Nitrate concentration in romaine lettuce leaves depends on
fertilizer type and dose rather than on the crop season and may
remain in Southern Greece far below the upper limits set by
the European Commission Regulation, even under high and
repeated fertilization treatments, a result attributed mainly to
the sufficient sunlight intensity and duration throughout the
year in this region. High application rates of sheep manure
maybeneededtoreachthelettucegrowthobtainedby
inorganic fertilizers under optimal climatic conditions only.
Increased inorganic fertilizer doses should be avoided since,
especially under suboptimal light conditions, they might
result in significant rise in nitrate accumulation in leaves but
marginal yield increase; onthe contrary composted sheep
manure may be safely applied even at high doses. High
residual availability of N, P and K may be obtained in the soil,
following the sheep manure applications, which relates to
manure dose; on the contrary no residual availability of N, P
and K was obtained by inorganic fertilization applied via
fertigation.