Silva (2010) reported linear increments in contents
and K accumulated in both leaves and stalks of ratoon as
results of K2O doses applied to the soil. Spironello et al.
(1986) report similar results. They also verified positive
and significant correlations between K levels in leaves
and productivity in six out of eight trials.
The soil application of K fertilizer affected stalk
production, in both ratoons, promoting quadratic increase
in first and second ratoons, reaching 87.5 and 107.5 t ha-1
with 120 and 123 kg K2O ha-1, respectively (Figure 5).
The maximum stalk yield in both ratoons was
associated with the highest K concentration in the leaf
+1, that is, 12.2 g kg-1 in first ratoon and 9.4 g kg-1 in
second ratoon, brought about by the fertilizer rate of 120
and 123 kg K2O ha-1, respectively (Figure 3). The values
found for K concentration in leaf +1 in the first ratoon are
considered adequate by Raij (2011), that is, between 10
and 16 g kg-1; however, in a second ratoon, values found
for K concentration were very close to those considered
appropriate (9.4 g kg-1), although these values are valid
for a harvest system in which sugarcane residues are
previously destroyed by fire.
Silva (2010) also reported that the highest stalk yield
in a harvest system without previous burning of residues
(119.5 t ha-1) was associated with the increment of K level
in leaf +1 (50.9 g kg-1), which resulted from the highest K
fertilizer dose (195 kg K2O ha-1).
These results are an indication of the importance of
providing ratoon sugarcane plants with adequate nutrient
levels, especially K, to have high stalk yields. Pancelli
(2011) verified that the highest yield of 127 t ha-1 resulted
from the application of 147 kg K2O ha-1 when leaves
showed a K concentration of 9.3 g kg-1.
The beneficial effect of K on productivity of sugarcane
cultivated in a harvest system without elimination of
residues by fire is reported in the literature. Rossetto et al.
(2004) reported significant effects of K in seven out of ten
evaluations in which K was applied to ratoon sugarcane
plants, since K is the nutrient more extracted by the crop,
mainly by ratoon (Korndörfer and Oliveira, 2005). In
most of their results productivity increased linearly with
K2O doses only in second or third ratoon. Uchôa et al.
(2009) reported quadratic adjustments between K2O doses
and stalk productivity. According to these authors, K2O
dose with maximum economic effect was between 94 and
165 kg ha-1. It is important to remember that the optimum
economical yield depends of changing economic factors
(prices of products and inputs), therefore they may change
year to year.