To obtain plants of different P status, maize and soybean seedlings were grown for several weeks in flowing nutrient solution culture with P concentrations ranging from 0.03-100/.~mol P L-1 kept constant within treatments. P uptake kinetics of the roots were then determined with intact plants in short-term experiments by monitoring P depletion of a 3.5 L volume of nutrient solution in contact with the roots. Results show maximum influx, I . . . . 5-fold higher in plants which had been raised in solution of low compared with high P concentration. Because P concentrations in the plants were increased with increase in external P concentration, Ima x was negatively related to % P in shoots. Michaelis constants, Km, were also increased with increased pretreatment P concentration, only slightly with soybean, but by a factor of 3 with maize. The minimum P concentration, Cmin, where net influx equals zero, was found between 0.06 and 0.3 /.~mol L -1 with a tendency to increase with pretreatment P concentration. Filtration of solutions at the end of the depletion experiment showed that part of the external P was associated with solid particles.It was concluded that plants markedly adapt P uptake kinetics to their P status, essentially by the increase of Imax, when internal P concentration decreases. Changes of Krn and Cmi n were of minor importance.

No abstract

To predict the contribution of soil K fractions of different mobility to K supply of plants, the kinetics of K release from soil was related to the kinetics of K uptake of young sugar beet and wheat plants. For this purpose K release rates from soil were measured by continuously percolating samples of a luvisol with 0.01 M CaC12 solution and effective diffusion coefficients, D e, were determined. Two soil K fractions of different mobility were obtained. D e values of the more mobile 'exchangeable K' and the less mobile 'non-exchangeable' K fraction were found to be 58.9x 10 -9 and 8.2×10-gcm2s -1, respectively. In a pot experiment, sugar beet and wheat plants were grown for 15 days and both root growth and K uptake were measured. K uptake kinetics of both crops was determined in a separate experiment using flowing solution culture. To integrate these data quantitatively, the simulation model of Claassen et al. (1986) was applied. Results show that calculated total K uptake agreed closely with real K uptake of the plants. On this basis, 64 and 79% of the K taken up by wheat and sugar beet plants was derived from the rapidly released 'exchangeable' and 21-36% from the less mobile 'nonexchangeable' soil K fraction.

Four tar-contaminated sites in Germany (Lünen, Castrop Rauxel, Stuttgart and Wülknitz) We are grateful to the Federal Ministry of Education and Research in Germany (BMBF, project numbers 02WN0366 and 02WR0763) for their support. Furthermore, we thank all members involved in the KORA-project for support, discussion and especially providing groundwater samples.

No abstract