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Ohio State University Extension


C.O.R.N. Newsletter 2013-35

Dates Covered: 
October 15, 2013 - October 22, 2013
Curtis Young

Keys to Phosphorus Results on a Soil Test and Developing a P Recommendation

One thing that has become very clear working with understanding soil test results over the past 18 months is when it comes to phosphorus we (universities, laboratories and practitioners) have made it very complicated. There are a variety of explanations that can be given for the different reporting but the bottom line is the collective “we” need and can do better. The discussion on environmental impacts will lead to more standardized soil test result reporting that will benefit agriculture for meeting both production and environmental goals. But until those discussions occur, there are a few key questions to ask when looking at phosphorus on your soil test results.

What is the reported extractant?

There are two primary extractants used for phosphorus in Ohio, the Bray P1 and Mehlich III. Either extractant works well for our soils to generate a soil test level. But we need to remind ourselves that the only reason to do a soil test is to be able to correlate yield response to the number on the soil test report. The Tri-state Fertilizer Recommendations is developed from a Bray P1 soil test result. Fortunately there is a relationship between Mehlich III and Bray P1 and a conversion can be made. An excellent discussion on this can be found in the factsheet Understanding Soil Test for Plant Available Phosphorus at  As a user of soil test results, you need to know whether the reported number is a Bray P1 or a Mehlich III result so you can appropriately use the number in relation to Tri-state Fertilizer Recommendations. It is not always clear on report forms. When in doubt call the laboratory.

What are the units used for the Phosphorus level result?

Another confusion point is the units of reporting. Check the report for pounds per acre or parts per million (ppm) which is also sometimes expressed as mg/kg on some reports. The Tri-state Fertilizer Recommendations charts provide recommendations for either of the units. The reader of the report just needs to know what the units are to get to the correct point on the chart. The conversion to ppm from pounds per acre is pounds per acre divided by 2.

What is the form of phosphorus reported?

For the most part soil test results are reported as the elemental P, expressed as P. This form is used for the Tri-state Fertilizer Recommendations chart and no conversion is needed. The other form that appears on an occasional report is oxidized form of phosphorus expressed P2O5. If both forms are on the report, just use the number expressed as P. If the only number is the P2O5 number, then it needs to be converted to P to use the Tri-state Recommendations. Just multiply P2O5 by 0.44 to get to the P number.

Tools to develop fertilizer recommendations.

Answering these three questions from the soil test report makes selecting the correct tools to get the fertilizer recommendation an easy task. If you are more comfortable with paper then download Developing Phosphorus and Potassium Recommendations for Field Crops at If soil test results are in Bray 1 Units then you can use the charts in the Tri-State Fertilizer Guide for the recommendation found at

If you have results in Mehlich III results, then supplemental sheets with both Bray P1 and Mehlich III have been developed to avoid the need for conversion between the two tests. If units are reported as parts per million use If units from your lab are pound per acre use this table .

If you would like to use a spreadsheet then check out Version 2013-4 of P, K and Lime Recommendation Spreadsheet released October, 2013.  If you have downloaded previous version please replace with this version that works with older versions of excel and correct errors in the previous releases. This can be found at Once again check the introduction page of the spreadsheet and make sure it has version 2013-4 and 10/2013 as the release date.

Finally if you are using precision applications the equations used for the printed tables are found in the Tri-State Fertilizer Guide on page 11.

Insect Home Invaders

Brown marmorated stink bugs are making their presence known to home owners and renters the past few weeks as the insects begin their yearly trek to overwintering sites.  We have been answering phone calls and emails, and also giving numerous interviews on these insects that are being attracted to houses.  These insects are attempting to find locations where they can spend the winter until next spring when they will return to their various host plants.  The main problem is that they are a nuisance; they are not harmful to either people or buildings.   Most recommendations are that as long as they are on the outside of the house, live with them.  Make sure all openings into the house are closed up, especially around windows.  If they do get into the house, collect them by hand and either throw them outside, or put them into plastic bag or glass jar and hold them until the die.  A quick death for the stink bugs would come if you put the bag into a freezer for a day.  Do not smash or crush them as they could leave a stain and smell (they are called stink bugs for a reason).  You could vacuum them up, but you will need to immediately empty it or they will remain alive.  If the vacuum tends to mash them up, they will probably begin to smell.  We do NOT recommend spraying insecticides, either outside or inside the house.  More bugs will just show up later, and their presence just does not justify spraying a toxin inside in the house.  As weather gets colder, you will see fewer and fewer of them except for a stray straggler. 

However, there is another possible home invader that might show up in large numbers within the next month, the multicolored Asian lady beetle.  This is normally a beneficial insect that builds up to huge numbers in soybean fields every other year, following the buildup of soybean aphids.  This past summer was a year where we saw large populations of aphids, and subsequently, large numbers of lady beetles in fields throughout Ohio.  Observations from past years suggest that these lady beetles will go to homes for overwintering, just like the brown marmorated stink bug.  We usually see this movement by the lady beetle following a long cold spell followed by a warm up.  Because we have not yet seen a good cold snap, we anticipated that the occurrence of these lady beetles will be sometime later this or next month.  Management is the same as with the stink bug: live with them as long as they stay outside, close up any openings, and if the get inside, hand pick them and throw them away.  Again, we do not recommend spraying insecticides.

Control Problem Biennial Weeds in the Fall

Poison hemlock has become a persistent problem in many roadsides, fencerows, wood edges, etc, and it occasionally becomes a problem in crop fields.  Fall is the ideal time to control poison hemlock and other biennial weeds such wild carrot, teasel, and common burdock.  Making an effort to control these weeds in areas that border crop fields or pastures can reduce potential for them to interfere with crop growth or cause problems in hay or animals.  The targets for fall herbicide treatments are the low-growing biennial plants that are at the end of their first year of growth.  These weeds are much more easily controlled when in this stage, compared with next spring/summer when plants are large with a well-established root.  There are ratings for pasture herbicides on page 154 of the current “Weed Control Guide for Ohio and Indiana”, including ratings for dicamba, 2,4-D, and glyphosate.  Two-way mixtures such as glyphosate + 2,4-D and dicamba + 2,4-D should be most effective on biennial weeds mentioned here.  While relatively low rates of these herbicides are effective on most winter annual weeds, consider increasing rates where biennials or perennials are a problem.  When applying herbicide in the fall to crop fields, one strategy for controlling weeds in fencerows, etc. would be to position the sprayer so that one of more nozzles are over the field border area.  Grass waterways within fields can be treated along with the rest of the field as long as the herbicide mixture does not contain glyphosate or another herbicide with significant activity on perennial grasses.

Forage Toxicity Issues with Frosts

We have had a beautiful fall so far, but Jack Frost will be visiting us soon. Now is the time to finish harvesting and grazing several forage species that can be extremely toxic soon after a frost. Those include primarily annual grasses in the sorghum family and other closely related species that contain compounds called cyanogenic glucosides, which are converted quickly to prussic acid (i.e. hydrogen cyanide) in freeze-damaged plant tissues.

Other species that can develop toxic levels of prussic acid after frost are Johnsongrass, shattercane, chokecherry, black cherry, indiangrass, and elderberry. It is always a good idea to check areas where wild cherry trees grow after a storm and pick up and discard any fallen limbs to prevent animals from grazing on the leaves and twigs.

The potential toxicity after frost varies by species. Sudangrass varieties are low to intermediate in cyanide poisoning potential, sudangrass hybrids are intermediate, sorghum-sudangrass hybrids and forage sorghums are intermediate to high, and grain sorghum is high to very high and is most likely to be toxic after a frost. Piper sudangrass has low prussic acid poisoning potential. Pearl millet and foxtail millet have very low levels of cyanogenic glucosides and rarely cause toxicity.

Animals can die within minutes if they consume forage with high concentrations of prussic acid. Prussic acid interferes with oxygen transfer in the blood stream of the animal, causing it to die of asphyxiation. Before death, symptoms include excess salivation, difficult breathing, staggering, convulsions, and collapse. Ruminants are more susceptible to prussic acid poisoning than horses or swine because cud chewing and rumen bacteria help release the cyanide from plant tissue.

Plants growing under high nitrogen levels or in soils deficient in phosphorus or potassium will be more likely to have high prussic acid poisoning potential. After frost damage, cyanide levels will likely be higher in fresh forage as compared with silage or hay. This is because cyanide is a gas and dissipates as the forage is wilted and dried for making silage or dry hay.

Young, rapidly growing plants of species that contain cyanogenic glucosides will have the highest levels of prussic acid. After a frost, cyanide is more concentrated in young leaves and tillers than in older leaves or stems. New growth of sorghum species following a non-killing frost is dangerously high in cyanide. Pure stands of indiangrass can have lethal levels of cyanide ifvthey are grazed when the plants are less than 8 inches tall.

Grazing Precautions

The following guidelines will help you avoid danger to your livestock this fall when feeding species with prussic acid poisoning potential:

*  Do not graze on nights when frost is likely. High levels of the toxic compounds are produced within hours after a frost, even if it was a light frost.

* Do not graze after a killing frost until plants are dry, which usually takes 5 to 7 days.

* After a non-killing frost, do not allow animals to graze for two weeks because the plants usually contain high concentrations of toxic compounds.

*  New growth may appear at the base of the plant after a non-killing frost. If this occurs, wait for a hard, killing freeze,then wait another 10 to 14 days before grazing the new growth.

*  Don’t allow hungry or stressed animals to graze young growth of species with prussic acid potential.

*  Graze or greenchop sudangrass only after it is 15 to 18 inches tall. Sorghum-sudangrass should be 24 to 30 inches tall before grazing. Never graze immature growth or short regrowth following a harvest or grazing (at any time of the year).

*  Do not graze wilted plants or plants with young tillers.

*  Green-chopping the frost-damaged plants will lower the risk compared with grazing directly, because animals be less likely to selectively graze damaged tissue. However, the forage can still be toxic, so feed greenchop with great caution after a frost.

*  Always feed greenchopped forage of species containing cyanogenic glucosides within a few hours, and don’t leave greenchopped forage in wagons or feedbunks overnight.

Hay and silage are safer

Prussic acid content in the plant decreases dramatically during the hay drying process and the forage should be safe once baled as dry hay. The forage can be mowed anytime after a frost if you are making hay. It is very rare for dry hay to contain toxic levels of prussic acid. However, if the hay was not properly cured and dried before baling, it should be tested for prussic acid content before feeding to livestock.

Forage with prussic acid potential that is stored as silage is generally safe to feed. To be extra cautious, wait 5 to 7 days after a frost before chopping for silage. If the plants appear to be drying down quickly after a killing frost, it is safe to ensile sooner after a frost.

Delay feeding silage for 8 weeks after ensiling. If the forage likely contained high levels of cyanide at the time of chopping, hazardous levels of cyanide might remain and the silage should be analyzed before feeding.

What about frosted alfalfa, clover, and grasses?

Other common forages such as alfalfa, clovers, and cool-season perennial grasses do NOT produce toxic compounds after a frost. However, the risk of bloat is higher when grazing alfalfa, clovers, or other legumes one or two days after a hard frost. The bloat risk is highest when grazing pure legume stands, and least when grazing stands having mostly grass.

The safest management is to wait a few days after a killing frost before grazing pure legume stands - wait until the forage begins to dry from the frost damage. It is also a good idea to make sure animals have some dry hay before being introduced to lush fall pastures that contain significant amounts of legumes.

Crane Flies Not Hessian Flies

Reports of numerous huge, mosquito-like insects flying in and near fields and grassy areas around fields have reached Extension Offices.  Some are concerned that these large flies might be Hessian flies (Mayetiola destructor, family Cecidomyiidae) especially when they are observed near wheat fields.  Fortunately, these flies are not Hessian flies, but rather are crane flies (Tipula spp., family Tipulidae).  In comparison to these "super mosquitoes," Hessian flies are relatively minute.  Also mosquito-like, the adult Hessian fly is tiny and fragile, long-legged, dark colored (females have an orange-red colored abdomen), and measures about 1/8 inch in length (Bugwood Image of Hessian Fly: 

It is the larval stage of the Hessian fly that is the damaging stage.  The legless maggot is red to orange in color when it hatches and turns greenish-white as it feeds and grows.  The maggots feed behind leaf sheathes on grasses and is especially damaging to wheat stems when wheat is infested.  When the maggot is finished feeding, they form a dark-brown puparium called the "flax seed" stage.  On infested winter wheat, these "flax seeds" are found inserted into the crown of young wheat plants in the fall and winter (fall generation) or just above stem joints in the spring (spring generation).

Young plants in the fall suffer the most serious injury.  Feeding injury results in stunting of plants, and secondary tillers that are infested fail to develop. Young plants that are infested are typically a darker green to bluish-green color than uninfested plants, and the leaves are thicker. When larvae feed on jointed stems, they become weakened and lodge at the feeding site.

The larvae of crane flies are called leatherjackets.  These leatherjackets are typically gray in color and have tough leathery skin.  Fully developed leatherjackets are up to 1 1/4 inches in length.  Many crane fly larvae are aquatic to semi-aquatic, however there are also multiple species that live in soggy to moist soils.  These leatherjackets feed on organic materials found in the soil.  There are a few species that feed directly on living plant material such as roots, crowns and leaves.  Those species that feed directly on living plant material can be especially destructive to seedlings.  Two species for which OSU Extension and ODA are on the watch are the European Crane Flies (Tipula paludosa and T. oleracea).  These two species are exotic invasive crane flies that are known to damage turfgrass, pasture grass and seedlings of a number of plants.

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Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.