Estimation of Available Nitrogen for Advisory Purposes in Southern Rhodesia

Summary In Southern Rhodesia the nitrogen mineralized on laboratory incubation of soil samples taken towards the end of the dry season appears to provide a good index of the nitrogen likely to be available for crop use under field conditions during the subsequent growing season. Incubation for 2 weeks at 35° C., at moistures near field capacity, has been adopted. The occurrence of some soils showing a time‐lag between the commencement of ammonification and the commencement of nitrification makes it desirable to determine both ammonia‐ and nitrate‐nitrogen. Owing to practical difficulties in obtaining representative samples of both soil and undecomposed organic matter which has been recently ploughed under, and because it has not been found possible to incorporate this organic matter with the soil sample for incubation in such a way as to reproduce, during the incubation period, its overall effect in the field, soil samples for advisory work are at present sieved before incubation and allowances made for the probable effects of such organic matter when interpreting the results obtained. The nitrogen mineralized during incubation has been shown to be of the same order as that mineralized in miniature lysimeters maintained in the field under bare‐fallow conditions for the duration of the growing season. The mineralized nitrogen obtained after incubation has also been shown to correlate well (r =+.83) with yields, at zero fertilizer‐nitrogen levels, of maize and tobacco obtained in the field during seasons of good rainfall. Responses to fertilizer nitrogen in Rhodesia have been found to be 1–2 bags of maize or 100‐zoo lb. of tobacco per 10 lb. of nitrogen applied, corresponding to an efficiency of uptake of about 30–60 per cent.