摘要:

SUMMARYMechanical impedance to root growth is one of the most important factors determining root elongation and proliferation within a soil profile. Penetrometers overestimate resistance to root growth in soil by a factor of between two and eight and, although they remain the most convenient method for predicting root resistance, careful interpretation of results and choice of penetrometer design are essential if improved estimates of soil resistance to root elongation are to be obtained. Resistance to root growth through pressurized cells containing ballotini considerably exceeds the confining pressure applied externally to these cells. Results from this work are reappraised. Existing models of soil penetration by roots and penetrometers are reviewed together with the factors influencing penetration resistance. The interpretation of results from mechanical impedance experiments is examined in some detail and root responses, including possible mechanisms of response, are discussed.

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00070.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYThe dissolution rates of feldspars in the presence of naturally occurring organic solutes were compared at pH 5.1. Natural silt or ground feldspar minerals were suspended in water, streamwater, soil water, water extracts of peat and mor and a citrate solution for approximately 100 d. The increase of major cations and silicic acid in the aqueous phase was determined by periodic sampling and rates of dissolution were calculated when concentration increased linearly with time.The dissolution rate for the major cations was 2.7±0.9 (n= 3) times greater for stream‐water and 2.4±0.4 (n= 5) times greater for mor and peat extract than for distilled water. For citrate, the rate was greater by 1.7±0.3 (n=3) times.By inoculating the suspensions with microorganisms weathering rates decreased to the value for distilled water. This suggests that the microorganisms consumed some ‘active’ fraction of the organic

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00071.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYEleven samples of terrestrial humus from different vegetational backgrounds were examined with solid‐state13C NMR using cross‐polarization and magic‐angle spinning (CP‐MAS). In addition, all the samples were run with a dipolar dephasing pulse sequence for non‐quarternary carbon suppression (NQS). The humus samples all appeared to contain small amounts of aromatic substances and larger amounts of aliphatic compounds. Most of the samples contained considerable amounts of hydroxyl groups and acetals, which originate mainly from carbohydrates. No correlations were found between vegetational background and chemical

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00072.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARY15N‐labelled poultry excrement was obtained by feeding chickens with15N‐labelled barley grain. The excrement produced was tested for uniformity of15N‐labelling; only small variations in the15N content were found, indicating uniform labelling. The excrement was mixed with straw to produce manures which were used in a pot experiment measuring uptake of labelled and unlabelled N by ryegrass from the mixture in both a fresh form and after a period of composting. Interactions of N pools differed between the two materials. Addition of composted manure to soil caused a larger uptake of unlabelled N by ryegrass, whereas fresh manure caused a smaller plant uptake of unlabelled N. The proportion of plant N coming from labelled N from fresh manure was 25.7% and from composted manure 3.8%. However, the contributions of the two manures to plant uptake of total N were 15.5% and 13.8%, respectively. The fresh manure, being a mixture of excrement and straw, is considered as a two‐component material, whereas composted manure is considered as a single component. It was concluded that the increase or decrease in the quantity of unlabelled, soil‐derived N in plants after manure addition was related to the amount of energy available in the differen

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00073.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYMeasurements were made of the volatilization of ammonia from mono‐ammonium phosphate (MAP), di‐ammonium phosphate (DAP), ammonium sulphate (AS), ammonium nitrate (AN) and urea, applied to the surface of five contrasting soils. The compounds were applied as solids, at a rate equivalent to 100 kg N ha−1, to samples of moist soil packed into columns (48 mm diameter) and placed individually in jars through which a stream of air was passed for a period of 8 d. Volatilization ranged from nil to 53% of the N applied, with both the nature of the compound and soil type having large effects. Taking all combinations into account, there was a close relationship between the extent of volatilization, expressed as a percentage of the ammonium or urea N, and the pH attained after 24 h by the corresponding mixtures of soil and compound. Using the results of these and other experiments, the proportion of fertilizer N volatilized as ammonia is estimated to be about 3.4% over the UK as a

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00074.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYThe kinetics of oxidation of iron in an aqueous suspension of a thoroughly reduced low‐humus tropical rice paddy soil were followed by measuring the extractable ferrous iron in the whole suspension and in the solution. Three‐quarters of the initial ferrous iron was oxidized rapidly (first‐order rate constant = 9.2 × 10−5s−1). The subsequent reaction was slow (first‐order rate constant = 9.4 × 10−7S−1) and was not studied in detail. The pH fell from 6.6 to 4.9 over the course of the fast reaction. In further experiments the rate of oxidation was followed at constant pH values in the range 6.5 to 4.5. It was concluded that the oxidation of adsorbed iron was much faster than solution iron, and that the adsorbed iron was oxidized at a rate that was nearly independent of the pH. During the reaction some ferrous iron is adsorbed on the ferric hydroxide formed. The proportion of the remaining ferrous iron adsorbed on ferric hydroxide rather than the original exchange surfaces was high at pH>6.0 and low at pH<5.0. The rate of oxidation of the ferrous iron was similar whether it was adsorbed on exchange sites or on the ferric hydroxide formed. Since the rate of oxidation of the iron adsorbed on ferric hydroxide was very much slower than that on ferric hydroxide formed in the absence of soil, it is suggested that the rate in soil may be controlled by diffusion of oxygen to the

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00075.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYKinetic equations are developed for a system in which a column of reduced soil is exposed to oxygen at one end. The equations are combined in a simulation model in which they are solved by finite‐difference methods. The model predicts the consequent diffusion of oxygen into the column; the diffusion of ferrous iron towards the oxidation zone; the rate of formation and concentration profile of the ferric hydroxide formed; and the diffusion by acid‐base transfer of the acidity produced in the oxidation reaction.A sensitivity analysis of the model, in which runs were made for a wide range of input parameters, showed that for most combinations of parameters, in water‐saturated soil, substantial amounts of iron are transferred towards the air‐exposed surface, leading to a well‐defined zone of ferric hydroxide accumulation. The profile of total iron in this zone is often banded. The pH in the zone falls by at least two units. A small amount of air‐filled pore space increases the depth of the oxidation front dramatically. The model indicates that coupled iron oxidation and diffusion reactions, which are very widespread in natural soils, may be understood qua

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00076.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYThe adsorption and desorption of SO4was investigated as a function of KCI and KNO3concentration using soils with contrasting surface‐charge properties. In the net negatively‐charged soils, additions of C1 or NO3of up to about 0.05–0.10 M increased the adsorption of SO4but at higher concentrations adsorption decreased. In contrast, adsorption by the net positively‐charged soils decreased with concentration increase over the entire range (0 to 1 M) investigated. The effects of CI and NO3on the adsorption of SO4were practically identical. The different pattern of SO4adsorption in the two groups of soils in response to addition of KCI or KNO3can at least partly be explained in terms of the effect of electrolyte on soil pH. The depression in pH of net negatively‐charged soils induced by an indifferent electrolyte favours adsorption of SO4; but, because pH changes in the opposite direction in positively‐charged soils, SO4adsorption decreases in these soils. The distribution of a pulse of 35S‐labelled SO4in soil columns after leaching with KCI solutions, ranging in concentration from 0 (H2O) to 0.10 M, clearly reflected the manner in which electrolyte concentration affected the adsorption of SO4. The distribution of35S was reasonably well simulated using the general transport equation combined with the Freundlich equation to describe the adsorption/desorption of SO4. In contrast to other inorganic anions (H2PO4and OH) applied in agricultural practice, C1 and NO3may have beneficial effects on the S economy of many soils by decreasing leaching

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00077.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYThe phosphate adsorption capacity (Pmax) of samples from various horizons of five Danish podzolized soils were investigated before and after organic matter removal. Removal of organic matter had no direct influence on Pmaxsuggesting that organic matter did not compete with phosphate for adsorption sites. In the soils investigated aluminium and iron oxides were the main phosphate adsorbents. Thus, more than 96% of the variation in Pmaxcould be accounted for by poorly crystalline aluminium and iron oxides (extractable by oxalate) and by well‐crystallized iron oxides (taken as the difference between dithionite‐citrate‐bicarbonate‐extractable iron and oxalate‐extractable iron).Organic matter affected phosphate adsorption indirectly by inhibiting aluminium oxide crystallization. The resulting poorly crystalline oxides had high Pmax. In contrast, the influence of organic matter on the crystallinity of the iron oxides, and therefore on their capacity to adsorb phosphate, seeme

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00078.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

SUMMARYWe propose a general model for soil pH measurement that includes instrumental drift, random measurement error, and random and correlated spatial variation. Methods for estimating these four components are described in detail. For soil pH in water, instrumental drift, random measurement error and random spatial variation (nugget effect) were greater than the corresponding quantities for soil pH in CaCl2. For both pH measurements, instrumental drift was quite marked. Measurement error and nugget effect were of a similar size. A modified kriging method is presented that takes into account the four‐component model proposed here. It is concluded that, for measuring soil chemical attributes, grid layouts should be supplemented by additional sites for the estimation of short‐range variation, that laboratory sampling designs should include controls, and that field measurements should be adjusted for instrumental drift prior to being used for spatial contouring or krig

ISSN:0022-4588    

DOI:10.1111/j.1365-2389.1990.tb00079.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY