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The New Agriculture Network's on-line newsletter with seasonal advice for field crop and vegetable growers interested in organic agriculture. Vol. 1, No. 7 - July 15, 2004 In this issue |
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Evaluating nitrogen availability in organically managed fields Sieglinde Snapp |
Organic amendments build up the capacity of soil to supply nutrients in slow release fashion. It is important to take into account that generally this pool of nutrients can be counted on to provide nutrients without supplemental applications, particularly if moisture is sufficient (e.g., during a high rainfall summer such as the one we are experiencing). There are exceptions. In these cases, indicators of low nutrient availability can be observed if monitoring is conducted. Leaf tissue nutrient analysis or soil pre-sidedress N tests should be conducted where feasible. Note that a pale-green leaf color may be due to flooding stress from excess water, rather than from the more common association of pale green with insufficient nitrogen. Measuring plants and soils is the best way to know if nitrogen is in short supply, however more research is needed to calibrate soil tests (and plant tests) for organic farms. Initial findings from research in California and Michigan indicate that organic management practices are often associated with an apparent low inorganic nitrogen status from typical soil tests such as PSNT (Pre-sidedress nitrate test), but that this does not necessarily mean that nitrogen is deficit. Where organically managed soils are tested for nitrogen mineralization potential, this “biological-nitrogen” pool is often much higher than that measured in conventionally managed soils. That is, organically managed soils tend to have a high “active” or available nitrogen pool, from past organic amendments and soil organic matter. Thus a test of nitrogen mineralization potential (NMP) may be more appropriate to organic farms than the “snap shot” of inorganic nitrogen status provided by PSNT tests. Further research is needed to develop tests that are inexpensive and widely available to measure soil nitrogen mineralization potential, as NMP tests are often confined to research laboratories—although diagnostic laboratories are currently testing amino-nitrogen and other ways to estimate the NMP pool. In the meantime, a PSNT can be measured and is recommended to provide information on soil inorganic nitrogen status. The PSNT in Michigan is based on the nitrate content of the top two feet of soil. It is recommended that soil cores be taken to a depth of 12 inches. The nitrate content of the 13 to 24 inch depth is estimated based on the content of the top 12 inches, thus a soil sample should be taken that is representative of this depth. See the article published in MSU’s Field Crop Advisory Team Alert newsletter on sampling for a PSNT test at http://www.ipm.msu.edu/CAT01_field/nitrogen3-01.htm for further information and the regional fact sheet description of the PSNT test for corn nitrogen at Missouri’s extension site: http://muextension.missouri.edu/explorepdf/miscpubs/sb1001.pdf. To submit a sample it is important to use the sampling procedure recommended by the lab that you use for the analysis. There are a number of private and public laboratories that perform this test, including the MSU Soil and Plant Nutrient Laboratory (website: http://www.css.msu.edu/SoilTesting/) If an organic farmer uses a PSNT, it is good to keep in mind that the MSU recommendation is to multiply by 6 to obtain the amount of available nitrogen in the soil for crop uptake, thus a 5 ppm NO3—N test result from a PSNT indicates that the available nitrogen in that soil is ~5*6 = 30 lb N per acre. A 25 ppm NO3—N test result would indicate that sufficient nitrogen (150 lb N per acre) is available to support a medium yield goal corn crop. See: http://www.ipm.msu.edu/CAT03_fld/FC02-06-04.htm#4 for a description of this calculation. However, in an organically managed field, there may be enhanced nitrogen mineralization potential and a higher multiplication factor such as 8 (rather than 6) may be appropriate, thus a 20 ppm NO3—N result of a PSNT test may be sufficient to provide sufficient corn nutrition (8*20 = 160 lb N per acre). This is an area that needs more research, so this is a suggestion rather than a recommendation, and farmers need to evaluate for themselves the appropriate calibration of a PSNT result. Organic farmers should keep in mind that a reserve of organic nitrogen that can be easily mineralized to inorganic nitrogen forms (nitrate and ammonium, the nitrogen forms that plants take up) is often very high on organically managed farms, and particularly in a wet year, may be relied upon to provide nitrogen late in the growing season. The bottom line is that we encourage farmers to monitor soil nitrogen status and make observations on the subsequent crop nutrition, to evaluate what a soil nitrogen test value means on a field-by-field basis, and for specific weather conditions. |
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| Are soybean aphids regulated by weather, natural enemies or both?
Doug Landis, Alejandro Costamagna, and Chris DiFonzo, |
Since its initial detection in the North Central U.S. in 2000, the soybean aphid has become a key pest of concern in soybean. In 2003, over 42 million acres of soybean in the North Central U.S. were infested and more than seven million acres were treated with insecticides to control the soybean aphid. While 2004 seems to be shaping up as a relatively low aphid year so far, we are just now entering the time when outbreaks typically occur. Many factors contribute to determining if soybean aphids reach outbreak levels or not. Among these are weather and the impact of natural enemies. In this article we focus on the interaction of weather and natural enemies in suppressing soybean aphid. There has been a longstanding debate among entomologists as to what factors have the greatest influence on pest populations. One school of thought is that weather is the main factor and all other influences play a minor role. Another school of thought contends that competition and the impact of natural enemies are the dominant factors regulating pest populations. As in most such debates the truth is probably somewhere in between. In the case of soybean aphid, a picture is beginning to emerge that suggests that both weather and natural enemies interact to regulate aphid outbreaks. Cool weather conditions favor soybean aphid outbreaks in several ways. Recent studies in Minnesota show that the soybean aphid reaches its maximum reproductive potential when temperatures are around 81 degrees and that even short exposures to temperatures over 90 degrees greatly decrease aphid reproduction. Coupled with this is the fact that many natural enemies are less active at cooler temperatures. For example, in 2002 many days exceeding 90 degrees and almost no aphid outbreaks were reported throughout the region. In contrast, 2003 was more moderate and aphid outbreaks were extensive. Observations of this sort tend to support the idea that weather is the driving factor. On the other hand, studies in Michigan and Indiana have documented 22 species of predators in soybean fields that readily feed on soybean aphid. Of these, lady beetles, minute pirate bugs, hoverflies, lacewings, carabid beetles and spiders are common in most fields. Parasitic wasps that are either native or previously introduced are just beginning to show up in some surveys but at present contribute little to soybean aphid biocontrol. Finally, certain predaceous aphid flies and midges lay eggs in the colonies and their larvae help reduce aphid numbers. Thresholds developed for soybean aphid generally suggest that action should be taken once aphids reach 250 per plant. All of these studies have been conducted in open fields with natural enemies present. Another way to think about this is that once aphids reach about 250 per plant, natural enemies are unlikely to stop them before they reach outbreak proportions. Thus, it is important that natural enemies act early to suppress soybean aphid populations rather than just respond to outbreaks. While it’s pretty hard to experimentally change the weather, we can alter natural enemy numbers by use of exclusion cages. In this way we can ask the question, given the weather conditions in a particular year, are natural enemies an important factor in suppressing soybean aphid? Our experiment consists of establishing 1-meter square plots in soybean fields and infesting them with 10-11 aphids per plant representing a typical early season infestation. We then cage one-half of the plots and leave the other half un-caged (fake cages that allow natural enemies to enter are always included to control for possible cage effects but for simplicity are not discussed here). Pairs of caged and un-caged plots are repeated 6-10 times throughout the field and aphid populations are counted weekly. If weather were the sole driving factor, we would expect to see nearly equal numbers of aphids in caged and un-caged plots over time. In 2002, aphid populations in open plots (i.e. exposed to natural enemies) never exceeded eight aphids per plant (See figure). However, when natural enemies were excluded, populations reached 160 aphids per plants by mid-July. Over the next two weeks these numbers reached over 2,000 aphids per plant in exclusion plots, while open plots never exceed 20 aphids per plant. Thus, natural enemies were critical to keeping populations below thresholds in 2002. In 2003, just one week without natural enemies resulted in rising aphid populations (7/21) and by two weeks (7/28) aphid populations in exclusion plots were out of control (See figure). However, also notice that in the open plots populations had reached about 200 aphids per plant on 7/28 (i.e. approaching the 250 aphid/plant threshold). We all remember that 2003 was a very bad year for aphid infestations. Thus, while natural enemies suppressed soybean aphid populations and delayed outbreaks, nonetheless, outbreaks did occur in many fields. In 2004, our data again indicate that natural enemies are very effective in suppressing aphid populations (See figure). Last week’s counts (7/6) showed over 300 aphids per plant in exclusion plots and less than 10 per plant in open plots. It remains to be seen if natural enemies will keep overall populations below thresholds as in 2002 or if conditions will once again favor aphid outbreaks as in 2003. In production fields, aphids numbers across Michigan and in surrounding states continue to be very low. The percentage of infested plants in the fields monitored by the MSU Field Crops Entomology lab is less than 5%, and often less than 1%. In other words, the vast majority of plants are still aphid-free, while only a few plants have one or several aphids. In comparison, aphid populations were already building in early July 2003, and by late July, fields were being treated. Even in 2002, the last "low" aphid year, most plants had aphids by the end of July, although in low numbers. Thus, each year from 2000 to 2004 has been a bit different! In conclusion, it’s both! Weather conditions appear to set the basic trajectory for soybean aphid population growth. Temperatures around 81 degrees favor maximum aphid reproduction, while temperatures over 90 degrees slow population growth. In 2003, aphid populations were growing slowly due to unfavorably hot conditions and natural enemies had no trouble suppressing their populations throughout the season. In contrast, in 2003 more moderate conditions favored the soybean aphid and while natural enemies suppressed and delayed aphid outbreaks for several weeks, they were unable to prevent the eventual build-up of heavy late season populations. Looking at all three years together it is clear that in the absence of natural enemies, soybean aphids reach outbreak levels regardless of the weather! Our current research is aimed at determining if we can enhance the suppressive effect or our existing predator-dominated communities by augmenting them with parasitoids. While growers cannot change the weather they can do things to encourage effective natural enemy populations. Most of the natural enemies mentioned above require food (pollen and nectar), alternate prey (aphids or other small insects) and shelter from adverse conditions. Managing habitats on your farm to promote natural enemies may be one way to help ensure that these helpers are always present. For more information on habitat management for natural enemies see the following resources Farmscaping to Enhance Biological Control. http://attra.ncat.org/attra-pub/farmscape.html 'Naturalize' Your Farming System: A Whole-Farm Approach to Managing Pests. http://www.sare.org/publications/farmpest/index.htm |
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Oilseed radish variety trial Dale R. Mutch |
Introduction Description
Cultivars, seed source and cost
In 2002, we received a grant from Project GREEEN (www.green.msu.edu) to evaluate four oilseed radish varieties for seeding rate, leaf and root biomass. Methods
Results References Fahey, J. W., A. T. Zalcmann and P. Talalay. 2001. The chemical diversity and distribution of glucosinolates and isothiocynates among plants. Phytochemistry 56:5-51. Tsao, R., C. J. Peterson and J. R. Coats. 2002. Glucosinolate breakdown products as insect fumigants and their effect on carbon dioxide emission of insects. BMC Ecology 2002, 2. URL: http://www.biomedcentral.com/content/pdf/1472-6785-2-5.pdf. Retrieved January 2004. |
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Organic corn variety/hybrid comparisons Dale R. Mutch |
Introduction Methods Experiment Results Higher plant populations seemed to help the hybrids, however, had little effect on the open-pollinated varieties. The high LSD number in this experiment suggested there must have been variability in the experiment because we would normally expect a significant difference between 99.6 bu/A corn and 61.1 bu/A corn. These data do demonstrate the organic corn seed NC + Organics can yield comparably to conventional hybrid seed (Great Lakes hybrid).
Means within columns followed by the same letter are not significantly different at the 0.05 level by the Least Significant Difference (LSD) test. |
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| Reports from organic growers | Indiana Dave also had the following questions: What experience have people had with different sources of gypsum as a soil conditioner? One conference call participant had tried mined gypsum and observed favorable results. He warned against using industrial by-products or crushed wallboard as a source of gypsum because of the potential for unknown content that might not meet organic standards. What experience do people have with fall seeding of berseem clover: how much germination, how much fall growth, degree of winter kill, how much N is fixed? Some conference call participants reported observing it overwinter, but unreliably. What sorts of disease problems are people seeing with spelt? One conference call participant reported possible rust. Dan Flotow in Northeast Indiana at Country Garden and Farm Market provided the following update: As we settle into mid-summer, three chores consume our days. The number one chore at this time is weed control. For us that means cultivation and mulching. Cultivation is either done by mechanical (Tillers) or by hand (hoeing or pulling by hand). We are trying less and less cultivation but there will always be some. We spend half of our time mulching. We use whatever materials we can get—grass clippings we vacuum here on the farm, leaves we acquired last fall or wood chips that tree trimmers bring us. We have been busy mulching our annual cut flowers and veggies – trying to stay ahead of the weeds. We will have this task completed in the next few days. The second chore during this period is planting and transplanting. Yes, we are still doing that. We plant lettuce every week to ten days, along with other succession crops (zinnias, sunflowers, and radishes). We are starting seeds of our fall crops such as cabbage, broccoli, cauliflower, beets, and turnips. In the next few days, we will do another planting of cucumbers, zucchini, and summer squash. The last major chore during the last couple of weeks has been harvesting. The vast majority of what we have been harvesting has been going to our CSA members. Even though we are planting a lot more than last year, we have very little in the store due to the fact that our CSA memberships doubled from last year. Another factor is the wet weather we had delayed planting in May and early June. But our crops are in and looking very good at this time. We have been doing some side dressing. We will be doing foliar feeding with compost tea and fish emulsion—this will be done weekly along with insect control (cucumber beetles, squash bugs, Japanese beetles, and cabbage moths). Illinois Central Illinois: There are always weather conditions that affect our cropping system. We had a super cell move through last nigh t-- received 0.7 inches of rain and had a lot of wind. Lately, it has been the frequency of the rain, not the amount that has made cultivation and primary tillage difficult. Current farming practices include trying to harvest onions and garlic. Frequent rains are threatening to rot the crop. Finished harvesting spring crops last week and have just begun harvesting cherry tomatoes, squash, peppers, and potatoes. Will miss a week or two of lettuce harvest because of weather and insect pressure form leafhoppers. Cultivating everything we can, whenever we can. We have a “bumper crop” of purslane. Once again, we have failed to keep up with mother-nature. In the next two weeks, we will be planting fall crops, carrots, broccoli etc. and will continue harvest Southern Illinois: Excess rain has continued to make cultivating corn and soybeans difficult. Corn and beans are not growing as fast as normal due to cooler than normal temperatures, but looks good overall. Alfalfa is growing back nicely after second cutting. All of the corn has been cultivated for the last time and all of the beans have been cultivated once, ahead of current rainy spell. Weed pressure is moderate in the corn and insignificant in the beans, so far. In the next two weeks, I will cultivate soybeans for the second time. Questions: I am looking for ideas for a cover crop to accompany oats to be planted into wheat ground. I am looking for a cover that is high in nitrogen that doesn’t cause volunteer problems. Considering chickling vetch (A.C. Greenfix), but not sure how costly the seed is and I am not aware of any potential volunteer problems with this crop. Has anyone controlled leafhoppers with soap? If so, what is your secret? Michigan John Simmons: They’ve been getting a little less rain lately. It’s still coming frequently, but usually only a 0.1 to 0.2 inches at a time. Soybeans and corn have been cultivated. With the wet weather they’re getting immediate regrowth of new weeds after cultivation. Beans were planted in late June and are at first and second trifoliate stage. Corn is at 10 to 12 leaf stage and is starting its accelerated growth phase. Spelt is turning and is probalby two weeks away from harvest. The clover cover crops in the small grains are doing well. Oats are doing well. They’ve taken the cool weather better than other small grains. The barley is pretty thin and the hard red spring wheat is a little thin. They planted millet and amaranth with underseeded clover a few weeks ago. They’re planning to plant buckwheat as soon as it gets dry enough. There’s quite a lot of scab in the wheats, so vomitoxin is a concern. They’re dealing with it in two ways. The scab that infested early has resulted in small kernels. The main defense there is to turn up the combine fans and blow it out. Larger kernels from later infestation will be harvested, and the grain will be artificially dried to prevent it from spreading after harvest. Matt Wiley: Earlier in the season the rain held up cultivation, but now he’s going steady with it. Soybeans are in their 5th trifoliate and he hopes to finish cultivatingwithin a couple of days. He’s been using a Bezzerrides cultivator with spinners and he’s really appreciating it now because it’s done a good job at weeding within the row, so the hand weeding is going much faster this year than it has in the past. One curious observation is that his last planted beans were planted on June 5th, he wasn’t able to touch them for 18 days, then he cultivated them with the Bezzerides spinners, and now they’re the cleanest beans on his farm. He’d like to know why, but nobody had a good answer. His spelt is a couple of weeks away from harvest. It looks like it may have some rust in it. |
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