|Yard & Garden Line News
Volume 4 Number 13 August 15, 2002
Soil Sampling, Soil Testing, and Soil Test Kits
Peter Bierman, Dept. of Soil, Water, and Climate
An adequate supply of plant nutrients is necessary to maintain optimum growth, but applying too much fertilizer can retard plant growth. Excessive fertilizer applications are also wasteful, and detrimental to the surrounding environment when nutrients like nitrogen and phosphorus move into groundwater, lakes, and streams. Soil tests are necessary to make accurate fertilizer recommendations for lawns, gardens, trees and shrubs.
Collecting soil sample.|
How many soil samples to collect from different areas of a home landscape, and the reliability of do-it-yourself soil test kits, are two commonly asked questions about soil testing. These questions are discussed below, along with a review of the procedures to follow when collecting soil samples.
How many samples?
Accurate interpretation of soil tests depends upon collecting representative soil samples. This frequently requires that you collect separate samples from different areas around your home. Soils are not uniform and several distinct soil types can occur even in an area as small as a city lot. Differences in soil color and texture (amount of sand, silt, and clay) can be used as guidelines to identify different soil types. Areas that differ in drainage also may need to be sampled separately, because of differences in nutrient leaching.
Soil is especially variable around homes, because of activities like excavation, filling, and grading during construction. Differences in the amounts of plant nutrients in the soil, as well as soil pH (which has strong effects on nutrient availability), can also result from differences in the way a soil is managed. Even when the soil type is the same, the kinds of plants previously grown, and previous applications of fertilizer, lime, and organic amendments like compost and manure will cause different areas of that soil to have different fertility requirements.
The need for multiple soil samples is also determined by what will be planted in an area or the type of plant currently growing there. This is due to differences in sampling depth for different plants. Standard sampling depths are: established turfgrass - 3 inches; flower and vegetable gardens or areas where a new lawn will be seeded or sodded - 6 inches; trees and shrubs - 12 inches.
Soil samples are easiest to collect with a soil probe, but equally good samples can be collected with available garden tools like a hand trowel or spade. Scrape off all surface vegetation and plant litter and take the sample to the required depth (see standard sampling depths for different situations in the previous paragraph). Because the amounts of plant nutrients in soil are not uniformly spaced, each soil sample should be a composite of sub-samples collected from randomly selected spots within the chosen area. Sub-sampling provides an average level of nutrients for that area. Take 5 to 10 sub-samples, mix them thoroughly in a clean plastic container (like an ice cream bucket), put about one pint of soil in a sample bag, label the sample bag, and keep a record of the area represented by each sample you collect. Sample bags and sample submission forms for the University of Minnesota Soil Testing Laboratory are available from your local county Extension office.
Further information on soil sampling, soil testing, and fertilizer recommendations can be found in:
Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens, and Landscape Plants,
University of Minnesota Extension Service - BU-1731-GO, is available on-line at: http://www.extension.umn.edu/distribution/horticulture/DG1731.html
Soil Test Kits
Soil test kits and individual soil testing components are available in garden centers and through gardening supply catalogs. Advantages of do-it-yourself kits include immediate results and the ability to test many different areas for a relatively low cost. There are a variety of products on the market, so it is difficult to generalize since some are undoubtedly better than others, but a lot of good, low cost technology has been developed and many of the kits can be very good monitoring tools. You can get portable, hand-held, inexpensive pH meters that are surprisingly accurate and as good as you need for gardening and similar uses (as long as you maintain and don't abuse them). The same goes for soluble salt meters. And many kits are good at measuring relative levels of plant nutrients like phosphorus and potassium.
(Ed. note: I asked the author what was "inexpensive" and he responded: "I certainly haven't tried every meter that is out there, but I think you need to spend more like $50 to get a reliable pH or soluble salts meter. Inexpensive compared to hundreds of dollars for a lab model...")
Kits can be useful in following trends and evaluating trouble spots where you suspect nutrient deficiencies, high salts, or soil pH that is out of the optimum range. The weakness of kits is the ability to make specific application recommendations from them. For example, you can measure soil pH and know you need lime, but soil pH alone doesn't tell you how much lime you need to apply to your soil to reach the desired pH. You can find tables and make estimates based on soil texture, but soil-testing labs can measure the buffer index of the soil and give a very accurate recommendation for how much lime to apply.
Similar reasoning applies to plant nutrients like phosphorus and potassium. Kits may be able to tell you if one soil is higher than another, or if levels are increasing or decreasing, but any soil test is only an "index" of relative availability and accurate recommendations depend upon calibration experiments to determine how much fertilizer to apply at a given soil test level. And these calibration experiments need to be done on the soil types that exist in a given region. You should be suspicious of a blanket recommendation to be followed wherever in the country the kit is used. So use test kits to monitor, follow trends, and identify trouble spots, but if the result of a soil test with a kit indicates low nutrient levels, I recommend sending a sample to a soil lab to get a specific fertilizer (or lime) recommendation.
Getting accurate results with soil test kits
Using soil test kits may require learning new skills, so it can take a little practice and experience to consistently get accurate results. Initially, it will probably be helpful to do duplicate tests each time you do a sample. A small error in procedures can make a big difference in results, so doing these duplicate tests on the same soil sample is a good double-check until you consistently get reasonably close results for each test. Another way of checking your results is to split a sample and send one-half to a soil-testing lab and analyze the other one-half with the test kit. Soil pH measurements can be compared directly, but for plant nutrients compare the relative levels indicated by the tests (low, medium, high) rather than the numerical values.
Instruments like pH and soluble salt meters require calibration for accurate results. This need for routine calibration is frequently overlooked. Follow the instructions with the instrument for both calibration frequency and proper maintenance procedures.
Any soil test is only as good as the sample you analyze, so the soil sampling guidelines discussed previously are just as important for do-it-yourself kits as for samples sent to a professional laboratory. You need a sample that accurately represents the area you are evaluating. The importance of thoroughly mixing sub-samples is especially important when you are doing duplicate tests or sending a portion of the sample to a soil-testing lab. If your composite sample isn't a homogeneous mixture, comparisons between split samples will be misleading.
See the following Yard and Garden Line articles for information on related topics:
Understanding soil test reports:
Soil Testing Lab:
Be Skeptical About Lime and Lawns
Deborah Brown, Extension Horticulturist
Homeowners who have called the Yard & Garden Line are reporting that a local Twin Cities company that provides "organic" lawn care is promoting lime applications for people's grass. They claim it's good for the lawn, and will act as a pre-emergent herbicide to prevent weed seeds from sprouting. They say it also will help take care of thatch build-up. This is really bad misinformation – on all counts.
To control weed seeds, you need to apply a pre-emergent herbicide in late April or early May, then water it lightly into the soil to activate it before crabgrass typically germinates. Some products recommend a late summer application as well, to prevent winter annuals from sprouting. There are many good pre-emergent herbicides on the market, both manufactured ("chemical") and natural (corn gluten-meal), but lime is not one of them!
Liming the soil will not prevent weed seeds from sprouting, nor will it improve grass growth unless your soil is quite acidic – perhaps with a pH below 4.5 or 5. Soils in the Twin Cities area are typically in the neutral to alkaline range, not acidic. We almost always have to acidify our soils in order to grow azaleas, blueberries, or other acid-loving plants. This is unlike soils in the eastern U.S. that are older and generally acidic rather than alkaline.
Turfgrasses are actually highly tolerant of a wide range of soil acidity and alkalinity. But adding lime regularly will make the soil more and more alkaline, eventually damaging turfgrasses by tying up nutrients in the soil and making them unavailable to the plants. To say liming will help the lawn just doesn't make sense in this area.
There is also no truth to the idea that lime will help decompose a thatch layer. To help get rid of thatch build-up you need to core aerate and power rake in early autumn – perhaps annually – until it is under control. You should aim for a thatch layer no deeper than ½ inch.
To help prevent thatch build-up, make a point of mowing the lawn regularly as soon as it's about an inch taller than you want it. Allowing the grass to grow very tall, then mowing it quite short, leaves plants with wiry stems that are slower to decompose when they become part of the thatch layer.
The bottom line: If someone tries to talk you into using lime on your lawn – or in your garden, for that matter – be sure to have a soil test run first. For accurate results, send your sample to the University's Soil Testing Lab (return to the Yard & Garden home page and click on &Soil Testing Lab& for details). Home test strips cannot always be relied upon to yield accurate pH readings.
Janna Beckerman, Extension Plant Pathologist
A few weeks ago, Minnesotans from across the state were reporting reduced yields in their raspberries patches this year. By now, most people have realized that the yields were just as good, but two weeks later than usual. Many people in the droughtier areas of Minnesota reported smaller than normal berries on the top of the plants, but regular sized berries on the bottom. This pattern of "disease" is a direct result of below normal rainfall. Thorough weekly watering should prevent this from re-occurring in fall-harvest raspberries.
Leaf on the left is infected with virus; the leaf on the right appears to be healthy.
Plant Disease Clinic.
Although most of the samples we received turned out to have disorders due to inclement weather, people actually do have yield problems that are directly due to disease problems. Symptoms being reported include small, berries that are crumbly and hard, or a complete failure to produce fruit. The first question we are repeatedly asked by anxious homeowners is: Is this virus? To date, none of the raspberries submitted had symptoms consistent with viral infection.
Raspberries are host to several viruses, all of which can cause serious damage to the plant for the remainder of its life. It must be stressed that once the plant is infected with the virus, the entire plant will be infected, always and forever - there are no cures. Because viral infections cause decreased productivity, it is important to start a planting with healthy plant stock obtained from a reputable nursery. Although your neighbor may have brambles enough to share, plantings such as these often have a low level of virus, which will increase over time.
Even if your neighbor started with virus-free stock 5 years ago, viruses can be introduced into the planting by various vectors. In Minnesota, these three vectors are aphids, nematodes, and pollen. Like other viral disease problems, the best management strategy is prevention. Be sure to eradicate nearby wild brambles that may serve as reservoirs of virus and host to the vectors that spread these diseases.
The three major raspberry viruses are:
Raspberry Mosaic Viral Complex: Several viruses are responsible for the disease called "Raspberry Mosaic". Infection by raspberry mosaic viral complex results in delayed bud break, yellow-green leaf mottling, leaf blistering, leaf tip browning, and/or stunting. Foliar symptoms often abate when the weather becomes hot. If infection becomes severe, shoot-tips may dieback, and infected canes produce multiple shoots from the same node. Plants usually die in a few years. If this panoply of symptoms wasn't confusing enough, infected plants may be asymptomatic; That is, raspberry mosaic virus can also infect plants without expressing any visual symptoms. The large raspberry aphid transmits the viruses that cause the raspberry mosaic disease complex. Severe infection causes a reduction in stand vigor, fruit quality, winter hardiness, and stand yield. Late spring frosts, powdery mildew, mite injury, fungicide and herbicide sprays, and boron deficiency can mimic symptoms of this disease. A conclusive diagnosis requires the laboratory testing.
Raspberry Leaf Curl Virus: As the name suggests, symptoms of raspberry leaf curl virus include curling and distortion of dark green leaves. Infected plants appear stunted and contain excessive branching. Infected fruit is usually small, crumbly, and seedy. This virus is vectored (spread) by the small raspberry aphid. Severe infection causes a reduction in yield, fruit quality, and winter hardiness. Injury to canes, or anthracnose may mimic this problem. This virus has a wide host range including many weeds such as dandelion.
Tomato Ringspot Virus: Tomato ringspot virus is vectored through the soil by the root feeding dagger nematode (Xiphinema sp.). Symptoms of this disease are yellow rings and/or line patterns on the leaves. The youngest growth is often asymptomatic. Severely infected plants may be stunted, produce poor yields of crumbly fruit, and/or show an overall reduction in plant health. Tomato ringspot virus can also be dispersed through infected seed. Tomato ringspot virus has a broad host range that includes all member of the Solanaceae (tomato, potato, pepper and eggplant, in addition to nightshade and flowering tobacco), and numerous annual and perennial plants.
Virus Disease Management
Management for viral diseases is aimed at removing sources of the virus from the vicinity of the raspberry planting. Wild brambles should be destroyed within 500-1000 feet of the planting. A good weed control program should be used to eliminate host plants for the viruses. Plants should be examined throughout the season for virus infection symptoms, with prompt removal if symptoms are detected. Strict aphid control should be maintained to prevent infection. Remember, once the plant is infected with a virus, the only management strategy to control this disease is eradication of the infected plant!
West Nile Virus Is Here
Jeff Hahn, Assistant Extension Entomologist
(The source for some of the following information in this article was obtained from the Centers for Disease Control and Prevention and the Minnesota Department of Health)
Map of spread.
Image credit: CDC
West Nile Virus was found in Minnesota last month when two infected crows were discovered, one in Hennepin county and one in Crow Wing county. Including Minnesota, there are now 36 states and the District of Columbia that have recorded a confirmed infection in birds, mammals, mosquitoes, or humans (as of August 12, 2002). The first West Nile virus case was found in New York in 1999.
There have not been any confirmed human cases in Minnesota (as of August 5, 2002). However, the virus has been verified in 17 birds. These infected birds have been discovered in seven different counties: Hennepin (6), Stearns (4), Ramsey (3), St. Louis (1), Swift (1), Mille Lacs (1) and Kandiyohi (1). Seven horses, four of which have died or been euthanized have also been confirmed to have the virus. Infected horses have been found in Beltrami, Clay, Grant, Hennepin, Lac Qui Parle, Roseau and Stearns counties. The number of different counties where the virus has been found indicates the disease has become widespread in the state.
Although this disease is present in Minnesota, people should not panic as the risk of human infection is low in Minnesota. Although there are no shortages of mosquitoes here, the two most common species, Aedes vexans and Coquilletidia perturbans are not competent vectors of West Nile virus. Culex pipiens/restuans and Culex tarsalis are much more efficient vectors of the disease. Although these mosquitoes are present in Minnesota, they are much less abundant, reducing the chances of an encounter with a West Nile virus infested mosquito.
Most humans infected with West Nile virus show no symptoms or may experience only a mild illness such as fever, headache, and body aches before recovering. Some people also develop a mild rash or swollen lymph glands. At its most serious, it can cause permanent neurological damage and can be fatal. Encephalitis symptoms include rapid onset of severe headache, high fever, stiff neck, confusion, coma, muscle weakness or possibly death. Fatal cases are more prevalent in people 50 years or older. There have been 145 human cases of West Nile virus reported in the U.S. this year, including eight deaths.
Although the risk of West Nile virus for humans is generally low, take common sense steps to minimize mosquito bites. Avoid being outside during dawn, dusk, and early evening when mosquitoes are most active. Wear protective clothing, including long-sleeved shirts and long pants when you are in areas of high mosquito numbers. Eliminate any sources of standing water in your yard to remove potential mosquito breeding sources.
The most effective tool in mosquito protection is the use of a repellent. The most effective product is DEET (N,N-diethyl-meta-toluamide). This result was verified in a scientific research study reported in the July 2002 New England Journal of Medicine. Apply DEET to clothes or skin but only enough to lightly cover the desired areas. Do not overapply repellents! Do not treat children with a product containing more than 15% DEET.
Be skeptical of traps or other devices that offer mosquito control. If the product's claim seems too good to be true, it probably is.
Treating an area routinely for adult mosquitoes to combat West Nile virus is not practical and should not be attempted. The most effective vicinity wide mosquito control is to treat the larvae. However, this requires a organized abatement program. For most areas, the effort needed to do this would be costly and logistically complex.
For information about West Nile virus and horses, follow this link to the University of Minnesota College of Veterinary Medicine web site:
An Interesting Beetle
Jeff Hahn, Assistant Extension Entomologist
People occasionally see small, ornately marked beetles in their yards defoliating shrubs. These insects are a type of leaf beetle known as Calligrapha. This genus of leaf beetles is characterized by having ornate patterns on their wing covers. They are generally yellowish to reddish brown with black and sometimes gold markings. The particular species people are likely to encounter is Calligrapha spiraeae. They are known to defoliate spiraea, ninebark, and Ribes. Adults are commonly seen in July and August.
Photo credit: Jeff Hahn
In most cases, these beetles can be ignored. Healthy, well-established shrubs can tolerate their feeding, even complete defoliation. Don't treat shrubs if they are already mostly defoliated. However, if recently transplanted shrubs or stressed plants are attacked and there is still at least 50% of the leaves remaining, it may be necessary to protect the shrubs from the insects. Any registered residual insecticide, such as permethrin, cyfluthrin, or acephate, should be effective against these beetles. Be sure the product you intend to use is labeled for the plant you wish to treat.
White Pine Weevil
Jeff Hahn, Assistant Extension Entomologist
White pine weevils, Pissodes strobi, damage the terminal shoots of trees. They prefer white pine, but will also feed on other species of pine and spruce. As larvae, they are small (up to 2/5 inch long), whitish, legless grubs. As adults, they are about 1/4 inch long with a brown body with patches of white and orangish brown scales. They also has a conspicuous snout, typical of weevils.
white pine weevil|
white pine weevil adult
white pine weevil larva (left), chip cocoon (right), and pupa
White pine weevils overwinter in plant debris underneath trees. In early spring, they crawl or fly to host trees to feed, mate, and lay eggs in pits in the bark on the terminal shoots just below the terminal bud cluster. Potentially hundreds of eggs can be laid. The eggs hatch in May or June and the larvae feed in the bark. By June and July, the larvae pupate. They take fine strips of wood to line their pupal cell, creating a chip cocoon. By late July and August, adult weevils emerge. They feed some on twigs before settling down to overwinter.
You can identify white pine weevil infested trees in several ways. Watch early in the spring for drops of pitch exuding from feeding wounds on the terminal shoots. Feeding by the larvae deforms and wilts the current year's growth into what is called shepherd's crook. Eventually the terminal growth is killed. Small trees (less than four feet high) can be killed by white pine weevil. Although larger trees survive attacks by this weevil, growth is reduced and the tree becomes crooked or bushy as one or more lateral shoots try to replace the killed terminal growth.
To help manage this insect, prune out and destroy infested terminals (i.e. when it is found wilting) as soon as they are discovered. Cut it below where the bark is discolored. In cases of heavy infestations, use a residual insecticide, such as permethrin. Time the treatment early in the spring when weevils are first moving to the terminal growth.
The wedding of two friends gave me an excellent excuse to visit British Columbia in mid-July. Between Seattle and BC, I saw 10 public gardens. It's amazing what plants overwinter there so effortlessly. (And there were no mosquitos to swat, either!) Shasta daisies are native to the area and grow like weeds.
Once the State Fair is over and classes have resumed, can a hard, killing frost be far behind? In the Twin Cities metro, that frost usually happens in early October, but hits earlier in the suburbs outside of the metro heat island. Soon we'll be hearing some gardener wishing aloud for a hard frost so they can cut back tired and tattered foliage.
Peter's article on soil tests came about due to suggesttions from two different Master Gardeners. In the Septemeber 1 issue, Meagan Keefe, from Forest Resources, will write about just how valuable is that black walnut in your yard. Coming in future editions: Integrated Pest Management: just what is it and how to apply to home gardens. I'm also tracking down authors to write about acid soil-loving landscape plants and breeding flowers for fragrance. These ideas were also suggested by Master Gardeners. Your ideas are welcome.
Please feel free to cut and paste any of the articles for use in your own newsletters. All we ask is that you give our authors credit.
Back issues Yard & Garden Line News are on the Yard & Garden Line home page at www.extension.umn.edu/yardandgarden/. Our home page has clickable links to most of the components of the Yard & Garden Line, such as Bell Museum of Natural History, INFO U and the Soil Testing Lab.
Deb Brown answers gardening questions on Minnesota Public Radio's (MPR) "Midmorning" program on the first Thursday of every month at 10 a.m. Katherine Lanpher hosts the program that is broadcast on KNOW 91.1 FM, and available state-wide on the MPR news radio stations.
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