June 2, 1923 
Effect of Manganese on Plant Growth 
789 
The preparation of a medium that will, when the test is applied, show 
no manganese, requires certain procedures and precautions which, in so far 
as the writer has been able to discover, have not previously been taken 
into consideration in experiments planned to show the effect the absence 
of this element may have on the growth of plants. Tests for manganese 
in many different samples of chemical reagents which are used for plant 
nutrients show that manganese-free chemicals are rarely if ever found. 
So intimate is the association of manganese with iron that a manganese- 
free iron compound could not be purchased from dealers in chemicals 
and it was necessary to prepare it in the laboratory. Therefore in order 
to eliminate all possible sources of contamination with manganese it is 
necessary to test and purify all the reagents that enter the nutrient 
medium. Even though manganese has been eliminated from the nutrient 
medium it is necessary to grow plants until they approach maturity 
before any definite conclusions can be reached with respect to the effect 
of this element on their growth. The seeds of those plants with which 
the writer has carried on experiments apparently contained enough 
manganese to maintain a normal growth for the first six or eight weeks. 
After having procured reagents that were proved by chemical tests to 
be free from manganese, a Knop’s nutrient solution was made for water 
culture experiments. 
Alaska garden pea seeds were germinated on moist cheesecloth in a 
porcelain-lined pan. When the seedlings were of the proper size they 
were transferred and held in place by means of plugs of cotton in holes 
made in squares cut from boards which were made to fit over the mouths 
of well-glazed one-half gallon earthenware jars. An attempt was made 
to waterproof the boards by keeping them submerged in paraffin kept 
near the boiling point for several hours. Although the squares of wood 
afford excellent means of support for the plants, they proved to be unsat¬ 
isfactory in these and in previous experiments from the fact that the lower 
surface of the board, which is close to the top of the nutrient solution, 
affords favorable conditions for the growth of molds which, unless daily 
attention is given to washing them away, very soon attack the roots of 
the plants. 
More recently supports have been made of well-glazed earthenware and 
no trouble from molds has been experienced. 
The earthenware jars were filled nearly to the mouth with the 
Knop’s solution and when the covers containing the seedlings were on, 
the roots were submerged in the solution. The pots containing the 
plants were kept at a suitable temperature for the growth of the peas 
in the greenhouse and the nutrient solutions were changed twice each 
week. The water lost by transpiration in the meantime was replaced 
with distilled water, thus keeping the concentration approximately con¬ 
stant. 
Previous to starting the experiment new pots were obtained and tested 
for soluble manganese in the following manner: Three of the new pots 
were taken at random and filled to near the mouth with a mixture of 
equal parts of i: i nitric and hydrochloric acids and placed over holes on a 
water bath and kept at the temperature of boiling water for 48 hours. 
The mouths of the pots were covered with watchglasses during the 
digestion. The solutions were then transferred to porcelain dishes and 
brought to dryness. The residues were tested for the presence of man¬ 
ganese, and only a slight trace was found from any of the pots. The 
writer considers this a negligible source of manganese as it was thought 
