CORN
Crop Observation and Recommendation Network

September 27 - October 11, 1999
C.O.R.N. 99-28

In This Issue:

A) Fall is the Best Time to Sample for Soybean Cyst Nematode
B) Lime Considerations
C) Yield Map Interpretation

A) Fall is the Best Time to Sample for Soybean Cyst Nematode - Anne Dorrance and Nancy Taylor CORN Questions

Fall is often the best time to collect soil samples for soybean cyst nematode. Soybean cyst nematode numbers will be highest when soybeans are almost mature to shortly after harvest. When samples are processed in the lab, the soil is washed away and the mature cysts are collected on screens. If the cysts are still attached to roots, as they would be earlier in the season, they will not be collected and numbers may be misleading.

A soil test is the best means to determine if soybean cyst nematode is present as well as ESTIMATE population densities. The population densities of soybean cyst nematodes in a field are used to recommend management guidelines. These are guidelines, if your soil is lighter, the soybean cyst nematode is going to have a bigger impact on soybeans than in fields where the soil is heavier. We have seen considerably more SCN damage this year due to dry conditions in many areas of the state.

Crop rotation, crop rotation and crop rotation are the best management procedures for SCN. Following a year of corn or wheat, the number of cysts in a field should drop by half. But if initial cyst numbers are very high- 5,000 or 10,000 cysts - a year of corn or wheat will only drop them to 2,500 or 5,000 cysts - which is still above the economic threshold. We have fields in Ohio in a corn - soybean rotation in which the numbers of cysts have continued to build over time. For these high population fields, a one-year rotation has not reduced the population below the economic threshold. Populations of cysts greater than 2,000 are too high for even cyst resistant beans. (Under these high populations, cyst resistant beans will have economic losses). We also do not want resistant beans in high population fields due to management considerations of these resistant genes.

A reminder that the Ohio Soybean Council (OSC) has announced it will pay for every soybean cyst nematode (SCN) soil sample that is sent in by Ohio soybean producers to the C. Wayne Ellett Plant and Pest Diagnostic Clinic at The Ohio State University. The SCN test will be paid for by Ohio soybean checkoff dollars.

Growers may submit the samples for free testing to: C. Wayne Ellett Plant and Pest Diagnostic Clinic, The Ohio State University, 2021 Coffey Rd, Columbus, Ohio 43210. Sample bags and reporting forms can be obtained from your local Ohio State University extension agent or by contacting the C. Wayne Ellett Plant and Pest Clinic at (614) 292-5006. Additional information about SCN is available in OSU Extension Fact Sheet AC-39-98.Or, check the web site: http://www.ag.ohio-state.edu/~ipm/scn/scn.htm.

B) Lime Considerations - Ed Lentz and Greg LaBarge  CORN Questions

Fall is an excellent time to apply lime. Proper soil pH is important for nutrient availability, herbicide activity, and crop development. For most soils, additional lime is not needed every year. Consider these points before liming your fields:

1. Do I need lime? Begin with a soil test. Soil pH is a measure of the Hydrogen ion content of the soil. Buffer pH or Lime Test Index should be used to determine lime requirements. At a buffer pH of 6.8 (68- LTI) or above no lime is required. The further below 6.8 (68- LTI) the greater the lime requirement to raise soil pH to target levels.
2. Does subsoil (soil at depths greater than 8 inches) pH affect lime requirements for crops? The sensitivity of a crop to 0-8 deep soil pH does depend upon the subsoil pH. In eastern Ohio where the subsoil pH is less than 6.0 for mineral soils, additional lime is recommend after the 0-8 inch soil pH drops to 6.2 for corn and soybean, and 6.5 for alfalfa. In western Ohio where the subsoil pH for mineral soils is greater than 6.0 and lime is not needed until the soil pH drops to 5.7 for corn and soybean and 6.2 for alfalfa. Private laboratories may not take in account the subsoil pH and use recommendations based on a subsoil pH less than 6.0 for all parts of the state, unnecessarily recommending lime applications for some areas.
3. What is the neutralizing ability of the lime? The type of lime used should be based on its ability to neutralize soil acidity and cost, not on its percent calcium. Most Ohio soils tend to be high in calcium and should not respond to additional calcium as long as exchangeable soil calcium levels are above 200 ppm. Adding lime to balance soil cations exchange capacities, especially calcium to magnesium is not necessary for maximum crop yields. Research at Ohio State University (Don Eckert and others) has shown crops will yield the same over a wide range of calcium to magnesium ratios. As long as the calcium to magnesium ratio is not less than one, additional calcium will not improve yields for corn, soybean and wheat. Thus, select a lime source based on cost per nuetralizing unit and not its calcium content. Dolomitic lime often has a higher neutralizing ability than calcitic lime and in many parts of the state is the lowest cost material.
4. What is the Effective Neutralizing Power of my lime? Ohio revised code requires materials sold as lime for agricultural purposes to be tested for percent Effective Neutralizing Power (%ENP). Effective Neutralizing Power considers the percent moisture and neutralizing power of a lime source. This information will allow a producer to determine the price per pound of a lime source that can be used to determine which lime source is the most cost effective way to neutralize soil acidity.
5. I have been offered some low cost lime product, should I use it? Principles of lime use are the same even when the product is low cost. If pH needs adjustment these sources of lime can provide a good alternative if the product can be spread with available equipment, and the analysis is consistent. In some cases we do not want to pass up a good deal, but if pH is at proper levels additional lime can cause problems. A grower can inadvertently raise their soil pH to 7.5 or above. At this elevated pH, certain nutrients may become limiting, reducing crop productivity to the point where special management practices to lower pH become necessary. It is expensive to add sulfur fertilizers to lower the soil pH and with proper practices, unnecessary. A soil analysis is the best step to determine if a field needs lime.

In summary, make sure you take a soil test, determine if lime is needed, know the historic pH of your subsoil, and then use the ENP to select the most cost effective lime material. A soil test every three to four years will determine the lime requirements for your fields. For additional information, refer to the Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat & Alfalfa http://ohioline.ag.ohio-state.edu/e2567/index.html and the Ohio Agronomy Guide http://ohioline.ag.ohio-state.edu/b472/fertile.html.

C) Yield Map Interpretation - Larry Lotz and Greg LaBarge  CORN Questions

The value of a yield map is in its interpretation. A yield map exhibiting yield variability will cause you to ask more questions than it will answer and can become a source of frustration rather than a source of information. A yield map only documents the spatial distribution of crop yield, not what caused the variation. A yield map does not indicate why yields vary, whether yield potential is reached anywhere in the field, or predict yield patterns in future years. A yield map is of value only when it leads to a management decision or validates management practices.

A good history of past field practices and conditions through the current growing season are essential to interpret yield maps. Farmers need to consider all possible reasons for yield response so the true reason can be identified. In some cases, observations over time maybe needed to isolate yield limiting factors. Below are common pattern combinations found in yield maps and potential cause for those patterns.

-Straight Line Patterns in Direction of Application change in planting date, change in hybrid/variety, change in chemical application, selected rescue treatment, chemical skips and misapplications, equipment errors, poor straw/chaff distribution, compaction

-Straight Line Patterns Against Direction of Application tile patterns, historically different fields, old traffic patterns, manure applications, pipelines/phone lines, underground, previous compaction, old laneways/fence lines

-Irregular Patterns in irregular lines topography changes, herbicide drift, border shading effects, insect infestation from bordering lands, improper manure application, waterways

-Irregular Patterns in Areas or Patches change in soil type, drainage patterns, weed infestations, soil fertility changes, previous crop activity, disease infestations, herbicide carryover, historic occurrences, insect infestations, changes in o.m., animal damage, wet areas

Yield maps do have limitations. While yield maps show variability in a field, meaningful comparisons to base decisions on can be hard to come by. Furthermore, variability in yield can be the result of several characteristics rather than one. Do not jump to conclusions! Some short-term decisions can be made, but longer-term decisions are tougher. The worst case scenario is that some fields may have several years of yield maps with no consistency from year to year. Also, yield maps of the same field from different mapping software companies can look very different. Changing yield ranges (increase or decrease ranges of yield) assigned to a specific map color can also change the look and therefore the subsequent interpretation of a map dramatically.

Yield maps can be a powerful decision making tool. Continual observation and adequate information are necessary to make sound management changes from this tool. Fact Sheet, AEX-550-97, Yield Monitors and Maps: Making Decisions is available for more information and can be found at http://ohioline.ag.ohio-state.edu/aex-fact/0550.html

Readers can subscribe electronically to this newsletter by sending an e-mail message to: corn-out-on@postoffice.ag.ohio-state.edu. A successful subscription message will receive by an automatic reply from the listserv. Contact your local Ohio State University Extension Office or e-mail labarge.1@osu.edu if you have problems subscribing.

Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/

C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.

Contributors to C.O.R.N. this week include: State Specialists: Anne Dorrance (Plant Pathology), Hal Willson (Entomology); District Specialist: Ed Lentz (Agronomy); Extension Agents: Roger Bender (Shelby), Glen Arnold (Putnam), Gary Wilson (Hancock), Larry Lotz (Fayette), Andy Kleinschmidt (Van Wert) Clark Hutson (Seneca) and Greg La Barge (Fulton).

Editor: Greg LaBarge        Web Editor: Nathan Watermeier

Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.

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