Effects of temperature and prior flooding on intensity and sorption of phosphorus in soil

Summary Effects of temperature and flooded-drained soil conditions on 0.01M CaCl₂ extractable phosphorus (soluble P) were investigated in four soils over the period of 42 days after fertilizer-P application. These soils show severe induced P deficiency problem in crops following flooded rice culture. The effects of temperature on the reaction rate constants were determined and activation energy was calculated. Increasing soil temperature as well as prior flooding of soil decreased soluble P concentration but the effect of the latter was dominant. The decrease in soluble P concentration in these soils with time followed a first order kinetics and the rate constant (K₁) increased as the temperature increased from 10°C to 30°C. The activation energy (Ea) for the kinetics of soluble P concentration in soil, as affected by temperature, was found to be 8.9 and 34.5 KJ mol⁻¹ for Meyers and Willows clay, respectively, over the temperature range studied.     

A comparison of minirhizotron,core and monolith methods for quantifying barley (Hordeum vulgare L.) and fababean (Vicia faba L.) root distribution

Root research has been hampered by a lack of good methods and by the amount of time involved in making measurements. The use of the minirhizotron as a quantitative tool requires comparison with conventional destructive methods. This study was conducted in the greenhouse to compare the minirhizotron technique with core and monolith methods in quantifying barley (Hordeum vulgare L.) and fababean (Vicia faba L.) root distribution. Plants were grown in boxes (80 cm long × 80 cm wide × 75 cm deep) in a hexagonal arrangement to minimize the effects of rooting anistrophy. Minirhizotron observations and destructive sampling to a depth of 70 cm using core and monolith methods were performed at the ripening growth stage. Total root length for the entire depth interval was generally higher in barley (159–309 m) than fababean (110–226 m). Significant correlation coefficients between monolith and core methods for root length density (RLD, cm cm^–3) was observed in both crops (p 0.01). A method and depth interaction showed no significant differences in fababean RLD distribution measured by core and monolith methods. However, the RLD was different for the uppermost 40 cm depth in barley. The relationship for RLD between minirhizotron and core methods was significant only in barley (r=0.77^*). For both crops, estimates of RLD in the top 10-cm layer by the minirhizotron technique were lower than those by core and monolith techniques. In contrast, estimates of RLD were higher in fababean at a depth >30 cm. Destructive sampling still remains the method to quantify root growth in the 0–10 cm soil layer. ei]B E Clothier     

Soluble carbohydrates in roots of leek (Allium porrum) plants in relation to phosphorus supply and VA mycorrhizas

Leek plants (Allium porrum L.) inoculated with Glomus mosseae were raised on sterilized soil/sand medium amended with Ca(H₂PO₄)₂.H₂O to test the hypothesis that high concentration of soil P inhibits formation of vesicular-arbuscular (VA) mycorrhizas by reducing concentration of soluble carbohydrate in the root. When P supply was increased, from either P addition or VA mycorrhizal infection, there was initially also an increase in concentration of soluble carbohydrate in the root. At the concentration of soil P at which infection was reduced, concentration of soluble carbohydrate was at its maximum. Therefore the above hypothesis is discounted. An increased delay in infection establishment and a greater number of abortive entry points would suggest that high concentration of soil P reduces VA mycorrhizal infection by changing the anatomy of the root to make it resistant to fungal penetration.     

Field performance of Solanum sisymbriifolium, a trap crop for potato cyst nematodes. II. Root characteristics

Hatching of potato cyst nematodes is induced by root exudates of Solanaceae, such as Solanum sisymbriifolium, and is therefore related to root length distribution of this crop. A mathematical model was derived to relate the hatching potential to root length density (RLD). A series of field experiments was carried out to study actual root length distribution of S. sisymbriifolium in relation to shoot properties and to provide input into the model. Using a modified Poisson distribution formula for the three‐dimensional distribution of roots in a volume of soil, the relation between the zone of influence of hatching agents and the RLD could be derived. On this basis, the minimal RLD was estimated, which is needed to expose 75%, 90% or 95% of cysts to root exudates, as a function of the length of the zone of influence of hatching agents on cysts. The logarithm of the total root length showed a linear relation with the logarithms of above‐ground biomass and with leaf area index. Root diameter distribution was the same for all crops examined and independent of soil depth. Fine roots (<0.4 mm in diameter) constituted around 50% of total root length. Using a zone of influence of 1.00, 0.75 and 0.50 cm around the centre of each root, a minimal RLD for sufficient soil exploration (75%) was estimated. Depth at which that minimal RLD was exceeded was linearly related to total root length (km m^?2) and to above‐ground crop biomass, enabling estimations being made of the potential hatching efficacy as related to measurable properties of S. sisymbriifolium crops. The proposed approach to derive potential hatching effects from crop properties needs further validation; particularly, the distance of influence of root exudates is a critical factor.     

Cultivar and planting date effects on soybean root growth

To avoid late summer drought, soybean [Gylcine max (L) Merrill] producers in many southern and border states of the USA modify their cropping systems. Options include use of unadapted cultivars and changing planting dates. Because root function is important to plant health and yield, this study was conducted to determine if planting date and soybean cultivar affect root growth and distribution. Seeds of one cultivar from each of four maturity groups (MG III, IV, V, and VI) were sown in mid-April, mid-May, and mid-June in 1992 and 1993 on a Tiptonville silt loam near Portageville, MO. Root observations were performed 30 and 60 days after emergence (DAE) using a minirhizotron system. Cultivars differed for root length density (RLD) only in the 15 to 28 cm depth in 1992 and in the 15 to 28 cm and 29 to 42 cm depths in 1993, but differences were not related to maturity classification of cultivar. Average RLD was 1.02 cm^–3 for MG III and IV cultivars and 1.21 cm cm^–3 for MG V and VI cultivars. Average RLD for the mid-June planting date was 1.65 cm cm^–3 but only 0.73 cm cm^–3 for the mid-April planting date. An increase in RLD between 30 and 60 DAE occurred at all soil depths. For both years, MG V and VI cultivars produced higher yields than the MG III cultivars. Earlier than normal planting dates inhibited early root growth, but did not reduce yield. Cultivars differed only slightly for the rooting characteristics measured in this study. These rooting characteristics may not be important criteria for cultivar selection.Abbreviations MG maturity group - VCR videocassette recorder - DAE days after emergence - RLD root length density - CRLD change in root length density Contribution from the Missouri Agric. Exp. Station Journal Series Number 12, 153Contribution from the Missouri Agric. Exp. Station Journal Series Number 12, 153     

Effects of red clover (Trifolium pratense) green manure and compost soil amendments on wild mustard (Brassica kaber) growth and incidence of disease

Using a soil bioassay technique, seedling growth and incidence of disease of wild mustard (Brassica kaber) and sweet corn (Zea mays) were assessed in soil from field plots that received either of two treatments: incorporated red clover (Trifolium pratense) residue plus application of compost (`amended soil'), or application of ammonium nitrate fertilizer (`unamended soil'). Soils were analyzed for percent moisture, dissolved organic carbon, conductivity, phenolics, and nutrient content. A trend toward greater incidence of Pythium spp. infection of wild mustard seedlings grown in amended soil was observed during the first 40 days after incorporation (DAI) of red clover and compost, with significant differences ( = 0.05) at two out of four sampling dates in 1997, and four out of four sampling dates in 1998. Incidence of Pythium infection was 10–70% greater in the amended soil treatment during that period. Asymptomatic wild mustard seedlings grown in amended soil were also on average 2.5 cm shorter ( = 0.05) at 5 DAI than those grown in unamended soil in one year out of two. Concentration of phenolic compounds in soil solution was weakly correlated with decreased shoot and root growth (r = 0.50, 0.28, respectively) and increased incidence of disease (r = 0.48) in wild mustard seedlings in one year out of two. Dissolved organic carbon concentration was weakly correlated with increased disease in wild mustard seedlings in both years (r = 0.51, 0.33, respectively). Growth of corn seedlings did not differ between the two soil treatments, suggesting that red clover green manure and compost may selectively reduce density and competitive ability of wild mustard in the field. Bioassay results corresponded well with emergence and shoot weight results from a related field study, indicating that this technique may be useful for screening potential soil treatments prior to field studies.     

A comparison of soil core sampling and minirhizotrons to quantify root development of field-grown potatoes

Root growth of potato (Solanum tuberosum L.) is sensitive to soil conditions. A reduced root system size can result in reduced uptake of water and/or nutrients, leading to impaired crop growth. To understand the mechanisms by which soil conditions affect crop growth, study of temporal and spatial development of roots is required.In field experiments, effects of soil temperature, soil compaction and potato cyst nematodes (Globodera pallida) on root growth of potato cultivars were studied using two methods: core sampling and vertically oriented minirhizotrons.Minirhizotrons showed relatively more roots in deeper soil layers than core sampling, probably because of preferential root growth along the tube. Spatial distribution of roots should therefore be analysed by core sampling.To eliminate differences in spatial distribution, total root systems as measured by both methods were compared. Nematodes, cultivars and time did not affect the relationship between both methods. Soil compaction, however, affected it because of a strong response of root length in bulk soil and small differences in root number against the minirhizotron, suggesting that soil coring has to be used to study effects of different bulk densities.With both methods, sequential measurements of roots give the net effect of root growth and decay. Data on root turnover can only be obtained with minirhizotrons by comparing video recordings of different dates. Other information obtained with minirhizotrons is the average orientation of roots. Moreover, the minirhizotron method has the advantage of demanding less labour.     

Soil profile gravel concentration and its effect on rainfed crop yields

Summary Soil characteristics in the crop root zone are critical to soil water and nutrient availability to rainfed crops and determine crop production in coarse textured soils. A four-year field study was conducted in the foot-hills of North Himalayas near Chandigarh (India) on a coarse textured soil (Gravelly udic ustocrepts) to evaluate the effect of varying soil profile gravel concentration on the yield of rainfed crops of Taramira (Eruca sativa Mill.) in winter followed by maize (Zea mays L.), sorghum (Sorghum vulgare Pers.), cowpea (Vigna unguiculata L.) and sesamum (Sesamum indicum L.) in summer. Taramira gave a mean grain yield of 683, 410 and 275 kg ha^–1 at gravel concentration (GC) of 18, 28 and 40 percent by volume in the surface one metre soil depth. The grain and forage yield of summer crops decreased with the increasing GC. The gross monetary returns decreased in the order: Sorghum fodder, cowpea, sesamum and maize. The dilution of soil mass with increasing GC and corresponding decrease in nutrient and water holding capacity of the soil appears to have depressed the crop yields. The results indicated that the legume which can also conserve rainwater with dense canopy like cowpea or crops having vigorous fibrous root system and are relatively drought tolerant like sorghum may provide better economic returns in light textured soil containing gravel upto 40 percent.     

Arbuscular Mycorrhizal Fungi Enhance Zinc and Nickel Uptake from Contaminated Soil by Soybean and Lentil

Generally, soils in Pakistan are deficient in P and N. Due to intensive cropping and irrigation, Pakistani soils have also become deficient in micronutrients such as Zn, Fe, Cu, and Mn. Arbuscular mycorrhizal fungi, which form symbiotic associations with roots of most land plants, are known to enhance uptake of P and trace elements such as Cu, Ni, Pb, and Zn. The present study was conducted to investigate the role of arbuscular mycorrhizae (AM) in uptake of nickel (Ni) and zinc (Zn) by crops viz. soybean (Glycine max (L.) Merrill) and lentil (Lens culinaris Medic). Zn and Ni were applied as ZnSO₄ 7H₂O and NiCl₂ respectively, in four concentrations (0.0, 1.0, 3.0, and 5.0 g kg^-1 soil). AM inoculum consisted of sand containing sporocarps, spores, and AMF infected root pieces from a pot culture of Glomus mosseae. Control plants received pot culture filtrate containing soil microflora minus AM fungal propagules. A significant difference (p < 0.05) was observed in the dry weights of roots and shoots of the mycorrhizal (M) and nonmycorrhizal (NM) cereal plants. The sievate-amended treatments did not stimulate plant growth to the same extent as the AM fungal amended treatments. Trace metals inhibited the extent of mycorrhizal colonization of the cereal roots. The concentrations of the trace metals in the plant tissues of 12-week old cereal plants were found significantly (p < 0.05) higher in M than NM plants. These results indicate that mycorrhize can be used as effective tools to supply sufficient Zn in generally Zn-deficient Pakistani soils and to ameliorate the toxicity of trace metals in polluted soils. The contents of Ni in mycorrhizal soybean plant tissues were higher than those in the mycorrhizal lentil plant tissues. The implications of these results in mycorrhizo remediation of agricultural soils are discussed.     

Effects of transgenic soybean on growth and phosphorus acquisition in mixed culture system

Transgenic soybean plants overexpressing the Arabidopsis purple acid phosphatase gene AtPAP15 (OXp) or the soybean expansin gene GmEXPB2 (OXe) can improve phosphorous (P) efficiency in pure culture by increasing Apase secretion or changing root morphology. In this study, soybean‐soybean mixed cultures were employed to illuminate P acquisition among plants in mixed stands of transgenic and wild‐type soybean. Our results showed that transgenic soybean plants were much more competitive, and had greater growth and P uptake than wild‐type soybean in mixed culture in both low P calcareous and acid soils. Furthermore, OXe plants had an advantage in calcareous soils when mixed with OXp, whereas the latter performed much better in acid soils. In soybean‐maize mixed culture, transgenic soybean had no impact on maize growth compared to controls in both acid and calcareous soils with different P conditions. As for soybean in mixed culture, OXp plants had no significant advantages regardless of P availability or soil type, while P efficiency improved in OXe in calcareous soils compared to controls. These results imply that physiological traits could be easily affected by the mixed maize. Transgenic soybean plants with enhanced root traits had more competitive advantages than those with improved root physiology in mixed culture.     

Influence of the phenolic compound bearing species Ledum palustre on soil N cycling in a boreal hardwood forest

The effects of the understory shrub Ledum palustre on soil N cycling were studied in a hardwood forest of Interior Alaska. This species releases high concentrations of phenolic compounds from green leaves and decomposing litter by rainfall. Organic and mineral soils sampled underneath L. palustre and at nearby non-Ledum sites were amended with L. palustre litter leachates and incubated at controlled conditions. We aimed to know (i) whether L. palustre presence and litter leachate addition changed net N cycling rates in organic and mineral soils, and (ii) what N cycling processes, including gross N mineralization, N immobilization and gross N nitrification, were affected in association with L. palustre. Our results indicate that N transformation rates in the surface organic horizon were not affected by L. palustre presence or leachate addition. However, mineral soils underneath L. palustre as well as soils amended with leachates had significantly higher C/N ratios and microbial respiration rates, and lower net N mineralization and N-to-C mineralization compared to no Ledum and no leachates soils. No nitrification was detected. Plant presence and leachate addition also tended to increase both gross N mineralization and immobilization. These results suggest that soluble C compounds present in L. palustre increased N immobilization in mineral soils when soil biota used them as a C source. Increases in gross N mineralization may have been caused by an enhanced microbial biomass due to C addition. Since both plant presence and leachate addition decreased soil C/N ratio and had similar effects on N transformation rates, our results suggest that litter leachates could be partially responsible for plant presence effects. The lower N availability under L. palustre canopy could exert negative interactions on the establishment and growth of other plant species.     

Phosphorus benefits of different legume crops to subsequent wheat grown in different soils of Western Australia

Certain legume crops, including white lupin (Lupinus albus L.), mobilise soil-bound phosphorus (P) through root exudates. The changes in the rhizosphere enhance P availability to these crops, and possibly to subsequent crops growing in the same soil. We conducted a pot experiment to compare phosphorus acquisition of three legume species with that of wheat, and to determine whether the legume crops influence growth and P uptake of a subsequent wheat crop. Field pea (Pisum sativum L.), faba bean (Vicia faba L.), white lupin (Lupinus albus L.) and wheat (Triticum aestivum L.) were grown in three different soils to which we added no or 20 mg P kg^–1 soil (P0, P20). Growth, P content and rhizosphere carboxylates varied significantly amongst crops, soils and P levels. Total P content of the plants was increased with applied phosphorus. Phosphorus content of faba bean was 3.9 and 8.8 mg/pot, at P0 and P20, respectively, which was about double that of all other species at the respective P levels. Field pea and white lupin had large amounts of rhizosphere carboxylates, whereas wheat and faba bean had negligible amounts in all three soils at both P levels. Wheat grew better after legumes than after wheat in all three soils. The effect of the previous plant species was greater when these previous species had received P fertiliser. All the legumes increased plant biomass of subsequent wheat significantly over the unplanted pots in all the soils. Faba bean was unparalleled in promoting subsequent wheat growth on all fertilised soils. This experiment clearly demonstrated a residual benefit of the legume crops on the growth of the subsequent wheat crop due to enhanced P uptake. Faba bean appeared to be a suitable P-mobilising legume crop plant for use in rotations with wheat.     

The effects of crop rotation and fertilization on soil xylanase activity in a soil of the southeastern United States

Summary Xylanase activity was determined in soils from a 3-year rotation with corn, (Zea mays L.), cotton (Gossypium hirsutum L.), and soybean (Glycine max (L) Merr.) as summer crops at Auburn, Alabama, in the southeastern U.S.A. Winter wheat (Triticum aestivum L.) followed corn and a period of winter fallow as maintained after soybeans. A combination of common vetch (vicia sativa L.) and crimson clover (Trifolium incarnatum Gibelli & Belli) followed cotton during the winter months to serve as green manure. Highest xylanase activity was detected after soybeans and lowest after cotton among the summer crops. The culture of wheat and winter legumes resulted in increased soil xylanase activity wheareas winter fallowing reduced the activity. The effect of various fertilization schemes superimeposed on the rotation on soil xylanase activity was also studied. Seasonal fluctuations in soil xylanase activity were not affected by the fertilization regimes and highest xylanase activities were observed during crop periods and with fertilization treatments that resulted in high root densities but not necessarily in high yields.     

Traits associated with improved P-uptake efficiency in CIMMYT's semidwarf spring bread wheat grown on an acid Andisol in Mexico

Phosphorus deficiency is a major yield limiting constraint in wheat cultivation on acid soils. The plant factors that influence P uptake efficiency (PUPE) are mainly associated with root characteristics. This study was conducted to analyze the genotypic differences and relationships between PUPE, root length density (RLD), colonization by vesicular arbuscular and arbuscular mycorrhizal (V)AM fungi and root excretion of phosphatases in a P-deficient Andisol in the Central Mexican Highlands. Forty-two semidwarf spring-bread-wheat (Triticum aestivumL.) genotypes from CIMMYT were grown without (−P) and with P fertilization (+P), and subsequently in subsets of 30 and 22 genotypes in replicated field trials over 2 and 3 years, respectively. Acid phosphatase activity at the root surface (APASE) was analyzed in accompanying greenhouse experiments in nutrient solution. In this environment, PUPE contributed more than P utilization efficiency, in one experiment almost completely, to the variation of grain yield among genotypes. Late-flowering genotypes were higher yielding, because the postanthesis period of wheat was extended due to the cold weather at the end of the crop cycles, and postanthesis P uptake accounted for 40–45% of total P uptake. PUPE was positively correlated with the numbers of days to anthesis (at −P r=0.57 and at +P r=0.73). The RLD in the upper soil layer (0–20 cm) of the wheat germplasm tested ranged from 0.5 to 2.4 cm cm^-3 at –P and 0.7 to 7.7 at +P. RLD was the most important root trait for improved P absorption, and it was positively genetically correlated with PUPE (at –P r=0.42 and at +P r=0.63) and the number of spikes m^-2 (at –P r=0.58 and at +P r=0.36). RLD in the upper soil layer was more important with P fertilizer application. Without P fertilization, root proliferation in the deeper soil profile secured access to residual, native P in the deeper soil layer. (V)AM-colonisation and APASE were to a lesser degree correlated with PUPE. Among genoptypes, the level of (V)AM-colonisation ranged from 14 to 32% of the RLD in the upper soil layer, and APASE from 0.5 to 1.1 nmol s^-1 plant^-1 10^-2. This revised version was published online in June 2006 with corrections to the Cover Date.     

Populations of Rhizoctonia solani in soil under crops in rotation with sugar beet

Using a soil debris isolation method, populations of Rhizoctonia solani were monitored over a 4 -yr period in four fields which were initially cropped to sugar beet and in which four areas of Rhizoctonia crown rot diseased beets (DA) and four areas of apparently healthy beets (AH) had been selected and precisely located. Soil from these areas was assayed during the subsequent crops, which included sugar beet, tomato, cucumber, maize and soybean. No significant differences in colony counts were found between the soils in DA and AH on any of 30 sampling dates. R. solani population counts were, in general, quite low, except under sugar beet and following tomato harvest. Areas of diseased beet and high R. solani soil populations that developed in subsequent sugar beet crops did not necessarily coincide with the previously selected diseased areas. High R. solani populations developed from parasitic activity on sugar beet or saprophytically on tomato crop residues. Of the other crops, both maize and soybean may have slightly increased the low R. solani residual populations in soil. The monitoring of R. solani populations in the season prior to, and during the early season of sugar beet cropping did not provide a basis for forecasting disease in fields or sites within fields. The initiation of disease patches in these sugar beet fields was therefore governed by factors other than inoculum density.     

Coordination between Bradyrhizobium and Pseudomonas alleviates salt stress in soybean through altering root system architecture

It is a well accepted strategy to improve plant salt tolerance through inoculation with beneficial microorganisms. However, its underlying mechanisms still remain unclear. In the present study, hydroponic experiments were conducted to evaluate the effects of Bradyrhizobium japonicum USDA 110 with salt-tolerant Pseudomonas putida TSAU1 on growth, protein content, nitrogen, and phosphorus uptake as well as root system architecture of soybean (Glycine max L.) under salt stress. The results indicated that the combined inoculation with USDA 110 and TSAU1 significantly improved plant growth, nitrogen and phosphorus contents, and contents of soluble leaf proteins under salt stress compared to the inoculation with the symbiont alone or compared to un-inoculated ones. The root architectural traits, like root length, surface area, project area, and root volume; as well as nodulation traits were also significantly increased by co-inoculation with USDA 110 and TSAU1. The plant-growth promoting rhizobacteria (PGPR) P. putida strain TSAU1 could improve the symbiotic interaction between the salt-stressed soybean and B. japonicum USDA 110. In conclusion, inoculation with B. japonicum and salt-tolerant P. putida synergistically improved soybean salt tolerance through altering root system architecture facilitating nitrogen and phosphorus acquisition, and nodule formation.     

Spatial distribution of roots of pearl millet on sandy soils of Niger

Root sampling in crop stands of low planting density requires reliable information on horizontal distribution of roots. This applies particularly to pearl millet in the Sahel, which is sown at a rate of less than two pockets of seed per m². The objective of this study was to investigate the spatial variability of root length density (RLD) among sampling positions in an improved management system with ridging and under traditional sowing. RLD between ridges (bR) was lower compared to sampling positions within ridges (wR) at soil depth layers from 0 to 80 cm soil depth. We found a highly significant, positive correlation between the sum of the root length (RL) of four sampling dates (tillering, booting, flowering, and maturity) with shoot dry mass (SDM) at maturity. The square of the correlation coefficient was highest when calculation of RL was based on RLD at all four sampling positions. While SDM exhibited significant differences among three pearl millet varieties, sole root sampling wR at a lateral distance of 60 cm relative to the pocket would not allow for the detection of varietal differences in RL, while all other sampling positions did. The correlation between RL and SDM was considerably improved if information of RLD bR was included. Under traditional sowing, RLD directly under the plant was lower compared to sampling positions at lateral distance 25 and 50 cm from the centre of the pocket, but this effect of sampling position was not significant. RLD estimates within deeper soil layers were not systematically affected by direction and lateral distance. To obtain accurate information about depth of rooting and RL under traditional sowing, samples should be taken from lateral distances between 20 and 40 cm from the pocket centre.     

Effects of liming and mycorrhizal colonization on soil phosphate depletion and phosphate uptake by maize (Zea mays L.) and soybean (Glycine max L.) grown in two tropical acid soils

The effects of liming and inoculation with the arbuscular mycorrhizal fungus, Glomus intraradices Schenck and Smith on the uptake of phosphate (P) by maize (Zea mays L.) and soybean (Glycine max [L.] Merr.) and on depletion of inorganic phosphate fractions in rhizosphere soil (Al-P, Fe-P, and Ca-P) were studied in flat plastic containers using two acid soils, an Oxisol and an Ultisol, from Indonesia. The bulk soil pH was adjusted in both soils to 4.7, 5.6, and 6.4 by liming with different amounts of CaCO₃.In both soils, liming increased shoot dry weight, total root length, and mycorrhizal colonization of roots in the two plant species. Mycorrhizal inoculation significantly increased root dry weight in some cases, but much more markedly increased shoot dry weight and P concentration in shoot and roots, and also the calculated P uptake per unit root length. In the rhizosphere soil of mycorrhizal and non-mycorrhizal plants, the depletion of Al-P, Fe-P, and Ca-P depended in some cases on the soil pH. At all pH levels, the extent of P depletion in the rhizosphere soil was greater in mycorrhizal than in non-mycorrhizal plants. Despite these quantitative differences in exploitation of soil P, mycorrhizal roots used the same inorganic P sources as non-mycorrhizal roots. These results do not suggest that mycorrhizal roots have specific properties for P solubilization. Rather, the efficient P uptake from soil solution by the roots determines the effectiveness of the use of the different soil P sources. The results indicate also that both liming and mycorrhizal colonization are important for enhancing P uptake and plant growth in tropical acid soils.     

Evaluation of Argemone mexicana for Control of Root-Infecting Fungi in Tomato

Argemone mexicana L. (Papaveraceae), a tropical annual weed, is known to be phytotoxic to many crop species. This study was designed to examine the possible impact of A. mexicana on root‐infecting fungi, changes in fungal community structure and the growth of tomato. A. mexicana decaying shoots in soil provided a marked decrease in the infectivity of Fusarium solani and Rhizoctonia solani but Macrophomina phaseolina remained unaffected. Plant height and shoot growth of tomato plants increased markedly though high concentration of A. mexicana (5% w/w) was deleterious to tomato plants. General species diversity of soil fungal communities increased in the amended soils over the controls and greater increase in diversity occurred at higher concentrations of decaying A. mexicana. Likewise, equitability and richness components of diversity increased in treatments compared to controls but declined with increasing sampling period. Aspergillus nidulans, Cephaliophora irregularis, Drechslera halodes, Paecilomyces lilacinus and Trichoderma viride were isolated exclusively from the amended soils. Aqueous extract of A. mexicana when applied in soil greatly suppressed all three of the above root‐infecting fungi, and at lower concentration actually enhanced plant growth. The influence of different levels of N‐fertilization with NH₄NO₃ on the modification of the effect of decaying A. mexicana on root‐infecting fungi was also investigated. N‐fertilization to some extent alleviated the phytotoxicity to tomato plants while suppressing the root‐infecting fungi. A. mexicana in conjunction with Pseudomonas aeruginosa, a plant growth‐promoting rhizobacterium, significantly suppressed root‐infecting fungi with concomitant increase in plant growth. Whereas P. aeruginosa was reisolated from the rhizosphere and inner root tissues of tomato, its population slightly declined in the amended soil but not to an extent that could reduce the biocontrol and growth promoting potential of the bacterium.     

Interaction of lime,organic matter and fertilizer on growth and uptake of arsenic and mercury by Zorro fescue (Vulpia myuros L.)

The Sulphur Bank Mercury Mine (SBMM) is an abandoned open pit mine located on the eastern shores of Clear Lake, California. Revegetation efforts have been difficult because the mine-soils at SBMM have low pH, low fertility and elevated As and Hg concentrations. In a greenhouse study, we examined the interactions of lime, N, P and OM additions with respect to plant growth, and As and Hg uptake. Three selected acidic mine-soils from the site containing high (164 mg/kg) (S-H), medium (123 mg/kg) (S-M) and low (31 mg/kg) (S-L) total As content were planted to the Eurasian annual grass, Zorro fescue (Vulpia myuros L.). The Hg concentrations for these soils varied between 1700 and 3000 mg/kg with S-L > S-H S-M. A factorial design used 3 soils, 2 lime, 2 N, 2 P and 2 OM treatments with treatments replicated three times. Multiple regression analyses indicated a strong relationship between As plant uptake, root length density (RLD) and soluble As. A highly significant linear relationship between Hg uptake and RLD for plants growing on the three soils illustrated the importance of plant root characteristics in influencing Hg uptake. Soluble As decreased in the order S-H > S-M > S-L in positive correlation with P and DOC but in inverse relationship to oxalate extractable Fe. Lime and OM additions correlated negatively with soluble Hg and Hg tissue concentration due to either Hg adsorption to OM or to inorganic surfaces. Addition of lime increased dry matter yield and Hg uptake in all three soils.