No abstract

A simplified procedure for extracting ions from soil with ion-exchange resins is reported. The resin, placed in nylon-netting bags, were immersed in soil suspensions and shaken. Compared to the conventional procedure, where the resin beads are freely suspended in the soil-water mixture, this bagprocedure allowed quicker separation of resin from the soil suspensions. Furthermore, the severe soil grinding as done in the conventional procedure was eliminated. Phosphate was extracted from 4 Danish soils using both procedures. The bag-procedure resulted in slightly higher values than did the conventional procedure.

Information on the depression storage capacity (DSC) of soil surfaces is needed for modelling surface runoff and erosion. However, time-consuming measurements of the soil surface microrelief in a two-dimensional grid are necessary to obtain accurate estimates. Therefore, estimation of DSC from roughness indices requiring fewer measurements is desirable. In this study a new simple and physically based roughness index was developed and tested. Thirty-two soil surface microreliefs were determined in Danish erosion study plots on two sandy loam soil types, Humic Acrosol and Haplic Luvisol according to the FAO soil classi®cation. The soil treatments were: cultivated up-and-down the slope (black fallow), mouldboard ploughed upand-down the slope, winter wheat (Triticum aestivum L.) drilled up-and-down the slope and winter wheat drilled across the slope. The slope was ca. 10% for all plots. Data were treated initially by removing the slope after which 20 arti®cial slopes (1± 20%) were introduced producing 640 data sets. The DSC was calculated for each data set and a number of roughness indices were related to the calculated DSC. The best correlation to DSC was obtained by the newly developed index, mean upslope depression, MUD (R 2 0.86), followed by the geostatistically based limiting elevation difference, LD, and limiting slope, LS (R 2 0.76), and some statistically based parameters as standard deviation adjusted for oriented roughness up-and-down the slope (R 2 0.76), random roughness, RR (R 2 0.67), and standard deviation adjusted for oriented roughness both up-anddown and across the slope (R 2 0.62). Furthermore, MUD appeared to give less biased estimates than other indices, which overestimated small DSC-values and underestimated larger DSC-values. Calculations showed that the MUD index required relatively few measurements. MUD based on less than 200 surface elevation readings gave a good prediction (R 2 > 0.7) of DSC of tilled soil surfaces. #

Some phosphate soil tests (P-tests) are evaluated for their suitability to assess the phosphate status (P-status) of contrasting soils. A P-test at best only provides a relative estimate of the soil P-status, which may be thought of as the ability of a soil to release P either to a crop, or more specifically to a given plant root for a given period under optimum conditions with respect to all other growth factors. A P-test gives no direct information on the crop response to added P.The P-tests were evaluated on the basis of data from 29 published papers where the anion exchange resin method was among the P-tests used, and where plant P-uptake in pot experiments had been used as the evaluation basis.The evaluations ranked the P-tests in three groups: best group: anion-exchange resin method, intermediate group : water, and sodium bicarbonate methods, worst group: all 'acid' methods: acetate buffer, lactate buffer, citric acid, Bray-1, Truog and Bondorff methods.

Phosphate sorption was studied on surface and subsurface soils sampled from the old field experiments at Askov, Denmark. Two empirical three-parameter equations. an extended Freundlich and an extended Langmuir equation are proposed and compared with the Langmuir, the Freundlich, the 'double' Langmuir. the Gunary, and the Fitter-Sutton equations.In the proposed equations the affinity parameter of the Langmuir equation and the exponent of the Freundlich equation are replaced by the term Bc-I) the value of which decreases with increasing phosphate concentration, c. On average the Freundlich equation thus modified yielded the closest fit to the sorption data. This was followed by the Fitter-Sutton, the modified Langmuir, the 'double' Langmuir, the Gunary, the Freundlich, and finally the Langmuir equation. Out of the three equations that yielded the closest fit to the sorption data the correlation between the parameters within the equations, furthermore, was least for the proposed extended Freundlich equation. Therefore, this equation may be generally suitable for describing phosphate sorption by soils. IntroductionB A R R O W (1978) gave two motives for studying sorption equations, first to understand the processes involved, and second to summarize many results in a few numbers.The objective of this paper is primarily concerned with the latter. Two three-parameter sorption equations, an extended Freundlich and an extended Langmuir equation are proposed and compared with five equations from the literature.