Undried Soil Research Articles

Soil Analysis by Mobile Multinuclear NMR: Quantification of Phosphorus, Aluminum, and Sodium.

Soil analyses are essential to ensure economically and environmentally sustainable crop production while maintaining the soil fertile for future use. Unfortunately, common soil analyses may be highly demanding in terms of time, chemicals, and costs. This applies, in particular, when total quantities of elements are desired. As an easy and fast alternative without consumption of chemicals, we here present mobile 31P, 27Al, 23Na, and 1H NMR for quantification of phosphorus, aluminum, and sodium contents in soil. This enables accurate on-site analysis and is suitable for direct measurement on fresh, undried soil samples since the water content is quantified as well. For demonstration, 40 various Danish agricultural soil samples were analyzed using a mobile NMR sensor, and the results were compared with external laboratory analyses for P, Al, and Na. The laboratory analyses were conducted with ICP-OES after four-acid digestion, which additionally were compared with aqua regia digestion, showing inadequacies in the performance of the latter. Good agreement between NMR and laboratory analyses (correlation coefficients 0.91 for P, 0.98 for Al, and 0.90 for Na, in the concentration ranges 250-1200 ppm P, 1.4-5% Al, and 0.3-1% Na) were obtained with high accuracy using NMR measuring times of 20 min to 1 h for P, 4-12 min for Al, and 6-20 min for Na. Additionally, the NMR measurements provide information on the amount of P associated with paramagnetic centers (e.g., Fe3+). Good correlations were also observed to other parameters such as the clay content, which is predictable from the intensity of the more fast-relaxing of three 27Al NMR components.

Survival of root-lesion nematodes (Pratylenchus thornei) after wheat growth in a vertisol is influenced by rate of progressive soil desiccation

The root-lesion nematode (Pratylenchus thornei) is a major pathogen of wheat in the subtropical grain region of eastern Australia. Experiments were conducted to learn whether soil desiccation can account for the rapid fall in peak P. thornei population densities noted in the field after wheat matures. The decline in population densities of P. thornei after growth of wheat was measured on progressive desiccation of soil with roots by fast and slow drying methods. The vertisolic soil of initial moisture content 45% w/w (or matric potential of pF 3.3) was dried in 5% decrements to an air-dried gravimetric moisture content of 15% (pF 5.6) taking 10.7 h for fast drying and 91.5 h for slow drying. After drying, live nematodes were extracted with Whitehead trays for 2 and 7 days and counted in four life stages (adults and juvenile stages J2, J3 and J4). Fast drying resulted in a sigmoidal decline in total P. thornei with only 5% of the population alive in soil at 15% moisture content, but slow drying had no significant effect on the population density. The percentage of nematodes extracted at 2 days compared with the total extracted over 7 days in undried soil (∼89% of total) declined quadratically on desiccation to be 48% (fast drying) and 78% (slow drying) at 15% moisture content. With fast drying, the proportion of adults and J2 decreased whereas the proportion of J4 increased as the soil dried. With slow drying, the proportion of J2 and J3 stages decreased while the proportion of J4 increased. Thus the J4 or pre-adult was the life stage most tolerant of soil desiccation. Time is required for P. thornei to go into a state of anhydrobiosis as a soil dries and this information can be used to model P. thornei survival in the field based on environmental parameters.

The effect of Cylindrocarpon destructans on the growth of Eucalyptus regnans seedlings in air-dried and undried forest soil

The growth of Eucalyptus regnans F.Muell. (mountain ash) seedlings is poor in natural forest soil, where purple coloration of the foliage indicates P deficiency and where the fungus Cylindrocarpon destructans (Zinsm.) Scholten is commonly isolated from the roots of the seedlings. When forest soil is air-dried, P acquisiton and growth of seedlings are markedly improved, although the degree of growth stimulation varies considerably at different times, as does the frequency of occurrence of C. destructans on the roots. C. destructans has been implicated as a possible reason for suppressed growth of seedlings in undried natural soil. To find out whether C. destructans contributes to growth inhibition of E. regnans seedlings in undried forest soil, the effect of three isolates of C. destructans on the root growth of E. regnans seedlings was tested in Petri dish experiments in vitro and the effect of C. destructans inoculation on seedling growth both in air-dried and undried forest soil was tested in pot experiments. The frequency of occurrence of C. destructans on the roots varied at different times, and was not consistently higher in undried than in air-dried soil, even though the growth of the seedlings was always poor in undried soil compared with that in air-dried soil. In vitro, C. destructans decreased the root growth significantly and caused blackening of root tips. This effect was removed by adding natural air-dried or undried soil. In pot experiments using undried forest soil, there was no evidence of either direct toxic effect or any other adverse effect on the roots when soil was inoculated with this fungus, even when the growth of the seedlings was reduced to ~1/2 of that in uninoculated undried soil. In air-dried soil, inoculation with the fungus did not significantly reduce seedling growth. Although potentially pathogenic and able to cause blackening of root tips, C. destructans is unlikely to be the main reason for poor seedling growth in undried forest soil. It appears to be antagonistic rather than pathogenic, suppressing seedling growth only under unfavourable conditions, such as in undried soil, possibly by competing for limited nutrients, or by suppressing other beneficial micro-organisms. The results are discussed in the context of field conditions.

Growth, nutrient acquisition and ectomycorrhizae of Eucalyptus regnans F. Muell. seedlings in fertilized or diluted air-dried and undried forest soil

Seedlings of Eucalyptus regnans (mountain ash) grow poorly in undried forest soil, where they develop purple coloration in the foliage, but their growth is markedly improved when forest soil has been air dried. Whether this growth promotion is purely due to improved nutrient status of the soil, as a result of air drying, was investigated. In several pot experiments, E. regnans seedlings were grown (i) in air-dried and undried forest soil with addition of different levels of complete fertiliser, (ii) in air-dried or undried soil diluted to different extents with sand, or (iii) in undried soil mixed with different amounts of air-dried soil. Seedling dry weight, P content and incidence of ectomycorrhizal root tips were determined. In all experiments, the dry weights of seedlings were 3–6 times greater in 100% air-dried soil than in 100% undried soil. Fertiliser application resulted in a significant increase in dry weight of seedlings in both air-dried and undried soil, but the dry weights in air-dried soil were always significantly greater than those in undried soil at the same level of fertiliser application. Even at the highest level of fertiliser application, the growth difference between seedlings in air-dried and undried soil remained. When air-dried soil was diluted with sand, there was a significant reduction in seedling dry weight only when soil was diluted to 20% or less (air-dried soil:total mix). Conversly, when air-dried soil was mixed with undried soil, there was a proportional decrease in seedling dry weight with increasing amounts of undried soil. In all experiments, the dominant ectomycorrhizal morphotypes in 100% air-dried soil were different from those in undried soil. Fertilisation and dilution of air-dried and undried soil did not result in a reduction in the overall incidence of ectomycorrhizal root tips, although the frequency of occurrence of different ectomycorrhizal morphotypes was affected. It is concluded that the growth difference between seedlings in air-dried and undried forest soils is not due solely to differences in the direct availability of nutrients in the soils, and different ectomycorrhizae may indirectly affect nutrient availability to the plant.

The growth and P acquisition ofEucalyptus regnans F. Muell. seedlings in air-dried and undried forest soil in relation to seedling age and ectomycorrhizal infection

The growth ofEucalyptus regnans seedlings in forest soil is enhanced when it has been air-dried. In undried forest soil seedlings grow poorly and develop purple coloration in the foliage, indicating P deficiency. This paper reports the results of pot experiments designed to investigate the relationship between growth and P acquisition, ectomycorrhizal infection and age of seedlings grown in air-dried and undried soil. The effect on seedling growth of their inoculation with air-dried or undried soil or with ectomycorrhizal roots from plants growing in air-dried or undried soil was also investigated. Ectomycorrhizal root tips were detected in 3-week-oldE. regnans seedlings in both air-dried and undried soil, and from then on the frequency of ectomycorrhizal root tips increased rapidly. In air-dried soil, seedlings were fully ectomycorrhizal at 9 weeks, and the occurrence of maximum ectomycorrhizal infection coincided with enhanced P acquisition and the initiation of rapid seedling growth. In undried forest soil, seedling growth remained poor, even though the seedlings had well-developed ectomycorrhizae and the incidence of ectomycorrhizal root tips was the same as in air-dried soil. The dominant ectomycorrhizae in airdried soil were associated with an ascomycete fungus, whereas in undried, undisturbed soil they were commonly associated with basidiomycete fungi. Inoculation of sterile soil/sand mix with washed ectomycorrhizal roots from air-dried soil increased the P acquisition and growth of the seedlings significantly compared with controls, whereas ectomycorrhizal inocula from undried soil had no effect on seedling growth, although both inocula resulted in a similar incidence of ectomycorrhizal root tips. Similarly, addition of a small amount of air-dried soil into sterile soil/sand mix resulted in a significantly greater increase in the P content and dry weight of the seedlings, compared with the control, than addition of undried soil. In both treatments, the incidence of ectomycorrhial root tips was similar. As (i) the differentiation in seedling growth between air-dried and undried soil occurred after seedlings became ectomycorrhizal, (ii) the dominant ectomycorrhizae in air-dried soil were different from those in undried soil, and (iii) inocula from air-dried soil, but not from undried soil, stimulated seedling growth in sterile soil/sand mix, it is concluded that development of particular ectomycorrhizae may be involved in seedling growth stimulation and enhanced P acquisition associated with air drying of forest soil.

Phosphorus mineralization kinetics and response of microbial phosphorus to drying and rewetting in a Florida Spodosol

Surface soils of a north central Florida Spodosol (sandy, siliceous hyperthermic Alaquod) from fertilized and unfertilized plantations of loblolly pine ( Pinus taeda L.) were conditioned, dried, rewet and incubated at 38°C for up to 26 d with periodic sampling for inorganic P and microbial P. Undried samples were also incubated and sampled periodically. Several kinetic models were evaluated to describe patterns of net P mineralization. Cumulative net mineralization of P in undried samples of both fertilized and unfertilized treatments was best described by zero-order kinetics. In contrast, a segmented model with two pools was most appropriate for describing cumulative net mineralization of P in dried and rewet samples, with one pool following zero-order kinetics, and the other following first-order kinetics. Net mineralization in rewet soils progressed in three stages: (i) An initial flush where inorganic P was brought into solution, the source most likely being turnover of the microbial biomass from the previous drying period and mineralization of organic substrates; (ii) a lag of a few days where there was no net release of P; and (iii) a period that followed similar kinetics to the undried soil, where the microbial biomass had recovered sufficiently to mineralize P from soil organic matter. Although more P was mineralized in fertilized soils, the kinetics of the reactions were similar to those in unfertilized soils. Generalized models were used to predict net P mineralization expressed as a percent of total P (specific P mineralization) for all data (both fertilized and unfertilized treatments). A zero-order model best described mineralization of P in undried soils ( R 2=0.884), and a segmented two-pool model was the best fit for soils that had been dried and rewet ( R 2=0.923). There was a negative relationship between inorganic P (as a % of total P) and microbial P in undried soils ( R 2=0.715). When soils were dried and rewet, microbial P increased over the entire incubation period but this relationship fluctuated with time and was not significantly correlated with P mineralization. The kinetic models proposed here should be useful in improving predictions of P mineralization in other sandy soils of low adsorption capacity.

A field method for quantifying ammonium picrate and picric acid in soil

A simple field method for the determination of ammonium picrate and picric acid in soil was devel- oped. Picric acid is a strong acid with a pKa 5 0.80, and is colorless when dissolved in an organic solvent, whereas its anion ( picrate) is bright yellow. Picric acid and picrate ions were extracted from undried soil by shaking with acetone; any picric acid extracted was rapidly converted to picrate in the wet acetone. Picrate was extracted from the acetone soil extracts by passing the solutions through a solid-phase anion exchanger to remove interferences. Acidified acetone was used to convert the picrate to picric acid and elute it from the ion exchanger. The absorbance of the solution at 400 nm was measured; then the picric acid was converted to the colored picrate ion by diluting the eluent with wa- ter. Absorbance at 400 nm was measured again and the concentration of picrate was obtained from the differ- ence in the absorbance measurements, corrected for di- lution. The method detection limit is 1.3 mg g 2 1 of soil. Field-contaminated soils were assayed, and the results compared favorably to those from HPLC analyses in the range of 10 - 4400 mg g 2 1 . The method is simple to use, can be implemented under field conditions, and com- plements on-site methods for TNT, RDX, and 2,4-DNT. © 1997 John Wiley & Sons, Inc. Field Analyt Chem Tech- nol 1: 165 - 170, 1997.

Effects of forest soil desiccation on the growth of Eucalyptus regnans F. Muell. seedlings

Abstract. The growth rate of Eucalyptus regnans seedlings in their first year can be much increased if the soil is first dried and then rewetted. The ratio of growth on predried soil to growth on undried soil (the Growth Ratio or GR) reaches a maximum at air‐dryness (pF 6.0–6.4). In E. regnans forest soil, GR is greatest in humus‐rich topsoil and declines with depth. The effect of air‐drying persists for several months after rewetting when soil is stored under glasshouse conditions. It is largely unaffected by repeated drying and wetting, by the rate of drying or by the season of collection. The mixing of dried and undried soil or the placement of a layer of dried soil above undried soil produces an enhancement of growth proportional to the amount of dried soil added. Firing of a litter layer above soil at wilting point increases subsequent seedling growth to that in air‐dried soil. The addition of ash from a litter fire to undried soil produces an increase in growth approximately equal to that caused by air‐dryingThe drying effect is most pronounced in soils from mature E. regnans forest and nearby brackenland and is less in dense younger forest, frost‐hollow grasslands and old grassy gaps in the mature forest. The effect is restored by the inoculation of E. regnans mycorrhizal roots from both dried and undried soil. The effect varies along an gradient from 500 to 1500 m a.s.l. and is a maximum in the wet E. regnans climatic zone and a minimum in zones or local aspects where forests are normally subject to frequent drying. The stimulatory effect on seedling growth in soils of the E. regnans zone may have an effect on the outcome of competition during regeneration in large gaps. Part of the growth responses previously ascribed to the ‘ash‐bed’ effect may be due to the desiccation effect in these soils.

The effect of soil desiccation on the nutrient status of Eucalyptus regnans F. Muell seedlings

The poor growth of young Eucalyptus regnans seedlings in undried soil from the mature forest of E. regnans can be overcome by previously air-drying the soil or by adding sufficient amounts of complete soluble fertilizer or equivalent concentrations of P (as NaH2PO4) and N (as NaNO3). A factorial pot experiment in which phosphate and nitrate were added to undried soil indicated that P was the primary deficiency for young seedlings and that response to N did not occur until this lack was satisfied. In dried soil, seedlings also responded to additions of complete fertilizer but most of this effect was due to N rather than P. Field trials in the mature forest also indicated greater growth in dried soil than undried soil and confirmed a response of young seedlings to superphosphate. In pot experiments, the concentration of P and N per g plant dry weight after four months was relatively constant irrespective of the final size of the plant. Seedlings in dried soil extracted up to 15 times more P than did those grown in undried soil. In general, chemical analysis of soil indicated more extractable P and N from dried soil although this was not always consistently so. Soil desiccation resulted in an increase in soil surface area due to the fragmentation of larger peds and to an increase in the number of microfractures which remained in the soil crumbs after rewetting. Mycorrhiza are likely to be important since the differentiation of the growth response of seedlings in dried and undried soil, which occurred at 5–6 weeks, corresponded with the establishment of full ectomycorrhizal development (80% root tips). The factors concerned with the increase in fertility after air-drying are discussed.

Chemistry of Cr in Some Swedish Soils

Abstract Retention and recovery of Cr(III) and Cr(VI) added to four mineral soils and their impact on pH and status of soluble Mn were investigated. Undried soil portions (≤4 mm) equal to 50 g dry matter were packed in glass columns and eluted with CrCl3 solution containing 6250 μg Cr(III) or K2Cr2O7 solution containing 1000 μg Cr(VI), displacing the retained Cr(III) and Cr(VI) with NH4Cl and KH2PO4 solutions respectively. Mn was measured in all effluents. Total Cr, Cr(III) and Mn were determined on an atomic absorption spectrometer with a graphite furnace and D2-background compensation and Cr(VI) on a photometer. Cr(III) strongly acidified and Cr(VI) weakly acidified the soils. The soils possessed a high affinity for retention of Cr(III) and a low one for retention of Cr(VI). Recovery of the retained Cr was very low and tended to be greater the lower the soil pH. Oxidation of Cr(III) was not detected. Reduction of Cr(VI) was substantial in the soils. It increased linearly with decreasing pH (R 2=0.71) an...

NITROGEN ISOTOPE FRACTIONATION ASSOCIATED WITH THE NO2− → N2O STEP OF DENITRIFICATION IN SOILS

The fractionation of the nitrogen isotopes 14N and 15N in the denitrification process has been studied in laboratory experiments for the step NO2− → N2O. This study has been carried out on natural soils under anaerobic conditions (helium atmosphere). Reduction of N2O is blocked by introduction of a small amount of acetylene in the incubator atmosphere. Variation of experimental conditions of incubation (temperatures, direct or after air-drying incubation of the soil, addition of glucose), greatly modify the reaction rate. 15N is enriched in the substrate during denitrification. The isotopic enrichment factor changes with experimental conditions from about −33 to −11‰. Greatest enrichment is obtained for the lowest reduction rate (low temperature, undried soil). For high rates of denitrification (higher temperature, addition of glucose), the isotopic fractionation decreases. An exponential relation is found between isotopic enrichment, εp/s, and the reaction rate.

Stability of urease in soils

Studies with surface samples of Iowa soils selected to obtain a wide range in properties showed that the following treatments of field-moist soils had no effect on urease activity: leaching with water ; drying for 24 h at temperatures ranging from 30 to 60°C ; storage for 6 months at temperatures ranging from −20 to 40°C; incubation under aerobic or waterlogged conditions at 30 or 40°C for 6 months. No loss of urease activity could be detected when field-moist soils were air-dried and stored at 21–23°C for 2yr, but complete loss of urease activity was observed when they were dried at 105°C for 24 h or autoclaved (120°C) for 2h. Inactivation of urease in moist soils was detected at temperatures above 60°C. Treatment of field-moist soils with proteolytic enzymes which cause rapid destruction of jackbean urease did not decrease urease activity, but jackbean urease was destroyed or inactivated when added to sterilized or unsterilized soils. Although no decrease in urease activity could be detected when field-moist soils were air-dried, an appreciable (9–33%) decrease in urease activity was observed when air-dried soils were incubated under aerobic or waterlogged conditions. This decrease occurred within a few days, and prolonged incubation or repetition of the drying-incubation treatment did not lead to a further decrease in urease activity. Treatment of incubated air-dried soil with urease or glucose initially increased urease activity to a level exceeding that of the undried soil, but this activity decreased with time and eventually stabilized at the level observed for the undried soil. The work reported supports the conclusions from previous work that the native urease in Iowa soils is remarkably stable and that different soils have different levels of urease activity determined by the ability of their constituents to protect urease against microbial degradation and other processes leading to inactivation of enzymes.

Sodium Availability in Nonalkali Soils

Ryegrass (Lolium multiflorum) was grown in the greenhouse on undried soil samples from the 0 to 15 and 30 to 45 cm depths of 15 Iowa soils. The NH4OAc exchangeable Na was 21 ppm or less in all soil samples except those from the 30 to 45 cm depth in southern Iowa soils, which contained 28 to 115 ppm exchangeable Na. The pH of these southern Iowa subsurface samples varied from 5.1 to 6.0. Percentages of Na in the first harvest of ryegrass were directly related to the exchangeable Na contents of the soils and inversely related to exchangeable soil K and K content of the plants. Plant contents of Na increased in succeeding harvests as the available K in the soil samples was decreased by crop removal. Total uptake of Na in six harvests of ryegrass was equal to the content of exchangeable Na initially present in the potted soil samples.

Plant Availability of Native and Added Potassium in Undried Soils

AbstractUndried surface and subsurface samples from 15 soils in Iowa were analyzed in the laboratory and used for a greenhouse experiment. K2SO4 was added to the soil samples at rates of 0, 50, 100, 150, and 200 ppm K. Increases in NH4OAc‐exchangeable K due to the K fertilizer additions were equivalent to 49 to 80% and 18 to 78% of the added K in the soil samples from the 0–15 and 30–45 cm depths, respectively. Percentages of K in the first harvest of ryegrass (Lolium multiflorum) were highly correlated, R2 = 0.89**, with the amounts of exchangeable K in the undried soil samples with and without additions of fertilizer K. Total plant uptake of K in six ryegrass harvests from soil samples without added K2SO4 was highly correlated with and generally greater than 2.67 times the amount of exchangeable K initially present in the soil samples. Added K2SO4 resulted in increases in total plant uptake of K equivalent to 42 to 97% of the added K. At a given level of exchangeable K, percentages of K in the first harvest of ryegrass grown on undried calcareous soil samples were higher than in grass grown on noncalcareous soil samples.

Control of Potassium Release and Reversion Associated with Changes in Soil Moisture

AbstractThe changes in exchangeable K that occur when soils are dried and rewet, and the affect of organic additives on these changes were determined with 20 soils from the North Central Region of the USA.The release of K associated with drying was essentially eliminated by adding dextrose to the soils before drying. The use of dried samples for the determination of the exchangeable K in field‐moist soils will be limited to some extent, however, because some soils fix K when they are dried and dextrose does not prevent fixation.In most cases, a decrease in exchangeable K occurred when air‐dry soils were rewet. A greater decrease in exchangeable K was attained with all soils when the air‐dry, rewet samples were redried in the presence of octanol. In some cases, only the reversion of K released by drying was involved, and the decrease was not enough to reduce the exchangeable K level to that in undried soil. In others, K fixation occurred and the heated soils contained less exchangeable K after treatment than they had in their field‐moist state. Nevertheless, the octanol treatment resulted in improved estimates of plant‐available K and of exchangeable K in undried soil samples over those obtainable with either air‐dry or air‐dry rewet samples.

Uptake of Native Potassium by Plants from Natural Soil Aggregates of Different Sizes

AbstractThe uptake of the native K by ryegrass (Lolium multiflorum) from undried soil aggregates > 9, 5–9, 3–5, 2–3, and 1–2 mm in diameter was studied in 14 soil samples. Yields of K and percentage K in the plants were highly correlated with the exchangeable K in the undried soils, irrespective of aggregate size. Early in the cropping period, plant yields were slightly higher, and slightly more K was taken up by the plants from the smaller aggregates than from the larger aggregates.

Changes in Exchangeable Potassium Observed on Drying Soils after Treatment with Organic Compounds: II. Reversion

AbstractUndried samples of Marshall subsoil that contained 27 ppm exchangeable (NH4OAc extractable) K were dried to release K and then treated in various ways to maximize K reversion. This was done to determine the possibility of reducing the exchangeable K in dried soil to the level of the undried soil and, thus, of using dry samples with released K to estimate the amount of exchangeable K that was present in the soil before drying.Much of the desired K reversion was attained by simply rewetting the dry soil. The rate and amount of this decrease in exchangeable K was enhanced by heating the rewet soil, but even so, the minimum values attained with rewet samples of air‐dry and oven‐dry soil were 54 and 62 ppm K, respectively. For complete reversion, it was necessary to mix the rewet soil with organic compounds and then oven‐dry the mixture.A decrease in exchangeable K was attained by drying the rewet soil, provided organic additives were present to prevent a release of K. Thus, the effects observed with various organic additives varied with the degree to which the additives were retained in the sample while the water was removed. Additions of high‐boiling‐point, normal alcohols; rewetting with a minimum of water; and slow drying gave the best results. When 10‐g samples of oven‐dry soil with 210 ppm exchangeable K were rewet with 7 ml of water, mixed with 7 ml of octanol and dried at 110C for 48 hours in a gravity‐draft oven, there was only 30 ppm exchangeable K present. Oven drying did not enhance K reversion when sucrose or glycerol was present.

The availability of organic matter in dried and undried soil, estimated by an anaerobic respiration technique

A study was made of the availability of the organic matter in gamma irradiated and non-irradiated soil, both dried and undried. It was concluded that the release of available soil organic matter during air-drying was the major contributor to the air-drying effect. It was also concluded that the change in microbial populations, which occurred during air-drying, did not contribute significantly to the air-drying effect. In addition it was observed that gamma irradiation affected the availability of soil organic matter in both dried and undried soil, the effect being most marked in the undried soil. It was considered that the anaerobic denitrifying technique which was used in this investigation had definite advantages over aerobic techniques which have been used to study the activity of micro-organisms in soil.

Comparisons of Laboratory and Greenhouse Tests for Nitrogen and Phosphorus Availability in Soils

AbstractGreenhouse techniques were developed for estimating nutrient availability in soils which would provide standards for comparison of different chemical soil tests. Plants were grown on undried soil samples to which nothing except water was added, but where other nutrients were supplied to the plants. Plant growth and P contents were highly correlated with the results of a chemical soil test. Plant uptake of N during the early period of growth was highly correlated with the initial NO3 content of the soils. Later plant uptake of N was highly correlated with the aerobic and anaerobic rates of N mineralization in the soils, but the relationships were different at different levels of P availability in the soils.

The hydrogen-ion concentration of heavy alkaline soils

(1) The colorimetric method is unsuited to the examination of heavy, alkaline soils owing to the turbidity of the suspension.(2) Where the nature of the suspension permits of colorimetric determinations being made, they agree with those obtained electrometrically: with the latter method practically identical results are obtained using soil-water mixtures or moderately clear extracts.(3) No disturbing effect is likely to be introduced by amounts of nitrate up to 500 parts per million of soil.(4) Owing to the effect on the pH of a soil suspension caused by varying the proportion of water and time of extraction, these conditions should be fixed for routine work. We have found 1 hour's extraction with 5 parts of water satisfactory.(5) On account of the amphoteric or buffer nature of clay, soil shifts the reaction of acids and alkalis in the direction of neutrality.(6) The effect of sodium salts on a soil is to displace aluminium and so reduce alkalinity: the residual soil after leaching is found to be more alkaline.(7) The effect of drying alkaline soil is to cause the pH of the extract to be lower than that obtained from the undried soil. If however the time of extraction is prolonged, the differences disappear almost entirely.