Yoshida - 1981 - Fundamentals of Rice Crop Science

122 FUNDAMENTALS OF RICE CROP SCIENCE
composition of the culture solution commonly used for rice (Table 3.6). The
comparison indicates that the leachate had adequate concentrations of all nutrients
except phosphorus. Analysis at maturity supported the observation that phosphorus
was extremely deficient in the plants grown in the leachate (Table 3.7).
A later experiment, however, demonstrated that rice can grow normally even at
0.1 ppm P in the culture solution provided a much larger pot is used (Tanaka
1962). This experiment indicated that rice can absorb phosphorus from a solution
with 0.1 ppm P and, hence, the concentration in the leachate can not be considered
limiting to rice growth; the total supply of phosphorus plays an important role.
The above two experiments lead to the following points:
• The soil can replenish phosphorus in the soil solution as plants absorb it.
This process increases the total supply of phosphorus even though the
concentration itself is low.
• In a well-mixed solution culture, the total supply of phosphorus is determined
by pot size, the phosphorus concentration in culture solution, and
the renewal frequency of the culture solution. To meet the plant’s phosphorus
requirement, the pot size must be large at low concentrations of
phosphorus, or, if it is small, the phosphorus concentration must be high.
If, however, the pot size is small and the concentration is low, the renewal
frequency of the culture solution must be increased.
3.3.3. Dynamics of nutrient availability
The important point in the above discussion is the maintenance of an adequate
concentration gradient between root surfaces and the soil solution. It follows that
the ability of the soil to maintain an adequate concentration in the soil solution is
critically important.
For most nutrients, an idealized reaction sequence can be formulated in terms of
transfer rates between forms (Bache 1977):
(3.6)
The equilibrium (a) between the unavailable and intermediate forms is established
slowly, perhaps over many decades. The intermediate forms are the long-term
reserves that can be replenished slowly from inert forms or, more rapidly, by
fertilizer reactions with soil minerals. Some examples of intermediate forms are
potassium ions in clay interlayers and phosphate ions in dicalcium phosphate
crystals. The equilibrium (b) between intermediate and labile forms is established
over a shorter time period, perhaps a few months to a year. Labile nutrient (loosely
held) is a fraction of a soil nutrient that comes to equilibrium with the solution
rapidly, within hours in laboratory experiments and perhaps longer in the field.
The amount of the labile pool is usually measured using a radioactive isotope such
as 32 P. If the soil contains a sufficient labile pool of a nutrient, it is able to
maintain an adequate concentration by releasing the nutrient into the soil solution