Yoshida - 1981 - Fundamentals of Rice Crop Science

NUTRITIONAL DISORDERS 183
the leaf sheath than in the leaf blade (Table 4.2). Therefore, it would be better to
select the leaf sheath for diagnosis.
The most important point in selecting which plant part to sample is knowing the
relationship of a nutrient in the selected plant tissue to plant performance such as
dry weight or tillering. The sample should be collected when the plants are
showing symptoms of a disorder. For example, symptoms of iron toxicity and zinc
and phosphorus deficiencies are usually seen 2–4 weeks after transplanting.
In plant tissue analysis, analytical errors are much smaller than sampling errors.
Yanagisawa and Takahashi (1964) studied sampling for a field-grown rice crop.
The coefficient of variation (CV) for 7 nutrients in the straw ranged from 9 to 21%
at maturity (Table 4.3). On the basis of the observed CV, the number of samples
that should be taken to give the desired precision at a given CV was computed. In
general, if a precision of 10% is desired, it is necessary to randomly collect 10–20
samples from the same field.
c. Soil contamination and sample preparation. Soil contamination is a serious
source of errors in plant tissue analysis (Mitchell 1960). How seriously the results
of plant tissue analysis could be affected by soil contamination depends on the ratio
of a nutrient content in the soil to that in a plant sample and the percentage of soil
contamination in the plant sample. For example, the soil-plant nutrient content
ratio ranges from 0.5 in phosphorus to 10,000 in titanium (Table 4.4). Assuming
that the soil contamination is 0.5% in the sample and the soil-plant nutrient content
ratio for a given nutrient is 10, the nutrient content in the sample would be affected
by about 5%, as illustrated below:
C p × 0.995 + C s × 0.005 = C p × 0.995 + C p , × 0.05
= 1.045 × C p ,
where C p and C s are nutrient contents in plant and soil, respectively. In other
words, when the soil-plant nutrient content ratio is less than 10, a 0.5% soil
contamination would not seriously affect the analytical results of the sample. It is
not difficult to maintain this degree of cleanliness in routine sample preparation.
However, if the soil-plant nutrient content ratio is more than 100, soil contamina-
Table 4.3. Coefficient of variation (CV) for straw analysis for 7
nutrients in a rice crop at maturity. a
Analytical CV of plant
Nutrient error (%) samples (%)
N 1.2 17
P 2 O 5 2.8 21
K 2 O 1.0 9
CaO 2 2.4 10
MgO 5.4 15
MnO 3.8 19
SiO 2
1.4 9
a
Yanagasiwa and Takahashi (1964).