b3b6 POTATO INVESTIGATIONS LABORATORY
Mimeo Report 62-6* October 6, 1961
LIMING AND FERTILIZATION OF POTATOES
D. R. Hensel
Need for Liming
The natural soil processes leach out the basic elements such as sodium,
potassium, calcium and magnesium. The loss of these elements is also stimulated
by heavy cropping where the elements are taken away by removal of crops. Also
replacement of the elements by use of fertilizers may have an acidifying effect.
The sum total of these processes will tend to lower the pH of the soil. To brinr,
the soil back to the proper pH, lime is applied periodically. It will also furnish
calcium and magnesium. The adjustment of the pH will make the nutrients, partic-
ularily the minor elements, in the soil more available to the plants. The pH
aC.justment will increase the activity of the microorganisms which will in turn
improve the nitrification processes occurring in the soil.
The most important item in the determination of pH is the collection of the
soil sample. The soil should be sampled in the fall immediately after the summer
cover crop has been turned under. In taking the soil sample, one should take 10-15
different borings in the area. These borings should be randomly spaced all over
the field and not from one corner. The same amount of soil should be collected
from each boring and placed in a bucket. After all the borings are placed in the
bucket, the soil should be throughly mixed and approximately one pound saved for
the final sample. The borings can be made with an auger, spade, or similar device.
One should take the soil from the upper six inches and be careful not to mix
different soil types. If different soil types exist one should take separate
samples for each soil type. The field should be divided into approximately ten-acre
areas for sampling. A chart of the field showing number and location of the samples
will be necessary in order to use results of the soil test.
The proper pH for growing potatoes is pH 5.5. After the existing pH has been
determined, then one must decide how much lime is needed for the correct pH adjust-
ment. This will be determined by what material is to be applied. Listed below are
some of the more important liming materials used in this area.
1. Dolomite is the most common liming material and contains magnesium.
This is slower in action and has a longer lasting effect.
2. High Calic Limestone is suitable when less time is available for
the correction of acidity.
3. Hydrated lime gives much faster correction of acidity than,the .
other two materials. Generally it is higher in price and'does not
maintain the soil at the proper pH as long.
*Revision of Mimeo Report 59-6.
Fertilization of Potatoes
The current recommended rate of fertilization for potatoes is 2,500 lbs. per
acre of 6-8-8. This basic rate supplies 150 pounds of N, 200 lbs. of P205, and
200 pounds of K20. Other fertilizer analyses may be used if they supply approx-
imately the same amount of nutrients. For example, 2,200 Ibs. of 7-9-9 contains
about the same amount of nutrients as 2,500 Ibs. of 6-8-8.
Nitrogen.- In general one-fifth of the nitrogen applied should be nitrate-
nitrogen. For newly cleared lands and in areas where nutritional leaf roll has
been prevalent the nitrate-nitrogen can be increased to one-third of the total
nitrogen in the fertilizer. Only limited amounts of insoluble, organic nitrogen
should be used. At the present time, not more than one unit of organic nitrogen
Phosphorus.- No attempt should be made to reduce the amount of phosphorus
applied. Latest tests showed that plants which were grown with less phosphorus
yielded significantly less potatoes than plants with recommended rates of phosphorus.
This was true even though potatoes grown on less phosphorus looked healthy and pro-
duced very high yields. For example, the check plots which received no phosphorus
yielded 296 cwt./A in 1961, whereas, 200 lbs./A P20; produced 338 cwt./A. Even
though 296 cwt./A is considered as a profit making yield, it is better management to
apply the recommended phosphorus. The extra cost of 200 pounds of P205 is
negligible compared to returns from 42 extra bags of potatoes. On new land,
increasing the amount of phosphorus applied to 400 Ibs./A of P205 increased the
yield more than 30 bags over the recommended rate.
Potash.- Not more than one-half of the potassium in the fertilizer should be
derived from muriate (the chloride form) of potash. The use of other forms such
as sulfate of potash tends to increase the specific gravity of potatoes.
Magnesium.- The recommended rate for magnesium is 2p soluble magnesia in the
basic fertilizer. Therefore 2,500 pounds of fertilizer containing 2% soluble
magnesia would supply 50 lbs. MgO per acre.
Sidedressing.- Recent experiments show that 30 pounds per acre of nitrogen
applied approximately 40 days after planting increased the yield of potatoes.
Results were obtained on soil which had received 2,200 Ibs./A of 7-9-9 at planting
and which was irrigated during the growing season. Results are more adaptable to
light, sandy soils. On dark, sandy soils, benefits of sidedressing are still
doubtful. The source of nitrogen used for sidedressing makes little difference in
yields. Studies show that 6-8-8, 7-9-9, 10-0-10, urea, nitrate of soda, nitrate of
soda-potash and ammonium nitrate did equally well. With this in mind the cheapest
source of nitrogen would be the most economical.