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XII. Relation of Yield and Tuber Composition to Plant and Soil Analyses
in the plants on plots without added phosphorus, soluble phosphorus was
less than 70 ppm. Soluble potassium was never less than 10,000 ppm.
The starch content decreased from 17 per cent in tubers produced without nitrogen to less than 13 per cent in tubers grown on plots receiving
210 lbs. nitrogen per acre. Throughout the season nitrogen was lower
in all parts of the plants grown on the plots lacking nitrogen than in
plants on plots receiving nitrogen, Phosphorus was low both in plants
grown without nitrogen fertilizer and in those without phosphorus. OmiPsion of nitrogen from the fertilizer greatly reduced the absorption of phosphorus and the amount in all parts of the plant analyzed. Potassium
was high in all samples and was unaffected by any of the fertilizer treatments. Calcium was highest in plants grown without nitrogen and lowest
in plants grown without phosphorus. Lorenz (1947) showed later that
the largest amount of growth and the greatest intensity of nutrient absorption occurred between about 75 and 110 days after planting or between 45 and 80 days after emergence. A yield of 658 bushels per acre
absorbed 139 Ibs. of nitrogen, 36 of phosphoric acid and 253 of potash.
The tubers alone removed 106 IbR. of nitrogen, 31 of phosphoric acid and
196 of potash.
Hawkins (1942) reporting from Maine, states that potato plants
capable of producing upwards of 400 bushels per acre, absorb about 120
to 160 lbs. of nitrogen, 25 to 30 lbs. of phosphoric acid, 200 to 250 lbs.
of potash, 60 llp. of calcium oxide, 30 Ibs. of magnesium oxide and about
10 to 12 Ibs. of sulfur per acre. The Green Mountain variety absorbed
about 70 per cent of its total consumption of plant nutrients between
50 and 80 days after planting. The earlier varieties, Cobbler and Chippewa, usually emerge and make more rapid growth early in tqheseason
than do the later-maturing varieties, and the period of more rapid absorption of nutrients occurred with these two varieties about one week
earlier than for the Green Mountain variety. The magnesium content
of the tops and roots of the Cobbler variety was considerably less than
that of the other varieties, but there was comparatively little difference
between varieties in the magnesium content of the tubers. During the
first 50 days after planting, the Green Mountain variety absorbed 9 per
cent of the total major nutrient elements while making 3 per cent of the
growth for the season. The later varieties, Green Mountain and Smooth
Rural, each absorbed about 100 lbs. more total major nutrient elements
during the season than did the earlier varibties, Cobbler and Chippewa.
The later varieties absorbed considerably more nitrogen, potassium and
calcium, and, as compared with the Cobbler variety, particularly more
magnesium. The Cobbler variety absorbed 22 per cent of its nitrogen requirements by the 50th day as compared with 8 per cent for the Rural
variety. By t,he 70th day, Cobblerv had absorbed 86 per cent of the
nitrogen absorbed during the season BS compared to 52 per cent for the
Rural and 56 per cent for the Green Mountain varieties. Hawkins (1946)
later 'reported that the proportion of the nutrients absorbed that were
translocated into the tubers was approximately as follows: 80 per cent
of the Pz05,67 per cent of the N, 60 per cent of the S, 50 per cent of the
KzO,40 per cent of the MgO, and about 5 per cent of the CaO.
Regardless of rate of fertilizer application, 1200 and 2400 lbs. 5-10-10
to the acre, method of placement, source of nitrogen or source of potash,
Smith and Kelly (1946) found no significant difference in phosphorus
content of leaf petioles at any time during the growing season. Petioles
of plants of all treatments contained large quantities of potassium and
evidently sufficient potassium was taken up by all treatments to result
in high yields. Correlation of nitrogen and magnesium content of petioles
with yields of potatoes was highly significant at all treatments. Jones
and Plant (1942) found, however, that deficiency symptoms of potassium and magnesium agreed with chemical analysis of the leaves.
Soil analyses, especially variations of the so-called quick soil tests,
have been made in attempts to establish relationship between such analyses and subsequent potato yields. These have been reasonably successful and show considerable promise 8s a tool for detecting deficiencies of
soil nutrients. Sparks and McLean (1946), working with alkaline soils in
Colorado, found a close correlation between the results obtained by soil
analyses and yields following applications of nitrogen, phosphorus and
potassium. Peech (1945) reported a marked accumulation of readily
soluble phosphorus in all soils studied from the important potato-producing areas along the Atlantic Coast. This accumulation varied in
different soils. I n general, the amount of readily soluble phosphorus
increased with the increasing degree of saturation of the soil with
phosphate. I n the light-textured soils containing large amounts of
readily soluble phosphorus in the surface layer, there has been appreciable
downward movement of phosphorus into the subsoil. Despite the low
cation-exchange capacity and the low p H value, the exchangeable potassium content of many of these soils has been greatly increased by fertilization. This accumulation is relatively small as compared with the
total amount of potassium applied over a period of years. The majority
of the soils had p H values below 5. The amounts of exchangeable
calcium and magnesium were very low. I n some areas the soils were
extremely deficient in magnesium. The organic matter content was low
and was quite variable in many of the soils even within the same series,
I n some of the areas the organic matter content of the soils has been
increased, whereas in other areas it has been decreased by cultivation.
Hawkins (1945) found that in Maine and North Carolina the increase in
yield of potatoes per pound of PzOa applied was usually lower a t locations where there were high amounts of residual phosphate in the soils.
In Maine no significant increases in yield were obtained from additional
potash above the 100 lb. base rate. Analyses of virgin and cultivated
Northern Wisconsin soils show that when potatoes are grown on these
soils from 10 to 30 years the available phosphorus and soluble manganese
content and acidity are increased, while the available potassium, caIcium
and magnesium contents are seriously depleted (Berger, 1948).
Nelson and Hawkins (1947) made a study to show the relationship
between the amounts of readily soluble phosphorus and exchangeable
potassium in the soil and the response of potatoes to applications of these
nutrients. Results from North Carolina shoved that applied phosphorus
gave significant increases in yield a t all six test locations. Significant
increases were obtained on 8 of the 9 experiments in Maine. The degree
of yield response to applications of P z O ~
was related to the amount of
readily soluble phosphorus in the soil. Yield increases from the first 80
lbs. of P205applied decreased as the amount of readily soluble phosphorus in the soil increased. The phosphorus content of the leaves in
the North Carolina experiments was related to the amount of readily
soluble phosphorus in the soil and to the amount of phosphorus applied.
Phosphorus was particularly important in influencing the number of
tubers per hill on soils low in readily soluble phosphorus. Significant
increases in yield from applied KzO were obtained in all experiments
in North Carolina and in 5 out of 8 experiments in Maine. The weight
of potatoes resulting from the first 60 lbs. of KzO tended to decrease as
the amount of exchangeable KzO in the soil increased. I n the North
Carolina experiments the KzO content of the leaves was related to the
amount of KzO in the soil, and to applied KzO up to 120 lbs. per acre.
The amount of potassium extracted from the rachises of the potato plants
in the Maine experiments was related to the exchangeable K20content
of the soil and to the amount of K20 applied.
During the past few years potato growers have shown growing interest
in killing potato vines preparatory to harvest.. The use of chemicals as
killing agents has increased steadily each year since the practice was
started on an appreciable scale. This may be attributed in part to the
increasing importance of varieties which remain veget.ative and green
over a longer period of time and to the intensive spraying programs now
practiced by good commercial growers which reduce insect and disease
injury to a relative minimum. It usually is the desire of any grower
to maintain strong, vigoroue top growth as long as feasible in order
to obtain greater total photosynthesis, translocation and increase in tuber
size. It may be desirable or necessary, however, to destroy the above
ground vegetation for one or more of the following reasons: (1) to avoid
the risk of having potatoes left in the ground after extreme weather
conditions arrive, (2) to avoid oversize tubers, (3) to prevent late virus
spread in fields certified for seed, (4) to prevent second growth of tubers,
(5) to prevent late blight infection of tubers, (6) to enable harvesting in
time to meet a favorable market, (7) to increase the proportion of tubers
within the size limits for good seed, (8) to enable completion of harvest
with the labor and equipment available, and (9) to destroy late weed
growth along with the vines. When it is desirable to begin harvesting before natural death of the yines occurs, some means of destroying top
growth is needed because: (1) commercial digging equipment will not
move heavy vines with green leaves and succulent stems over the conveyor belt in a satisfactory manner, (2) tubers will stick or hang onto
green vines thereby making picking extremely difficult, and (3) tubers
will not mature until after the vines have died and will skin or bruise
easily when in an immature state.
Wilson and Boyd (1947) and Wilson e t al. (1947) found that certain
tar-acid compounds provided the most efficient noncorrosive substitute
for sulfuric acid which is the killing agent of most widely accepted usage
in England. Sodium chlorate gave fair leaf kills, but poor stem kills.
Calcium cyanamid dust provided inefficient kills even when used a t very
heavy rates of application. Dinitrocresol derivatives were only moderately effective while copper sulfate and sodium chloride gave fairly
efficient kills on senescent haulms only. Young haulms kill readily but
become more resistant as the season advances and finally again susceptible during senescence. Varietal differences in susceptibility were attributed largely to differences in stage of maturity. Complete kill was
rarely accomplished in less than 10 days. Comprehensive tests indicated
no differences in flavor, texture, nr internal color of tuber attributable t n
vine killing treatments.
Bates and Martin (1935), MacDowell (1935), Main and Grainger
(1947),Small (1935), and some British workers (Anonymous, 1945), have
reported experimental results which indicate that destruction of vines by
chemical means is very important in the control of tuber infection by late
blight. Chemical destruction is preferable to cutting of the vines (Small,
1936). Bonde (1935) reported satisfactory kills with 6 to 7 per cent sulfuric acid, and later found Sinox with ammonium sulfate to be very effective as a vine killer in the control of late blight infection of tubers (Bonde
and Schultz, 1945). Schultz et al. (1944) found that 2 gallons of Sinox
plus 10 lbs. of ammonium sulfate per 100 gallons of water a t 135 gallons
per acre could be used as a less tedious substitute for pulling the vines
in early harvested seed plots in Maine. Such early harvesting materially
reduced the spread of virus infection as compared to plants remaining
green to a later date. Findlay and Sykes (1936, 1937) reported no
significant effect of vine killing treatments on yield in 1935, but a
reduction of yield in 1936. Kraus (1944) reported little, if any,
reduction in yield from vine killing in 1943 and 1944, but stated that
this would depend primarily on relative maturity. Samuel (1944) found
that killing of the vines may reduce potato yields and states that even
in September, crops with green tops can put on as much as half a ton
of tubers per acre per week. He considered 10 days or more necessary
between killing date and harvest date for satisfactory setting of the skins.
Idaho Experiment Station workers (Anonymous, 1940) also concluded
after microscopic examination of tubers dug a t frequent intervals that
normal ripening changes occurred after killing of the vines and that
potatoes were sufficiently matured in 10 days to prevent abnormal surface
damage during harvesting.
Steinbauer (1945) reported that sprays were more effective as vine
killers than dusts under Maine conditions. Dilute sulfuric acid was the
most rapid killing agent studied, but Dowspray 66 Improved and Sinox
gave fairly satisfactory rates of kill. Varieties such as the Sebago were
more difficult to kill than others. Plants making rapid growth with
abundant soil moisture and moderately high temperatures were more
readily killed than when they were toughened by low soil moisture or
low temperature. A yellowish discoloration of the xylem vessels was
reported in tubers from Sinox-killed vines. Greenhouse tests indicated
that this did not impair planting behavior. Some tuber growth takes
place after vines are sprayed with a slow acting vine killer and the killing
date may be advanced a few days with such chemicals without large
yield reductions. I n lQter work, Steinbauer (1947) reported that oil
preparations were much more expensive than most other vine killers
with ordinary sprayers and using 125 t o 150 gallons per acre. Products
mch as Stoddard Solvent were fairly satisfactory as defoliants but not
for complete kill of vines. Of the compounds studied those with a high
content of aromatics were most effective. Otis (1946) stated that maturing yellow vines were more easily killed than green vigorously growing
ones. Low humidity or low temperature a t time of application was unfavorable to both dusts and sprays. Varietal differences did not govern
speed and completeness of kill as much as maturity and size of vine
growth. Kunkel e t al. (1948) state that killing the vines prematurely
resulted in lower specific gravity and increased stem end discoloration.
Hoyman (1947) found in field experiments in North Dakota that
discoloration in and near the vascular tissue of tubers was positively
correlated with rapidity of kill and was less when applications were made
later in the season. Such discoloration may or may not be a direct effect
of vine killing chemicals. Tubers from vines cut a t ground level showed
the same type of discoloration as those from chemically killed vines.
The effectiveness of Dowspray 66 Improved was increased by the use
of aluminum sulfate as an activator, Brentzel (1944)obtained unsatisfactory kills with dusting treatments in North Dakota, but a 95 to 99
per cent kill resulted from 2 gallons of Sinox plus 10 lbs. of ammonium
sulfate in 100 gallons of water applied a t the rate of 173 gallons per acre.
Fernow and Smith (1944) stated that experience of New York growers
indicates that the vines die more rapidly when application is made during
comparatively warm weather or is followed by fairly high temperatures.
Ennis et al. (1946) found that 2,4,5-trichlorophenoxyaceticacid applied to the foliage of potato plants caused pronounced stunting and distortion of vegetative growth and scab-like injury on the tubers. Smith
et al. (1947) found potatoes resistant to spray applications of 10 ppm
of 2,4-D and 10,000ppm naphthaleneacetic acid methyl ester when applied during August. Injury similar to that reported by Ennis et al.
occurred on many tubers from plants which had been sprayed with the
Callbeck (1948) found that tubers from untreated cut vines showed
a greater incidence of stem-end discolorat,ion than tubers from untreated
plants or from plants destroyed by slow acting herbicides. Tuber vascular discoloration appears to be correlated with rapidity of kill of the
tops. The amount and intensity of discoloration in tubers from plants
killed a t different stages of development with dinitro ortho secondary
butyl phenol increased quite regularly with the age of the plants; those
killed late in the season exhibited the greatest injury. This is in accordance with the work of McGoldrick and Smith (1948)and of McGoldrick
(1948). Hoyman (1947), in North Dakota, found less discoloration when
the applications were made later in the season. Callbeck (1948)detected
no differences in flavor or texture of the cooked tubers from the several
treatments, McGoldrick and Smith (1948) and McGddrick (1948),
however, found that those killing agents which destroyed top growth
most efficiently and rapidly reduced specific gravity of the tubers compared to poorer kills and controls. Discoloration of the vascular region
of tubers was decidedly increased by killing injury to top growth.
Further results of comprehensive field and greenhouse experiments by
McGoldrick (1948) showed that Penite 6 plus an activating compound
resulted in the most efficient kill of vines. Dowspray 66 Improved,
Penite 6 without activator, Sinox General plus diesel oil, and sizz flame
treatment also gave satisfactory kills. 2,4-dichlorophenoxyacetic acid
a t the rate of 6% lbs. plus 118 gallons diesel oil to the acre or 125 lbs.
sodium nitrite to the acre resulted in satisfactory kills on relatively
senescent plants at the, last killing date only. Fairly satisfactory vine
destruction, especially of leaves, resulted from spray applications of
Cyanamid x-1 and x-5. The following materials at concentrations named
were worthless as vine killers: 2,4-D, 6% and 25 lbs. to the acre; methyl
ester naphthaleneacetic acid, 21/2 gallons per acre, and a petroleum
product a t 6% gallons to the acre. 2,4,5-trichlorophenoxyaceticacid as
a spray a t the rate of 25 lbs. to the acre caused some leaves to turn yellow
and die alt.hough there was no death of the plants. Increase in tuber size
during late stages of maturity warrants delaying the application of vine
killers as long as possible unless greater size is objectionable. Some discoloration of the vascular region was found in tubers from untreated
plants as well as in those from treated plants. This browning of the
vascular bundles, however, was decidedly increased by killing injury to
top growth. Sprouting and shrinkage loss other than sprouting in subsequent storage was not significantly affected by killing agents. Early kill
resulted in significantly greater loss than later kill. Neither killing agents
nor application dates appeared to have a significant effect on the value
of tubers as a source of seed the following season. More effective kills
were obtained a t 100 per cent relative humidity than a t approximately 75
per cent. Drying and shriveling of killed tissue, however, was faster a t
the lower humidity level. Artificial rainfall applied a t intervals up to 10
hours after killing treatment reduced the effectiveness of Sinox General
and Dowspray 66 Improved.
Recent experience with machines such as the “Roto-Beater,” which
chop the vines to small pieces, indicates that this method of vine destruction will gain wide acceptance by growers.
The potato crop is infest,ed with many insects which result in tremendous losses in yield and quality. Programs for combating them cost
millions of dollars annually. The most important insects are aphids,
leafhoppers, flea beetles, Colorado potato beetles and wireworms. Several
of these insects were not satisfactorily controlled until after the general
adoption of DDT as an insecticide for potatoes. This occurred mainly
in 1945 and 1946. After an amazingly short period of testing, this one
material has largely repIaced the various insecticides formerly recommended for potato insect,s throughout North America. Several other in-
secticides introduced even more recently than DDT are especially lethal
to aphids and appear very promising for their control in potato fields.
I n many areas the potato leafhopper, Ernpoasca fabae Harris, probably is the worst pest of potatoes. Leafhopper injury results in premature death of vines and a resultant decrease in starch content, mealiness and yield. Until the advent of DDT, control of this insect was
erratic and incomplete. Under such conditions it was extremely difficult
to evaluate potato varieties for anything other than leafhopper resistance.
It has been found repeatedly that almost perfect control can be obbained
with DDT without injury to the plants (Granovsky, 1944; Apple and
Arnold, 1945; Wilson and Sleesman, 1945; Heuberger and Stearns, 1946 ;
Wolfenbarger and Heuberger, 1946 ; Wilson and Sleesman, 1947).
Potato varieties vary widely in their susceptibility to hopper-burn
(Sleesman and Bushnell, 1937; Allen and Rieman, 1939; Allen et al.
1940; Sleesman and Stevenson, 1941 ; Sleesman and Bushnell, 1945;
Maughan, 1947) ranging from the very susceptible Bliss Triumph and
Pontiac to resistant Sequoia. There is no agreement, however, as to why
some varieties are more severely injured than others. It has been claimed
that early varieties are more susceptible than later varieties but Allen
et al. (1940) state that relative earliness or lateness is not the prime
factor; early varieties such as Bliss Triumph and Warba were more
attractive to the insects than the later varieties, Katahdin and Houma.
The correlation between earliness and susceptibility may not be a true
relationship as it is difficult to differentiate between natural maturity and
that caused by leafhoppers (Sleesman and Stevenson, 1941). Allen and
Rieman (1939) d a t e that leafhopper tolerance of Katahdin and Houma
might account in part for the heat, and drought resistance attributed to
these varieties. Varietal response to leafhopper control has been measured by adult and nymphal populations, severity of hopperburn and yield
increases with but little emphasis on increase in starch content of the
tubers. Sleesman and Wilson (1943) report a high negative correlation
between nymphal populations and yield, between dead foliage and yield,
and a high positive correlation between nymphs and dead foliage. Apple
and Arnold (1945) found a highly significant correlation coefficient. of
-0.87 between nymphal populations and specific gravity of the tubers.
Linn et al. (1948) measured the effects of leafhopper control with D D T
on length of season, quality and yield of seventeen new and old varieties.
Control of leafhoppers increased the length of season most in the case of
very early varieties. Yields of all varieties were increased except Erie
and Sequoia in one location. Control of leafhoppers also increased the
specific gravity of tubers of most varieties.
Post et al. (1948) obtained significant increases in yields from spray