Yoshida - 1981 - Fundamentals of Rice Crop Science

120 FUNDAMENTALS OF RICE CROP SCIENCE
3.5. Nutrient-depletion pattern at
the immediate vicinity of the root
for a soil with a high nutrient level
and one with a low nutrient level in
the bulk soil (Mengle and Kirkby
1978).
root surface is greater (Fig. 3.5). The higher nutrient level in the soil solution also
gives a higher concentration at the root surface, causing a more rapid uptake rate.
The larger gradient allows this uptake rate to be maintained.
The relative importance of mass-flow and diffusion in nutrient transport in the
soil (Barber 1962) can be illustrated by the following computation. The computation
assumes that the phosphorus concentration in the soil solution is 0.5 ppm and
the transpiration ratio is 300 g/g. The critical phosphorus concentration in rice
tissue is 2,000 ppm (0.2%). The transpiration ratio is the number of grams of water
transpired per gram of dry matter produced. Thus, the amount of phosphorus that
is available to the root surface from the soil solution is:
(3.3)
If this amount of phosphorus is absorbed by 1 g of dry rice tissue, the phosphorus
concentration in the tissue will be:
0.15 mg P×1,000 = 150 mg P / 1,000 g dry wieght
= 150 ppm P .
(3.4)
Thus, mass-flow can only account for less than one-tenth (150 ppm/2,000 ppm) of
the phosphorus content necessary for normal growth. It follows that diffusion is the
dominant process for transport of phosphorus in soil solution.
A similar calculation can be made for potassium. Assume that the potassium
concentration in the soil solution is 80 ppm K, and the critical potassium concentration
in rice tissue is 1.5%. Then, the potassium concentration in rice tissue that
can be accounted for by mass-flow will be:
(3.5)