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Click here to download a printable version of Virginia Soybean Update Newsletter - PDF format
I focusing this issue on things that we can do to insure that next year's crop doesn't suffer from things in our control. Fall is an excellent time to sample soils and diagnose other problems. Also, please consider entering this year's soybean yield contest. I imagine that you can win with a relatively low yield.
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Soil testing is an important agronomic tool for determining crop nutrient needs. Soil testing evaluates the fertility of the soil to determine the basic amounts of fertilizer and lime to apply. Fall is a good time for sampling. Regardless of the time of year that sampling takes place; one should try to sample at the same time each year. This allows the tracking of fertility levels over time. The dry summer and resulting low yields may mean that fewer nutrients were used, but one cannot tell unless a soil analysis is performed. pH levels may be lower than expected due to the dry summer, but rain this fall can bring them back up. Previous soil tests should probably be consulted to insure that the field is not over limed. Below are some guidelines to sampling soils.
Sampling Instructions - Collecting the sample is one of the most important steps in the soil-testing program. When one considers that a 2-lb soil sample must adequately represent 10 million or more lbs of soil in the area being sampled, the importance of doing a good job of sampling becomes apparent. Here are instructions for collecting a good representative soil sample:
Get Soil Sample Information Sheets and Soil Boxes. These may be obtained from your local Extension office, from the Virginia Tech Soil Testing Lab, or the commercial lab of your choice. Follow the directions they provide
Divide Farm into Areas or Fields. If the field is uniform, one sample will do. But most fields will have been treated differently, or the slope, drainage, or soil type will make it desirable to divide the field into small areas of 5 to 10 acres each
Obtain a Good Sample of Soil. The soil test can be no better than the sample. Take the sample from 20 or more places in the field. Zigzag across the field or area as shown in the diagram. When taking the sample, avoid unusual places such as old fence rows, old roadbeds, eroded spots, where lime or manure have been piled, or in the fertilizer band of row crops
Use Proper Sampling Tools. Sampling may be made with a soil auger, soil tube, or spade. The desired depth for cropland is plow depth (6 to 8" or more), and 2-4" for pasture land or no-till crop fields. Place the sample in clean, non-metal container.
Mix Well in Clean Plastic Pail. From the 20 or more stops you have made, you may now have one gallon or more of soil. Mix it thoroughly, and then send about 1/2 pint of the mixed soil to the lab for analysis
Fill out a Soil Sample Information Sheet (Cooperative Extension Information Sheet for Commercial Crop Production, #452-124 or available from a commercial lab) for each Sample. It is essential that your name, address, and sample number be plainly written on the sheet you send with each sample. As a guide in making recommendations for each of your numbered areas, it is important that the history of fertilization, liming and any unusual treatments be stated.
Mail to Soil Testing Laboratory. Place the Soil Sample Information Sheet inside the top flap of the soil box and mail to the Soil Testing Laboratory, Department of Crop & Soil Environmental Sciences, 145 Smyth Hall (0465), Virginia Tech, Blacksburg, VA 24061. (540) 231-6893 or mail to a commercial lab of your choice.
CHARGES per sample at the Virginia Tech Soil Testing Lab (No charge for in-state commercial farm samples) are listed below:
Soil sampling for precision farming applications.
Precision farming, also know as site-specific management, typically employs intensive soil sampling to map fertility in a field. Fertilizer and lime may then be applied at variable rates, according to the needs of the particular areas in the field. To begin, fields are first divided into grids, with typical grid size being 2.5 acres. For small fields, a one-acre grid size will give a more precise representation of the fertility across the field, but for larger fields, this is usually not economically feasible. The grid is established using the Global Positioning Satellite (GPS) system. A GPS receiver connected to a handheld or portable computer is used to receive satellite coordinate signals to map the field and locate the grid points. Soil samples are then collected at each grid point, with 6-8 subsamples being taken in a 10-foot circle around this point, and mixed together for a composite sample. After lab analysis, soil test information from the field can then be fed into a computer mounted in a variable-rate fertilizer/lime spreader with GPS receiver, and material can then be applied according to the needs of the various areas in the field.
"Smart sampling" techniques may also be employed to sample a field. This may be done in lieu of grid sampling or as a follow up to an initial grid sampling of the entire field. "Smart sampling" consists of sampling those areas of the field that are obviously different, such as with respect to topography (hilltops, low areas), or if a combine yield monitor has been used, high-yielding and low-yielding areas in the field. If yield data are not available, one often times has a good idea of the high-yielding and low-yielding areas in his field. These should be sampled separately. If the field has been intensively mapped (Order 1 soil survey), it can be divided by soil series (i.e., soil mapping unit). Grid points should be located or grouped in those areas with unique visible or measurable differences.
Using the Soil Test Report - The Soil Test Report will contain the laboratory test results plus fertilizer and lime recommendations. Additional information regarding time and method of fertilizer and lime applications will also be provided in the form of a Soil Test Note or Notes which will accompany the report. When several samples have been collected from the same field, the soil test reports should be compared to determine the best rates of fertilizer and lime to use for the field. Large differences in the reports may call for fertilizer and/or lime at two or more different rates. Advice on how to best fertilize a given field can be obtained from you local Extension agent, Certified Crop Advisor, or fertilizer dealer
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If nematodes are a problem in your fields, now is a very good time to diagnose these problems. Symptoms of nematode injury include stunting, nutrient deficiencies, root lesions, root galls caused by root-knot nematodes, and a general reduction in the size of the roots. Small lemon-shaped cysts from Soybean Cyst nematode can be found on the roots. However, one should not assume that nematodes are causing the above symptoms. Accurate diagnosis requires a thorough examination of root and soil samples.
Nematode Diagnostic Services. Virginia Tech's Plant Diagnostic Clinic provides root and soil analyses without charge for identification of crop damage by nematodes. Growers should work with their County Extension Agent in collecting plant and soil samples. Each sample must be submitted to the clinic with a diagnostic form in which the agent and grower describe the problem and list important aspects of crop management and field history.
Plant and soil samples for diagnosis of a problem should be collected from the affected area in the field. Always select plants that exhibit symptoms of the problem, and are still living. Samples of dead plants have little value for detection of nematodes, since problem species only feed and reproduce on living plants. Soil samples should be collected to a depth of eight inches from the root zone in the planted rows where symptoms of disease are expressed. Both plant and soil samples must be protected from direct sunlight and heat, because temperatures near 95 F or higher can be lethal to nematodes. It is best to ship plant samples in loosely folded plastic bags to retard drying. Soil samples should be securely closed to retain moisture.
Predictive Nematode Assay. This program requires soil samples only and is used to evaluate the risk of nematode damage in the fall of the year before planting. The best time to collect samples is following harvest of the previous crop, but no later than November 20. Local extension agents have information on proper sampling procedures, sample bags and forms for this program. A fee of $11 for routine analysis and $19 for samples requiring cyst counts is charged for each sample. Wherever soybean cyst nematodes are suspected, the routine and cyst assay is recommended.
Soil samples should be collected from areas measuring up to 4 acres in size. Collect 20 to 25 soil cores with a soil sampling tube to a depth of 8 inches in a zigzag pattern. Each sample area should have a similar soil type and the same crop history. Large fields should be sub-divided into 4-acre sections and sampled. If only portions of a field are sampled, preference should be given to sampling the lighter, sandy-textured areas where nematodes are most likely to cause crop damage.
Nematode Thresholds for Soybean. Based on results of on-farm tests over several locations and years, nematode population thresholds are available for soybean, cotton, corn and peanut in Virginia. The following table lists the crop to be planted and three levels of risk for yield loss according to population densities of nematodes in a 500 cc sample of soil. The risk thresholds are based on numbers of nematodes present during late summer or early fall (August 1 to November 20). A similar listing of thresholds for other field crops is available on the web at http://ipm-www.ento.vt.edu/nipmn/VA-IPM/updates/nematode/frames.html
Risk levels are assigned according to the numbers and kinds present in a given soil sample (see Table of Nematode Thresholds). When nematodes do not pose any risk of crop damage, the numbers are within the "A" level (little or no risk). The "B" level (moderate risk) denotes a borderline situation in which nematode control measures may be profitable depending on the growing season and the existence of other stress factors. When more than one nematode is present at the "B" level, the likelihood of a profitable response to nematode control increases. In cases where one or more nematode counts are at the C level (high risk), nematode control practices are almost always profitable
| Nematode Thresholds for Soybeans | |||
|---|---|---|---|
| Nematode | Risk code based on nematodes/500 cc soil (August 1 to November 20)* | ||
| A (Little or no risk) | B (Moderate risk) | C (High risk) | |
| Root knot | <50 | 50-170 | >170 |
| Cyst juveniles | <20 | 20-60 | >60 |
| Dagger | <100 | 100-300 | >300 |
| Sting | 0 | 10-20 | >20 |
| Lesion | <100 | 100-300 | >300 |
| Ring | <200 | 200-700 | >700 |
| Stunt | <300 | 300-1000 | >1000 |
| Spiral | <1000 | >1000 | - |
| Stubby root | <90 | >90 | - |
| Lance | <300 | 300-500 | >500 |
| * Based on counts of soil populations sampled between August 1 and November 20 of the year before planting the crop. A = nematodes are not likely to cause crop damage, B = borderline populations in which crop damage may occur if other factors stress the crop, and C = populations that are likely to cause crop damage and a significant loss of yield. | |||
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One can check for the presence and depth of soil compaction by probing the soil and looking at crop roots. Roots are usually the telltale sign of compaction. Don't pull up the roots, but use a spade to dig up roots near the existing row. Or, dig straight down immediately beside of the row and examine the root growth in the soil. Roots that are turned upward, flattened, or stubby are clear indications of compactions. However, stubby roots are also an indicator of nematode or herbicide injury. One can usually tell the depth of the compaction by examination of the roots, regardless of whether they were dug or not. Other methods for determining compaction includes probing with a soil sampling tube, a spade, a wire flag, or specialized tools such as a penetrometer that measures the soils resistance in pounds per square inch. Regardless of the instrument used, it is recommended that such probing be done under wet or saturated conditions. In addition, one should first probe in a relatively un-compacted area of the field such as a fencerow or the edge of a wooded area. Then, they should check the most compacted area of the field such as field entrances or turn rows. By doing this, one will have references to judge the field by. Care must be taken to not confuse a clay subsoil layer with compaction.
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Two unusual problems are occurring in soybeans. First, there have been at least 3 fields in the last three days where large numbers of corn earworms escaped (survived) treatments of either Karate, Fury, or Asana at rates that should have killed most. We have worm samples and will determine if any are tobacco budworms (typically more resistant to pyrethroids), but preliminary indications are that most are corn earworms. Field tests earlier in the season did not reveal this level of resistance. Is this late season 'gene pool' more resistant? It appears so. We may be in for more problems in future years and may have to begin recommending non-pyrethroids like Larvin, Tracer and Steward. North Carolina has already taken that step - not recommending pyrethroids for use in soybeans. They did it because of evidence of a gradual loss or activity over the years, and to protect/prolong efficacy for their cotton crop.
The second unusual problem is soybean looper. They have migrated from the south and are in very high numbers in many North Carolina soybean fields. They have been reported in several fields in Virginia,at least 3 with high numbers. In high numbers, soybean loopers can totally strip a field. Despite what is listed in the PMG, only Steward and Tracer offer any solution. I am hearing of a tentative threshold in North Carolina of 22 loopers per 15 sweep net sweeps and 15% or more defoliation. This would apply to a full canopy, reasonable yield potential fields. Fields with moderate to poor canopy would have to be protected if 5-10% defoliation has occurred and worm numbers are high. There will be a lot of hard calls regarding control of this late season pest. If fields are near senescence (leaves beginning to yellow) or senescence is expected within the next 2 weeks, treatment would probably not be economic. But the many fields that are still in the pod fill stage are susceptible to yield loss if defoliation exceeds those percentages. It will be tough to make the call to spend more money to protect fields, as some may have already been treated once, even twice, but large numbers of loopers can ruin a crop. Steward is labeled at 5.6 ‚11.3 oz per acre for soybean looper. The choice of rate should be dependent on the number of worms and their sizes. Large numbers with a fair proportion of medium and large sized worms will require 9 to 10 oz. Six to 7 oz should be sufficient for fields with fewer worms, or if they are predominantly small sized. Steward will also clean up any corn earworm that may be lingering.
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To estimate yields, follow the steps below. Be sure to sample in 5 to 7 different areas of the field.
| Row Width | Length of a single row to equal 1/1000th of an acre |
|---|---|
| 6 | 87' 1" |
| 7.5 | 74' 8" |
| 10 | 52' 3" |
| 15 | 34' 10" |
| 20 | 26' 2" |
| 30 | 17' 5" |
| 36 | 14' 6" |
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Sincerely,
David L. Holshouser
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