xt76dj58f84b https://nyx.uky.edu/dips/xt76dj58f84b/data/mets.xml   Kentucky Agricultural Experiment Station. 1962 journals 122 English Lexington : Agricultural Experiment Station, University of Kentucky Contact the Special Collections Research Center for information regarding rights and use of this collection. Kentucky Agricultural Experiment Station Progress report (Kentucky Agricultural Experiment Station) n.122 text Progress report (Kentucky Agricultural Experiment Station) n.122 1962 2014 true xt76dj58f84b section xt76dj58f84b I ROTATION AND NITROGEN EXPERIMENTS
WITH
CORN AND WHEAT
BY E. C, DOLL, H. F. MILLER, AND L_ A. LINK
PROGRESS REPORT 122
(Filing Code 1)
UNIVERSITY OF KENTUCKY
AGRICULTURAL EXPERIMENT STATION
DEPARTMENT OF AGRONOMY
Lexington

 ROTATION AND NITROGEN EXPERIMENTS WITH CQ_R_N_ANl)_WIj_EAT_  
E. C. Doll, H. F. Miller, and L. A Link -
Several. long—time rotation experiments have been conducted by the Kentucky Agricul-
tural Experiment Station. Earlier results were published by Roberts (1937), Karraker
(1936,1951), and Doll and Link (1957). Results of other recent fertilizer experiments . ·
with corn and wheat have been published by Doll (1961, 1962). In this publication, data are
reported from two long—time experiments, one at the Western Kentucky Substation in
Caldwell county and the other from the Greenville Soil Experiment Field in Muhlenberg _
county, together with the results of three recent experiments at the same locations.
WESTERN KENTUCKY SUBSTATION .
Legume Rotation Experiment
A three—year rotation of corn, wheat, and various legumes and timothy was system- ·
atically arranged on three series of plots so that each crop in the rotation was grown every
year. This experiment was conducted from 1927 to 1954, inclusive, on Crider silt loam, .
a residual soil of limestone origin. Soil treatments and legume and timothy management A
practices are given in Table 1. Ground limestone, where used. was applied at the rate of
2 tons per acre in 1927, again at 1 ton per acre as each series was planted to corn from
1936 to 1938, and again at the rate of 1 ton per acre from 1945 to 1947 From 1927 to
1937, 300 pounds per acre of 16% superphosphate was applied for each corn and each
wheat crop; after 1937, 300 pounds of 20% superphosphate was applied Beginning in 1937.
75 pounds of muriate of potash was applied to each corn and each wheat crop: after 1941,
the same amount of 60% muriate of potash was used On one series of plots where timothy
was grown. 100 pounds per acre of sulfate of ammonia was applied to each crop. On one
series df plots from which clover hay was harvested. manure was applied for corn at a l
rate equivalent to 3 tons of manure per acre for each ton of clover hay harvested. During
the experiment, 26 corn, 26 hay, and 25 wheat crops were harvested All corn stover and
wheat straw were removed from the plots
The largest differences in yields were obtained from applications of phosphorus, `
potassium, and ground limestone. as shown by a comparison of treatments 1, 3, and 4
(Table L) When a legume was plowed down rather than harvested for hay, yields of
both corn and wheat were increased slightly (Treatments 2. 3. 11, 12) Yields were
lowest when cowpeas and soybeans were used as the third—year crop. Manure appeared
to have about the same effects on corn and wheat yields as plowing down the clover
(Treatments 12, 13). Yields obtained from Treatment 14. where timothy was grown,
are not representative of yields with a nonlegume hay crop since volunteer lespedeza
was always present on these plots.
Changes in soil nitrogen (Table 2) were not great. but, in general. nitrogen tended
to derrease on plots which were not limed or fertilized (Treatment 1) and on plots where
cowpeas and soybeans were grown (Treatments 5. 6. 7). and to increase on plots which
were fertilized and where lespedeza. alfalfa. or clover were grown
-2-

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The results from most of the treatments in this experiment were reported in detail
by Doll and Link (1957).
( Effect of Nitrogen Fertilizer on Corn and Wheat Yields
A After the long-time experiment discussed above was terminated, another experiment
was conducted on the same soil type from 1955 to 1960, inclusive, in which corn and wheat
were grown in a two-year rotation. Two series of plots, each arranged in a Latin square
design, enabled each crop to be grown each year. A cover crop of red and Ladino clover
and Korean lespedeza was seeded in the wheat each spring, and plowed down the following
‘ spring for corn. All corn stover and wheat straw was removed from the plots.
Corn yields, given in Table 3, show a significant response to nitrogen in 1956 and
1957, but after the first two years, no further significant responses were obtained. How-
, ever, the first 30-pound increment of nitrogen did tend to result in higher yields in 1958
and 1959. The wheat yields, given in Table 4, show a significant response for nitrogen
each year. However, part of the yield increases shown for wheat was probably due to
‘ residual effects of nitrogen applied to the preceding corn crop. Significant yield increases
were obtained for each additional increment of nitrogen except in 1955 and 1959, when no
significant increases were obtained when the rate of nitrogen was increased from 30 to 60
pounds of nitrogen per acre. The higher corn and wheat yields obtained in this experiment
’ (Tables 3 and 4) than in the preceding long-time experiment (Tables 1 and 2) can be pro-
bably attributed mostly to the higher rates of fertilization, particularly phosphorus (Doll
and Link, 1957).
TABLE 3. - EFFECT OF NITROGEN ON YIELD OF CORN FOLLOWING WHEAT AND LEGUME COVER FROM 1956 to
1960, INCLUSIVE, AT PRINCETON, KY.
Com Yield, Bushels per Acre
' Pounds N per Acre 1956 1957 1958 1959 1960 Average
0 77. 3 99. 2 77. 0 85. 7 75. 8 83. 0
30 84. 3 103. 8 81. 1 92. 3 74. 9 87. 3
_ 60 90.6 101.1 83.5 91.6 75.9 88.5
120 90.0 107. 1 88.6 93.6 76. 3 91.1
L. S.D. ,05 6. 1 2. 9 ns ns ns -*5
. TABLE 4. — EFFECT OF NITROGEN APPLIED TO WHEAT AND PRECEDING CORN ON YIELD OF WHEAT AT PRINCETON,
KY., FROM 1955 1:6 1960, INCLUSIVE
Pounds N per Acre Wheat Yield, Bushels per Acre  
Com Wheat 1955 1956 1957 1958 @59 1960 /)·.1cra¢)_c_____,_
0 0 15.5 20.7 16.5 24.9 19.7 33.6 21..8
30 15 23. 9 36. 3 24. 2 32. 4 29. 9 47. 4 32. 4
60 30 31. 8 48.1 26. 3 36. 0 32. 4 54. 6   2
120 60 36. 6 58. 0 30. 9 45. 8 35. 7 62. 1 44. 9
L.S.D. .05 6.1 8.5 4.0 7.0 6.1 5.6

 -6- '
GREENVILLE SOIL EXPERIMENT FIELD
Lespedeza Rotation Experiment
Korean lespedeza was used as the third-year hay crop in rotation with corn and ’
wheat in an experiment conducted on Zanesville silt loam from 1936 to 1957, inclusive. g ‘
The plots were arranged systematically, and three series of plots enabled each crop to
be grown each year. I
The fertilizer and limestone treatments were as follows:
Limestone = 2 tons of ground limestone applied in 1936. ~
Phosphorus = 50 pounds of PZO5 preceding corn and 50 pounds preceding wheat, or
100 pounds total of P205 per acre for each round of the rotation begin-
ning in 1936. In 1942 this was reduced to 60 pounds of P205 per acre ‘ —
for each round of the rotation applied before seeding the wheat. ·
Potassium = 150 pounds of 50% muriate of potash (75 pounds of K20) per acre for
each round of the rotation, beginning in 1939. Starting in 1942, the »
amount of Kg0 was increased to 90 pounds through the use of a 60% I
muriate of potash.
The most marked yield differences were due to applications of lime, phosphorus,
and potassium (Table 5). The use of rye cover following lespedeza in the third year
(Treatment 3) had very little effect on yields of succeeding corn and wheat crops. When
wheat was grown in both the second and third years of the rotation (Treatment 4), yields
of wheat in the third year were only slightly better than those from the check plots in the
second year (Treatment 1), and yields of corn and second-year wheat appeared to be ‘—
slightly lower (Treatment 4). When lespedeza was cut for hay following wheat harvest in
the second year as well as in the third year (Treatment 5), yields of both corn and wheat
were lowcr than those obtained in Treatment 5, although total hay yields were about 50%
higher. Seeding timothy with lespedeza (Treatment 6) appeared to increase corn yields
slightly when compared with Treatment 5 and did not affect wheat or third-year hay yields, _
but decreased second-year hay yields. However, yields of both corn and wheat were very `
low throughout this experiment.
TABLE 5. - EFFECT OF FERTILIZER TREATMENT AND KIND OF ROTATION ON YHZLDS OF CORN, WHEAT, AND
HAY AT GREENVILLE FROM 1936 to 1957, INCLUSIVE
  F
First Year Second Year Third Year
Treatment Fertilizer Corn Wheat Hay Wheat Hav
 —--E~U¢¥
1 None C—W-L 15. O 3. 2 —- —- 17. $
2 L P K C-w-L 36.4 17. 1 -- -- 35. 3
3 L F R C-W-I-r 37. 7 17. 1 -- -- 36. 2
4 L P K C-W-WLr 35. 2 15. 9 -- 6. 4 34. 8
5 L P K C-WI.-Lr 32. 2 14. 3 24. 6 -- 27. 7
  I
*C : corn, W: wheat, L : lespedeza Hay, 'I` = timothy hay, r = rye cover crop. ~’" W

 3
. * “7—
The changes in nitrogen content of both the surface and subsoil (Table 6) are also
indicative of the low yield levels. Total nitrogen in the top 18 inches decreased on all
plots during the experiment; however, on all fertilized plots the nitrogen content of the
surface soil tended to increase while that of the subsoil decreased. This was probably
A due to stimulation of root growth and consequent accumulations of organic matter in the
fertilized plow layer.
TABLE 6. - EFFECT OF FERTILIZER TREATMENT AND KIND OF ROTATION ON SOH. NITROGEN AT DIFFERENT
_ · DEPTHS AT GREENVILLE, KY., FROM 1936 to 1955
)
` Pounds Nitrogen (lj) per Acre
Fertilizer Surface Soil (0-6") Subsoil (6-18") Total
Treatment Rotations* 1936 1955 Change 1936 1955 Change Change
None C-W-L 1, 785 1, 770 - 15 1, 875 1, 600 -275 -290
L P K C-W-L 1, 780 1, 945 165 1, 965 1, 650 -315 -150
L P K C-W—Lr 1, 865 1, 960 95 2, 005 1, 745 -260 -165
L P K C-W—WI.r 1, 805 1, 980 175 1, 950 1, 690 -260 - 85
L P K C-WL-Lr 1, 805 1, 890 85 1, 965 1, 740 -225 -140
L P K C-WLT—LT 1, 800 1, 865 65 2, 000 1, 645 -355 -290
 
` * C : corn, W= wheat, L: lespedeza hay, T = timothy hay, r = rye cover crop
Nitrogen and Rotation Experiments
` In 1956, the lespedeza rotation experiment discussed above was terminated, and a
new 3-year rotation experiment in which various rates of nitrogen were applied to corn
and wheat grown in two different rotations was initiated on the same plots. The new plots
were arranged in a Latin—square design. In the first rotation, corn was grown for the
first two years with an unfertilized wheat cover after the first corn crop, and wheat fol-~
1 lowed by a sweet clover—lespedeza cover crop was the third-year crop. The second
rotation was a 3—year rotation of corn, wheat, and red clover-orchardgrass hay. Three
series of plots enabled each crop in the rotation to be grown each year.
All plots were limed at the rate of 4 tons ground limestone per acre, and 120 pounds
. of P205 and 120 pounds of K20 per acre were broadcast on all plots each year for the first
three years, and at one-half that rate (60 pounds per acre) thereafter. All nitrogen lor
corn was broadcast prior to seeding, and for wheat, one-half of the nitrogen and all the
phosphorus and potassium were applied at seeding; the remainder of the nitrogen was top-
dressed in early spring. All crop residues (corn stover and wheat straw) were reinoxwerl
V from the plots. Stalk population for corn was 14,000 per acre.

 -8-
Corn Yields —— The yields of corn following lespedeza (the third year crop in the *
preceding lespedeza experiment) in 1956, 1957, and 1958 are given in Table 7. Fertilizer
nitrogen increased yields significantly in 1956 and 1957, but the higher yields when nitro-
gen was applied in 1958 were not satistically significant. Average yields showed signi- ·
ficant increases in yield for each additional increment of nitrogen. Yields of corn with-
out nitrogen which were about twice as high as those obtained in the preceding experiment A. _
(Table 5) were probably due largely to higher rates of phosphorus and potassium applied. `
l TABLE 7. - EFFECT OF NITROGEN ON YIELD OF CORN FOLLOWING LESPEDEZA AT GREENVILLE, KY.
~
Bushels per Acre
Pounds N pgr Acre 1956 1957 1958 Average ‘ ·
0 71. 1 64.0 101.8 79.0
30 81. 7 69. 1 109.0 86. 6 .
60 87. 4 74. 5 110. 9 90. 9
 
L.S.D. .05 6.1 5.7 ns 3.7 ’
. 01 8. 2 7. 6 ns 4. 9
CV % 8. 4 9. O 7. 9 8. 4
 
Yields obtained in 1959 and 1960 following either a sweet clover-lespedeza cover
crop or a red clover—orchardgrass hay crop are given in Table 8. No significant increases
in corn yields were obtained for nitrogen applications following the sweet clover—lespedeza
cover crop, although yields tended to be slightly higher each year. However, marked in- ’·
creases were obtained for each additional increment of nitrogen applied to corn following
red clover-orchardgrass hay. The highest corn yields were similar for each rotation,
indicating that lack of nitrogen was the limiting factor for yields of corn following red
clover-orchardgrass.
TABLE 8. - EFFECT OF PRECEDING LEGUMEZS AND NITROGEN FERTHJZER ON YIEID OF CORN AT GREENVILLE, A
KY., IN 1959 and 1960
 
Bushels per Acre
Sweet Clover—Lespedeza Cover Crop Red Clover-Orchardgrass Hay
Pounds N EY Acre 1959 1960 Average 1959 1960 Average
0 77.6 73.7 75.6 61. 1 62. 3 61.7
30 79. 6 77. 3 78. S 67. 4 68. 6 68. 0
60 86. 5 78. 0 82. 3 80. 1 78. 3 79. 2
 
L.S.D. .05 ns ns ns 13.5 8.9 7,7
. O1 ns ns ns 18. 9 ns 10. 4
Cv 96 9.2 10.4 9.8 14.2 9. 3 12.0
 

 The yields of corn following corn (second—year corn) grown in the corn—corn-wheat
(with sweet clover—lespedeza cover) rotation are given in Table 9. A marked response
· was obtained for nitrogen each year. Except in 1958, residual effects of nitrogen applied
to first—year corn did not significantly effect the yields of second-year corn. In 1958, the
( yield from plots which received no nitrogen on first—year corn and 30 pounds on second-
year corn was higher than that from plots which received 60 pounds of nitrogen for first-
_ year corn and 30 pounds for second—year corn. No significance is attributed to this dif-
ference since similar differences were not noted in other years. Each year, the maximum
yields of second—year corn were consistently lower than the maximum yields of first-—year
. corn (Tables 7, 8, and 9). The lower yields of second—year corn may have been due to
either insufficient fertilizer nitrogen or deterioration of soil structure under the intensive
cropping system.
TABLE 9. — EFFECT OF APPLIED AND RESIDUAL NITROGEN ON YIELD OF CORN FOLLOWING CORN AT GREEN-
VILLE, KY. , FROM 1957 to 1960, INCLUSIVE
. Pounds N per Acre on Com
First Second Bushels per Acre _______
Year Year 1957 1958 1959 1960 Average ______
l 30 O 40.9 25. 3 53. 1 37. 0 39. 1
30 30 45.7 70. 0 68. 8 59. 6 61. 0
30 60 4-8. 8 75. 1 74. 5 70. 4 67. 2
— 0 30 41. 1 72. 2 70. 0 55. 1 59. 6
60 30 47. 7 55. 5 65. 0 63. 8 58. 0
L. S.D. . OS   ‘j· 17. 0 7. 6 6.1  
l .01 ns 23.8 10.7 8.5 6.,3
_ CV % .10. 1 20. 6 8. 4 7.7 13.1
Wheat   -— The effect of fertilizer nitrogen on the yield of wheat follo‘· .1*.1 Ln.: e ilr- im
in the corn—wheat-—red clover»orchardgrass rotation is given in Table 10. Nitrogeti .1;;;-
plications to wheat resulted. in yield increases each year. The second 15—pound im·wi‘e»x;ii1,
(15 to 30 pounds per acre) resulted in a significant yield increase only in 1957, 2ii.1E1¤<.»·r;·l;.
the average yields also showed a significant increase as the rate of nitrogen was jr··¤·; *Vl’%*.`.¥.l
from 15 to 30 pounds per acre. No constant trends were noted due to the residum ..f"; vs
of nitrogen applied to the preceding corn crop.

 -10..
TABLE 10. - EFFECT OF NITROGEN APPLED TO WHEAT AND ALSO TO THE PRECEDING CORN ON YIELD OF
WHEAT AT GREENVILLE, KY. , IN 1957, 1959, and 1960. WHEAT GROWN IN ROTATION OF CORN,
WHEAT, AND RED CLOVER-ORCHARDGRASS g
V
Pounds N per Acre Wheat Yield, Bushels per Acre ’
Com Wheat 1957 1959 1960 Average ’
30 0 13.5 23.7 31.2 22.8 .
30 15 19. 5 30. 0 34. 9 28. 0
30 30 23.9 34.0 35.7 31. 2 .
0 30 22. 0 33. 6 32. S 29. 4
60 30 22. 6 31. 7 33. 4 29. 2 A
 
L.S.D. .05 3.7 4.7 3. 1 2.2
. 01 5. 1 6. 6 ns 2. 9
CV % 14. 1 11. 2 7. 1 10. 4 Il
  -
The yields of wheat grown as the third crop in the corn—corn-wheat rotation are _
given in Table 11. No residual effects of the nitrogen applied to the preceding corn crops
were noted.
TABLE 11. - EFFECT OF NITROGEN ON PRECEDING CORN CROPS ON YIEID OF WHEAT AT GREENVILLE, KY. , I.N
1959 and 1960. WHEAT GROWN IN ROTATION OF CORN, CORN, AND WHEAT, WITH SWEET CLOVER— 1
LESPEDEZA COVER CROP
 
Poimds N per Acre Wheat Yield, Bushels per Acre
Corn Wheat 1959 1960 Average
First Second
Year Year A V
0 30 30 25. 5 26. 3 25. 9
30 30 30 23. 2 26. 9 25. 1
60 30 30 21. 6 27. 8 24. 7 ·
30 0 30 19. 6 25. 3 22. 4 L
30 60 30 22. 7 28. 3 25. 5
 
L. S. D. . 05 ns ns ns
. 01 ns ns ns
CV 96 13. 6 10. 8 13. 2
 

 ':
‘ -11-
Hay Yields -- The yields of red clover-orchardgrass hay in the corn-wheat-red
clover-orchardgrass rotation (Table 12) were not affected by nitrogen applications to the
_ preceding corn and wheat crops.
TABLE 12. - EFFECT OF NITROGEN APPLIED ON THE PRECEDING CORN AND WHEAT ON TI-IE YIEID OF RED
CLOVER-ORCHARDGRASS HAY AT GREENVILLE, KY. , FROM 1958 to 1960, INCLUSIVE
 
Poimds N per Acre Hay Yield, Cwt gr Acre
A Com Wheat 1958 1959 1960 Average
30 0 88. 3 84. 7 70. 9 81. 3
30 15 80. 8 81.9 69. 8 77. 5
30 30 79. 7 81. 5 79. 0 80. 1
O 30 79. 4 86. 5 82. 2 82. 7
. 60 30 83. 6 84. 4 73. 0 80. 3
 
L.S.D. .05 ns ns ns ns
.01 ns ns us ns
CV % 8. 3 4. 3 23. 9 15. 7
 
Two -year Rotation with Corn and Wheat
A two—year rotation of corn and wheat followed by a sweet clover-lespedeza cover
crop was conducted from 1952 to 1960, inclusive, on Zansville silt loam. Adequate rates
of phosphorus and potassium were applied to all plots. Corn was thinned to a uniform
population of 10, 500 stalks per acre. All corn stover and wheat straw was removed
from the plots.
2 Corn yields were increased in six of the nine years by the first 25-pound increment
of nitrogen, but additional nitrogen applications did not further increase yields except in
1958 (Table 13). These results are similar to those given in Table 8 for corn following
a sweet clover-lespedeza cover.
TABLE 13. - EFFECT OF NITROGEN ON YIELD OF CORN GROWN IN ROTATION WITH Wl-IEAT FOLLOWED BY
SWEET CLOVER-LESPEDEZA COVER CROP
Pounds N per Acre Com Yield, Bushels per Acre _
1952 1953 1954 1955 1956 1957 1958 1959 1960 Average _
0 28. 8 38. 7 67. 3 51.9 75. 5 72. 7 75. 0 64. 8 48. 3 58.1
25 33.7 43.2 67. 3 53. 3 81.5 85.6 74.4 76.5 60.0 63.9
' 50 35. 5 45. 3 63. 6 S3. 4 82. 8 86. 3 79. 2 75.1 62. 7 64. 9
75 33.2 36.6 60.1 53.2 83.9 88.4 81.3 69.2 60.5 62.9

 -12- . V
I
Wheat yields, given in Table 14, show a response for the first 15-pound increment
of nitrogen, but very little increase for additional nitrogen. These results are similar
to those given in Table 10 for yields of wheat following corn.
TABLE 14. - EFFECT OF NITROGEN APPLIED TO WHEAT AND TI-IE PRECEDING CORN CROP ON YIELD OF WHEAT
CROWN IN ROTATION WITH CORN AND FOLLOWED BY A SWEET CLOVER-LESPEDEZA COVER ' _
 
Pounds N pgr Acre Wheat Yield, Bushels per Acre*
  I
0 0 14. 3 25. 0 23. 2 13. 1 14. 5 20. 2 21. 2 18. 8
25 15 23. 5 36. 0 35. 6 18.4 18.1 24. 4 29.7 26. 5 °
50 30 26. 6 38. 0 37. 6 18. 9 20. 5 27. 5 28. 4 28. 2
75 45 31. 3 41.9 36.9 21.1 24.6 39.0 27.8 31.8 I
 
*No yields obtained in 1956 or 1958.
CONCLUSIONS O
The results of the long-time rotation experiments indicate that even though the nitrogen ¥
content of the soil may be maintained or even slightly increased when moderate rates of ‘
fertilizers are applied, adequate phosphorus and potassium must be applied to properly
limed soils and must be supplemented by proper amounts of fertilizer nitrogen if optimum
yields are to be obtained. The removal of corn stover and wheat straw from the plots in
these experiments undoubtedly influenced the degree of response to fertilizer nitrogen, V
particularly with respect to second-year corn. Data reported by Doll, Miller, and Todd
(1962) indicate that appreciable quantities of nitrogen are removed in corn stover and
wheat straw.
Experiments with nitrogen fertilizer for corn indicate that approximately 30 pounds
of N should be applied for corn following a good legume crop. For second-year corn (corn
following corn), much higher rates would be necessary, since yields given in Table 9 in-
dicate that 60 pounds of nitrogen was not enough for optimum yields. In summarizing the .
results of numerous field experiments, Doll (1962a) concluded that 100 to 120 pounds of
nitrogen would be necessary for optimum yields of continuous corn.
When wheat was grown following corn, consistent yield increases were obtained.
Very little residual effect of nitrogen applied to preceding corn was noted (Tables 10
and 11). Based on a summarization of the results of all wheat experiments, the optimum
rate of nitrogen fertilization seems to have been between 30 and 45 pounds per acre.

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LITERATURE CITED
1. Doll, E. C. , and Link, L. A. Influence of various legumes on the yields of succeeding
corn and wheat and nitrogen content of the soil. Agron. J. 49:307-309 1957.
2. Doll, E. C. Fertilizer experiments with corn in Kentucky. Ky. Agr. Exp. Sta. Progress
Report 118. 1962.
3. Doll, E. C. Effects of fall—applied nitrogen fertilizer and winter rainfall on yield of wheat.
` Agron. J. 54:471-473 1962.
4. Doll, E. C., Miller, H. F., and Todd, J. R. Effect of phosphorus fertilization and liming
on yield and chemical composition of corn, wheat, and red clover. Ky. Agr. Exp. Sta.
Bull. (in press)· 1963.
5. Karraker, P. E. Effect of certain management treatments on the amount of nitrogen in a
soil. J. Amer. Soc. Agron. 28:292-296. 1936.
6. Karraker, P. E. Effects of certain cropping and management practices on soil nitrogen
content. Ky. Agr. Exp. Sta. Bull. 561. 1951.
7. Roberts, G. Legumes in cropping systems. Ky. Agr. Exp. Sta. Bull. 374. 1937.
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