The flux of water in unsaturated soils can be determined indirectly by using Darcy's Law. The critical part of this approach is the determination of the hydraulic conductivity at a given depth and time. In this paper we analyze the relative errors of hydraulic conductivities determined from a transient drainage field experiment. Knowing the errors of this method would be useful in planning future experiments. It allows putting limitations on the conclusions to be drawn from such experiments and it further allows re‐examination of already published field data.In the wet range of the conductivity function errors are 20–30% of the k‐value. In the range where k‐values are small, the relative errors may be > 100%. Errors stemming from tensiometer readings are significant when the hydraulic gradient is < 0.3 mbar · cm‐1. During the early stage of the transient drainage experiment these errors are always considerable and are more important than other errors. On the other hand, the errors in the measured water content changes are dominant when drainage is slowed down due to desaturation of the soil.The errors contained in the conductivity, k, are calculated by means of error propagation equations. These errors are also simulated using the Monte Carlo technique. The simulation shows that the conductivity errors predicted by explicit error propagation equations are optimistic minimum estimates.

A previously derived model for disappearance of nitrite with depth in an open soil system was tested with an especially built column filled with a sand‐soil mixture. Apparent rate constants for nitrification increased with an increase of population of nitrifiers as was expected. Nitrifiers increased almost logistically during two months in the upper portion of the column and reached a maximum population, but numbers fluctuated below the maximum population at lower depths. A decline in population density after a maximum was reached at a lower depth is attributed to partial starvation, owing to nearly complete oxidation of nitrite in the upper regions and to oxygen depletion.

A mathematical model based on Michaelis‐Menten kinetics for oxidation of ammonium to nitrate during downward flow in a column of soil mixed with sand has been tested. First the column was perfused with nitrite to stimulate the growth of nitrite oxidizing microorganisms in order to decrease the concentration of nitrite at any time in the column during subsequent perfusion with ammonium. The nitrite oxidizers multiplied in a quasilogistic fashion to nearly maximal numbers, exceeding ammonium oxidizers by at least a factor of 102. After a population density of 103/cm3 was reached for NO2‐ oxidizers, the column was perfused with ammonium solution; in a steady state the ammonium concentration decreased with depth (proportional to time of flow) in accord with Michaelis‐Menten kinetics. The rate constants for oxidation of NH4+ were proportional to the numbers of ammonium oxidizing microorganisms extant at any given time of observation; these numbers increased exponentially at first but leveled off after about 3 weeks of continuous perfusion of the column.As expected, the decline of ammonium concentration with depth during solution flow just equalled the appearance of nitrate with very low concentrations of nitrite in the steady state. From the data the normalized rate constant is 4.5 × 10‐3 ppm cm3/hour per microbe at room temperature, about five times greater than the corresponding figure for nitrite oxidation.

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Calculation methods based on Schubert's ion‐exchange equilibrium method were developed for determining stability constants of metal‐soil organic matter complexes. The methods, which were free of certain assumptions and errors inherent in the ion‐exchange technique as applied previously to soil organic matter, were verified using Mn(II)‐citrate and Mn(II)‐oxalate systems. Apparent stability constants (log K) of Zn (II)‐humic acid complexes ranged from 3.13 to 5.13 at pH 6.5.