POTATO INVESTIGATIONS LABORATORY
Mimeo Report 58-6 October 1, 1957
LIMING AND FERTILIZATION OF POTATOES1,2
Donald L. Myhre
Most virgin and cultivated sandy soils in the Hastings section are naturally
acid, due to the nature of the parent material from which they were formed and also
to the soil-forming process in a humid climate which leached out the basic elements
such as sodium, potassium, calcium and magnesium. Periodic liming as indicated by
a pH test is necessary to replace the calcium and magnesium taken up by the potato
crop and lost by leaching, neutralize the acidifying effect of fertilizers, make the
nutrients in the soil and fertilizer more available to the plants, and make aluminum
more unavailable. Liming also increases nitrification which is a precautionary
measure against nutritional leaf roll of potatoes.
Soil Sampling.- Soil samples for pH determinations in indicating liming re-
quirements should be collected on old land in the fall immediately after the summer
cover crop has been turned under and on new land as soon as the land has been
cleared and leveled but before the cover crop is planted. However, if nutritional
leaf roll has occurred in potatoes grown on old land, the soil should be sampled in
the spring soon after the potatoes have been dug. In all cases, lime should be
applied as soon as possible after the soil samples have been collected and the pH
determined in order to provide additional time for the lime to react.
It is extremely important to obtain a representative soil sample from the field
being sampled. A tube, auger, or spade may be used to collect the sample. Each
type of soil should be sampled separately by taking from eight to ten plugs or
slices of the surface six inches over the area and mixing them together to make one
sample. Several such samples should be taken from each field, usually about one or
two per ten acres.
Soil Acidity (pH).- A soil pH of 5.5 is currently recommended for growing
potatoes in the Hastings section. Potatoes will grow at a higher pH than is recom-
mended for them, but generally scab is less troublesome in acid soils below pH $.5.
A pH of 6.0 or higher should be avoided because certain microelements such as boron,
copper, manganese and zinc become less available on most soils at high pH levels.
For average soils lime may be used on the basis of 200 pounds per acre of ground
limestone (dolomite or high calcic limestone) or 120 pounds per acre of hydrated
lime for each 0.1 pH below 5.5, as shown in Table 1.
1Revision of Mimeo Report 57-6.
2Based on Fla. Agr. Exp. Sta. Bul. 504, "Effect of Liming and Fert'i
Yield and Correlation of Nutritional Leaf Roll of Irish Potatoes" by Dj M. Vo2
and Nathan Gammon, Jr.; Proc. of Fla. State Hort. Soc., Vol. 68, pp. -235,"Irriga
tion, Fertilization and Seeding Rates on Potato Yields in 1955 at Ha asiFlorida"
by Drs. Donald L. Myhre, E. N, McCubbin and G. M. Volk. I 9 157
TABLE 1.- Amount of Lime Required to Adjust Soil pH to 5.5.
Original Pounds per Acre Lime Required to
Soil pH Adjust Soil pH to 5.5
Dolomite or High Calcic Hydrated
4.3 2400 1440
4.4 2200 1320
4.5 2000 1200
4.6 1800 1080
4.7 1600 960
4.8 14oo 804
4.9 1200 720
5.0 1000 600
5.1 800 480
5.2 600 360
5.3 400 240
5.4 200 120
Kind of Lime.- Dolomite is preferred for general use to maintain proper soil
pH for potatoes because ofhe magnesium it contains. High calcic limestone is
preferred where more rapid correction of acidity is deslra'le.~f'drated limo gives
still faster correction of acidity but is more costly and difficuTi to use. It may
be used in emergencies where liming of now lands or those subject to leaf roll has
been delayed until late fall.
Amount of Fertilizer.- About 2,500 pounds per acre of 6-8-8 fertilizer is
currently recommended for growing potatoes in the Hastings section. Proportionately
less of higher analysis materials can be used. For example, 2,200 pounds of 7-9-9
contains approximately the same amount of plant nutrients as 2,500 pounds of 6-8-8
and is usually cheaper per unit of plant nutrient.
Contents of Fertilizer.- All fertilizers should contain about 2 percent soluble
magnesia. For average conditions, one fifth of the nitrogen should be nitrate
nitrogen. For new lands or where leaf roll has been prevalent, the nitrate nitrogen
can be increased up to one third of the total nitrogen present. Generally, the use
of more than one unit of nitrogen from insoluble sources cannot be justified for
potato fertilizers. The chlorine content should be not more than three percent, the
lower the better if a dry season is encountered, or if harvest is to be delayed bo-
yond the average digging dates. Tests in 1955 and 1957 showed that Sebago potato
yields were increased about 15 percent by using fertilizer which derived its potas-
sium from sulfate of potash rather than muriate (chloride) of potash.
Side Dressing.- New lands or ones previously showing leaf roll should be side-
dressed with about 150 pounds of nitrate of soda-potash or its equivalent per acre
at about 40 days after planting. This sidedressing is also recommended after heavy
leaching rains have occurred.