NUTRIENT AND GROWTH RESPONSES OF LEERSIA ORYZOIDES,RICE CUTGRASS,TO VARYING DEGREES OF SOIL SATURATION AND WATER NITROGEN CONCENTRATION

Leersia oryzoides (rice cutgrass) is an obligate wetland plant common to agricultural drainage ditches. The objective of this greenhouse study was to expose plants to various flooding and aqueous nitrogen (N) concentrations and then to quantify the allocation of nutrients and biomass to plant components. Plants in the continuously flooded treatment (CF) had the highest tissue concentrations of copper (Cu), sulfur (S), zinc (Zn), potassium (K), sodium (Na), and manganese (Mn) in one or more plant components. Plants in the partially flooded treatment (PF) had the highest concentrations of magnesium (Mg) in leaves. The N input affected phosphorus (P) and S concentrations in roots. Leaf, stem, and root biomass were highest in PF plants. Rhizome biomass was the lowest in CF plants. These results indicate that L. oryzoides may significantly affect elemental concentrations in surface waters by its ability to uptake various elements and subsequent sequestration in various biomass components.     

EFFECT OF LONG-TERM SEWAGE SLUDGE APPLICATION ON THE DISTRIBUTION OF NUTRIENTS IN MAIZE (ZEA MAYS L.)

The distribution of the nutrients nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and iron (Fe) in maize (Zea mays L.) was studied in a long-term sewage sludge field application in an acid coarse sandy soil at Bordeaux, France. Sewage sludge had been applied since 1974 at 100 t (dry weight) ha^?1 per 2 years (SS 100) in plots cultivated annually with maize. Treatment with farmyard manure (FYM) at 10 t (dry weight) ha^?1 per year and mineral fertilization served as control. Five plants per treatment were investigated at six different growth stages. The plants were separated into their different organs and the distribution of nutrients was determined in up to 12 different plant parts. Sludge application did not significantly influence the nutrient partitioning in maize, but improved slightly the plants' nutrient status compared with the FYM treatment. Grain yield was similar for both treatments. The values for N, P, K, Ca, Mg and Fe content in sludge-treated plants were in the recommended range for optimal growth. Thus, sewage sludge would be a valuable source for maize nutrition even after long-term application, if the critically high copper (Cu) and zinc (Zn) concentrations, previously reported in these sludge-amended plants, could be avoided by the use of sludge low in these elements.     

Compositional Nutrient Diagnosis in Maize Grown in a Calcareous Soil

ABSTRACT

Preliminary compositional nutrient diagnosis (CND) norms for maize (Zea mays L.) were developed from a small database as means and standard deviations of row-centered log ratios V _X of five nutrients [nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg)] and a filling value R, which comprised all nutrients not chemically analyzed and quantified in 72 leaves of hybrid ‘Pioneer 3044.’ Norms were derived from maize grain yields higher than 7.7 t ha^?1. Principal-component analysis performed using CND nutrient indexes allowed us to identify an antagonism between N and Mg explained by the calcareous nature of the soil and the relevant N fertilization with ammonium (NH₄ ⁺), and a positive interaction between P and K, and P-Ca and K-Ca antagonisms. Maize plants tended to take up lower amounts of N and K and higher amount of Mg in our calcareous soil than the concentrations of these nutrients reported to be optimum.     

水分状态、土壤类型和氮素种类对氧化亚氮和甲烷排放的影响

Specific management of water regimes, soil and N in China might play an important role in regulating N2O and CH4 emissions in rice fields. Nitrous oxide and methane emissions from alternate non-flooded/flooded paddies were monitored simultaneously during a 516-day incubation with lysimeter experiments. Two N sources （^15N-（NH4）2SO4 and ^15N-labeled milk vetch） were applied to two contrasting paddies： one derived from Xiashu loess （Loess） and one from Quaternary red clay （Clay）. Both N2O and CH4 emissions were significantly higher in soil Clay than in soil Loess during the flooded period. For both soil, N2O emissions peaked at the transition periods shortly after the beginning of the flooded and non-flooded seasons. Soil type affected N2O emission patterns. In soil Clay, the emission peak during the transition period from non-flooded to flooded conditions was much higher than the peak during the transition period from flooded to non-flooded conditions. In soil Loess, the emission peak during the transition period from flooded to non-flooded conditions was obviously higher than the peak during the transition period from non-flooded to flooded conditions except for milk vetch treatment. Soil type also had a significant effect on CH4 emissions during the flooded season, over which the weighted average flux was 111 mg C m^-2 h^-1 and 2.2 mg C m^-2 h^-1 from Clay and Loess, respectively. Results indicated that it was the transition in the water regime that dominated N2O emissions while it was the soil type that dominated CH4 emissions during the flooded season. Anaerobic oxidation of methane possibly existed in soil Loess during the flooded season.     

深淹对狗牙根根际土壤酶活性及肥力的影响

以三峡库区消落带生境和非消落带生境生长的狗牙根(XC、FC)为试验材料, 研究了不同生境狗牙根经不同深度水淹胁迫后, 植株根际土壤酶活性和土壤肥力状况。结果表明: 未淹对照植株根际土壤的蔗糖酶活性明显大于不同深度淹水处理, 说明淹水处理不能给植物提供与未淹对照同样多的营养源; FC 根际土壤的脲酶和酸性磷酸酶活性都较XC 的高; 淹水处理后, FC 根际土壤的全氮含量明显低于XC; 淹水前FC 根际土壤的全磷含量明显高于XC, 表明在未进行淹水处理之前的生长时期, XC 可能较FC 利用更多的土壤营养进行生长, 储备更多的能量, 从而为淹水期间植株提供能量供应, 为增强其耐淹能力奠定较好的能量基础。以上结果说明不同生境狗牙根在不同水淹胁迫下, 植株根际土壤酶活性和土壤肥力的变化与其耐淹能力有关。     

Effect of soil pH and nitrogen source on nutrient status in peach: I. Macronutrients

Following 13‐year treatments of soil pH and nitrogen (N) source in a peach orchard of North Carolina, the concentration of calcium (Ca), magnesium (Mg), N, phosphorus (P), and potassium (K) in leaves, shoots, trunks and roots, as well as soil pH, soil exchangeable Ca, Mg, and K content, were determined. Through liming, higher soil pH treatment enhanced soil Ca and tissue Ca level. Among six N sources examined, the highest values of soil pH and soil Ca, Mg, and K were detected following poultry manure application. Compared to ammonium sulfate [(NH₄)₂SO₄], calcium nitrate [Ca(NO₃)₂] increased soil pH and soil Ca and K content, but reduced soil Mg. For most of macronutrients examined in peach tissues, the highest levels were found in manure treatment. Mineral N sources containing Ca(NO₃)₂ resulted in high tissue Ca and low tissue N. In the above‐ground tissues, Mg concentration was relatively low following application of mineral N materials containing Ca, K, or sodium (Na). Acid‐ forming N, especially (NH₄)₂SO₄, reduced tissue Ca and P. The magnitude of impact of liming and N source on macronutrients was tissue‐type dependent, with leaves and other new growth the most sensitive ones while trunks seldom responded to the treatments.     

Mineral‐nutrient synergism and dilution responses to nitrogen fertilizer in field‐grown maize

Nitrogen (N)‐fertilizer applications to field‐grown maize may result in a dilution response whereby essential mineral‐element concentrations in shoots would decrease as shoot‐dry‐matter accumulation increased. To investigate this, the effect of N‐fertilizer treatments (no N or fertilizer rate based upon 5.3 or 8.5 t ha^–1 yield goal) on maize (Zea mays L.) shoot dry weight and shoot mineral concentrations (N, P, K, S, Mg, Ca, and Mn) at the sixth leaf (V6), twelfth leaf (V12), and tassel (VT) development stages were investigated in a 2‐year study conducted at Brookings, South Dakota (USA). With increasing N‐fertilizer application rates, shoot dry weight was greater and shoot P and K concentrations decreased. A possible explanation of this dilution response is that planting‐time P and K fertilizers, which were applied in a band near the seed furrow, may have enhanced the uptake of P and K in a manner that was independent of N‐fertilizer treatments. Increased shoot‐dry‐weight production due to the application of N fertilizers, if P and K uptake were similar across N‐fertilizer treatments, would lead to decreased shoot P and K concentrations in N‐sufficient compared with N‐deficient plants. Conversely, N‐fertilizer‐induced increases in shoot dry weight were accompanied by increased shoot concentrations of N, Ca, and Mn. This synergistic response between dry‐weight accumulation and shoot N concentration was present at all leaf developmental stages studied, while that for Ca was present only at VT. Thus, N fertilizer applications that increase shoot dry weight can affect the dilution and synergistic responses of specific mineral nutrients in maize shoots. Crop developmental stage as well as the location of these specific mineral nutrients in the soil profile might play important roles in mediating these responses.     

Dryland corn response to tillage and nitrogen fertilization. II. P,K, CA,MG

Abstract

Nitrogen fertilization and tillage practices may influence the availability and uptake of essential plant nutrients other than N. This study was conducted to assess the interactive effects of N rate and timing and tillage practices on uptake and concentration of P, K, Ca and Mg in corn grown under dryland conditions. Potassium accumulations in no till (NT) soils were greater than in conventional till (CT) near the surface and lower than CT in the subsoil. Phosphorus and Ca levels decreased with soil depth, while Mg tended to accumulate in the subsoil. Phosphorus uptake and concentration of 5‐leaf stage corn was increased as tillage intensity decreased. Nitrogen rate at planting increased 5‐leaf P uptake but reduced P concentration; however, by silking no effect of tillage or N fertilization practice on ear leaf P concentration was obtained. Increases in 5‐leaf corn K uptake and concentration as tillage intensity decreased may have reduced Mg and Ca concentrations via cation antagonism. Ear leaf Mg and Ca concentrations were increased by N rate, probably as a result of solubilization of Ca and Mg and improved crop growth. Distribution of essential elements in the soil due to tillage in combination with varying N fertilization practices can influence temporal nutrient uptake, thereby altering plant nutrient diagnosis.     

Physiological differentiation of flooded N2-fixing grass-diazotroph associations from non-flooded, non-N2-fixing ones

N fixation by grass-associated diazotrophs is more active under flooded than under nonflooded conditions. Roots cut from Spartina alterniflora Loisel, rice or oats that were flooded exhibited nitrogenase activity (NA, assessed by acetylene reduction) that was at least ten times as high as that of roots from plants growing in non-flooded soils. O₂ inhibition of NA or lack of appropriate NA-supporting nutrients did not account for the low activity associated with the roots of the non-flooded plants. The roots of the plants from the non-flooded soils contained a factor that inhibited NA. This factor was not detected in roots of plants from flooded soils. The factor was soluble, heat stable (100°C) and passed through 1000 m.w. cut-off dialysis membrane. Analogous NA-inhibiting factors may serve to differentiate other N₂-fixing systems from non-fixing ones.     

NUTRIENT PARTITIONING IN POTTED PEACH (PRUNUS PERSICA L.) TREES SUPPLIED WITH MINERAL AND ORGANIC FERTILIZERS

The aim of the experiment was to evaluate the effect of organic fertilization on nutrient uptake and partitioning in potted peach trees. The study was carried out on 72 peach (Prunus persica L.) plants. The following treatments were compared: 1) unfertilized control, 2) mineral fertilization [713 mg nitrogen (N), 119 mg phosphorus pentoxide (P₂O₅), 476 mg potassium oxide (K₂O) pot^?1], 3) cow manure (119 g dw pot^?1) and 4) compost (119 g dw pot^?1). Compared to the untreated control, plant biomass was improved by supplying cow manure and compost. Potassium concentration in fine roots and leaves was higher in compost treated trees. Leaf concentrations of calcium (Ca) and magnesium (Mg) were decreased by applications of compost and cow manure. Nutrient concentrations in fine roots were always positively correlated with nutrients in coarse roots, only N showed a negative correlation. With the exception of Ca and Mg, macronutrients removed by trees were higher after compost fertilization than in unfertilized controls.     

Salt tolerance of grain crops in relation to ionic balance ability to absorb microelements

A greenhouse experiment was conducted to investigate the effect of exchangeable Na on the growth and absorption of metal elements in barley, rye, and maize. The plants were cultivated in soils whose exchangeable sodium percentages (ESP) were 6.6 (saline soil: Saline), 17.4 (saline-sodic soil: Sodic 1), and 39.6 (sodic soil: Sodic 2), which were prepared from Tottori sand dune soil (Control). The dry weight (DW) and concentrations of metal elements Ca, Mg, Mn, Zn, and Cu) in shoots were analyzed. The shoot DW was smaller with higher ESP, but in barley the difference between all the treatments was no longer observed with time. In Sodic soils, the growth of barley was vigorous, whereas rye growth was poor, and maize plants died by 5 weeks after planting. The Na concentration in shoots of all the species was higher with higher ESP. The K concentration in shoots was low at the early growth stage, but in barley it was higher in the Saline and both Sodic soils than in the Control at the subsequent stages. The concentrations of Ca and Mg in shoots of barley and maize in the Saline and both Sodic soils were higher than those in the Control, but in rye the concentrations were lowest in Sodic 2. The concentrations of Mn, Zn, and Cu in barley shoots in the Saline and bothSodic soils tended to be higher than those in the Control, whereas in rye they were lower than in the Control in both Sodic soils. Barley showed a higher ability to absorb low available microelements than rye and maize. These results indicate that barley is tolerant to sodicity as well as salinity, maize is tolerant to salinity, but is very sensitive to sodicity, and rye is moderately sensitive to both stresses. We suggest that the tolerance of grain crops to ESP involves a tolerance to a high Na concentration in shoots, the ability to keep suitable concentrations of essential cations in the presence of a high concentration of Na in shoots and the ability to absorb low available microelements.     

Nitrogen,calcium, and magnesium fertilization affects growth and leaf elemental content of ‘dormanred’ raspberry

’Dormanred’ raspberry (Rubus species) plants grown in sand culture were subjected to varying concentrations of N, Ca, and Mg over a two‐year period. Increasing nitrogen fertilization resulted in linear reductions of leaf Ca, K, Zn, Fe, and Mn but did not affect leaf Mg. Leaf Ca and K increased linearly with Ca fertilization, but applied Ca had an antagonistic influence on leaf Mg. Magnesium fertilization had a positive influence on leaf Mg but negatively affected leaf K, Ca, and Mn. Plant growth was negatively correlated with leaf Ca and leaf K, but had a positive correlation with leaf Mg and Mn. Nitrogen fertilization increased plant growth up to the mid‐level of applied N, but additional N reduced plant growth.     

Concentration Profile of Nitrogen and Phosphorus in Leachate of a Paddy Plot during the Rice Cultivation Period in Southern Korea

Abstract

The relationships between nitrogen (N) and phosphorus (P) concentrations in surface flooding water and those in the leachate of various soil depths were monitored, and temporal variation of leaching losses of N and P from a paddy plot during rice cultivation was estimated under the conditions of southern Korea. Even flooded conditions nitrification in subsurface soil was identified, but nitrate concentrations in leachate were less than 10 mg/L, the standard drinking water nitrate concentration set by the World Health Organization (WHO). The NO₃‐N and ortho‐P concentrations in the leachate were generally higher than those in the surface flooding water. Field data implied that leaching losses would not be accurately estimated under the flooded conditions of the paddy field when using the N and P concentrations of surface flooding water and infiltration depth. The leaching losses of NO₃‐N from paddy fields were high immediately after fertilization. The study results suggested that proper fertilization and irrigation strategies are required to reduce leaching losses of NO₃‐N from paddy fields.     

Soil P and cation availability and crop uptake in a forage rotation under conventional and reduced tillage

Long‐term conservation tillage can modify vertical distribution of nutrients in soil profiles and alter nutrient availability and yields of crops. This study aimed to evaluate the effect of 14 yr of conventional (CT) and reduced tillage (RT) on soil macronutrient availability (0–5, 5–15, 15–30 cm) and uptake by Italian ryegrass and maize in a forage rotation under a temperate–humid climate (NW Spain). Soil contents of total C, plant available Ca, Mg, Na, K and P and their uptake by plants were evaluated over 2 yr. The three‐way ANOVA showed that tillage and its interactions with soil depth and sampling date have little influence on soil C and macronutrients contents (<13% of variance explained). In the topsoil layer, all studied variables (except K) increased in RT compared with CT, but they remained unchanged (C, Ca and Na) or decreased (Mg, K and P) in deeper layers. Crop yields were greater with RT than CT during the year with soil‐water‐deficit periods, while limited tillage effect was found in the other year. Whereas no differences were obtained for maize, nutrient concentration (Mg, Na, K and P) in ryegrass increased under RT. Conservation tillage improved surface soil fertility, maize yield and ryegrass nutrient content.     

Correlations Between Soil Exchangeable Ca2+, Mg2+, K+ and Foliar Nutrient Concentrations in Mature Biological Olive Groves (Olea europaea L., cv. ‘Chondrolia Chalkidikis’)

Leaf and soil samples were taken and analyzed from two mature biological olive groves (Olea europaea L., cv. ‘Chondrolia Chalkidikis’), in Thessaloniki, Macedonia, Northern Greece, in order to determine the correlations between soil exchangeable cations and foliar calcium (Ca), magnesium (Mg) and potassium (K) concentrations, and the interrelations among leaf nutrients. Τhe nutritional requirements of trees for both biological groves were exclusively based on patent kali supply and nutrient recycling (via pruning material and weed cut recycling). Foliar K, Ca and Mg were positively correlated with soil exchangeable K, Ca and Mg, in the 40–60 cm layer, then in the 20–40 cm layer. Synergistic uptake mechanisms among Ca²⁺, Mg²⁺ and K⁺ probably exist. Leaf N was negatively correlated with foliar K, and positively with leaf Ca, Mg and manganese (Mn). Foliar P was negatively correlated with leaf Ca, Mg and Mn, while foliar Ca was positively correlated with leaf Mg and Mn. Foliar Mg was positively related with leaf Mn. High phosphorus (P) may decrease leaf Ca, Mg and Mn. Enhanced Ca may increase leaf Mg and Mn, while high Mg may also enhance foliar Mn. Finally, based on the determination of foliar nutrient concentrations, the nutritional requirements of olive trees in Ca, Mg, K, P, Fe, Zn were sufficiently (or over-sufficiently) satisfied. However, additional organic fertilization is needed, in order to achieve optimum levels of N, B and Mn (since their foliar concentrations were slightly insufficient). The correlations between leaf and soil exchangeable Ca, Mg and K, as well as among foliar nutrients should be taken into consideration, in order to achieve successful organic fertilization for mature biological olive groves, and to avoid nutritional imbalances and disorders.     

Impacts of biochar and processed poultry manure,applied to a calcareous soil,on the growth of bean and maize

Direct use of poultry manure on agricultural lands may cause environmental concerns, so there is a need to establish the suitability of the application of biochar derived from poultry manure for calcareous soil chemical properties and plant growth. The purpose of this study was to evaluate the effects of processed poultry manure (0, 5, 10 and 20 g/kg) and its biochar (0, 2.5, 5, 10 and 20 g/kg) on soil chemical properties of a calcareous soil and growth of bean (Phaseolus vulgaris) and maize (Zea mays) plants. In the incubation experiment, both processed poultry manure (PPM) and biochar decreased pH and the concentration of plant‐available Fe of soil but increased plant‐available P, Zn, Cu and Mn concentrations. PPM and biochar increased the concentrations of exchangeable cations (K, Ca and Mg) in soil. PPM and biochar applications increased the growth of maize and bean plants. PPM and biochar resulted in increased concentrations of N, P, K, Ca, Fe, Zn, Cu and Mn in bean plants. In maize plants, PPM and biochar applications increased the N, P, K, Zn, Cu and Mn but decreased the Ca and Mg concentrations. Results of this study reveal that poultry manure biochar can be used effectively for agricultural purposes.     

Aluminum toxicity in maize: Biomass production and nutrient uptake and translocation

The effect of increasing aluminum (Al) concentrations on root nutrient contents along with the concurrent translocation to the shoot of C₄ plants prompted this study. Two‐week‐old maize (Zea mays cv XL‐72.3) plants were therefore submitted for 20 days to Al concentrations ranging from 0 to 3.00 mM in a medium with low ionic strength were used as a test system. Aluminum concentrations in root tissues showed a 3‐fold increase between 0 and 3.00 mM Al treatment, and was not detected in the shoot. Root plasma membrane‐H⁺ ATPase activity decreased after the 0.33 mg L^‐1 Al treatment, while membrane permeability increased up to 1.00 mM Al treatment. Root and shoot biomass decreased after the 0.33 mM Al treatment. All elements in the roots, except potassium (K), manganese (Mn), and zinc (Zn) were highest for plants treated with 0.33 mM Al. Potassium increased continuously between 0 and 3.00 mM Al treatments, and iron (Fe) decreased above 0.33 mM. Only a slight decrease in nitrogen (N) was observed. All the measured nutrients in shoots, except N, Mn, and Fe decreased above 0.33 mM, but calcium (Ca) and magnesium (Mg) had little variation as Al varied. Data indicated that maximum net uptake for mineral nutrients, except Mn, occurred up to 0.33 mM Al. Translocation of phosphorus (P), K, Mn, and Zn decreased above 0.33 mM Al, N, and Ca decreased when any Al was added, and no clear trend was observed for Mg and Fe. Between the 0 and the 3.00 mM Al treatments, electrolytic conductance did not increased significantly indicating that the observed inhibitions of translocation from roots to shoots were not directly related to increasing membrane degradation.     

Effects of prolonged flooding on soybean at the R2 growth stage. I. Dry matter and N and P accumulation

Prolonged flooding of soybean [Glycine max(L.) Merrill] reduce yields. One explanation for lower yields in legumes has been the reduction in N fixation associated with the reduction in 0₂ supply in the flooded soil. This work investigated the effects of prolonged flooding at the initiation of reproductive growth on the dry matter and N and P accumulation in soybean. The field study was conducted on a Crowley silt loam (Typic Albaquaif). Forrest soybean were flooded at R₂ for 7 consecutive days at a flood height of 2.5 cm. Dry matter accumulation and concentrations of N and P of the stems, leaves, branches, and pods were determined by nodes on both flooded and non‐flooded plants for six sampling periods of 0, 7, 14, 21, 36, and 62 days after flooding. The results showed that, in terms of concentration and total amounts accumulated, flooding at R₂ adversely affected N nutrition in soybean. The soybean recovered from this effect two weeks after the flood was removed. As compared to the non‐flooded soybean, flooding improved P nutrition. The flooded soybean had greater P concentrations and total amounts of P accumulated. Flooding also prolonged vegetative growth in the upper portion of the canopy.     

EFFECT OF THE ECTOMYCORRHIZAL FUNGUS PAXILLUS INVOLUTUS ON GROWTH AND CATION (POTASSIUM,CALCIUM, AND MAGNESIUM) NUTRITION OF PINUS SYLVESTRIS L. IN SEMI-HYDROPONIC CULTURE

A semi-hydroponic culture was used to compare growth and cation nutrition of mycorrhizal (Paxillus involutus) and non-mycorrhizal Scots pine seedlings. When roots and hyphae grew together, concentrations and contents of macronutrients in needles and roots were not significantly different between mycorrhizal and non-mycorrhizal plants. When grown in two separate compartments, root potassium (K) concentrations, concentrations and contents of calcium (Ca) in needles and roots, needle nitrogen (N) concentrations, total N content and contents of root K and Mg were significantly reduced in mycorrhizal plants. Whereas ¹⁵N abundance increased in roots of mycorrhizal plants. The results indicated that the extraradical mycelium of the fungus strain used was able to transport N to the plant but did not contribute to long-term cation uptake and growth of host plants. An insufficient supply of macro-elements [N, phosphorus (P)] may account for the reduced growth of mycorrhizal plants and the differences in cation uptake between mycorrhizal and non-mycorrhizal plants.     

Toxicity and tolerance of aluminium in vascular plants

Elevated levels of Al in soil solution may be toxic to plants. The activity of monomeric inorganic Al species is the best measure of the phytotoxic fraction. Decreasing soil pH increases soil solution Al exponentially as the pH falls below 4.5. In the presence of organic matter and organic acids the availability of A1 is reduced, due to complexation. The Al concentration is also dependent on dominant clay mineral and clay content. Symptoms of Al toxicity usually first appear in the roots becoming structurally and functionally damaged and inefficient in absorbing water and nutrients. This might be due to disturbance of mitosis and changed permeability of the root membranes. Aluminium also affects DNA replication, protein synthesis, and activity of some enzymes. Symptoms on shoots usually develop at a later stage of Al poisoning. There is an antagonism between the macronutrients P, Ca, Mg, and K on the one hand, and Al on the other. Aluminium may induce deficiency of mineral nutrients and high nutrient concentrations reduce al toxicity. There is considerable variation in Al tolerance between species and between varieties of the same species. Several mechanisms for Al tolerance are suggested. Tolerant plants may possess an active exclusion mechanism or may be able to inactivate Al within their tissues. pH increases induced by the plant in the rhizosphere, exudation of chelating agents, a low C.E.C. in the roots and an ability to withstand or compensate Al induced nutrient deficiency are other possible tolerance mechanisms.