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Time Factor in Utilization of Mineral Nutrients by Hemp

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SISTER MARY ETIENNE T IBEAU
(WITH NINE FIGURES)
Introduction
Certain recent studies in mineral nutrition of plants have shown that the
time of fertilizer application, in relation to the plant's ability to use nutrients,
is as important as the balance among the nutrients themselves. Since
it is also known that the physiology of the staminate and pistillate plants is
in sharp contrast, especially in dioecious plants, the writer has undertaken
to determine the physiological changes in hemp (Cannabis sativa L.) by varying
the composition of various nutrient solutions, and the time of growth in
these cultures, in order to test the effect of the treatment upon the ultimate
sex expression.
Procedure
Hemp (Cannabis sativa) was chosen for this investigation because it
shows marked sexual dimorphism and grows well under laboratory conditions.
Seeds were planted in the greenhouse. When the seedlings were
about 2 inches tall, they were removed from the soil and their roots were
washed in distilled water. They were then transferred to 2-gallon jars containing
white quartz sand which had been washed free from all solutes.
Eight seedlings were planted in each of twenty-seven pots, on May 27, after
being watered with distilled water. Four days later the pots were marked
off in sets of three, and to each pot of each set there was added 500 cc. of a
specially prepared nutrient solution. The plants were grown under continuous
light for 12 days to stimulate vegetative development. As soon as
they had attained sufficient growth, 6 plants of uniform size were selected
from each set for observation.
All pots were kept constant in weight by the addition of distilled water,
and 500 cc. of the nutrient solution was given as needed until 6 liters had
been used.
The vegetative growth of the plants was rapid due to the length of the
photoperiod. When the first photographs were taken on June 25, the plants
being then 29 days old, very striking contrasts in growth and general appearance
were apparent.
Knop's four-salt solution was used as the basic solution for supplying
nutrients to the control plants. Three types of experiments, in which the
basic nutrient solution was modified, were performed: (1) a high content
series in which the plants were supplied with a high content of the essential
elements K, Mg, Ca, and N, respectively, added to a complete (Knop 's) nutrient
solution (table I) ; (2) a deficiency series in which the plants were sup-
TABLE I
COMPOSITION OF KNOP 'S AND HIGH-CONTENT CULTURE SOLUTIONS
GRAMS OF EACH SALT IN 14 LITERS OF SOLUTION
GIVING WEIGHT IN
CULTURE
SALTS
KNOP'S 8K 8MG 8CA 8N
gm. gm. gn. gin. gi.
KNO3 .......p...... ..g.00 4.00 2.00 2.00 2.00
KH2PO4 .2.00 4.00 2.00 2.00 2.00
MgSO4.... 2.00 2.00 8.00 2.00 4.00
Ca(NO3*.-'.).'---.-..--2'''''.--- 8.00 8.00 8.00 32.00 8.00
K2SO4 .... 9.10 .........
KEC ...................... .. ........ 5.74 ...... ..... .........
MgC 12 6.34 .......
Ca 12 ....................... 21.64
NH4NOS. 32.80
plied with a nutrient solution in which K, Mg, Ca, and N, respectively, were
omitted from the complete nutrient solution (table II); and (3) a series in
TABLE II
COMPOSITION OF DEFICIENCY CULTURE SOLUTIONS GIVING WEIGHT IN GRAMS OF EACH SALT
IN 14 LITERS OF SOLUTION
CuLTURE
SALTS
-K -MG -CA -N
gm. gin. gm. gn.
KNO, 2.00 2.00
KH,P04 ... 2.00 2.00 2.00
MgSO4 ....... ........ 2.00 ......... 2.00 4.00
KC1..l........ ........ 1.48
CaC12 ......... ... ....... 5.50
Ca(NO*.....).....2.... 8.00 8.00
NaNO .1.68 ....... 8.32 .....
Na2,S 4 ........ . ......... 2.38 ......... .......
NaHP04.1.8 ....... ......... .......
which the plants after first undergoing periods of starvation of a single element,
were then supplied with a nutrient solution having a high content of
this element.
Investigation
EXPERIMENTS I AND II
The plants supplied with a high-potassium (8K) solution showed an
increase in growth compared with those supplied with Knop 's solution, while
the plants supplied with a potassium-deficient (- K) solution exhibited a
retarded development (fig. 1). These differences in height, however, were
r. n) (- j) S
FIG. 1. Comparative growth of vegetative hemp in sand cultures variously treated
with potassium. Age 29 days.
not maintained. At the end of 47 days, although the plants supplied with
high-potassium (8K) solution were somewhat taller, plants supplied with
Knop's solution had attained almost the same height; and the plants supplied
with a potassium-deficient (- K) solution were lagging far behind,
FIG. 2. Comparative growth of vegetative hemp in sand cultures variously treated
with magnesium. Age 29 days.
GAl0
early stages of its growth (2, 4).
The plants supplied with a high-magnesium (8Mg) solution, at this time,
were taller than those supplied with a magnesium-deficient solution (fig. 2),
but the most striking contrast was in the color, the latter showing
chlorosis (9).
The plants supplied with a high-calcium (8Ca) solution were shorter than
any of the high-content series so far considered; yet they were dark green in
color and looked healthy. The plants supplied with a calcium-deficient
(- Ca) solution were stunted in growth, the eleventh node scarcely reaching
the eighth node of the plants grown in a high-calcium (8Ca) solution (fig. 3).
FIG. 3. Comparative growth of vegetative hemp in sand cultures variously treated
with caleium. Age 29 days.
FIG. 4. Comparative growth of vegetative hemp in sand cultures variously treated
with nitrogen. Age 29 days.
The plants supplied with a high-nitrogen (8N) solution and those supplied
with a nitrogen-deficient (- N) solution showed by far the most striking
contrasts (fig. 4). At this stage of growth, the best plant of the set
supplied with a high-nitrogen (8N) solution was equal in development to the
plants supplied with a high-potassium (8K) solution (fig. 1) and had produced
an abundance of dark green foliage; but shortly after the plants were
photographed brown spots covered the leaves, and they developed brownrimmed
shot-hole markings. In a short time the leaves rolled laterally and
the whole plant showed signs of wilting. The marginal rolling up of the
leaves may have been a means by which the plant checked excessive transpiration,
since the weather was unusually hot. The plants supplied with a
nitrogen-deficient solution were very small and of a pale yellowish-green
color.
The height of hemp plants grown in high-content series of nutrient solution
was in the order: 8K> 8Mg> 8Ca> 8N; and the height of those plants
grown in the deficiency series of solutions was in the order: - Mg> - Ca>
-K> -N.
Observations on the hemp plants were recorded weekly during the period
of vegetative growth and the early reproductive stage (table III). In
general appearance the plants supplied with the high-potassium (8K),
Knop's, and high-calcium (8Ca) solutions were healthy looking and dark
green, while the plants supplied with a high-magnesium (8Mg) solution
were tall, and had long internodes, but were pale green. An excess of nitrogen
produced a green leafy plant, but the nitrogen proved to be toxic.
The leaves of the plants supplied with a high-magnesium (8Mg) solution
showed brown spot, shot hole with brown rim, and tip scorch; those supplied
with a high-nitrogen (8N) solution, brown spot and shot hole with a brown
leaf margin; while those plants supplied with potassium-deficient (- K) and
the calcium-deficient (- Ca) solutions showed only brown spot, which JAMES
(8) has designated as "coppering," and defined as a characteristic spot
deficiency of potassium.
The plants supplied with a high-potassium (8K) solution produced the
largest leaves. They had approximately fourteen times more leaf surface
than the plants supplied with a nitrogen-deficient (- N) solution. The
leaves of the latter plants were the smallest produced.
The most striking contrast of these two experiments was between the
plants supplied with a high-nitrogen (8N) solution and those supplied with
a nitrogen-deficient (- N) solution. The plants of the former series (8N)
produced an abundance of dark green foliage, and all plants had begun to
differentiate into females before they wilted and died; the latter series
(- N) had small, pale green leaves, and all plants were males. Sex ratios
(table III) in the other cultures ranged from 100 per cent. male plants in
those that were supplied with a nitrogen-deficient (- N) solution to 100 per
cent. female plants in those that were supplied with a high-nitrogen (8N)
solution.
A group of plants grown in distilled water had the same characteristics
as those supplied with a nitrogen-deficient solution: viz., pale green, stunted
growth, red stems, and a preponderance of males. It seems that in the
absence of nitrogen the other essential nutrients are not utilized. Furthermore,
the fact that the flowers of hemp (Cannabis) are sometimes perfect
suggests that both sex potentialities are present, but only one is normally
expressed. The sex of the plant would, then, seem to depend, in large
measure, upon the presence or absence of certain nutritional factors (10).
In the groups under consideration, the plants supplied with high-content and
nitrogen-deficient solutions, the presence of nitrogen apparently was the
factor which determined the female sex of the plants supplied with a highnitrogen
(8N) solution; and the absence of nitrogen may have been the
factor which determined the male sex of the plants supplied with the nitrogen-
deficient (- N) solution.
EXPERIMENT III
Experiment III was performed to determine the feeding capacity of the
hemp plant: that is, to learn how long an essential element might be withheld
from a plant without permanent injury; to what extent it would
recover; and how rapidly such recovery would take place after a more or
less prolonged period of starvation.
For this experiment four set of plants were used. The first set was
designated as - K1, - Mg,, - Cal, and - N1 and each pot was first supplied
with two liters of a solution lacking the element indicated. The plants were
grown in this solution 27 days. Following this each pot was supplied with
a solution containing a high content of the same element. For example,
in the case of the - K deficiency series, 2 liters of the - K solution were
followed by 6 liters of 8K solution.
The plants of the second set were designated as - K2, - Mg2, - Ca2, and
- N2,, and each pot of the set was first supplied with 4 liters of a solution
lacking the element indicated. The plants were grown in this culture for
44 days. Following this they were supplied with 4 liters of a solution containing
a high content of the same element.
The plants of the third set were designated as - K3, - Mg3, - Ca3, and
- N3, and each pot of the set was supplied with 6 liters of a solution lacking
the element indicated. The plants were grown in this culture for 57 days.
Following this the plants were then supplied with two liters of a solution
containing a high content of the same element.
The plants of the fourth set were designated as - K4, - Mg4, - Ca4, and
- N4, and each pot of the set was supplied with 8 liters of the solution
lacking the element indicated.
On July 18, at the age of 27 days, as previously stated, the deficiency
culture solutions were replaced by high content solutions of the essential
elements previously lacking.
Two weeks later (fig. 5) there was a marked improvement in the appearance
of the plants supplied with the 8 K solution. There was an increase
Q.~~1 FIG. 5. Recovery of 6-weeks-old hemp plants after a 27-day potassium starvation
period. - K4 plants without potassium for 6 weeks; - K1 plants without potassium until
27 days old; 8K plants supplied with potassium for 6 weeks.
in height, in size of the leaves, and in vigor of growth. The hemp was growing
rapidly at this time owing to the length of the photoperiod (Mazda lights,
1000 watts per square yard 1 foot above the tops of plants were used daily
from 6: 00 P.mi. to 11: 00 P.M.). In two weeks the average height of the
plants of the - K1 group increased from 11.4 to 35.2 cm., while the plants of
the - K4 group, grown in a solution lacking potassium, increased from 13.5
to 30.2 cm. This shows a growth difference of 7.1 cm. or a daily increase of
0.5 cm. more for the plants of the - K1 group than for those grown in the
potassium-deficient (- K) solution.
In the magnesium groups the difference in height was not so significant,
all of the plants having attained a growth almost equivalent to that of the
control. Chlorosis, however, was evident in the plants grown in the magnesium-
free solution. Recovery here was slow but at the end of 2 weeks the
new growth at the apical meristem was darker green, and 4 days later the
lower leaves showed the same recovery.
Recovery in the caleium groups was apparent in the leaves, which became
darker green, and in the disappearance of white stripings which had been
quite general.
A very striking recovery was made by the plants of the nitrogen-deficient
(-N1) group (fig. 6). After 27 days of nitrogen starvation, these plants,
FIG. 6. Recovery of 6-weeks-old hemp plants after a 27-day nitrogen starvation
period. - N4 plants without nitrogen for 6 weeks; - N1 plants without nitrogen until 27
days old; 8 N plants, with nitrogen for 6 weeks
on receiving a high-nitrogen nutrient solution, were able to make rapid
growth, increasing daily 1.1 cm. more in height than plants growing in a
nitrogen-deficient solution. Recovery was also apparent in the leaves which,
in the new growth, were much larger and dark green. The lower leaves
recovered slightly but the new growth remained quite distinct from the old.
The stems increased in diameter throughout their entire length and the red
color was either masked by the chlorophyll or disappeared. This rapid recovery
may have been due to the fact that the nitrogen-free solution used in
this experiment contained potassium chloride and monopotassium phosphate
(table II). Since, in the absence of nitrogen, potassium is not absorbed, it
seems reasonable that as soon as the high-nitrogen (8N) nutrient was applied
an abundance of potassium became available to the plant and it was utilized
quickly.
After 44 days of potassium shortage, the nutrient solution of the -K?
group was changed to a high potassium (8K) solution. In the 2 weeks which
followed, the daily increase in growth was 1.27 cm. more than that of the
plants grown in the potassium-free solution (fig. 7). At the same time a
marked improvement was apparent in the appearance of the foliage and in
the color of the plants. Likewise the plants of the -KK3 group were sup-plied with a high-potassium nutrient after a starvation period of 58 days,
after which they increased in height 0.6 cm. more daily than the plants grown
in the potassium-free solution.
The daily increases in the height of the plants owing to the addition of K
to the nutrient after various periods of shortage were as follows:
After 27 days of shortage ........ 0.50 cm.
4:4 " ". 1.27 "
58 ". ".. 0.60
It is evident from these data that, for hemp, approximately 44 days of
initial shortage gives optimum increase in vegetative growth. However, the
final data show that while some of the plants in each initially deficient group
were able to recover from the deficiency and attain a height equal to that of
plants continuously supplied with potassium, yet the average height of the
plants was considerably less.
Final observations on these plants showed that the plants grown in a
high-potassium (8K) solution (fig. 7) were tall and had large leaves, but
FIG. 7. Recovery of 64-day-old hemp plants from K starvation. 8K plants supplied
with high-potassium nutrient 64 days; - K, plants supplied with K-deficient nutrient 27
days and high-potassium 37 days; - K, plants supplied with potassium-free solution 44
days and high potassium 20 days; -E, plants supplied with potassium-free nutrient 58
days and high potassium 6 days; - K4 plants without potassium.
meristematic activity had ceased. The plants of the - K1 and the -K2
groups were equally tall, in some cases even taller, and were still growing.
The plants of the - K3 group showed some improvement in growth but the
leaves were still small and the color was notably paler green than those of the
plants already described. The plants of the - K4 group were stunted and
the leaves were generally covered with brown spot.
Apparently potassium is necesary in the early life of the plant; and hence
it seems reasonable to the writer that the greater the requirement for potassium,
at least to a limited extent, the more rapidly it will be absorbed and
utilized (6). The extreme heat and the lateness of the season prevented the
continuation of the experiment to a point where potassium starvation would
result in permanent injury and the plant would be unable to utilize any
potassium.
Recovery of the plants in the magnesium groups was somewhat different
from the others. In the - Mg1 group of plants, from which magnesium had
first been withheld for 27 days, no increase in growth attributable to magnesium
was observed until 2 weeks after the high-magnesium (8Mg) nutrient
solution had been supplied. The plants of the - Mg2 group recovered more
rapidly than those of the - Mg1 group, but the plants of the - Mg3 group
were unable to recover from initial shortage of 58 days. The results of
recovery expressed in daily increase in height of the plants owing to the addition
of Mg to the nutrient after various periods of shortage are as follows:
After 27 days of shortage .................... 1.93 cm. (4 wk. av.)
44 " " "..................... 1.08 " (2 wk. av.)
58 "" ........................ .....
Since all of the plants of these groups attained considerable height, even
those grown in magnesium-free solutions, it appears that magnesium has
more to do with the actual life of the plant than with the vigor of its
growth. Recovery in this case involves the production of chlorophyll. In
general, the final data showed that the tips of the plants grown in the highmagnesium
(8Mg) solution were dead and that there was some growth at the
nodes. The leaves of the plants of the - Mg1 group were dark green; the
lower leaves of the plants of the - Mg2 group were yellowish-white and dropping
off, and the upper parts of the plants were green; while the plants of
the - Mg3 and - Mg4 groups died.
Recovery in the calcium group (fig. 8) was more striking than in that of
any group previously considered. In contrast with the potassium- and magnesium-
deficient series, these plants were not only able to recover from the
initial shortage (3), but they were able to make a better and a more rapid
recovery the longer the calcium was withheld. It was observed in the final
data (fig. 8) that both the plants grown in the high calcium (8Ca) and the
calcium-deficient (- Ca) solutions were stunted, while the plants of the
- Ca,, - Ca2 and - Ca1 groups, from which calcium had been withheld for
various periods, were taller and more vigorous.
The daily increases in growth of plants imputable to the addition of
calcium after various periods of shortage were as follows:
FIG. 8. Recovery of 64-day-old hemp plants from calcium starvation. 8Ca plants
supplied with high calcium nutrient 64 days; - Ca1 plants supplied with calcium-free
nutrient 27 days and high calcium solution 37 days; -Ca2 plants supplied with calciumfree
nutrient 44 days and high-calcium solution 20 days; - Ca3 plants supplied with calcium-
free nutrient 58 days and high calcium solution 6 days; - Ca4 plants without calcium.
After 27 days of shortage.0.23cm. "44 " t i
.............. .......... 0.43"
" 58 t cc i 2.60 "
Since the plants grown in a calcium-free nutrient had practically ceased
meristematic activity when the last data were taken, the results in terms of
daily increase in height are as follows:
After 27 days of shortage....1.5 cm.
" 44 "i it " ........................................................................ 2.2
58 " . 2.6
These data indicate that the limit for recovery from calcium starvation for
this particular group of hemp plants is approximately 60 days, after which
there is a cessation of meristematic activity followed by death of the apical
meristems.
In the plants of the nitrogen group, those which had been grown in a
high-nitrogen (8N) solution died early (7). The plants of the -N1 group
made a good recovery, but after receiving a high-nitrogen solution for three
weeks they began to show signs of wilting. The plants of the - N, group had
large, dark green leaves on the upper part of the plant and small, pale green
leaves on the lower part. Apparently, the application of nitrogen after 44
days of shortage benefited only the new growth. The plants of the - N3
group showed slight improvement in size and color of the upper leaves (1).
The recovery of plants in the nitrogen groups, upon the addition of N
after various periods of shortage, is given in centimeters of daily increase in
height as follows:
After 27 days of shortage ..................................... 1.080 cm. "44 "i "" .. 1.007 " "58 t it i
....0.700 "
The above data support the conclusion that the longer nitrogen is withheld,
the slower is the recovery that the plant is able to make (5).
As stated before, the plants grown in a high-nitrogen nutrient solution
tended to be 100 per cent. female while those deprived of nitrogen were 100
per cent. male. After an initial shortage of 27 days, plants of the - N1 group
were able to utilize the available nitrogen and the plants were all females.
On the other hand, plants of the - N2 group were apparently unable to use
adequately the nitrogen which they received after 44 days of starvation, and
male plants resulted. The plants of the - N3 group had already begun to
differentiate into males when the high-nitrogen solution was given to them
after a shortage of 58 days. At the end of 64 days, plants of the - N4 group
were still hardy, growing slowly, and beginning to differentiate into male
plants.
An examination of cross-sections of stems and leaves of hemp plants variously
treated revealed certain significant differences. The stem tips of both
FIG. 9. Cross-sections of stem tips of 54-day-old hemp plants Right, cross-section
of stem of plant grown in high-potassium (8K) nutrient; left, cross-section of stem of
plant grown in potassium-deficient (- K) nutrient
the plants grown in the high-potassium (8K) and the potassium-deficient
(- K) solutions (fig. 9) were fluted, and had mechanical tissue largely in the
areas that bulged. Well defined cortex, phloem, cambium, and xylem were
in evidence in both, although the stem of the plants grown in the high-potassium
(8K) solution was much larger, measuring 1.9 mm. more in diameter
at node 6 than that of the plants grown in the potassium-deficient solution
(- K). Calcium oxalate crystals were numerous in the stem of the plants
grown in the high-potassium (8K) solution but there were few of them in the
stem of the plants grown in the potassium-deficient solution. Large inclusions,
staining deeply red with haematoxylin, were found in the sieve tubes
of both stems but they appeared more granular in the stem tip of the plants
grown in the potassium-free (- K) solution. Inclusions of various sorts
occurred in the cortex of both stems.
The stem tip of the plants grown in the high-magnesium (8Mg) solution
had solid pith in contrast with the hollow pith of those grown in the magnesium-
deficient (-Mg) solution, in which calcium oxalate crystals were
more numerous. Large brownish inclusions, the resin of hemp, appeared in
the phloem of the stem tip of plants grown in the high-magnesium solution
and in the xylem of the stem tip of those grown in the magnesium-deficient
solution.
The stem tip of the plants grown in the high-calcium (8Ca) solution
showed numerous calcium oxalate crystals in the solid pith, which also contained
a few calcium carbonate crystals. Mechanical tissue appeared largely
in the areas of the stem that bulged. The bast fibers were better developed
in the stem tip of plants grown in the high-calcium (8Ca) solution than in
those grown in the calcium-deficient (- Ca) solution.
The stem tips of the plants grown both in the high-nitrogen (8N) and in
the nitrogen-deficient (- N) solutions had a hollow pith; the former having
some calcium oxalate crystals near the vascular tissue, and the latter considerable
resin. More mechanical tissue had developed in the absence of
nitrogen.
Examination of the vegetative stem tips of male and female plants failed
to reveal any consistent outstanding variations, but the vegetative leaf of the
female was slightly thicker than that of the male. The flowering stem tip
of the male plant was 2.5 mm. in diameter and it had extensive xylem and
little phloem. This contrasted with the female stem tip which was 5 mm. in
diameter and had phloem as well as xylem well developed. Both had hollow
pith with numerous calcium oxalate inclusions.
Summary
1. This investigation was concerned with the physiological effects of
various nutrients on the hemp plant and the results of withholding single
essential nutrients for varying intervals of time.
2. High-potassium (8K) nutrient (Knop's solution having 8 times the
usual amount of potassium) produced the tallest and most vigorous plants
which also had the largest and thickest leaves. Potassium-deficient nutrient
(complete Knop's solution except for potassium) caused stunting of the
growth of the plant and copper mottling on the leaves.
3. After variously prolonged periods of potassium starvation, hemp
plants recovered rapidly but failed to attain a growth equal to that of plants
which had a continuous supply of potassium.
4. Concentration of magnesium in the nutrient did not affect the growth
of the plant, but magnesium deficiency resulted in chlorosis.
5. Recovery from magnesium starvation was slower the longer the magnesium
was withheld.
6. Excess of calcium in the nutrient solution retarded growth. Calcium
deficiency caused paleness in color, necrotic spots on the leaves, and early
cessation of meristematic activity.
7. Recovery from calcium shortage in the hemp plant was more rapid
after longer periods of starvation. Recovered plants attained greater height
than those grown continuously in high-content solution.
8. High-nitrogen nutrient produced a dark green, leafy plant which did
not survive. Plants grown in nitrogen-free solution were stunted and pale
green.
9. After a short period of nitrogen starvation, hemp plants made a rapid
recovery but died soon after. From longer periods of nitrogen shortage,
recovery was slower.
10. An abundance of nitrogen at the time of fruit bud differentiation
obviously leads to the production of female flowers, while the absence of
nitrogen at that time tends to the production of male flowers.
Grateful acknowledgment is made to Dr. W. F. LOEHWING for his direction
and interest in this investigation.
MouNr MERCY JUNIOR COLLEGE
C(EDAR RAPIDS, IOWA
LITERATURE CITED
1. BREAZEALE, J. F. The effect of one element of plant food upon the
absorption by plants of another element. Arizona Agr. Exp. Sta.
Tech. Bull. 19: 465-480. 1928.
2. DASSONVILLE, CH. Influence des sels mineraux sur la forme et la structure
des vegetaux. Rev. Gen. Bot. 10: 335-344. 1898.
3. DAY, D. Some effects on Pisum sativum of a lack of calcium in the
nutrient solution. Science, n. s. 68: 426-427. 1928.
4. DEUBER, C. G. Influence of mineral elements upon the development of
chloroplast pigments of soybeans. Bot. Gaz. 82: 132-153. 1926.
5. GARNER, W. W., BACON, C. W., BOWLING, J. D., and BROWN, D. E. The
nitrogen nutrition of tobacco. U. S. Dept. Agr. Tech. Bull. 414.
1934.
6. GERICKE, W. F. The beneficial effect to plant growth of the temporary
depletion of some of the essential elements in the soil. Science, n. s.
59: 321-324. 1924.
7. HORNER, JOHN M. A study of the composition of pineapple plants at
various stages of growth as influenced by different types of fertilization.
Univ. Hawaii Agr. Exp. Sta. Bull. 13. 1930.
8. JAMES, W. 0. Studies of the physiological importance of the mineral
elements in plants. I. The relation of potassium to the properties
and functions of the leaf. Ann. Bot. 44: 173-198. 1930.
9. PETTINGER, N. A., and HENDERSON, R. G. Some nutritional disorders
in corn grown in sand cultures. Phytopath. 22: 33-51. 1932.
10. SCHAFFNER, J. H. The influence of the substratum on the percentage
of sex reversal in winter-grown hemp. Ohio Jour. Sci. 25: 172-176
1925.


Source: Time Factor in Utilization of Mineral Nutrients by Hemp
 
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