Finnish Mineral Soils Research Articles

Estimating cation exchange capacity and clay content from agricultural soil testing data

Clay content and the ability to reversibly retain cations affect many essential chemical and physical properties of soil, such as pH buffering and carbon sequestration. Cation exchange capacity (CEC) and base saturation are also commonly used as criteria in soil classification. However, determination of CEC and particle-size distribution is laborious and not included in routine soil testing. In this study, pedotransfer functions including soil test cations (STCat; Ca2+ + Mg2+ + K+), pH and soil organic carbon (SOC, %) as explanatory variables were developed for estimating CEC, titratable acidity (TA; H+ + Al3+) and clay content (clay, %). In addition, reference values for potential CEC and its components were determined for Finnish mineral and organic soils. The mean of potential CEC extracted by 1 M ammonium acetate at pH 7.0 ranged from 14 (range 6.4−25) in coarse soils to 33 (21−45) cmol(+) kg-1 in heavy clay soils, and from 42 (24−82) in mull soils to 77 (25−138) cmol(+) kg-1 in peat soils. The average CEC of clay and SOC were 27 and 160 cmol(+) kg-1, respectively. Titratable acidity occupied 53% and around 40% of the CEC sites in organic and mineral soils, respectively, evidencing that it is a prominent component of the potential CEC in these predominantly acidic soils. STCat, pH and SOC explained 96% of the variation in potential CEC. STCat and pH can be used in estimating the clay content especially for soils containing over 30% clay. In coarse textured soils, in contrast, SOC hampers the STCat based estimation of clay content.

Methane oxidation in soil profiles of Dutch and Finnish coniferous forests with different soil texture and atmospheric nitrogen deposition

We studied methane oxidation capacity in soil profiles of Dutch and Finnish coniferous forests. The Finnish sites ( n = 9) had nitrogen depositions from 3 to 36 kg N ha −1 a −1. The deposition of N on the Dutch sites ( n = 13) was higher ranging from 50 to 92 kg N ha −1 a −1. The Dutch sites had also limed counterparts. Methane oxidation rates were determined by incubating soil samples in the laboratory at + 15°C with 10 μl CH 4 l −1 (10 ppmv CH 4). In general, CH 4 oxidation rates were highest in the uppermost mineral layers. The average CH 4 oxidation rate in the Finnish mineral soils was three times higher than that in the Dutch soils. The litter layers did not oxidize CH 4. In the Netherlands all organic horizons had a negligible capacity to oxidize CH 4. However, some Finnish organic horizons showed high CH 4 oxidation capacity. In the Netherlands, in contrast to Finland, there were some soil profiles lacking CH 4 oxidation. Higher contents of nitrate and ammonium, as well as greater production of nitrous oxide (N 2O) and lower production of carbon dioxide in the Dutch than in the Finnish forest soils reflected the high N deposition rate in the Netherlands. Not only the N deposition, but also the highly sorted soil texture (fine sand) with low amounts of both coarse and fine particles is an important reason for the low CH 4 oxidation in the Dutch soils. The proportions of fine and coarse particles, both well represented in moraine soils typical in northern Europe, correlated positively with the CH 4 oxidation. Fine particles provide a good surface for microbial growth. Coarse particles, on the other hand, enhance diffusion of CH 4 and oxygen into the soil. Methane oxidation in the Dutch mineral soils was slightly enhanced by liming.

Retention of vanadium (V) by three Finnish mineral soils

SummaryRetention of V(V) by three Finnish mineral soils from 10−4m and 10−5m sodium vanadate solutions was investigated at room temperature in the pH range 2.3–7.5. In adsorption experiments, 0.02 m KCl was used as ionic medium, and the samples were left to equilibrate for 72 h. The solid: solution ration was 1:100 (m: v). Maximum adsorption occurred at pH ∼4, where 70–80% of the added V (350–400 μg g−1 from 10−4m solution and 35–40 μg g−1 from 10−5m solution) was retained by each soil. Retention was significant even at pH 6, where adsorption of a similar anion, molybdate, is negligible. Desorption of the retained V was measured by adding 50.0 cm3 of 0.02 M KCl into weighed soil samples and shaking for 17 h in a mechanical stirrer. Only a minor part of retained V could be removed from each soil by the KCl treatment.Retention of different ionic species of V from 10−5m solution is approximated using values of log K obtained from the literature. In these calculations, kaolin is used as a reference material.

Desorption of phosphate from three Finnish mineral soil samples during adsorption of vanadate, molybdate and tungstate

Adsorption of V(V) and Mo(VI) from 10-4 M and 10-5 M solutions and W(VI) from a 10-4 M solution (in 0.02 M KCI) by three Finnish mineral soils was studied in two series of experiments. In the first experiment, the adsorption of V, Mo and W by soil and the desorption of P were measured at the soils’ natural pH after an equilibration time of 3, 5, 22, 29, 46 and 70 h. Adsorption of molybdate occurred mainly within the three first hours, whereas adsorption of vanadate and tungstate were slower processes. During the first few hours, the presence of molybdate seemed to increase the desorption of phosphate most effectively, but after a longer equilibration period, the differences between additions of V, Mo, and W became smaller. In the second experiment, the adsorption process was followed as a function of the acidity of the suspension (pH 2.3-7.5; for W pH 2.8-7.5). Adsorption of V(V), Mo(VI) or W(VI) resulted in a statistically significant increase in the amounts of P desorbed from all three soils over the pH range studied. The aqueous chemistry of V(V), Mo(VI) and W(VI) is briefly discussed.

Retention of Tungsten(VI) by Three Finnish Mineral Soils

Abstract Retention of W(VI) by three Finnish mineral soils from 10−4 M solutions for 72 h was studied at pH 2.8–7.5, using an adsorbent: solution ratio 1:100 (w: v) and 0.02 M KCl as a support medium. Retention was greatest from the most acidic samples, and at pH 3, 75–87% of the added W was retained. The W adsorption curves obtained for these mineral soils had different shapes compared with those reported for activated carbon, ferric oxide and kaolin. Retention of W by soils was strong, and only a small amount of the retained W was desorbed during a 17 h treatment with 0.02 M Kl.

Retention of Molybdenum(VI) by Three Finnish Mineral Soils

Retention of molybdate by three Finnish mineral soils from 10−4 and 10−5 M sodium molybdate solutions at room temperature was investigated at pH 2.3–7.5. In the adsorption experiments 0.02 M KCl wa...

Sulphate sorption by Finnish mineral soils

Sulphate sorption by 38 Finnish cultivated mineral soils was determined and its correlation with soil properties was studied. Sulphate sorption was correlated with soil pH (r =—0.46**) and with phosphate sorption (r =0.69***). With increasing soil pH, sulphate sorption decreased in relation to phosphate sorption. Phosphorus status was decisive in explaining the sulphate sorption of the soils. Even if both anions are sorbed by the same soil component (amorphous Al compounds), the sites are not available for sulphate if they are already occupied by phosphate. Sulphate sorption was negligible in soils very rich in easily soluble phosphorus. This was reflected in a close negative correlation between sulphate sorption and acid ammonium acetate (pH 4.65) extractable phosphorus (r =—0.70***). During the last few decades, phosphorus fertilization has increased the amount of easily soluble phosphorus in Finnish fields, which obviously has decreased the capacity of the soils to retain sulphate.

Base-neutralizing capacity of Finnish mineral soils

The base-neutralizing capacity, BMC7 (OH- as meq kg-1 needed to raise soil pH to 7), was determined graphically from curves obtained in KOH titration (at a constant ionic strength of I = 0.1). In 84 soil samples, BMC7 amounted to 0—316 meq kg-1, being highest in the heavy clay soils and lowest in the non-clay soils. In different textural groups, BMC7 seemed most markedly to be dependent on the initial soil pH, followed by organic C or oxalate soluble Al, in the coarser clays also on clay content. The results evidence that in determination of lime requirement, attention should be paid to the capacity of soil acidity. In routine soil testing, detailed lime recommendations for various soil types are needed.

Effect of Acidification on Cation Exchange Capacity of Eight Finnish Mineral Soils

Abstract The effect of acidification on cation exchange capacity was studied with eight Finnish mineral soils by adding HCl 0-320 meq/kg soil. Without acid the effective CEC ranged from 29 to 343 meq/kg soil, pH from 5.0 to 6.2. The acid treatment lowered the pH to 1.3–2.5 and reduced CEC to 23–169 meq/kg soil. The decrease of CEC ranged from 6 to 174 meq/kg soil, corresponding to 21–59% of the initial value. Two pH-regions with enhanced decrease in CEC were observed at pH 6-5 and 4-2, possibly being connected with buffering of organic matter and amorphous oxides, respectively. The quantity of acid needed to reduce the soil pH to 3.8 (ANC3.8) ranged from 18 to 140 meq/kg soil and decreased CEC by 2–105 meq/kg soil. The acid-neutralizing ability of a coarse soil had a maximum between 10 and 80 meq/kg soil and was nearly broken up by acid treatment 160 meq/kg soil. In two soils the maximum was between 160 and 320 meq/kg soil.

Acid‐ and base titration behaviour of finnish mineral soils

AbstractThe acid‐ and base‐buffering properties of 84 non‐calcareous surface soil samples were studied by batch titration with HCI or KOH at a constant ionic strength of I = 0.1. The soil samples were classified according to their pH of zero point of titration (ZPT). Differential buffer values, dB(H) or dB(OH) (H+ or OH− as meq kg−1 needed to reduce or increase the soil pH sequentially by 0.5 units, respectively), were introduced to describe the course of titration curve and the intensity of buffer action.In all soils, the first acid‐buffer value, dB(H)0→0.5, varied from 8 to 78 meq kg−1 and the second one, dB(H)0.5→1, from 10 to 138 meq kg−1. The corresponding base‐buffer values, dB(OH)0→0.5 and dB(OH)0.5→1, ranged from 10 to 48 and from 14 to 44 meq kg−1, respectively. The most acid soils were most strongly buffered against acid, and the soils with the highest initial pH against base. The results reveal the acid‐buffering by exchange reactions to be very important. In the soils with ZPT≦5.4, the first acid‐buffer value was dependent on the content of organic matter and oxalate‐soluble Al, whereas in the more acid soils the role of clay became significant. Thus, it was concluded that at higher pHs the foremost inactivation of H+ is attributable to soil components of pH‐dependent charges, and the significance of constituents of permanent charges to increase with proceeding acidification. In strongly acid soils (ZPT≦4.8) the very effective buffering seemed to be primarily due to the dissolution of Al‐hydroxides and, thus, to exert detrimental effects on the edaphic environment. The general rank of soil factors explaining the variation in the base‐buffer values was in accord with the neutralization sequence, i.e. the strongest acid in the soil being neutralized first. In the strongly acid soils (ZPT≦4.8) the base‐buffer values seemed to depend on the clay as well as KCl‐ and NH4OAc‐extractable Al, whereas in the soils with higher initial pH mostly on organic C.

Acid-neutralizing capacity of Finnish mineral soils

The acid-neutralizing capacity (ANC) was determined graphically from curves obtained in HCI titration (at a constant ionic strength I = 0.1) and was expressed as a quantity of acid (meq kg-1) needed to reduce the soil pH to 3.8. The relationship between ANC3.8 g and soil characteristics was studied statistically. In 84 soil samples, ANC3.8 ranged from 12 to 184 meq kg-1. The average ANC3.8 was highest in the heavy clay soils and lowest in the non-clay soils, but the differences between the various textural soil groups were not significant. In all soil groups the initial pHCaCl2 was relatively the most important factor explaining the variation in ANC3.8. Organic C was also a significant variable; this was considered to indicate the importance of cation exchange reactions of organic matter in acid-buffering. With the exception of heavy clay soils, oxalate-soluble Al significantly explained the variation in ANC3.8, suggesting that dissolution of Al hydroxides acted as a sink for H+ ions and contributed to the neutralizing capacity at the reference pH of 3.8.

Relationship between phosphorus intensity and capacity parameters in Finnish mineral soils: II Sorption-desorption isotherms and their relation to soil characteristics

The relationship between P intensity and capacity parameters in 104 mineral soil samples was studied by means of sorption-desorption isotherms of two types. In the isotherm A the P exchange was expressed as a function of P concentration in the initial solution, in the isotherm B as a function of P concentration in the final equilibrium solution. Both isotherms conformed to the equation y = a + bx, where y stands for the amount of P sorbed or desorbed and x the P concentration in the solution. In the isotherm A the constant a is the intensity factor expressing the amount of water soluble P at a given soil-solution ratio. The term a in the isotherm B, on the contrary, was only poorly related to water soluble P in soil. In both isotherms the slope b of the line seemed to be most effectively affected by oxalate extractable Al. The relative importance of oxalate soluble Fe appeared to be greater in affecting the effectiveness of sorption-desorption reactions than in affecting the buffer reactions. However, the slope b of both isotherms was found to be a semi-intensive parameter: it was quite markedly dependent also on soil characteristics which control the level of water soluble P in soil. The ratio of the term —a to b (termed as EBS or EPC), expressing the zero point of net P exchange, varied from 0.003 to 13.89 mg P per liter, the lowest values tending to be in the heavy clay soils and the highest ones in the non-clay soils. The practical significance of this quantity was discussed.

Relationship between phosphorus intensity and capacity parameters in Finnish mineral soils: I Interpretation and application of phosphorus sorption-desorption isotherms

The interpretation and application of two types of P isotherms were elucidated by means of sorption-desorption graphs of two surface soil samples. The isotherm expressing the retention or removal of P as a function of P application indicates the direction and extent of reactions at a given initial stage. The isotherm describing the sorption or desorption as a function of P concentration in the final solution after events illustrates the P buffering power of the soil. By using simultaneously the isotherms of both types, all information included can be integrated and the graphs can be used as a dynamic model describing P exchange.

Watersoluble phosphorus in Finnish mineral soils and its dependence on soil properties

Water soluble phosphorus ranged from 0.2 mg to 117.8mg/kg in 104 surface soil samples studied. On the average, water extracted less P from the heavy clay soils (4.8 ± 2.2mg/kg) than from the coarser clays (12.8 ± 4.6 mg/kg) and non-clay soils (13.3 + 7.2 mg/kg). Water extraction seemed to illustrate ’’the effective” P status, it is that determined by the quantity and quality of sorption components in soil, soil pH and the content of organic carbon. These factors did not affect the amounts of P dissolved in water directly but inderectly by controlling the nature of P bonding which, in turn, seems to be of decisive importance in the extractability of P into water. The P supplying power of a given fraction is obviously controlled by the quantity of corresponding sorption agent. Water extractable P correlated most closely with the molar ratio of NH4F soluble P to oxalate extractable Al (r=0.93***, n= 103). However, according to the theory presented, with progressing desorption, P starts to mobilize also from the NaOH soluble fraction, its significance being the more apparent the greater the corresponding molar ratio NaOH-P/Fe is. In addition, the role and significance of other inorganic P fractions were discussed.

Magnesium-supplying power of some Finnish mineral soils

Uptake of magnesium from a sand, fine sand, muddy clay, silty clay and heavy clay soil under exhaustive cropping with perennial rye grass was studied in green house. An application of 0.5 g Mg as MgSO4 ∙ 7 H2O per the 5-liter pots increased slightly the total yield of rye grass shoots and markedly the amount of Mg harvested in the shoots from the sand and fine sand soils with an initial content of only 14 and 37 ppm exchangeable Mg, respectively. No respond to the application of Mg was detected in the silty clay and heavy clay soils which contained exchangeable Mg 226 and 910 ppm, respectively. The muddy clay soil contained 137 ppm exchangeable Mg, and the application of Mg markedly increased the amount of Mg harvested in the shoots, but brought about a decrease in the yield of shoots. The amount of Mg harvested in the shoots without the application of Mg was only in the sand and fine sand soils higher than the original content of exchangeable Mg. Yet, in all soils except in the muddy clay, the decrease in the content of exchangeable Mg during the cropping was lower than the amount of Mg harvested. This was taken to indicate that some release of nonexchangeable Mg did occur during this trial. According to a rough estimation this mobilization of Mg varied from 0 to 60 ppm, whereas the corresponding release of nonexchangeable K was 500–1000 ppm, except in the sand soil. The »exhaustion Mg», or the sum of Mg harvested in the shoots of rye grass and the exchangeable Mg in the soil after cropping, was in all soils of the same order as the amount of Mg extracted by 0.05 N or 0.1 N HCI from the original soil samples.

Basic exchangeable cations in Finnish mineral soils

The content of exchangeable Ca, Mg, K and Na replaced by neutral ammonium acetate was determined in 470 samples of mineral soils from various parts of Finland, except from Lapland. The amount of all these cations tended to increase with an increase in the clay content, but variation within each textural class was large, and the ranges usually overlapped those of the other classes. The higher acidity of virgin surface soils was connected with a lower average degree of saturation by Ca as compared with the corresponding textural classes of cultivated soils. No significant difference in the respective contents of other cations was detected. The samples of various textural groups from deeper layers were usually poorer in exchangeable Ca and K than the corresponding groups of plough layer. The mean content of exchangeable Mg was equal or even higher in the samples from deeper layers than in the samples from plough layer, except in the group of sand soils. The percentage of Mg of the effective CEC increased, as an average, from 9 in the sand and fine sand soils of plough layer to 30 in the heavy clay soils; in the heavy clay soils from deeper layers its mean value was 38 ± 4 %. In the samples of plough layer, the mean ratio of Ca to Mg in sand and fine sand soils was about 9, in silt and loam soils about 6, in the coarser clay soils about 4, and in heavy clay about 2.

Effective cation-exchange capacity in Finnish mineral soils

Effective CEC of 230 mineral soil samples was estimated as sum of (Ca + Mg) and (AI + H) displaced by N KCI. The mean values as me/100 g of soil were, in the surface samples, 15.9 ± 2.0 in 46 clay soils, 8.9 ± 1.3 in 21 silt and loam soils, and 8.3 ± 1.1 in 39 sandy soils. In samples from the deeper layers the corresponding means were 16.3 ± 2.3 in 54 clay soils, 5.6 ± 0.9 in 21 silt and loam soils, and 2.5 ± 0.5 in 49 sandy soils. In surface samples of clay soils the mean effective CEC was about two thirds, in sandy soils of deeper layers about one third, and in all other groups about one half of the corresponding average potential CEC determined by neutral ammonium acetate. In the total material in which clay content ranged from 0 to 95%, organic C from 0.1 to 8.7 %, soil pH from 3.3 to 7.5, and oxalate soluble Al from 1.4 to 47.9 mmol/100 g, the »effective CEC» depended mostly on clay content: the partial correlation coefficient r = 0.90***, and the standard partial regression coefficient β = 0.84. The corresponding coefficients for the relationship between the »effective CEC» and the content of organic C were r = 0.55*** and β = 0.29, soil pH r = 0.35*** and β = 0.16, and oxalate soluble Al r = –0.13 and β = –0.06. The positive effect of liming on effective CEC, particularly, in coarser textured acid soils high in organic matter was emphasized.

Release of nonexchangeable potassium from Finnish mineral soils

Release of nonexchangeable potassium by treatment with 1 N HCI at 50°C was studied on basis of a material consisting of 330 samples of Finnish mineral soils. The results ranged from 1 to 830mg K/100g. The mean content of nonexchangeable acid-soluble potassium was in the surface samples of sand and fine sand soils 95±26 mg/100 g, in loam soils 165±31 mg/100 g, in silt soils 195±52 mg/100 g, in clay loam soils 258±32 mg/100 g, in silty clay soils 283±43 mg/100 g, and in heavy clay soils 345±126 mg/100 g. In the subsoil samples of loam, clay loam, silty clay and heavy clay soils the mean content was significantly higher than in the surface samples, or 283±51 mg/100 g, 404±56 mg/100 g, 535±53 mg/100 g, and 580±37 mg/100 g, respectively. The results seem to be high as compared with data reported from Sweden, Norway and Germany. The content of nonexchangeable potassium released by acid was to some extent connected with the clay content: the correlation coefficient in the whole material was r = 0.74***, but only about 0.5*** both in the separate groups of the 178 nonclay samples and the 152 clay samples. There was only a very low correlation between the contents of nonexchangeable acid-soluble potassium and readily exchangeable potassium. A somewhat higher correlation, r = 0.65***, was found for the relationship between the former and fixation of added potassium under »wet» conditions, but it was markedly decreased by the elimination of the effect of the clay content. Nonexchangeable acid-soluble potassium usually represented a lower part of the total potassium in the surface samples than in the subsoil samples, and also the proportion tended to be higher in the clay soils than in the coarser soils. It varied from 0.2 to 26.3 per cent in the small material studied. In most cultivated soils less nonexchangeable potassium was released from the samples of plough layer than from samples of deeper layers. In a podsol profile the minimum content of nonexchangeable and exchangeable potassium and the maximum of fixation of added potassium was found in the A2 horizon; in a brown podsolic soil all these test values decreased fairly regularly with depth. From some silt and silty clay soils incubated for three months at room temperature a large part, even more than 40 per cent of the added potassium was not recovered by the acid extraction. Ammonium acetate extracted from 9 to 85 per cent of the potassium applied before incubation, and the part of added potassium found as nonexchangeable acid-soluble form varied from 5 to 53 per cent. The equilibrium between the different potassium fractions in soil was discussed. It was supposed that differences in the ability of plants to utilize nonexchangeable potassium may partly depend on the level to which plant roots are able to decrease potassium concentration in the solution around the minerals.

Fixed ammonium in some Finnish soils

The nonexchangeable or fixed NH4-N was determined by the method of SCHACHTSCHABEL in 130 samples of Finnish mineral soils. In this relatively heterogeneous material consisting of 44 samples from the surface layer down to 20 cm and 86 samples from the deeper layers, the pH values ranged from 3.3 to 7.5, the contents of clay from 0 to 96 per cent, organic C from 0.03 to 6.6 per cent, and Kjeldahl-N from 0.13 to 4.53 mg/g. In the 46 finesand, loam and silt soil samples, the content of nonexchangeable NH4-N was, on the average, 90 ± 30 ppm ranging from 0 to 190 ppm. In the 84 clay soil samples it ranged from 40 to 620 ppm, with an average of 290 ± 30 ppm. The heavy clay soils of the deeper layers were particularly rich in fixed NH4-N containing it averagely 400 ± 40 ppm. Because of these soils the mean value in the samples of the deeper layers was as high as 260 ± 30 ppm and thus significantly higher than the corresponding figure in the surface soils, 140 ± 40 ppm. In the clay soils the amount of nonexchangeable ammonium was correlated with the clay fraction less than 0.6 μ (r = 0.74***), but not at all with the coarser clay. In the finesand, loam, and silt soils no correlation between the contents of fixed ammonium and clay was detected. No close relationship existed between the indigenous fixed ammonium and the capacity of the soil to fix applied ammonium. If the sum of these quantities would represent the total ammonium fixing capacity of the soil, about one half of this capacity would be saturated by the native fixed ammonium in the topsoil. In the subsoils the rate of saturation would be much lower. In the topsoil samples the amount of nonexchangeable NH4-N corresponded in the non-clay soils to 4 ± 2 per cent and in the clay soils to 9 ± 3 per cent of the Kjeldahl-N. In the subsoil samples the corresponding average percentage was 52 ± 5. The rather low ratios of organic C to Kjeldahl-N in the clay soils of the deeper layers may be taken to indicate that a part of the Kjeldahl-N was not in organic compounds. The C/N-ratios obtained when the amount of nonexchangeable NH4-N is subtracted from the Kjeldahl-N may be too high to characterize the soil organic matter, since it is likely that all the fixed NH4-N was not determined by the Kjeldahl-procedure.

Fractions of inorganic phosphorus in Finnish mineral soils

Inorganic phosphorus in 363 samples of Finnish mineral soils was fractionated by the procedure of CHANG and JACKSON. The average content of total inorganic phosphorus determined as the difference of the total phosphorus and organic phosphorus tended to increase from sand to clay soils. The sand and fine sand soils appeared to be richer in fluoride-soluble phosphorus but poorer in acid-soluble phosphorus than the other groups of soils. The part of phosphorus extracted by alkali seemed to be almost independent on the soil texture. In about 60 per cent of the samples the acid-soluble phosphorus was the dominant inorganic phosphorus fraction, in spite of the often high acidity of the soil. This, in connection with the rather low content of reductant soluble phosphorus, was taken to indicate the relatively low degree of weathering in these soils. The higher contents of fluoride-soluble and alkali-soluble phosphorus in the surface samples of the cultivated soils as compared with the corresponding kind of virgin soils or soils from the deeper layers may be mainly attributed to the application of phosphorus fertilizers and to a somewhat higher degree of weathering. The soil pH did not seem to play any important role among the factors related to the distribution of inorganic phosphorus into various fractions in the present material. This was particularly true in the cultivated surface soils. It is likely, that in our soils the variation in the contents of active iron and aluminium will to a higher degree than pH explain the variation in the fractions of alkali-soluble and fuoride-soluble phosphorus. The relatively high content of the latter fraction in the sand and fine sand soils as compared with the soils of the finer texture could be related to the higher ratio of ammonium oxalate soluble aluminium to iron in the former soils.