Greg Schwab, Lloyd Murdock, and John
Grove, Plant and Soil Sciences
This year we have
seen a higher number of corn and soybean fields exhibiting potassium
deficiency symptoms. It is generally appears first on the lower
leaves of corn. The leaf edges turn yellow from the tip and
eventually lead to necrosis (tissue death) beginning with the leaf
tip. In soybeans, you will also see chlorosis followed by necrosis of
the leaf edge. On young soybean plants, it occurs on leaves lower in
the canopy, but can appear on upper leaves if the plants are stressed
later in the season. See the images for potassium deficiency
deficiency symptoms in corn. Click on image to see larger picture.
deficiency symptoms in soybean. Click on image to see larger
The primary cause of
K deficiency is low levels of available soil K. If you are doing a
good job at routine soil testing, and applying the proper amount of
fertilizer, soil test K should not be an issue. However, the high
price of K fertilizer this spring might have caused many producers to
apply less than the recommended rate. In a normal year, skimping on K
fertilizer might not have been a problem, but this year has not been
normal. The K deficiency we have observed this year is probably a
result of several factors.
First, soil test
levels might not be as high as you think. Keep in mind that the past
two seasons produced exceptional yields in many parts of the state,
and K removal increases as the yield increases. Therefore, it might
have been a mistake to use soil test information that was more than a
couple of years old. Also, keep in mind that fields testing adequate
can have small areas that are deficient. In some cases one sample
represents 80 or more acres and deficiencies are only present in parts
of the field. Samples should not represent more than 20 acres and
should represent similar soil types and past management histories.
A compounding factor
is anything that restricts or slows root development. An example is
soil compaction, which can cause K deficiency symptoms to appear first
in wet areas of fields. Because of the rooting restriction,
additional K fertilizer in these areas may not help. There is still
time this season to assess soil compaction in areas showing K
deficiency. If you determine that compaction is the culprit, you may
want to consider deep tillage this fall (depending on the size of the
area and severity), and you should definitely try to limit future
This year however,
dry soils might also be a contributing culprit. Potassium is carried
to the plant root with the soil water. Droughty soils are less able
to supply K and compacted droughty soils will further limit root
acquisition of K. A secondary factor under any form of
conservation/shallow tillage soil management system is that much of
the available K is in the surface few inches of soil. As K is not
that mobile within the soil, K availability is further reduced when
this surface layer dries. Surface waster loss can be minimized by
maintaining as much soil coverage with crop residue as possible.
Shallow surface tillage encourages degradation of crop resides and
should be avoided. Additional rain will benefit both root growth and
soil K mobility.
While these areas
look bad now, it is impossible to determine how much (if any) yield
reduction will be associated with midseason K deficiency.
Nevertheless, such symptoms demonstrate the importance of routine soil
testing, and give you an indication of problem fields where soil
sampling this fall is essential.
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Famine - How Much Rain is Enough?
Dennis B. Egli, Plant
and Soil Sciences
After two “feast”
years in Kentucky
are we looking at a “famine” year in 2005? Lots of rain in 2003 and
2004 produced record or near-record corn and soybean yields, but this
year is not off to a good start. Rainfall in May and June was below
normal, but there may be some grounds for optimism after Hurricane
Dennis brought good rains in mid-July. Dry periods that reduce yields
often occur in Kentucky in spite of the fact that we have a humid
climate with between 45 and 50 inches of rain in an average year†.
With so much rain, it’s surprising that the most common factor
limiting yields is probably water.
How much rain do we
need? More than you might think because crops use a lot of water – a
field of corn or soybean with enough leaves to completely cover the
soil surface will use 0.20 to 0.25 inches or more on a warm sunny day
in July if plenty of water is available. That amounts to 1.00 inch
(27,000 gallons per acre) every four or five days which totals roughly
15 to 25 inches for the crop (depending on weather, planting date and
variety or hybrid maturity). The water moves into the roots and
evaporates from the leaves (through pores called stomata). The rate of
evaporation depends on the weather and the amount of water in the
soil. Evaporation will be higher on warm sunny days with low relative
humidity and the wind blowing and lower on cloudy humid days.
Evaporation will decrease as the soil dries.
Crops use water
every day, but it only rains occasionally, so the water stored in the
soil must provide the buffer to match the intermittent water supply
with the constant use. When most of the soil water is gone, plants are
stressed and yields reduced. The amount of water stored in the soil is
very important – crops growing in a soil that stores a lot of water
are less likely to be stressed than those in soils with little
storage. Many soils in Kentucky hold about 5 to 6 inches of water,
which is less than the deep loess soils in Iowa and Illinois (one of
the reasons why their yields are usually higher than ours), but more
than shallow sandy soils in some southern states. Erosion on hillsides
reduces soil depth and water holding capacity, while soils in bottom
areas may be deeper and hold more water. The water holding capacity of
a soil helps define its yield potential. Kentucky’s soils are not the
best in this respect, but we can be thankful that they are not the
We depend upon rain
to keep water in the soil reservoir. When it rains is almost as
important as how much it rains. Too much rain at one time will fill
the soil to overflowing and the extra water will drain out the bottom
and be lost as far as the crop is concerned. Too much time between
rains and the soil reservoir will empty, the plants will be stressed
and yield may be reduced. It will take 20 days to empty the soil if
your soil holds 5 inches of water and the crop is using 0.25 inches
per day. Unfortunately, the crop will probably be stressed before all
of the water is used, so you don’t have 20 days to get the next rain.
In a perfect world we would get a nice rain (1 to 1.5 inches) every
week, and crop yields would always be high. We are often far from this
perfect world in Kentucky as we are seeing so far this year.
is probably not enough for maximum corn and soybean yields – but a lot
depends on your soil and when it rains. High yields in
(2003 for example) are often associated with above-average summer
rainfall (Table 1), but we can get good yields with near-average
rainfall (e.g., 2004) if it occurs at the right time. Below-average
rainfall is definitely bad news (e.g., 1999).
Rain is more
valuable if it falls after the crop starts flowering. Water stress
during flowering or seed filling causes larger yield losses than
stress during vegetative growth. Low rainfall or a poor distribution
before flowering may not reduce yield (but it’s always a matter of
degree, if it’s too dry early, yield will be reduced), but
near-perfect amounts and distribution after flowering are the key to
What is going to
happen this summer? We are not off to a good start, but the remnants
of the hurricanes have been a big help in July. No one knows for sure
what the rest of the summer will bring – we can only hope that the
rain will be enough for good yields. Check the University of Kentucky
Agricultural Weather Center website (www.agwx.ca.uky.edu/)
to keep abreast of the rainfall and drought situation in
This year, as always, corn and soybean yields will depend, in large
part, upon when it rains, how much it rains and the water holding
capacity of the soil. All we can do now is watch the Weather Channel
Hill, Jerry. 2005. Kentucky Weather. The University Press of Kentucky.
Lexington, KY. 198 pp.
Table 1. Corn and soybean yields and rainfall in
1999 to 2004.
1 Average yields for western
reporting districts 1 and 2).
Total rainfall for June, July and August (western climatological
Amount above (+) or below (-) the 20 year-average rainfall.
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on Corn Yields
Chad Lee, Plant and Soil Sciences
conditions faced earlier this growing season in Kentucky have
certainly impacted the corn crop across the state. The real question
is how much yield was lost due to the drought stress? The answer is:
it varies, depending on crop growth stage at the time of drought
stress, and duration of the drought.
Drought Stress at
Dry conditions will
have the greatest impact on corn that was tassling and pollinating
during the drought. Pollination requires the release of pollen from
the tassels and capture of that pollen by the silks. The pollen then
travels down each silk to fertilize the ovule. Dry weather will reduce
pollination in a couple of ways. First, dry weather will delay silking
and could result in pollen dropping before silks are exposed. Second,
dry weather will cause the silks to dry out quicker and reduce the
ability of the silks to capture and move pollen to the ovules. Nothing
can be done to regain unfertilized ovules after pollination. Adequate
moisture following poor pollination will help the fertilized ovules
develop kernels, but yield losses are certain with poorly pollinated
corn. Yield losses from a drought during pollination can be as high as
100% but more often the yield losses are much less.
A quick way to
determine how many ovules are fertilized within about 10 days after
pollination is with the ear shake method. With this method, use a
sharp knife to cut through the husk, but not the cob, from the base of
the ear to the tip. Gently remove the husk from the ear, taking care
to not remove any silks. Once all husks are removed, gently shake the
ear. The silks of fertilized ovules should fall off. If an ovule was
not fertilized, then the silk will remain on the ear. Repeat this
method several more times at different areas in the field to determine
how well the field of corn pollinated.
By about two weeks
after pollination, the corn should reach the blister stage and
fertilized kernels should be visible. From the blister stage on,
pollination success can be determined by examining the number of
developing kernels on each ear.
drought stress prior to pollination still has many chances to regain
most of the yield potential. Ear size, kernel rows and potential ovule
numbers are starting to be determined by leaf stage V9 through V12.
Drought stress can reduce these components. However, if adequate
moisture occurs by pollination, the corn plant probably will recover
and yield losses can be as little as 5 or 10%.
develop into kernels and the first stage of this development following
pollination is the blister stage. Dry conditions during this stage
could result in aborted kernels. Aborted kernels are shrunken and
white compared to plump, developing kernels. Kernels at the tip of the
ear are most susceptible to abortion.
kernels will progress through the blister, dough and dent stages
before reaching physiological maturity. The kernels are gaining weight
during the dough and dent stages. Water is a key component to kernel
weight gain. Dry weather during the dough and/or dent stages will
reduce final kernel weight and reduce yields. Dry weather will reduce
yields more during the
dough stage than during the dent stage.
The rains that came
with and followed Hurricane Dennis certainly have alleviated some of
the water stress. Corn that survived the dry weather will recover, but
specific yield loss levels are difficult to determine because
the final yield
depends so much on the amount and timing of stress. Water stress is
never good but stress closer to pollination will result in the
greatest yield losses, compared with water stress at other growth
stages. Reductions in ear length, kernel row and/or kernel numbers can
be offset by adequate moisture during seed fill, resulting in larger
kernels. However, larger kernels cannot compensate fully for large
losses in kernel number. Stress during seed fill will reduce seed size
but typically will not reduce yields as much as stress occurring
Nielsen, R.L. 2002. A Fast & Accurate Pregnancy Test for Corn.
Chat 'n Chew
Ritchie, S.W., J.J. Hanway, G.O. Benson, and J.C. Herman. 1993. How a
Corn Plant Develops. Special Report No. 48. Iowa State University
Press. URL: http://maize.agron.iastate.edu/corngrows.html
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Abortion Limit Soybean Yield?
Dennis B. Egli, Department of Plant
and Soil Sciences
Anyone watching a
soybean crop closely will notice that many of the flowers abort – they
do not develop into mature pods containing seeds. Agronomists believed
for many years that flower and pod abortion limited yield. Some
researchers looked for a miracle spray that would prevent abortion
because surely that would increase yield.
It was easy to
conclude that flower abortion was bad; after all, flowers produce the
pods that contain the seeds we harvest. It just seems logical that
more pods should equal higher yield, and this logic is borne out in
the field where high yields are almost always associated with more
pods and seeds on plants than when yields are low. The fallacy in this
logic is that more pods and high yields do not necessarily mean less
flower abortion. Plants in high yield environments grow faster and are
larger with more nodes and more flowers; so, with more flowers,
abortion levels do not have to go down to produce more pods. We have
measured - by counting flowers and pods– 50% abortion when yield was
60 bushels per acre. Most would agree that 60 bushel is a pretty good
yield, but half of the flowers failed to produce mature pods.
The soybean plant
seems to be an eternal optimist; it always produces more flowers than
it needs. Flower and pod abortion (most abortion seems to occur as
flowers or small pods) is always a part of soybean production.
Producing too many flowers is actually an advantage because it means
that yield will not be limited by a lack of flowers, as can happen
when corn is grown at low populations. There is no danger that a
soybean plant will produce only enough flowers for a 40 bushel yield
in an 80 bushel environment. Taking this viewpoint, flower abortion is
not bad; it simply represents the plant’s adjustment to its
flowering and pod set can cause large reductions in yield and it may
increase flower and pod abortion. But, in this case, flower and pod
abortion is not really the problem; the problem is the drought that
reduces photosynthesis and plant growth. Abortion is just a symptom.
So don’t worry about abortion – pray for rain or do a rain dance,
depending on your inclination.
Would a magic spray
that stopped abortion increase yield? If it worked there would be more
pods on the plants, but most research indicates that there would be no
more yield. Yield is determined by the variety and the environment –
soil fertility, rainfall, temperature, sunlight, pests – which
determines how fast the plants can grow. Simply adding more pods
without changing the growth rate will probably not increase yield. The
mostly likely result will be the same yield but smaller seeds.
is true that high soybean yields require a lot of pods and seeds, but
it is not true that flower and pod abortion must be reduced to get
high yields. It is no secret that the key to high yield is good
management and lots of rain after flowering – flower and pod abortion
are just part of the process.
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Chad Lee, Plant and Soil Sciences
rules and forms for the 2005 Kentucky Extension Corn Production
Contest are available online and at your County Extension Office. The
online address for the contest rules is:
which is also the University of Kentucky Extension Grain Crops home
page. The rules for eligibility, classes, awards and harvesting are
similar to the rules in previous contests. There will be three
non-irrigated classes: 1) conventional or minimum till, 2) no-till and
3) white corn. There is one division for irrigated corn. Deadline for
submission of entries is December 9, 2005. Contact your County
Extension Agent for more information.
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