(millimeters per meter of soil)Very coarse texture:
very coarse sands33–63Coarse texture:
coarse, fine, and loamy sands63–104Moderately coarse texture:
sandy and fine sandy loams104–146Moderately texture:
very fine sandy loams, loams, and silt loams125–192Moderately fine texture:
clay, silty clay, and sandy clay loams146–208Fine texture:
sandy clays, silty clays, and clays133–208Peats and mucks167–250Soil StructureDetermined partially by soil type, soil structure is the arrangement of particles into aggregates. Because it’s larger and it doesn’t stick together as well as a clay soil, sand has a simple structure; the pore spaces between each sand particle allow air and water to move through it quickly. Fine clay particles aggregate too well, forming large, impervious clods. However, amendments of organic matter (decomposed plants or humus) can change clay’s soil structure; the larger particles of organic matter impede the clay from bonding itself into huge clumps. Adding organic matter to sandy soil clogs it up a little, so water and nutrients don’t pass through it as easily. Organic matter also enhances soil structure because it attracts insects and earthworms that work themselves through the soil, mixing particles and creating pore spaces as they go. (The earthworms’ waste also enhances the soil.)
Soil ChemistrySoil chemistry, as expressed in pH, is crucial to the ability of plants to absorb and use soil nutrients. The extreme chemical reactions caused by pH levels below 4.5 and above 8 make nitrates unavailable to plants. In addition, many soil microorganisms that facilitate nitrogen fixation (the transformation of atmospheric nitrogen to a form available to plants) and nitrification (changing NH4+ into NO3-) cannot live in extreme pH conditions. A soil test is crucial for determining the pH of a site. If the soil pH is out of the “normal” range of 6.5 to 7.5, a decision must be made to either amend the soil, coax it toward a normal pH, or choose plantings that will survive in the existing pH range. Unfortunately, few plants survive an alkaline soil, and only a handful of cover crops (vetch, rye, Sudan grass, bent grass) will tolerate even moderate acidity.Finely ground limestone (able to pass through a 60-mesh screen) is commonly used to raise a soil’s pH, and, depending on the soil type, dozens of pounds can be required to raise the pH to plant-tolerable levels (Table 3).
In addition to limestone, various other sources of calcium carbonate (CaCO3) can be used. Keep in mind that limestone’s equivalent percentage of CaCO3 is 100, so the pounds per 1,000 ft2 figures above need to be adjusted if using shell meal (95%), hydrated (120%–135%) or burned (150%–175%) lime, dolomite (110%), sugar beet lime (80%–90%), or calcium silicate (60%–80%).Table 3. Limestone Needed to Raise the pH
of a 7-inch Soil LayerSoil Texture Pounds of Lime per 1,000 ft2 of soilFrom pH
4.5 to 5.5From pH
5.5 to 6.5Sand, loamy soil2328Sandy loam 3760Loam5578Silt loam 6992Clay loam 87106Muck174197A variety of amendments can be added to lower the pH of alkaline soils; however, many can be used only if the soil contains lime or free calcium carbonate. Testing for lime is simple: If a few drops of muriatic or sulfuric acid on a clod of soil cause bubbling or fizzing, the sample contains carbonates. If this occurs, any of the amendments listed in Table 4 can be used. (Purity percentage is listed on the product’s packaging.) If lime is absent from the soil, use only amendments containing soluble calcium.Table 4. Equivalent Amendment ValuesMaterial
(100% basis)Pounds of Amendment Equivalent to:100 lb pure gypsum100 lb soil sulfurGypsum100538Soil sulfur 19100Sulfuric acid (conc.)61320Ferric sulfate 109585Lime sulfur68365Calcium chloride86—Calcium nitrate106—Aluminum sulfate129694Ammonia polysulfide423As is the case with changing acidic soils, the amount of amendment needed to neutralize an alkaline soil can be enormous. For example, Table 5 shows the amount of 95% pure soil sulfur needed to increase the acidity of a 6-inch-deep layer of soil. Table 5. Soil Sulfur Needed to Lower pH of Alkaline SoilsChange pH to 6.5 From:Pounds of Sulfur per 1000 ft2 SandLoamClay7.02.33.46.97.511.518.423.08.027.534.445.98.545.957.468.8Once the soil is brought to an optimum pH level, plants will be able to glean the nutrients it contains. If the subsequent soil-sample test reveals nutrient deficiencies, fertilizer should be applied.“If you want to save money, test how much nitrate is left in the soil,” notes Gary Gao, associate professor at Ohio State University and a Clermont County Horticulture Extension agent. “For example, if the soil contains 20 pounds per acre, you will apply less in spring. For best all-round feeding for shrubs and wildflowers, a 10-10-10 fertilizer works well.”What Does the Soil Need?Plants require 16 essential elements as nutrients. Carbon (C), hydrogen (H), and oxygen (O), which plants take from the atmosphere, are considered “primary.” The remaining nutrients usually exist in the soil. Nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) are considered “secondary” nutrients; of these, N, P, and K usually have to be added to soils, as plants use these elements in greater volume. These elements are also more easily leached from the soil, or they may exist in forms the plants cannot use. Many commercial fertilizers contain only those three elements.Plants also use the elements zinc (Zn), iron (Fe), manganese (Mn), copper (Cu), boron (B), molybdenum (Mo), and chlorine (Cl) in small amounts. It’s often advised, however, that unless a soil test indicates deficiencies of the last 10 (Ca through Cl), fertilizers for those elements should not be applied, as toxic levels can easily build up in the soil.Before applying any fertilizer, soil testing is essential; discovering what the site really needs reduces waste and avoids possible toxic effects if too much of a nutrient is applied. In some areas of the United States, the test’s timing is important as well. Ohio State University Extension warns farmers that in humid-climate regions, mainly east of the Mississippi River, soil testing for nitrogen prior to planting generally doesn’t provide useful information, but a fall soil test for phosphorus and potassium levels is useful because those elements’ levels won’t change much before spring. An exception is clay soils subject to freezing and thawing, which can increase potassium levels 5%–10%.“Simple” fertilizers contain only one nutrient; “mixed fertilizers” or “complexes” contain more than one—usually nitrogen (N), phosphate (P2O5), and potash (K2O). Complexes are “graded,” or given a three-number designation representing the content of each nutrient in terms of percentage (by weight). The grade listing is always in the same order: N, P, K.By reading the grade, you can determine the ratio of each nutrient. For example, a 10-10-10 grade is a 1:1:1 ratio, and a 15-5-10 is a 3:1:2 ratio. A zero in a grade means that particular nutrient is not included; a 20-10-0 fertilizer contains a 2:1 ratio of N to P, and no K.Research has indicated that both nitrogen forms—nitrates (NO3-) and ammonium (NH4+)—are required for plant health. Many plants grow best with additions of 50%–60 % nitrates and 40%–50% ammonium. If N alone needs to be applied, you can select from a variety of sources, which are chosen depending upon the site (its proximity to surface water, residential areas, etc.), and product and application costs:Anhydrous ammonia (82-0-0) (usually only for agricultural use)Aqua ammonia (20-0-0)Ammonium nitrate (34-0-0)Ammonium sulfate (21-0-0-24S)Ammonium nitrate-sulfate (30-0-0-6.5S)Calcium nitrate (15.5-0-0-19Ca)Nitrate of soda (16-0-0)Urea (46-0-0)Note the fourth number in the grade; when an additional nutrient is included, its elemental symbol and its percentage are added to the labeling.If the soil suffers from phosphorus deficiency, you can add phosphoric acid (0-52-0), superphosphoric acid (0-70-0), or normal (0-20-0-12S) or concentrated (0-45-0) superphosphate. However, to ensure the best plant nutrition as well as expediency, many agronomists use a complex fertilizer containing N, P, and K. The ratio chosen depends on specific site or plant needs; for example, higher levels of phosphorus might be applied to stimulate flowering.Each of the nutrients plays a specific role in the development of healthy plants, as shown in Table 6. Table 6. Role of Nutrients in Plant GrowthNitrogen (N) Phosphorous (P) Potassium (K)PhotosynthesisPhotosynthesisPhotosynthesisProtein synthesis RespirationProtein synthesis Nutrient uptake Energy transfer, storageSucrose/carbohydrate movementCreation of amino acidsCell division and enlargementCell extensionUtilization of sunlightEarly root formation, growthStomatal movement (increases water use efficiency, reducing drought stress)Energy systems, vitaminsTransfer of hereditary traits, bloomingMetabolic functions, increasing winter hardiness Chlorophyll production Enzyme activationWeather, or NotNoting weather conditions around the time of a scheduled fertilizer application is a crucial step, not only to receive the best possible application, but also to ensure the fertilizer doesn’t get washed or blown off-site.“Fertilizing in fall can be a bad thing, as many areas get a lot of rain during that season,” Gao says. “Because nitrogen is highly soluble in water, it will dissolve and be leached out. However, if you apply nitrogen in spring, as farmers do, by October there’s likely not a whole lot left in the soil. To avoid potential pollution, if you apply 1 pound nitrogen per 1,000 square feet, but you know it’s going to rain soon after application, you should perhaps break that into two applications. If you know the autumn is going to be wet, you might put down two-thirds of the needed application in fall, applying the other third in spring.”However, in the erosion control industry, the site usually gets only one application of fertilizer, seeds, and possible amendments. Gao points out that those amendments may alleviate some N wash-off. “Organic matter helps retain nutrients. Ideally, you’d amend the soil first, then place the seeds on top. You don’t want to bury the seeds much at all, as they won’t get established as well.”Industry practices of hydroseeding (applying seed, fertilizer, and mulch in one application) and placing cover straw over fertilized, seeded areas serve this purpose, as a two- or three-step process of incorporating fertilizer, adding organic matter, and seeding would not be economically feasible for most projects. Plowing current vegetation under can also help, because the decomposing plants add organic matter to the soil.Cover Crops: Let the Plants Do the WorkMany cover crops fix nitrogen in the soil; what grew on the site in previous years can have an impact on how much nitrogen already exists. If the site had been a farm, various nitrogen-fixing crops might have been grown there. Soybeans can fix 30 pounds N per acre; grass sod or pasture, 40 pounds; an annual legume cover crop, 30 pounds; and an established forage legume can add 40 pounds, plus 20 pounds per acre times the number of plants per square foot, for a maximum of 140 pounds per acre.Unfortunately, not many nitrogen-fixing crops—legumes, hairy vetch, winter peas, clovers —make attractive ground cover for commercial sites, and some plants, such as vetch, have been labeled as invasive plants by many states. However, in roadside projects, especially those incorporating wildflowers, nitrogen-fixing grasses and blooming plants might blend in more easily. “Winter rye is a pretty good nitrogen-fixer, as well as red clover and alfalfa,” Gao notes.Mycorrhiz-ing ExpectationsIf the organisms are not already present on the site, injections of soil mycorrhiza, beneficial fungi that help plants absorb nutrients, might be needed; opinions vary. “It depends on who you talk to; there can be very mixed results,” Gao says. “If your soil is really bad—like a former strip mine site—would mycorrhiza help? Yes. But in an urban setting? Maybe, maybe not.”A bright spot on the horizon: In 2002, scientists at the Sainsbury Laboratory in Norwich, UK, discovered a plant gene essential in controlling the interactions between plants and mycorrhiza. Their findings suggest it may be possible to design plants that are able to make their own nitrogen fertilizer. It was previously believed that the nitrogen-fixing symbiosis between legumes and rhizobia bacteria was a unique relationship; however, genes that control the plant-fungal partnerships are widespread.
