Plant-available Forms Research Articles

Phosphorus availability under annual cropping, alley cropping, and multistrata agroforestry systems

The hypothesis that agroforestry systems conserve P in forms that are more crop-available than those found in annual cropping systems was tested on two tropical sites: an Ultisol in Yurimagas, Peru, and a volcanically derived Inceptisol in Turrialba, Costa Rica. In both sites, the Hedley P fractions were compared in annual cropping, alley cropping, multistrata agroforestry and old forest systems. On the Ultisol, the effect of P fertilization on the soil P fractions was also examined. Under non-fertilized conditions, the multistrata and forest systems maintained more and a greater proportion of P in plant-available resin form than the annual or alley cropping systems. Greater resin P was associated with greater amounts and percentages of P in bicarbonate fractions and less P in residual fractions. The latter may be caused by greater and more temporally constant organic matter additions, less mineralization of labile soil organic matter fractions due to lower soil temperatures, and the long-term mobilization of residual P due to P immobilization in the biomass of the multistrata or forest systems. With P fertilization, resin P was related to inorganic P fractions. However, resin P increased even more when inorganic P fertilizers were combined with organic residue additions.

The orthophosphate content of bicarbonate soil extracts

The Olsen P and Colwell P bicarbonate extraction procedures are empirically derived tests that provide an estimate of the soil P that is available for plant uptake. This paper examines each procedure using high performance liquid chromatography with flow injection analysis detection (HPLC-FIA) to specifically measure orthophosphate in bicarbonate extracts. Extract solutions from 3 soils of contrasting plant-available soil P contents were analysed for orthophosphate, total filtered (<15—45 µm) P (TFP), and either Olsen P or Colwell P. The amounts of P extracted by the Olsen procedure were not statistically different from orthophosphate (HPLC-FIA) (P > 0.05), suggesting the Olsen P test was a good measure of orthophosphate, the most immediately plant-available form of P. However, the average amount of P extracted by the Colwell procedure was 14% higher (P < 0.01) than the corresponding average orthophosphate concentration, presumably due to the presence of labile organic/condensed P, colloidal associated orthophosphate, or high molecular weight reactive P. These results suggest that there is a pool of non-orthophosphate P present in the molybdate reactive Colwell P extract that is potentially plant available. Future work should focus on specific identification of P compounds extracted from soils and soil solutions to examine their role as a source of P to plants.

Soil acidification in Pinus caribaea forests on Brazilian savanna Oxisols

The transformation of large areas in the Brazilian savanna into Pinus plantations may have resulted in soil acidification. The objective of this study was to determine if afforestation with Pinus caribaea Morelet (PI) influenced soil acidification by comparing the metal status of the soil solid phase and the course of metal concentrations in soil solution during one rainy season with those under natural savanna vegetation ( Cerrado, CE). Surface soil samples (0–0.15 m) from three spatially disconnected plots in CE and PI were sequentially extracted. At each plot, soil solution at 0.15, 0.3, 0.8, 1.2, and 2 m depth and in PI additionally litter leachate were collected between October 1997 and April 1998. Extracts and soil solution were analyzed for Al, Ca, Cu, Fe, K, Mg, Mn, Na, Si, and Zn concentrations, soil solution also for pH and electrical conductivity (EC). Total concentrations in soil solid phase ranged between 94 000 and 155 000 mg kg −1 (Al), 29 and 39 (Ca), 21 and 31 (Cu), 44 000 and 61 000 (Fe), 91 and 141 (K), 21 and 29 (Mg), 28 and 57 (Mn), 73 and 94 (Na), and 13 and 21 (Zn). Whereas 27–83% of total K, Ca, Mg, and Na concentrations were exchangeable, >83% of total Cu, Fe, Mn, Zn, and Al were bound in hardly plant-available forms. There were no significant differences in concentrations and partitioning between CE and PI. At the beginning of the rainy season (October to December), metal concentrations in soil solution were 2–5 times larger and more variable than between December and April because of the mineralization of organic matter which accumulated during the dry season. Metal concentrations in CE soil solution between December and April were extremely small at all depths (Al: not detected (n.d.)–3.26 μmol l −1, Ca: 2.8–11.7, K: 0.9–6.7, Mg: 0.7–8.3, Mn: n.d. −0.14, Na: 5.5–40.1, Zn: 0.13–1.02). Up to 1.2 m soil depth, soil solution pH was significantly lower in PI than in CE. Concentrations of Ca, K, Mg, and Na were up to two times larger than in CE, those of Mn and Al 9 and 60 times, respectively. Whereas the soil solid phase did not show significant differences in metal concentrations and partitioning, the soil solution composition clearly indicated enhanced acidification in PI. This resulted in increased leaching of plant nutrients from the topsoil.

The effects of slash burning on ecosystem nutrients during the land preparation phase of shifting cultivation

The most commonly observed change in soil following slash-and-burn clearing of tropical forest is a short-term increase in nutrient availability. Studies of shifting cultivation commonly cite the incorporation of nutrient-rich ash from consumed aboveground biomass into soil as the reason for this change. The effects of soil heating on nutrient availability have been examined only rarely in field studies of slash-and-burn, and soil heating as a mechanism of nutrient release is most often assumed to be of minor importance in the field. Few budgets for above and belowground nutrient flux have been developed in the tropics, and a survey of results from field and laboratory studies indicates that soils are sufficiently heated during most slash-and-burn events, particularly in dry and monsoonal climates, to cause significant, even substantial release of nutrients from non-plant-available into plant-available forms in soil. Conversely, large aboveground losses of nutrients during and after burning often result in low quantities of nutrients that are released to soil. Assessing the biophysical sustainability of an agricultural practice requires detailed information about nutrient flux and loss incurred during management. To this end, current conceptual models of shifting cultivation should be revised to more accurately describe these fluxes and losses.

Effect of calcined magnesite on soil and Pinus radiata foliage magnesium in pumice soils of New Zealand

Magnesium (Mg) deficiency is common in a number of forest regions in the world. It has been linked to a condition in P. radiata called ‘upper mid crown yellowing’ (UMCY) in New Zealand and ‘new type forest decline’ in Europe. Mg concentrations are low in many of New Zealand"s forest soils. With increases in the number of rotations and increased growth rates through tree breeding, Mg deficiency is expected to increase. This study was conducted to determine the fate of calcined magnesite (calmag) fertiliser applied at 150 kg Mg/ha at 2 sites in the Kaingaroa Forest near Rotorua, New Zealand. It also investigated the effectiveness of calmag in increasing the soil solution and soil exchangeable Mg in pumice soils and Mg concentrations in the pine needles, and in reducing the likelihood of UMCY 2 and 3 years after fertiliser application. In both sites and for both years of sampling the application of calmag fertiliser resulted in a significant increase in soil exchangeable and soil solution Mg in the 0–5 cm soil layer. Soil and soil solution pH had also been increased in the top 5 cm soil layer. Two years after application about 90% of the fertiliser applied had dissolved and about 70–80% of the Mg remained in a plant-available form (ammonium acetate exchangeable Mg) in the top 10 cm of soil. Calculations suggest that 3–10% of applied fertiliser had been lost due to leaching. Magnesium fertiliser application also resulted in significant reduction in the exchangeable K: Mg ratio and reduced exchangeable Al in the 0–5 cm soil layer. After 3 years, foliar Mg concentrations increased at all sites in the fertilised trees compared with the control trees, although differences were not yet significant. UMCY severity in the trees was also not significantly affected by the application of Mg fertiliser.

Availability of zinc and cadmium to different plant species

A pot study was conducted to investigate whether differences in the amounts of Zn and Cd taken up by a number of plant species (canola, capeweed, subterranean clover, lettuce, swiss chard, ryegrass, and wheat) growing in a single soil could be explained as the result of plants having access to different soil pools of Zn and Cd. Prior to potting, the soil was spiked with carrier-free 65Zn and 109Cd. Total shoot concentrations of both Zn and Cd varied greatly across the plant species studied. The concentration of Cd in the shoots ranged from 0·075 mg/kg in wheat cv. RAC to 2·27 mg/kg in capeweed, while the concentration of Zn in the shoots ranged from 33 mg/kg in wheat cv. RAC to 259 mg/kg in swiss chard. However, the specific activity of Zn in the shoots was found to be the same in all cases. Hence the experiments showed that all plant species were accessing the same pool of Zn in the soil, despite differences in the total amounts of Zn taken up. In contrast, the specific activity of Cd in canola was found to be nearly double the specific activity of Cd in the other plants, suggesting that canola was unable to access a pool of soil Cd that was available for uptake by the other species. The percentage of total soil Zn and Cd that was in bioavailable pools was calculated to be approximately 12 and 36 (or 20% for canola), respectively, implying that significant percentages of these metals exist in non plant available forms in this soil.

Extractable zinc and manganese as related to applied cadmium in contrasting Indian soils

Soil chemical interaction of applied cadmium (Cd) and native zinc (Zn) and manganese (Mn) was studied in four contrasting Indian soil types, namely, an ultisol, an alfisol, an entisol and a vertisol with and without sewage sludge amendment. The soils after incubation periods up to 16 weeks were sequentially extracted with 0.5 M KNO3, deionised water, 0.5 M NaOH, 0.05 M Na2EDTA and 4 M HNO3; these extracted fractions represent the exchangeable, adsorbed, organic-bound, carbonate and sulfide/residual forms of Zn and Mn, respectively. Results indicated that in both Cd-treated and Cd-enriched sludge treated soils very small amounts of plant-available Zn could be extracted and the sulfide/residual fraction was the predominant Zn fraction extracted throughout the incubation period of 16 weeks. High amounts of organic-bound Zn present in sewage sludge were transformed into sulfide/residual or bound forms during the incubation period. Data on the extraction of Mn from different Cd-and Cd-enriched sludge treated soils revealed that high amounts of exchangeable Mn were extracted in ultisol and alfisol throughout the period of 16 weeks of incubation, indicating its presence in plant-available forms for a longer period of time in these two soil types. In the case of entisol and vertisol, the extraction pattern was very much different and only very small amounts of plant-available Mn could be extracted, possibly due to the higher pH, cation exchange capacity and silt/clay loam texture of these soils.

Chemical speciation of cadmium in contrasting Indian soil types

Chemical speciation of cadmium (Cd) in four contrasting Indian soil types, namely, an ultisol, an alfisol, an entisol and a vertisol with and without sewage sludge amendment was studied for a period of 16 weeks, utilising sequential extraction methods involving different chemical extractants, namely, 0.5 M KNO3, deionised water, 0.5 M NaOH, 0.05 M Na2EDTA and 4 M HNO3, representing the exchangeable, adsorbed, organic-bound, carbonate and sulfide/residual forms of Cd, respectively. Results indicated that the extractability of Cd in the four soil types varied according to the pH and CEC of soils. At acidic pH and low CEC, high amounts of plant-available forms of Cd (exchangeable + adsorbed + organic-bound fractions) could be extracted, whereas, alkaline pH and high CEC favoured the presence of fixed or bound forms of Cd (carbonate + sulfide/residual fractions). Further, irrespective of the soil types, the sewage sludge and Cd-enhched sewage sludge treatments resulted in the predominant extraction of the carbonate fraction at different periods of incubation.

Plant Nutrition and Heavy Metal Aspects of Compost Use

Depending on the materials used to produce a compost, it will contain lower or higher levels of nutrients and metals. If composts have been appropriately matured, nutrients are in plant-available forms for crop production, and the compost pH will be near neutral. After 25 years of research and development of regulations and advice for biosolids and compost utilization, pretreatment of industrial wastes allows biosolids composts, and composts prepared from biosolids mixed with municipal solid wastes or yard debris to contain levels of microelements needed for plant nutrition but not high levels that could cause phytotoxicity. Composts can supply N, P, K, Ca, Mg, Fe, Zn, Cu, Mn, B, Mo, and Se required by plants or animals. When used in potting media, supplemental N fertilization is usually required, depending on crop requirements. Use of compost can replace other forms of microelements used as fertilizers in media or fields. Detailed evaluation of potential food chain transfer of Cd, Pb, and other elements in composts clearly shows that consumption of 60% of garden foods produced on pH 5.5 soils with 1000 t compost/ha would not comprise risk over a lifetime of consumption, nor would ingesting the composts at 200 mg/day for 5 years. Potentially toxic organic compounds are either destroyed during composting, or bound very strongly by the compost so that plant uptake is trivial. Compost use can be a safe and wise choice for both home and commercial use to replace peat or uncomposted manures, etc. Many states have developed regulatory controls to assure that pathogenic organisms are killed during composting, and that product quality standards are attained that allow marketing for general use in the community.

Aboveground biomass and nitrogen nutrition in a chronosequence of pristine Dahurian Larix stands in eastern Siberia

Measurements of aboveground biomass and nitrogen (N) nutrition were made during July 1993 in 50-, 130-, and 380-year-old stands of Larixgmelinii (Rupr.) Rupr. in eastern Siberia. Constituting six forest types based on understorey plants, the stands were representative of vegetation throughout the Yakutsk region. Average tree height, diameter, and density ranged from 2 m, 23 mm, and 50 800 stems/ha in the 50-year-old stand to 11 m, 160 mm, and 600 stems/ha in the oldest stand. Aboveground biomass in the 50-year-old stand was 4.4 kg•m−2, and the aboveground N pool was 1.1 mol•m−2. This was slightly higher than the N pool in a 125-year-old stand with a Ledum understorey (1.0 mol•m−2), despite its higher biomass (7.2 kg•m−2). The highest observed aboveground biomass in a 125-year-old stand (characterized by the N2-fixing understorey plant Alnasterfruticosa) reached 12.0 kg•m−2, but the corresponding N pool was only 1.6 mol•m−2. In the oldest stand, aboveground biomass was 8.9 kg•m−2 and the N pool was 1.1 mol•m−2. There was thus a relatively constant quantity of N in the aboveground biomass of stands differing in age by almost 400 years. We postulate that N sets a limit on carbon accumulation in this boreal forest type. Trees were extremely slow growing, and there was essentially no aboveground biomass accumulation between the ages of 130 and 380 years because of a lack of available N. This conclusion was supported by graphical analysis indicating that the self-thinning process in our stands was not governed by the availability of radiation according to allometric theory. Much of the available N was used in the production of tree stems where 86% of the aboveground N (and 96% of aboveground biomass) was immobilized in the oldest stand. N in wood of the old stand exceeded the N pool in the litter layer and was 20% of the N pool in the Ah horizon. The processes of carbon and N partitioning were further explored by the estimation of carbon and N fluxes during three periods of forest development. We calculated a loss of ecosystem N during the period of self-thinning, while in the mature stands the N cycle appeared to be very tight. The immobilized N is returned from the wood into a plant-available form only by a recurrent fire cycle, which regenerates the N cycle. Thus fire is an essential component for the persistence of the L. gmelinii forest.

A literature review and evaluation of the. Hedley fractionation: Applications to the biogeochemical cycle of soil phosphorus in natural ecosystems

The Hedley fractionation recognizes plant-available forms (Resin Pi, Bicarb Pi, and Bicarb Po) and refractory forms (NaOH Pi, NaOH Po, sonic Pi, sonic Po, HCl Pi, Residual P) of soil phosphorus. This updated survey of the recent literature shows that the sequential fractionation proposed by Hedley et al. can also be used to separate forms of organically bound soil phosphorus from the geochemically bound fractions. We consider that biological P includes all the extracted organic fractions (Bicarb Po, NaOH o, sonic Po) and geochemical P includes the remaining fractions (Resin Pi, Bicarb Pi, NaOH Pi, sonic Pi, HCl Pi) and the Po and Pi in the Residual fraction. Data from the Hedley fractionation suggest that the contribution of geochemical versus biological processes to soil phosphorus availability varies with pedogenesis. The pool of primary phosphate declines and the NaOH and sonicated-NaOH phosphorus fractions increase as phosphorus becomes geochemically fixed to the iron and aluminum oxides in more highly weathered soils. The sum of organic-P fractions — biological P — is an increasing proportion of total available P as a function of soil development. Therefore, the Hedley fractionation provides a valuable index of the relative importance of biological processes to soil phosphorus content across a soil weathering gradient.

Soil organic matter quality as influenced by tillage, lime, and phosphorus

In Eastern Canada, cereal yields are often restricted by soil acidity and low fertility. Continuous cereal production can also lead to soil structural degradation. The addition of lime and fertilizers and the adoption of conversation tillage practices are proposed solutions which may have a positive impact on soil quality. The objective of the present work was to assess the impact of 3 years of different tillage practices and P additions, and of a single lime addition on organic C and total N, microbial biomass C, and on N mineralization at the surface layer (0–7.5 cm) of a Courval sandy clay loam (Humic Gleysol). The easily mineralizable N, total amount of N mineralized in 22.1 weeks, the rate of N mineralization, and microbial biomass C were significantly greater in the minimum tillage than in the moldboard plow treatment. Chisel plow treatment showed intermediate values. The ratios of potentially mineralizable N and of easily mineralizable to total soil N were also significantly larger under minimum tillage and chisel plowing than under moldboard plowing. The lime and P treatments had no significant effect on the measured soil quality parameters. The total amount of N mineralized per unit of biomass C decreased as the tillage intensity increased, suggesting a decrease in the efficiency of the biomass in transforming organic N into potentially plant-available forms and thus a loss in soil organic matter quality. The results of this study indicate that conservation tillage practices such as rototilling and chisel plowing are efficient ways of maintaining soil organic matter quality when old pastures are brought back into cultivation.

Interpretation of soil tests: a review

Soil tests tell how much nutrient is extractable, some of which may be present in a plant-available form, and/or the amount that may become available to the crop or pasture. This is a quantitative measure of nutrients. Interpretation depends on the skill of interpreters, and a major factor in interpretation is experience. Accurate interpretation is only possible when the sample collected from the field is representative of the site, and this can best be achieved using monitor plots rather than the traditional zig-zag method of sampling. One of the main deficiencies in experimental data used to interpret soil tests is the poor characterisation of experimental sites and of the climatic conditions during the experiment. Better characterisation would allow better interpretations to be made. With the development of equipment that is able to analyse multiple elements (e.g. ICP-AES), investigation of multiple extractants should be expanded in Australia. Of these, the Mehlich No. 3 and AB-DTPA extractants would seem the most appropriate. Because of the frequent relocation of agriculture advisers, experience is often lost; hence, there is a need for GIS/Expert systems to gather this experience. The development of such systems will allow new personnel to understand more rapidly the characteristics of soils, climate, and crops in an area. The final decision on fertiliser rates depends on the financial situation of the farmer. Soil testing and interpretation of these results is just 1 factor used to make the fertiliser decision.

EFFECT OF ORGANIC MATTER ON THE DISTRIBUTION OF MANGANESE, COPPER, IRON, AND ZINC IN SOIL FRACTIONS

Organic matter can accumulate in surface soil when reduced tillage methods are used. This in turn can influence the plant-availability of micronutrients. This paper reports an incubation study carried out to determine the effects of organic matter and micronutrient addition (Mn, Cu, and Zn) on the distribution of Mn, Cu, Fe, and Zn among soil fractions. Eight topsoils were treated with 0, 20, and 50 g/kg of ground straw and none or one level of the above combined metals. After 5 mo, soils were extracted sequentially to determine metals in the exchangeable, organic, Mn oxide, amorphous Fe oxide, crystalline Fe oxide, and residual fractions. Metals were also extracted using Mehlich 1, Mehlich 3, and DTPA. Increasing organic matter caused Mn and Fe to move from the less soluble forms to more plant-available forms (exchangeable and organic). Oxidation-reduced effects are the probable mechanism of this movement. Copper was not affected significantly. Increasing organic matter caused Zn to increase in the Mn and Fe oxide fractions at the expense of the Zn in the other fractions. Released Zn may have been occluded in these fractions or been strongly adsorbed. Added metals were evident in the exchangeable and organic fractions. Metals extracted with Mehlich 3 increased with increases in organic matter, but the other extractants did not consistently show this pattern. Correlations between metals in the fractions and that extracted by the extractants showed that the soil test extractants were solubilizing the metals from the exchangeable and organic fractions. These results demonstrate that accumulated organic matter near the soil surface can increase plant availability of Mn and Fe and possibly decrease the availability of Zn by causing a redistribution of elements between soil fractions.

CHANGES WITH TIME IN THE FORM AND AVAILABILITY OF RESIDUAL FERTILIZER PHOSPHORUS ON CHERNOZEMIC SOILS

A sequential phosphorus (P) fractionation procedure was used to measure the changes in the labile and stable forms of inorganic and organic P following single broadcast P applications to Canadian Chernozemic soils under cereal cropping. Approximately half of the fertilizer residues remained in plant-available forms (resin, NaHCO3). In a Black Waskada clay loam 8 yr after the application of 200 and 400 kg P ha−1, residual fertilizer P consisted of resin-P, 30–40%; HCl-P, 25–30%; residue-P, 10–15%; NaOH-P, 10–15%, NaHCO3-P, 10%; and aggregate protected P, 3%. The residues in a Dark Brown Sutherland clay 5 yr after the application of 160 kg P ha−1 were: resin-P, 35%; NaOH-P, 30–40%; NaHCO3-P, 15%; HCl-P, 0–5%; H2SO4-P, 5%; and aggregate protected P, 5%. The soils differed in the quantity of fertilizer recovered in inorganic HCl-extractable forms. In the Sutherland soil the change from wheat-fallow to continuous wheat cropping produced a build-up of organic P which occurred with and without the addition of P fertilizer. Key words: Residual P, P transformations, Labile Pi; labile Po, stable Pi stable Po

Transformations in plant-available forms of iron and manganese as influenced by manuring and cropping under irrigation

Vertisols from field plots fertilized continuously with P, K, FYM alone and in combination at the rate of 60 kg P2O5, 30 kg K2O and 1.5 ton FYM per ha were tested for different forms of Fe and Mn as a consequence of continuous cropping with sorghum and wheat. The transformations that occurred in different forms of plant-available iron were greater than those of Mn. Exchangeable and easily reducible Fe contents were markedly influenced by treatments. In manganese, transformations of easily reducible to exchangeable forms were conspicuous.

Nitrogen and land reclamation

Nitrogen supply is the major limiting factor in most reclamation schemes. A fully functional nitrogen cycle is dependent on the accumulation of resident soil organic matter from which nitrogen may be released slowly by microbial decomposition. Soil nitrogen transformations including nitrification, mineralisation and immobilisation are discussed, with particular reference to nitrogen transformations in reclaimed land. The use of legumes in revegetation programmes, their contribution to soil organic matter and transfer of fixed nitrogen to associated grasses is also discussed. The importance of evaluating the capacity of treated spoil to supply nitrogen in plant-available form is emphasised.

Manganese deficiency in sugar beet and the incorporation of manganese in the coating of pelleted seed

A laboratory study, three glasshouse tests and eight field experiments on commercial farms in East Anglia during 1972 to 1974 tested the effect of incorporating manganese in the coating of pelleted seed on the manganese nutrition and yield of sugar beet. The pelleting material readily absorbed manganese from solution but most of the manganese was held in plant-available forms. Tests in the glasshouse showed that manganese sulphate or manganous oxide were likely to be the most effective seed-pellet additives, but manganese sulphate was harmful to sugar beet in slightly acid potting compost.

Fertility changes in soils cultivated for wheat in southern New South Wales.

A study has been made of red-brown earth�s cultivated for wheat on a narrow rotation of fallow-wheat-volunteer pasture in the Ariah Park district of Xew South Wales. Surface soil samples from 55 cropped paddocks currently under fallow and from matched adjoining areas with no history of cultivation were examined in laboratory, pot culture, and field studies. Many of the soils had carried at least 20 crops since being brought under cultivation and had lost an average of 30% of their organic matter during this period. However, only 17% of the organic phosphorus had been mineralized. The mean ratio of carbon : nitrogen : sulphur : organic phosphorus in the uncultivated soils was 178.3 : 10 : 1.37 : 0.63 while that for the cultivated soils was 170.9 : 10 : 1.41 : 0.74. As a result of the decrease in organic matter content the cation exchange capacity decreased under cultivation. Losses of the exchangeable cations also occurred, but the relative proportions of the metal ions in the cultivated soils were the same as those in the uncultivated. Removal of cations in crops could not account for all of the losses observed. There had been a net loss of sulphur from the surface soils over and above that likely by crop removal. The nitrate content of the surface samples was insufficient to provide the nitrogen uptake by the crop either in pots or in the field. Evidence is presented suggesting that other sources, including leached nitrate, were utilized in the field. Mineralization of organic phosphorus and fertilizer addition had increased the amounts of inorganic, plant-available forms, but the total phosphorus was unchanged in the cultivated soils. However, the phosphorus status of' the cultivated soils was still extremely low and responses to phosphorus were obtained at all sites. There seems little prospect of reducing current superphosphate rates on these soils.

The relationship between phosphate response and base saturation in acid soils

In three groups of field experiments totalling fiftysix observations phosphate responses were found to be very significantly and inversely related both to the percentage saturation of the b.e.c. and exchangeable calcium as a percentage of exchangeable calcium plus hydrogen. The availability of the native soil phosphate was significantly and directly related to these values. The relationships between phosphate response or availability and the amounts of acid-soluble, adsorbed and water-soluble phosphate in the soils were much less conclusive.In acid soils considerable amounts of phosphate can be retained in a plant-available form associated with exchangeable bases, mainly calcium. Since practically all the soils from the field experiments lay within the range pH 4·7–6·3, it is believed that this form of phosphate is operative in these soils. The presence of such phosphate in the soil is, however, a function of the degree of base saturation in so far as this governs (probably through its influence on soil pH) the equilibrium distribution of the native soil phosphate between the available base-linked (saloid-bound) form and the less available colloid-bound form. With decreasing pH and base saturation increasing amounts of colloid-bound phosphate are formed at the expense of the saloid-bound phosphate. The degree of base saturation is therefore directly related to phosphate availability and, consequently, inversely related to phosphate response.In view of the significant regressions obtained, and the association of the exchangeable bases with a specific form of available phosphate, it is suggested that until methods are worked out for determining the amount of saloid-bound phosphate exchangeable base data may be used for assessing phosphato availability and response in acid soils. This method is contrasted with the more conventional methods of determining acid-soluble and adsorbed phosphates which are not always specific regarding tho form of phosphate extracted and the values for which, since they may represent the sum of a number of forms of different availabilities, aro difficult to interpret.