Phosphorus uptake and P concentration of plants as governed by soil supply and plant demand Nutrient uptake from soil by plant roots may depend on the supply capacity of the soil or may depend on the nutrient demand of the plant imposed on the root. The later will depend on shoot growth rate, size of the root system and on the nutrient concentration of the shoot. The basic aspect of the nutrient uptake and plant growth relationships can be stated as follows: assuming then (1) Where W is total plant dry weight (g), X nutrient concentration of the shoot (mol g-1), U the total nutrient content of the plant (mol), t the time (s). Also (2) Here RL is root length (cm), In the net influx (mol cm-1 s-1). Combining Equation (1) and (2), gives Equation (3): (3) Defining the growth rate per unit root ( ) as a new parameter SDR (Shoot Dry matter Rate) and introducing it in Equation (3) gives: (4) The objective of this research was to study the relationships of equation (4) for the nutrient P. For this sugar beet, maize and groundnut were grown in a growth chamber on a fossil Oxisol containing P mainly bound to Fe and Al. Three P levels of 30, 100 and 1000 mg kg-1 soil were obtained by applying Ca(H2PO4)2.H2O. Three harvests were performed at around every two weeks after sowing. Thereby we had two growth periods for analysing the rates of In and SDR and X. Phosphorus fertilization affected plant growth. At low P level the dry matter yield as percent of the maximum at 2nd and 3rd harvests were 44 to 27 for groundnut, 35 to 41 for maize and 68 to 84 for sugar beet. Fertilization increased the yield and P influx but the increased P influx mainly increased SDR (Equation 4) as long as the yield remained below 70% of the maximum. Once this level was surpassed also X was increased. With age, i.e. from the first to the second growth period, in all species P influx at the medium level was reduced in second growth period down to 40 to 20% of that of the first growth period. This reduced P influx in maize and groundnut was because of a reduced SDR and X, while in sugar beet it was only because of a reduced SDR, while X remained constant.

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