangeable Al fraction represented from 12–17% of the total Al content in the root. Evidence for solid-phase co-precipitates of Al with PO₄ and oxalate was indicated from selective dissolution of the root tissue. Sufficient quantities of PO₄ and oxalate exist in the roots to tie up 20–40% of the Al present in the roots of the Oa and E horizons, but only 9% of that present in the Bhs horizon. Species differences in the distribution of Al between the above-ground and below-ground components may be dictated by these retention processes in the fine roots.     

Influence of summer rainfall on root and shoot growth of a cold-winter desert shrub,Atriplex confertifolia

Summary The influence of irrigation and nitrogen fertilization in early summer on root and shoot growth of Atriplex confertifolia, a C₄ shrub species, was examined in a cold-winter desert community in northern Utah. Soil water and xylem pressure potentials were monitored during the summer period.At the time of watering the surface soil (0–30 cm) was dry but there were turgid fine roots in this horizon. Watering of the soil reduced plant water stress from-30 to-15 bars (dawn values) indicating that roots near the surface were capable of absorbing water, and induced root growth in the 0–30 cm zone. The addition of N to the water treatment did not further increase root production. However, watering and watering +N fertilizer failed to stimulate shoot elongation or any dry weight increase of shoots. This shoot dormancy during summer is not typical of C₄ plants and is probably associated with adaptation to the cool arid environment.This work was carried out while the senior author was on study leave from CSIRO     

Vertical root distribution in relation to soil properties in New Jersey Pinelands forests

Vertical distribution of root density (length per unit soil volume) and abundance (length per unit ground surface area) to a depth of 1.5 m or to the depth of the water table and their relationships with soil properties and tree basal area were examined in 36 soil profiles of pine-oak and oak-pine forests of the New Jersey Pinelands. Soil morphology were almost uniform within the forest type and characterized by the presence of high coarse fragment contents in the C horizon in oak-pine uplands; by the spodic B horizon and water table in the C horizon in pine-oak lowlands; by the sandy soil throughout the profile in pine-oak uplands; and by the firm argillic B horizon in pine-oak plains. Root density decreased from ranges of 44423–133369 m m^-3 in the 0–5 cm depth in all the forest types to 1900–5593 m m^-3 in the 100–150 cm depth in all the forest types except in pine-oak lowlands. Total profile root density and abundance was in the order: oak-pine uplands>pine-oak lowlands>pine-oak uplands>pine-oak plains. Root density correlated positively with organic C, total N, water soluble P, exchangeable Ca, Mg, K, Al, Fe, and cation exchange capacity, and negatively with bulk density, coarse fraction content, and pH, whereas root abundance correlated positively with organic C, total N, water soluble P, exchangeable Ca, Mg, K, and Fe, and negatively with bulk density. No correlation existed between root density and abundance with tree basal area. Higher root density in the E horizon of oak-pine uplands as compared to the other forest types was associated with high nutrient content; higher root density in the C horizon of pine-oak lowlands was associated with a shallow water table beneath the horizon; and lower root densities in the B and C horizons of pine-oak plains were associated with the presence of a firm clay layer in the B horizon.     

Biogeochemical controls on aluminum chemistry in the O horizon of a red spruce (Picea rubens Sarg.) stand in central Maine,USA

This study examined the biotic and abiotic processes controlling solution chemistry and cycling of aluminum (Al) in the organic horizons of a northern coniferous forest ecosystem. A mass balance budget indicated that aboveground inputs of Al to the O horizon averaged 0.9 kg ha^–1 1 yr^–1, with major inputs accounted for by litterfall (69%), followed by precipitation (21%), and net canopy throughfall plus stemflow (10%). Estimated leaching losses of Al from the O horizon averaged 2.1 kg Al ha^-1 yr1. We hypothesize that the difference between measured Al inputs and outputs can be accounted for by Al release from weathering of soil minerals admixed into the O horizon. Variations in O horizon solution Al chemistry were influenced by a number of factors, including pH, Al equilibria with different solid-phase organic exchange sites, and Al complexation with humic ligands in soil solution.     

Effects of soil structural discontinuity on root and shoot growth and water use of maize

The role of roots penetrating various undisturbed soil horizons beneath loose layer in water use and shoot growth of maize was evaluated in greenhouse experiment. 18 undisturbed soil columns 20 cm in diameter and 20 cm in height were taken from the depths 30–50 cm and 50–70 cm from a Brown Lowland soil, a Pseudogley and a Brown Andosol (3 columns from each depth and soil). Initial resistance to penetration in undisturbed soil horizons varied from 2.5 to 8.9 MPa while that in the loose layer was 0.01 MPa. The undisturbed horizons had a major effect on vertical arrangement of roots. Root length density in loose layer varied from 96 to 126 km m^-3 while in adjacent stronger top layers of undisturbed horizons from 1.6 to 20.0 km m^-3 with higher values in upper horizons of each soil. For specific root length, the corresponding ranges were 79.4–107.7 m g^-1 (on dry basis) and 38.2–63.7 m g^-1, respectively. Ratios of root dry weight per unit volume of soil between loose and adjacent undisturbed layers were much lower than those of root length density indicating that roots in undisturbed horizons were produced with considerably higher partition of assimilates. Root size in undisturbed horizons relative to total roots was from 1.1 to 38.1% while water use from the horizons was from 54.1 to 74.0%. Total water use and shoot growth were positively correlated with root length in undisturbed soil horizons. There was no correlation between shoot growth and water use from the loose layers.     

Effects of waterlogging on soil aeration and on root and shoot growth and yield of winter oats (Avena sativa L.)

Summary Winter oats were grown outdoors in lysimeters containing monoliths of a sandy loam soil. The soil was either freely-drained throughout the experiment or waterlogged to the soil surface from mid-January until mid-April. After the start of waterlogging the oxygen flux density decreased most rapidly nearer the soil surface and in the upper 50 cm declined to zero. At 80 cm depth the oxygen flux density at the end of the waterlogging still had not diminished to zero. While the soil was waterlogged root growth was negligible in the 20–50 cm zone of the soil profile, whereas below that depth root growth continued, reaching 95 cm by the end of the treatment. During the latter part of the waterlogging period root growth resumed in the upper 10 cm, and in the upper 2.5 cm was greater than in the freelydrained treatment.At the end of the waterlogging period, the total root length and shoot dry weights were 77 and 60% of those in the freely-drained treatment, tillering was restricted and leaf area index diminished. However, by anthesis, root length and shoot weights of the plants that had been waterlogged were only 10 and 12% less respectively than for the freely-drained plants. At harvest, total dry matter and grain yields were only 9% less, the latter largely through fewer grains per panicle.     

Amelioration of growth reduction of lowland rice caused by a temporary loss of soil-water saturation

Decreases in nutrient availability after loss of soil-water saturation are significant constraints to productivity in lowland rainfed rice soils. The effectiveness of soil amendments like lime and straw in ameliorating these constraints are poorly understood. This pot experiment was conducted in Cambodia to investigate changes in soil chemical properties and nutrient uptake by rice after applying lime or straw to continuously flooded or intermittently flooded soil. In continuously flooded soils, exchangeable Al decreased to below 0.2 cmolc/kg. Liming (pH 6.5–6.8) the continuously flooded soil decreased the levels of acetate extractable Fe and P, plant P uptake and shoot dry matter, but had no effect on either Bray-1 or Olsen extractable P values. By contrast, the addition of straw (3.5 g dry straw/kg soil) increased Bray-1, Olsen, and acetate extractable P, plant P uptake, shoot P, and shoot dry matter. The non-amended soils became strongly acidic after loss of soil water saturation: extractable Al increased to 1.0 cmol_c/kg, a potentially harmful level for rice. By contrast, extractable P decreased markedly under loss of soil water saturation as did plant P uptake, shoot P, and shoot dry matter. With loss of soil water saturation, liming substantially depressed the levels of Al but it did not increase plant P uptake, shoot P, and shoot dry matter. Straw addition not only decreased extractable Al levels to well below 0.6 cmol_c/kg under loss of soil water saturation, but it also increased extractability of soil P, plant P uptake, shoot P, and shoot dry matter. Thus, in rainfed environments, the incorporation of straw may be more effective than liming to pH 6.8 for minimising the negative effects of temporary loss of soil-water saturation on P availability, P uptake, and growth of rice.     

Influence of soil solution aluminum on root elongation of wheat seedlings

Wheat (Triticum aestivum L.) seedlings were grown for 4 days in an acid soil horizon treated with 10 levels each of Ca(OH)₂, CaSO₄ and CaCl₂. The treatments resulted in a wide range of Al levels and Al speciation in soil solution. Seedling root length in the Ca(OH)₂ treatments was significantly related (p<0.01) to calculated Al³⁺ activity in soil solution. The Al–SO₄ complex in soil solution had a negligible effect on the root growth of Hart wheat, thus confirming the previously reached conclusion concerning the nonphytotoxicity of Al–SO₄. The short-term seedling root growth technique used in this investigation allowed for separation of Al effects on root elongation from those on plant nutrition and should be useful for studying Al toxicity relationships in soil.     

Contribution of micro-organisms to the carbon dynamics in black spruce (Picea mariana) forest soil in Canada

In order to clarify the role of micro-organisms in the carbon cycle of the boreal forest ecosystem, the vertical distribution of soil carbon, soil microbial biomass and respiratory activity was studied in a black spruce forest near Candle Lake in Saskatchewan, Canada. The total amount of carbon contained in moss and soil layers (to the depth of 50cm beneath the mineral soil surface) was 7.2kgm^–2, about 47% of which was in the L and FH horizons of the soil. Soil microbial biomass per dry weight of soil was largest in the L horizon, while the biomass per ground area was largest in the FH horizon. Soil respiration rate, measured using a portable infrared gas analyzer, was highest in the FH horizon, exceeding 50% of the total soil respiration. Low but significant CO₂ emission was detected even in deeper soil horizon (E horizon). We also examined the respiration rate of cut roots and the effect of root excision on respiration. The contribution of root respiration to total soil respiration, calculated from root biomass and respiration rate of cut roots, was about 54%. The amount of carbon evolved through microbial respiration during the snow-free season (June–October) was estimated as 221gCm^–2. Micro-organisms in the L horizon showed high respiratory activity as compared with those in deeper soil horizons.     

Microscale heterogeneity of acidity related stress-parameters in the soil solution of a forested cambic podzol

Acid related stress in soils might be caused by high concentrations of H⁺ and Al³⁺ in soil solution. Sampling of soil solution so far integrates over a relatively large soil volume, in the range of dm³. In order to study the microscale heterogeneity of acidity related stress-parameters the soil profile of a podzolic cambisol was covered by a 10×6 matrix of micro suction cups with a grid distance of 2 cm. The soil solution collected at 10 sampling events was analyzed for free cations and anions by capillary electrophoresis and for total metal content by a micro injection technique on ICP-OES. pH and UV absorption were also measured.There was a general trend of increasing pH and decreasing UV absorption with increasing soil depth, however without a clear correlation of concentration isolines to soil horizon borders. The latter was also true for total Al (Al_tot) and Al³⁺, with the exception of the soil horizon border A_he/B_h,which was very well reflected by Al³⁺ and also by the fraction of bound Al. In the A_he horizon less than 30%, in deeper mineral soil less than 50% of Al_tot were present as free Al³⁺. This fact is critical when calculating Ca/Al ratios as a stress parameter, because total metal content measured by ICP clearly overestimates the risk of root damage, even in deeper horizons of acid forest soils, where organic complexation of Al is of minor importance. The heterogeneity of soil solution chemistry and toxicity parameters on the cm-scale was found to be significant, for example with gradients of more than 0,5 pH-units within 2 cm. Because plant roots also experience soil on a microscale, high resolution investigations of soil solution chemistry offer a new approach for looking at the chemical environment relevant for root growth and plant nutrition.     

Structure,ecology and physiology of root clusters – a review

Hairy rootlets, aggregated in longitudinal rows to form distinct clusters, are a major part of the root system in some species. These root clusters are almost universal (1600 species) in the family Proteaceae (proteoid roots), with fewer species in another seven families. There may be 10–1000 rootlets per cm length of parent root in 2–7 rows. Proteoid roots may increase the surface area by over 140× and soil volume explored by 300× that per length of an equivalent non-proteoid root. This greatly enhances exudation of carboxylates, phenolics and water, solubilisation of mineral and organic nutrients and uptake of inorganic nutrients, amino acids and water per unit root mass. Root cluster production peaks at soil nutrient levels (P, N, Fe) suboptimal for growth of the rest of the root system, and may cease when shoot mass peaks. As with other root types, root cluster production is controlled by the interplay between external and internal nutrient levels, and mediated by auxin and other hormones to which the process is particularly sensitive. Proteoid roots are concentrated in the humus-rich surface soil horizons, by 800× in Banksia scrub-heath. Compared with an equal mass of the B horizon, the A₁ horizon has much higher levels of N, P, K and Ca in soils where species with proteoid root clusters are prominent, and the concentration of root clusters in that region ensures that uptake is optimal where supply is maximal. Both proteoid and non-proteoid root growth are promoted wherever the humus-rich layer is located in the soil profile, with 4× more proteoid roots per root length in Hakea laurina. Proteoid root production near the soil surface is favoured among hakeas, even in uniform soil, but to a lesser extent, while addition of dilute N or P solutions in split-root system studies promotes non-proteoid, but inhibits proteoid, root production. Local or seasonal applications of water to hakeas initiate non-proteoid, then proteoid, root production, while waterlogging inhibits non-proteoid, but promotes proteoid, root production near the soil surface. A chemical stimulus, probably of bacterial origin, may be associated with root cluster initiation, but most experiments have alternative interpretations. It is possible that the bacterial component of soil pockets rich in organic matter, rather than their nutrient component, could be responsible for the proliferation of proteoid roots there, but much more research on root cluster microbiology is needed.     

Shoot and root growth and nutrients uptake of wheat as affected by soil layers

The effect of soil layering on the growth and nutrient content of wheat shoots and roots was studied. PVC containers (120 cm long and 25 cm inside diameter) were filled with layers of loam and loamy sand. Both roots and shoots dry weight increased as the thickness of loam layer increased. The root:shoot ratios decreased throughout the growing season. The N, P and K content of the shoots peaked at two weeks before anthesis, while shoot dry weight peaked at anthesis. The ranges of shoot content of N, P and K at anthesis for the different treatments were 6–25, 8–25 and 5–25% of the total plant nutrients, respectively. Late in the season the translocation rate of nutrients from the shoots to the seeds were in the following order N>P>K.     

Vertical distribution of biological and geochemical phosphorus subcycles in two southern Appalachian forest soils

We measured Al, Fe, and P fractions by horizon in two southern Appalachian forest soil profiles, and compared solution PO₄ ^–1 removal in chloroform-sterilized and non-sterilized soils, to determine whether biological and geochemical P subcycles were vertically stratified in these soils. Because organic matter can inhibit Al and Fe oxide crystallization, we hypothesized that concentrations of non-crystalline (oxalate-extractable) Al (Al₀) and Fe (Fe₀), and concomitantly P sorption, would be greatest in near-surface mineral (A) horizons of these soils.Al₀ and Fe₀ reached maximum concentrations in forest floor and near-surface mineral horizons, declined significantly with depth in the mineral soil, and were highly correlated with P sorption capacity. Small pools of readily acid-soluble (AF-extractable) and readily-desorbable P suggested that PO₄ ^3– was tightly bound to Al and Fe hydroxide surfaces. P sorption in CHCl₃-sterilized mineral soils did not differ significantly from P sorption in non-sterilized soils, but CHCl₃ sterilization reduced P sorption 40–80% in the forest floor. CHCl₃ labile (microbial) P also reached maximum concentrations in forest floor and near-surface mineral horizons, comprising 31–35% of forest floor organic P. Combined with previous estimates of plant root distributions, data suggest that biological and geochemical P subcycles are not distinctly vertically stratified in these soils. Plant roots, soil microorganisms, and P sorbing minerals all reach maximum relative concentrations in near-surface mineral horizons, where they are likely to compete strongly for PO₄ ^3– available in solution.     

Root:shoot ratios of old and modern,tall and semi-dwarf wheats in a mediterranean environment

A field study tested the hypothesis that modern wheat varieties invest a lesser proportion of the total dry matter (root plus shoot) in the root system compared to old varieties. The study was carried out on a duplex soil (sand over clay) at Merredin, Western Australia in a Mediterranean type environment. We also compared the root:shoot dry matter ratios of near-isogenic lines for Rht dwarfing genes.Root:shoot ratios decreased with crop growth stage and were closely related to the developmental pattern of a variety. All varieties appeared to accumulate more dry matter into shoots after the terminal spikelet stage. For the modern variety Kulin this occurred as early as 55 days after sowing (DAS), but did not occur until 90 DAS in the old variety Purple Straw. For all varieties, root dry matter reached its maximum at anthesis, while shoot dry matter continued to increase till maturity. At anthesis there were no significant differences in shoot dry matter between varieties, but from Purple Straw to Kulin root dry matter and thus root:shoot ratio decreased.The tall and dwarf isogenic lines had similar developmental and root:shoot dry matter accumulation patterns.At anthesis, the old variety Purple Straw had significantly higher root dry matter and root length density in the top 40-cm of the profile than modern variety Kulin. There were no varietal differences in rooting depth, water extraction or water use. At maturity about 30% of the total dry matter was invested in the roots among wheat varieties. Grain yield, harvest index (HI) and water use efficiency of grain (WUE_gr) increased from old to modern varieties.The reduced investment of dry matter in the root system and thus the lower root:shoot ratio from early in the growing season may partly explain the increased HI and WUE_gr of modern compared to old varieties.     

新垦红壤坡地土壤水分有效性研究

针对南方红壤地区降雨时空分布不均的特点,以未开垦的自然植被为对照,对桂西北环境移民示范区不同季节(早季和雨季)一次性降雨前1d及降雨后4h、2d、4d、6d及8d新垦蔗地(中坡、下坡、谷地）0-20cm,20-40cm、40-60cm3个土层的土壤水分含量进行了测定．结果表明,雨后谷地蔗地的土壤有效水分增量几乎与降雨量相同,而中坡蔗地与未开垦的自然植被土壤有效水分增量仅相当于降雨量于80％．雨季雨后0-60cm土壤层次中土壤有效水分分布均匀,早季主要集中在表层．雨季一次性降雨后各哩理及各土层土壤有效水分饱和度均有显著差异;而早季3个新垦蔗地间无明显差异,3个土层以表层土壤有效水分饱和度最高,亚表层与心土层差异不明显．无论雨季还是旱季,0-60cm土层土壤有效水分的消耗速率都以自然植被处理为最低,由于雨季正是作物生长旺季,其0-60cm土层土壤有效水分的消耗速率比旱季快,按照早季雨后8d土壤有效水分的平均消耗速率,15d内0-60cm土层的有效水分将消耗殆尽。     

Fibrous carrot root responses to irrigation and compaction of sandy and organic soils

Field experiments were performed in Southern Finland on fine sand and organic soil in 1990 and 1991 to study carrot roots. Fall ploughed land was loosened by rotary harrowing to a depth of 20 cm or compacted under moist conditions to a depth of 25–30 cm by three passes of adjacent wheel tracks with a tractor weighing 3 Mg, in April were contiguously applied across the plot before seed bed preparation. Sprinkler irrigation (30 mm) was applied to fine sand when moisture in the 0–15 cm range of soil depth was 50% of plant-available water capacity. For root sampling, polyvinyl chloride (PVC) cylinders (30 × 60 cm) were installed in the rows of experimental plots after sowing, and removed at harvest. Six carrot plants were grown in each of in these soil colums in situ in the field.Fine root length and width were quantified by image analysis. Root length density (RLD) per plant was 0.2–1.0 cm cm^-3 in the 0–30 cm range. The fibrous root system of one carrot had total root lengths of 130–150 m in loose fine sand and 180–200 m in compacted fine sand. More roots were observed in irrigated than non-irrigated soils. In the 0–50 cm range of organic soil, 230–250 m of root length were removed from loosened organic soils and 240–300 m from compacted soils. Specific root surface area (surface area divided by dry root weight) of a carrot fibrous root system averaged 1500–2000 cm² g^-1. Root length to weight ratios of 250–350 m g^-1 effectively compare with the ratios of other species.Fibrous root growth was stimulated by soil compaction or irrigation to a depth of 30 cm, in both the fine sand and organic soils, suggesting better soil water supply in compacted than in loosened soils. Soil compaction increased root diameters more in fine sand than it did in organic soil. Most of the root length in loosened soils (fine sand 90%, organic soil 80%) and compacted soils (fine sand 80%, organic soil 75%) was composed of roots with diameters of approximately 0.15 mm. With respect to dry weight, length, surface area and volume of the fibrous root system, all the measurements gave significant resposes to irrigation and soil compaction. Total root volumes in the 0–50 cm of soil were 4.3 cm³ and 9.8 cm³ in loosened fine sand and organic soils, respectively, and 6.7 cm³ and 13.4 cm³ in compacted sand and organic soils, respectively. In fine sand, irrigation increased the volume from 4.8 to 6.3 cm³.     

Interacting effects of pH,aluminium and base cations on growth and mineral composition of the woodland grasses Bromus benekenii and Hordelymus europaeus

The influence of base cation concentrations on pH and aluminium sensitivity of the woodland grasses Bromus benekenii and Hordelymus europaeus was studied in flowing solution culture experiments. Plants were exposed to low pH (3.9, experiment 1) and Al concentrations of 19 and 37 M (experiment 2) at two base cation (Ca+Mg+K) levels, all within the ranges measured in natural forest soil solutions. Elevated base cation concentrations ameliorated both H and Al toxicity, as indicated by increased root and shoot growth. In the third experiment, interactions between pH (4.3 and 4.0) and Al (0 and 19 M) were investigated. It was shown that the combined toxicity effects of H and Al were not greater than the separate H or Al effects. Tissue concentrations of base cations and Al increased with increasing concentrations in the solution, but were also influenced by the base cation : Al ratio. Relating the experimental evidence with the composition of forest soil solutions suggests an important role of soil pH and Al in controlling the distribution of the two species. Growth conditions also differ at various soil depths. Concentrations of free cationic Al were higher and base cation concentrations lower at 5–10 cm than at 0–5 cm soil depth. Increasing base cation concentrations may protect roots from both H and Al injury during periods of drought when concentrations of most elements increase in the soil solution, whereas molar ratios between base cations, H and Al remain unchanged.     

Effects of forest clear-cutting on the carbon and nitrogen fluxes through podzolic soil horizons

Effects of clear-cutting on the dissolved fluxes of organic C (DOC), organic N (DON), NO₃ ^– and NH₄ ⁺ through surface soil horizons were studied in a Norway spruce dominated mixed boreal forest in eastern Finland. Bulk deposition, total throughfall and soil water from below the organic (including understorey vegetation and, after clear-cutting, also logging residues), eluvial and illuvial horizons were sampled weekly from 1993 to 1999. Clear-cutting was carried out in September 1996. The removal of the tree canopy decreased the deposition of DOC and DON to the forest floor and increased that of NH₄ ⁺ and NO₃ ^– but did not affect the deposition of total N (DTN, <3 kg ha^–1 a^–1). The leaching of DOC and DON from the organic horizon increased over twofold after clear-cutting (fluxes were on an average 168 kg C and 3.3 kg N ha^–1 a^–1), but the increased outputs were effectively retained in the surface mineral soil horizons. Inorganic N deposition was mainly retained by the logging residues and organic horizon indicating microbial immobilization. Increased NO₃ ^– formation reflected as elevated concentrations in the percolate from below the mineral soil horizons were observed especially in the third year after clear-cutting. However, the changes were small and the increased leaching of DTN from below the illuvial horizon remained small (<0.4 kg ha^–1 a^–1) and mainly DON. Effects of clear-cutting on the transport of C and N to surface waters will probably be negligible.     

Rapid bioassay of aluminum toxicity in soil

Fifty-five acid soil horizons from 19 profiles were evaluated for aluminum toxicity using root elongation as a criterion in a two-day petri dish bioassay. The method proved to be simple, efficient, and precise enough to clearly distinguish aluminum toxicity differences among horizons within and between profiles. Although toxicity patterns within profiles differed, it was common for surface horizons to be less toxic even when very acid. The R² for correlations of relative root lengths with pH in H₂O, pH in KCl, soluble and exchangeable aluminum and percent aluminum saturation were only 0.42, 0.45, 0.52, 0.66, and 0.54, respectively, which indicates the need for a bioassay. In a further use of the method, and to demonstrate its efficiency, 243 horizons from 26 profiles were screened. Approximately half of the horizons with a pH of 5.0 or below showed Al toxicity. When used by different operators, with a variety of soil and treatment parameter changes, the two-day bioassay in petri dishes gave consistent rankings of soils by degree of aluminum toxicity. Journal Paper 11690, Purdue University, Agricultural Experiment Station, West Lafayette, Indiana, USA. Contribution from the Agronomy Department.     

Effect of phosphorus availability on basal root shallowness in common bean

Root gravitropism may be an important element of plant response to phosphorus availability because it determines root foraging in fertile topsoil horizons, and thereby phosphorus acquisition. In this study we seek to test this hypothesis in both two dimensional paper growth pouch and three-dimensional solid media of sand and soil cultures. Five common bean (Phaseolus vulgaris L.) genotypes with contrasting adaptation to low phosphorus availability were evaluated in growth pouches over 6 days of growth, and in sand culture and soil culture over 4 weeks of growth. In all three media, phosphorus availability regulated the gravitropic response of basal roots in a genotype-dependent manner. In pouches, sand, and soil, the phosphorus-inefficient genotype DOR 364 had deeper roots with phosphorus stress, whereas the phosphorus-efficient genotype G19833 responded to phosphorus stress by producing shallower roots. Genotypes were most responsive to phosphorus stress in sand culture, where relative root allocation to the 0–3- and 3–6-cm horizons increased 50% with phosphorus stress, and varied 300% (3–6 cm) to 500% (0–3 cm) among genotypes. Our results indicate that (1) phosphorus availability regulates root gravitropic growth in both paper and solid media, (2) responses observed in young seedlings continue throughout vegetative growth, (3) the response of root gravitropism to phosphorus availability varies among genotypes, and (4) genotypic adaptation to low phosphorus availability is correlated with the ability to allocate roots to shallow soil horizons under phosphorus stress.