AGRICULTURAL RESEARCH CENTER.
Hastings Research Report PR-1973-5 November 1973
S FERTILIZATIO"lOF POTATOES BY INTENSITY
AND BALANCE METHODS
D. R. Hensel, Professor, Sojls Chemist
Intensity and balance methods of monitoring the soil fertility status
have shown potential with other vegetable crops in Florida. The amount of
soluble salts in the soil is measured by determining the electrical conduc-
tivity of the saturated extract. This is called the intensity factor and is
related to amount of fertilizer available to the crop. However, the intensity
factor can be affected by salty irrigation water and other factors such as fer-
tilizers with high salt indexes. The balance factor is the ratio of the in-
dividual elements to each other and to the total salts. By using this method
the fertilizer status of the crop can be estimated during the growing season.
During the last two seasons, 1972 and 1973, potatoes were fertilized with
three rates of 6-8-8. They were 1250, 1875, and 2500 pounds per acre. Side
dressings of nitrogen and potassium were applied at 30 days at 20 and 40
pounds per acre rates. One treatment had high salt index materials (sodium
nitrate and muriate of potash) while the two others had relatively lower salt
indexes (calcium nitrate with sulfate of potash and ammonium nitrate with sul-
fate of potash).
From the data in Table 1, the soluble salt levels increased as basic fer-
tilizer and side dressing rates increased. Also, an increase in salt level was
found when higher salt index materials were used. A second application of
side dressing treatments was applied at 50 days on the plots which received
the 1250 pound rate of basic fertilizer since all the soluble salt readings
were low. Yields were increased by increased fertilizer and increased side
dressings. However, yield response to high salt index materials was less than
those with low index materials. There was a significant increase in chloride
levels of the plots side dressed with high salt index materials (sodium nitrate
and muriate of potash). A significant increase in potassium level of soil ex-
tract was found as basic fertilizer rate increased, but no difference between
side dressing treatments or levels. No significant differences in nitrate or
calcium levels were found. Late application of side dressing (50 days) failed
to produce greater yields than other treatments which received approximately
the same amount of nitrogen and potassium, but at earlier dates.
The intensity and balance method can be used to estimate the fertility
status of a growing potato crop. The method becomes more useful if one has
knowledge of the fertilizer materials that were applied to the crop. Refine-
ment of the method will be continued.
Table 1. Effect of fertilizer rates and sidedressing application on soil
soluble salt levels and yield of potatoes during 1972 and 1973.
Sidedressing Salt Fertilizer rates Ibs/A 6-8-8
Treatments Index 1250 1875 2500
1. None 1862 1878 2242
2. 20-0-20 High 1908 2372 2738
3. 40-0-40 High 2048 2700 2676
4. 20-0-20 Low 1872 2143 2396
5. 40-0-40 Low 1852 2260 2463
1. None 120 148 178
2. 20-0-20 High 129* 168 177
3. 40-0-40 High 148* 176 175
4. 20-0-20 Low 150* 178 181
5. 40-0-40 Low 162* 185 200
* Plots received second application
planting due to low salt readings.
of side dressing at 50 days after