Agriculture Reference
In-Depth Information
(a)
Maximum yield, A
90% of A
Control yield, A-B
Critical value for plant or
soil test determined here
Fertilizer N (kg N/ha)
(b)
B
X
C
Y
A
Optimum rate
Fertilizer N (kg/ha)
Figure 5.4
(a) Generalized yield response to N fertilizer (b) Relationship between product values and
fertilizer cost showing optimum yield.
These figures clearly demonstrate the distinction between
maximum yield
,
where further additions of fertilizer do not produce an increase in yield (and yield
may even start to decrease due to a nutrient imbalance or toxicity), and
optimum
yield
, where the increase in product value relative to the cost of additional fertil-
izer is at a maximum. The concept of optimum yield is particularly relevant to
the wine and grape industry because high yields per ha may be produced at the
expense of fruit quality. This is especially true of N fertilization, and explains why
vignerons aim to restrict the supply of N to vines at critical stages of growth (sec-
tion 5.4.1.4).
Nutrient Requirement Converted to Fertilizer Requirement
A nutrient requirement, as assessed from a response curve such as in figure 5.4,
must be converted into a fertilizer requirement. Choosing a fertilizer depends on
5.3.4
