Chapter 17: Horticultural uses of soil.
After completing this chapter, you should be able to:
* state how to select soils for horticultural crops
* describe fertilization practices for horticultural crops
* describe how growers manage their soils
* solve the special problems of container soils
* describe how soil influences landscaping
TERMS TO KNOW
perched water table
soil-based potting mix
soil-less potting mix
Merriam-Webster's Collegiate Dictionary (10th edition, 1993) defines horticulture as the "science and art of growing fruits, vegetables, flowers, or ornamental plants." The information presented so far in this text about using soils applies to horticultural crops as well as to other crops. Let's see how soil is used by growers of different crops, starting with vegetable growers.
Vegetables are the most important horticultural crop in terms of total value. They make an important contribution to the human diet, supplying starch, fiber, and minerals and vitamins missing in grains and meats. There is also increasing interest in vegetables as a source of antioxidant chemicals shown to be important for human health. Vegetables are grown throughout the United States, but growing areas are concentrated in regions best suited to the economic production of vegetables, like California.
Soil Selection. Vegetable growers often select a specific soil type that suits the needs of the crop to be grown and marketing needs. Many growers in northern areas choose coarse soils because they warm up rapidly in the spring, allowing early planting and early harvest when prices are best. In general, vegetable growers favor coarser soils than do other farmers. Following are the soil types and their uses:
* Coarse-textured soils are best for early crop growth, especially for cool-season crops like lettuce or carrots. Several crops, like melons, grow best on sandy loams. For best yields, these soils are usually irrigated.
* Medium-textured soils are good for all crops. Where yields are more important than an early harvest, medium soils are better than coarse ones.
* Fine-textured soils are less desirable for vegetables. They tend to stay wet too long, hampering field operations, and crusting can inhibit germination of fine seeds. Heavy soils are very poor for root crops.
* Organic soils, especially mucks, are favorites for cool-season and root crops (figure 17-1), such as carrots, onions, and celery. Highly loose, porous mucks are especially favorable for growth of popular long-tapered carrots. Warm-season crops like tomatoes seldom thrive in organic soils because they tend to be cold and are often in "frost pockets."
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Soils selected for growing vegetables should be loose, friable, and high in humus. For most crops, a slightly acid soil is best (except for potatoes, which suffer less disease between pH 4.8 and 5.4). Many vegetables are also sensitive to high soil salts.
The most essential factor for success is good drainage. Poorly drained soil warms up slowly and cannot be planted early. Vegetable crops keep their quality for a very short time after they are ready for harvest. Thus, they must be picked regardless of soil conditions. In this case, growers cannot afford a muddy soil.
Soil Management. Vegetable growers prepare their fields much as do other growers. If needed, drainage is installed. Most growers use conventional tillage; that is, the field is plowed and harrowed before planting, or the field is prepared for furrow irrigation. There is some trend by vegetable growers toward conservation tillage. However, many vegetables have very fine seeds that are not well suited to the rough seedbed and high residue levels of conservation tillage. Also, live plants are usually planted for tomatoes, cabbages, and other crops, and transplanters do not handle the thick residues well. However, some strip-till planters have been adapted for use in vegetables. Herbicides and/or frequent tillage controls weeds.
Even when not furrow irrigating, vegetable growers often bed or list their fields (figure 17-1). Raised beds improve drainage and because the soil is warmer, promote rapid early growth. The loosened, deeper rooting zone that beds provide is also ideal for a well-formed root crop.
It can be difficult for vegetable growers to keep organic matter levels high, because many vegetables simply do not grow the bulk of green matter that field crops do. A large part of the plant may even be harvested, as in lettuce or cabbage. Growers may harvest both roots and tops, such as in green-top carrots or beets. To make up for the lack of crop residues, many growers use animal or green manures as often as possible.
Not only do many vegetable crops leave little residue, but many are also inefficient fertilizer users. This raises the potential for percolation of nitrates into groundwater. Cover cropping, as described in chapter 16, presents great potential for preserving organic matter levels, reducing runoff and erosion, and for sopping up leftover nutrients in vegetable crops.
Plant nutrition presents problems because of the unusual needs of many vegetables and the soils they are grown in. The irrigated sands favored by many growers leach readily and are often short of growth elements. Mucks are generally rich in nitrogen, but phosphate, potash, lime, copper, and others may be in short supply. The edible part of a vegetable is usually an enlargement of normal plant tissue--like the enlarged leaf stalk that is celery--and movement of nutrients into these tissues may be slow. Calcium never moves readily in plants and is often a problem in vegetables. Blossom-end rot of tomatoes, cracked stems in celery, and other problems result. Thus, fertilization with secondary and trace elements is common in vegetable production.
In general, growers feed vegetable crops by the same methods as those described in chapter 14. One difference is the use of starter solutions on vegetable transplants. Crops like tomatoes and peppers are transplanted into the field rather than seeded. Transplanting equipment has a tank to hold a weak fertilizer solution that soaks the planting hole of each plant as it is transplanted. Starter solutions are high in phosphate to overcome phosphate immobility and stimulate rapid rooting. Starter fertilizers typically have ratios of 1-2-1.
Fruits are important in the human diet because they are rich in vitamins, particularly ones missing from many other foods, and because they are a rich source of antioxidants. Unlike most crops, fruit are usually long-lived woody plants that remain in place for many years; this influences their soil management.
Soil Selection. Tree fruits require soil well-drained to at least three or four feet deep. Shallow soils cannot support a tree crop during dry periods, unless irrigation is provided. Soils with a high water table also restrict the deep rooting of fruit trees. In poorly drained soils, fruit trees may experience a serious health decline as soil-borne fungi attack weakened roots.
Fruit plants tolerate a wide pH range. The recommended pH range is 6.0 to 6.5, because it suits both the fruit plant and any cover crops that may be grown with the fruit. Blueberries, however, need a pH of 4.3 to 4.8.
Many fruits are clean cultivated to remove competition from weeds or sod. The plants must be grown on fairly level ground that is unlikely to erode, unless a slope is terraced. Apples, when grown in sod, can be grown on rather steep land. In fact, orchardists favor hillsides for growing apples because heavier cold air drains off hills on frosty nights.
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Fruits also tolerate a wide range of soil textures. Apples and pears do best on moderately fine-textured soils, while stone fruits, such as plums and peaches, prefer a coarser texture. Grapes grow on any soil, but the sweetest grapes are grown on sandy or gravelly soil (figure 17-2). Berry plants do best on moderately coarse soil.
Soil Management. All fruit crops are perennials and occupy the same ground for ten to twenty years. Therefore, the soil must be properly prepared before planting--there is no way to try again without a major financial loss. The site should be carefully selected. Any major soil changes like terracing, drainage, or leveling should be completed. Based upon soil-test results, potash, phosphate, and lime are broadcast and plowed into the soil.
Nitrogen is the most important nutrient for the established fruit crop. However, excess nitrogen or shortages of other elements causes poor fruiting, poor fruit quality, and late ripening. In addition, the tree is more likely to be injured by diseases or cold. Growers usually fertilize by topdressing in the late fall or early spring with a high-nitrogen fertilizer. Plant tissue testing can be useful in determining if elements other than nitrogen are needed on an established crop.
Many fruits are sensitive to low levels of trace elements. The best method of determining crop needs is to perform a soil test followed by a tissue test. Trace element problems can be solved by mixing trace elements into the base NPK fertilizer or by foliar feeding with chelated elements. Chapters 12 and 14 suggest some trace element fertilizers. In some cases, liming or acidifying the soil solves micronutrient problems.
Many fruit crops are clean-cultivated, which results in a loss of organic matter, poor tilth, and erosion. To overcome the problem, growers use manures, cover crops, mulches, and sod. Figure 17-3 shows an example of how these treatments are used. The figure shows part of a dwarf apple tree planting. The grower wants to ensure that apples do not compete with weeds or sod for nutrients and water. One approach keeps the soil surface bare, or clean-cultivated. Another way, which causes less soil damage, is to keep a strip of ground in the tree row free of weeds or grass by mulching, cultivation, or herbicides while the row middles are sodded. The vegetative cover between the rows protects the soil and is a source of organic matter. Mulching in the row also provides benefits like saving moisture, adding organic matter, and protecting the soil and fruit.
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Mulching can also be useful for small fruits (figure 17-4) to suppress weeds and save moisture. By mulching, pickers can work in the field after rain or irrigation; mulch also keeps fruit off the ground.
Nursery Field Culture
Soil management is a constant challenge for the nursery grower. The very process of growing nursery stock is very hard on the soil. The soil is clean-cultivated, and the only crop residues are a few leaves (figure 17-5).The soil itself is dug up and hauled away when trees and evergreens are balled and burlapped. For example, using the figures for root ball sizes from the American Standards for Nursery Stock and average soil bulk densities, it can be determined that digging up an acre of five-year-old evergreens removes 165 tons of soil.
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Site Selection. Soil is one of several factors to consider in selecting a nursery site. The land should be level, so erosion can be controlled. Sloping land may be used if it is terraced and rows are planted across the slope. It should be well drained, because wet soils hamper nursery operations. A pH between 5.5 and 6.5 is preferred for most nursery crops.
Soil texture is an important concern for nursery crops. Plants to be harvested bare-root (soil shaken off the roots) should be grown on sandy or sandy loam soil. Coarse soils most easily shake off plant roots without damaging them. In nurseries where stock is balled and burlapped by hand (soil ball stays on roots), finer-textured soils are better. Soils high in sand do not stick together, so it is hard to dig up an intact soil ball in sandy soils (figure 17-6). Silt loam or clay loam soils are best for balling and burlapping. Using machines to dig soil balls allows a wider range of soil textures.
Soil Management. Nurseries, perhaps more than any other operation, need to make a serious effort to maintain organic matter levels. It is typical to rotate nursery crops with green manure. Sudangrass, alfalfa, or other vegetation is used to build the soil. Many growers apply animal manures and/or sewage sludge. A few growers plant cover crops between nursery rows or mulch in the rows with chopped hay or wood chips. Some growers are beginning to sod row middles.
A new option could be called a "living mulch." Winter rye is planted in the nursery in early fall, allowed to grow overwinter, and develop in the spring. When the trees begin putting on growth, the rye is killed with an herbicide. The stand of dead rye acts like a mulch, protecting soil and moisture. Even more, chemicals given off by the rye suppress weed growth (allelopathy).
Before planting nursery stock, a soil sample should be taken from the top two feet of soil. Based upon the results of the soil test, the correct amount of potash, phosphate, and lime (or sulfur if the pH is too high) is mixed into the soil. Each year thereafter, high nitrogen fertilizers with ratios of 4-1-2 or 3-1-1 may be top-dressed in the fall or spring. Figure 17-7 suggests how much nitrogen should be applied.
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One of the most demanding ways to grow a plant is to grow it in a pot. A containerized plant requires constant attention to watering, fertilizing, and other practices. Despite this, more and more plants are being grown in containers. Not only are greenhouse growers growing flowers in pots, more and more nurseries grow shrubs, evergreens, and trees in containers (figure 17-8). The container grower has complete control over soil conditions, making it easier to grow a large, uniform crop of quality plants. More recently, the business of landscaping the interiors of buildings with potted plants has grown rapidly. Apartment dwellers and even homeowners now garden in containers.
Growing plants in containers differs from growing plants in the ground in one key way: the plant's root system is confined to a small soil volume that must supply all the plant's water and nutrient needs. This means the container grower waters and fertilizes far more than those who till the ground. This section examines six major topics: the naturally poor drainage of potted soil, types of potting soil, soil sterilization, soluble salts and alkalinity, soil temperature, and water pollution.
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Container Drainage. A pot of soil is by nature poorly drained because of the shallow soil profile. Compare soil in a pot with soil in the ground. Recall from the discussion in chapter 7 that capillary action "pulls" water into the drier soil below a wetting front. In a deep soil profile, then, lower layers of soil pull water downward.
In a pot, the soil column ends abruptly at the bottom of the pot. The bottom layer of soil has no capillary connection to deeper soil and the last bit of water cannot drain away after watering. Thus, in spite of drainage holes, a layer of soil on the bottom of the pot remains saturated after drainage ceases. This layer is called a perched water table (figure 17-9). As a result, potted soil is wetter and has less air after drainage than the same soil in the ground.
The difficulty is the short water column--no "depth" to pull water down, no heavy mass of water pushing down by gravity. Therefore, the taller the pot, the less severe the problem (figure 17-9). A six-inch pot filled with a standard greenhouse mix has an air-filled porosity of about 20 percent. Some greenhouse containers are no more than an inch deep. With the same mix, such a pot has a porosity of perhaps 2 percent.
Because of poor drainage, a potting mix must be highly porous, with very large pore spaces. Large pore spaces apply less capillary force to hold the water in the pot. There are two approaches to making a porous mix. One is to mix materials into a field soil to make a porous soil-based potting mix. The second method omits soil altogether, making a soil-less potting mix of other materials.
The key to making potting mixes is to use large particles to create large pore spaces. For example, one simple soil-less mix is half sand and half peat. When fine sand is used to prepare the mix, the percentage of total soil volume filled with air after watering is about 5 percent. The same mix with coarse sand has an air capacity of 16 percent. Thus, the mix with coarse sand holds more than three times as much air as the fine sand mix.
Potting Mixes. Good potting mixes, or media, have a high holding capacity for both air and water. To accomplish this, the mix needs large particles that can absorb water. Picture a pot full of shredded sponge. Each piece of sponge can soak up water, and the empty spaces between the pieces can hold air. Growers may not use shredded sponge, but they produce media that behave the same way. The mixes contain varying amounts of three materials:
* Soil is the main part of soil-based mixes. It cannot be used alone, but must be mixed with the other two materials. Soil helps a mix hold water and nutrients, and helps buffer the medium from rapid changes. However, the fine particles retard drainage and lower aeration. Avoid soil in shallow pots, or use only a small percentage of soil. In pots large enough to have a deep soil column, soil may be used more freely. Soils used in potting mixes should be loamy with good structure and be free of pesticide residues.
* Coarse aggregates are large, inorganic particles used to create large pores in the mix. Coarse aggregates include coarse sand, perlite (large granules of lightweight expanded volcanic glass), vermiculite (expanded mica), shredded plastics, and other materials.
* Organic amendments hold water and may themselves help porosity. Shredded sphagnum peat, mined from peat bogs, is most common. Many growers shred and compost tree bark or sawdust. Rice hulls, shredded coconut hulls, and wood chip/sludge compost are also being used.
Growers combine these materials into various mixes to suit their needs. Standard soil-based mixes follow the model set by the John Innes mixes developed in England in the 1930s. These mixes consist of loam, peat, a coarse aggregate, and fertilizers. Later, soil-less mixes based on mixing peat with a coarse aggregate were developed at the University of California (UC mixes) and Cornell University (Peat-lite mixes). Figure 17-10 briefly summarizes these mixes. Many growers now use mixes based on composted hardwood bark chips or pine bark. Bark chip mixes have very high porosity and seem to suppress many harmful soil organisms.
Soil Sterilization. Soil-based mixes contain weed seeds, insects, nematodes, and parasitic fungi and bacteria. Of special concern are fungi that destroy young seedlings or cause root rots. To kill these organisms, some growers treat the soil with chemicals. The soil must "air out" for several weeks after such a treatment to avoid injuring crops planted in the mix. More commonly, soil is sterilized by heat. Many growers use steam--normally a temperature of 212[degrees]F. However, this high temperature may cause three problems:
* High heat kills the bacteria that convert ammonia to nitrates (nitrification). This break in the nitrogen cycle can cause a buildup of ammonia to harmful levels.
* High heat raises the solubility of manganese, which can result in toxic levels.
* High heat creates a "biological vacuum," destroying organisms that could compete with pathogens if the soil were reinfected.
Several methods avoid these problems. First, lime can be used in a potting mix to maintain the pH between 6.0 and 7.0. At this level, manganese is insoluble, reducing toxicity problems. Second, potting mixes should be used soon after treatment, before high levels of ammonia can build up. Third, "live steam" (temperature 212[degrees]F) can be replaced by "aerated steam" (temperature 160[degrees]F to 180[degrees]F). Special devices inject air into live steam to lower the temperature. This temperature reduces ammonia and manganese problems, and allows some helpful organisms to survive while killing pathogens.
Soluble Salts and pH. To grow potted plants successfully, a great deal of fertilizer (fed through irrigation water or incorporated into the potting mix) must be poured into a small soil volume. Most irrigation water contains dissolved salts, and most fertilizers are salts. Therefore, a troublesome problem of growing in pots is the buildup of soluble salts.
Controlling soluble salts in potted plants means frequent testing of both irrigation water and the medium itself. Growers send samples of potting media to testing labs, as described in chapter 12. However, they also monitor it themselves with a conductivity meter (figure 17-11).The more ions dissolved in water, the more easily an electrical current will pass. Therefore, measuring the conductance of a standardized mixture of medium and water also measures its soluble salt concentration. Conductivity meters provide growers with critical and timely information needed to manage soluble salts and fertility. These other practices also help growers avoid problems:
* Saline water can be treated with special devices, but the process can be expensive for large amounts of water. A common water softener merely replaces calcium ions with sodium ions and does not lower salinity. Some California nurseries, contending with salty irrigation water, have installed very large--and expensive--systems to remove salts from their water supply.
* All pots should have good drainage. When watering, enough water is added so that some water leaks from the drainage holes. Drainage water leaches out excess salts.
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A major difficulty encountered by container growers is dramatic changes in soil pH. In many areas of the country, high levels of dissolved carbonates--lime--in irrigation water raise pH of the potting mix. In other areas, the water has so little dissolved lime that leaching calcium from the potting mix during watering lowers pH. For most containerized crops, pH should be between 5.8 and 6.5.
The carbonate content of water, termed alkalinity, should be measured by a testing laboratory. If it is too high, the most common treatment injects acid into the water supply to neutralize the alkalinity. Water could also be treated to remove the ions, and some have even suggested mixing irrigation water with rainwater collected from the greenhouse roof. Crops irrigated with low alkalinity water may need to be treated with lime to counteract falling pH.
Soil Temperature. Plants growing above ground in containers, especially when exposed to hot sunlight, suffer large swings in soil temperature that can damage roots.
More severe is the difficulty of overwintering containerized plants in cold climates. Plant roots are less hardy than plant tops, and a potted root system exposed to subzero temperatures will be damaged. Northern nurseries must protect container plants over winter by covering them with straw or some protective structure. Similarly, landscape plants in containers, like urns by the doorstep or trees in planters on city streets, experience the same freezing. Northern gardeners and landscapers should use only the root-hardiest plants for this purpose or plant only annuals that do not need to survive the winter. Insulating containers with rigid insulation also helps.
Water Pollution. The public is concerned about water pollution; this issue strongly affects nurseries and greenhouses. For example, container nurseries have pots sitting on the ground, and fertilize through overhead sprinklers. Most of this water--as much as 70 percent--lands between, rather than in, the pots. Because a container field is watered and fertilized daily, the potential for water pollution is great.
One answer, where feasible, is to use drip irrigation (figure 17-12). Little water or nutrients are wasted this way. Second, greater use of slow-release fertilizers added to the pots could reduce the amounts added to irrigation water. Third, pots should be set on a sealed surface that stops leaching into the soil and the land surface graded so water runs off into sealed holding ponds, where the water can be pumped for reuse.
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Landscaping makes our surroundings more beautiful and more livable. While most of us cannot live in a setting like the one shown in figure 17-13, even modest landscapes improve our lives. Landscapers must know about soil so that landscape plants will remain healthy, attractive, and easy to care for.
Most transplanted shrubs and trees survive--yet many fail to thrive, and too many actually die. Some have estimated that the average life of a new urban tree is less than ten years. These failures cost money, in replacements and damaged customer relations. Often building contractors are responsible, sometimes homeowners fail to provide proper care, and other times a landscape professional is at fault.
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When plants fail to thrive, more often than not, roots hold the answer. That means an understanding of soils is essential. Here are just two examples:
* A few inches of soil are added to the yard to raise the grade. The blanket of new soil cuts off oxygen to tree roots, trees decline for several years, then die. Many trees are very sensitive to raised grades.
* Many soils are not acid enough for the trees being planted in them. In the Midwest, pin oaks are planted by the thousands, yet many suffer iron chlorosis from unsuitable soil pH.
More than most, landscapers must understand how soils, roots, and water interact. Landscape soils are complex compared to field soils, because they are radically altered by construction and landscaping. Soils of different textures are placed together, and as chapter 7 points out, that affects water movement.
Now let's see how our knowledge of soil can help us understand proper landscape practices.
Site and Plant Selection. Too often, landscape designers pick a plant only because it would look pleasing in a certain spot, not because it would grow well in that spot. Mismatches between plants and site lead to unhealthy plants and endless maintenance. Landscape designers should know which plants grow well in their area and select plants that match soil drainage, pH, salinity, and degree of compaction on the landscape site. Appendix 6 lists soil preferences for selected trees.
A thorough designer will actually examine the soil on a site. Salinity and pH can be checked with a soil test or portable testing device. Compaction can be checked with a penetrometer or by seeing how hard it is to shove a screwdriver into the ground, taking soil type and moisture into account. Soil color provides guidance on drainage, or a very simple, crude percolation test can be done. A foot-deep hole is filled with water, and after it drains empty, filled again. Drainage can be inferred from the amount of water that drains in an hour. Experience makes these simple tests most valid.
Avoiding Compaction. Most landscape sites are slightly to severely compacted during construction. Studies on the effects of compaction on tree growth showed interesting results. Compaction caused shallow root systems. Often, roots from deep in the planting hole actually grew upward along the sides of the hole toward the surface, rather than outward into the surrounding soil. Failure to explore the native soil results in drought damage, nutrient shortages, and poor anchorage.
Tillage helps tear up the compaction, but often little can really be done about it. However, landscapers can avoid adding to compaction while working. Landscapers who drive on a site with trucks full of rock mulch only compound the problem. Landscapers should especially take care with wet, fine-textured soils.
While no method exists to completely relieve compaction in established landscapes, techniques are available that can be helpful. In vertical mulching, many holes are drilled in the ground and filled with organic matter or coarse material like sand. This allows some movement of air and moisture into the root zone of trees. Devices have also been developed to inject air or water into the soil under very high pressure, fracturing the compacted soil. The fractures are then filled with a coarse material. The long-term effectiveness of these treatments has not yet been fully determined.
Deep Planting. Recent work at the University of Minnesota has uncovered long-term problems associated with deeply planted trees. Trees planted too deep often fail because roots are deprived of oxygen, but those that survive face a longer-term problem called stem-girdling roots. When planted too deep, even by a few inches, tree roots often develop in ways that cause some to grow across the stem. As the tree ages, those roots prevent proper development of the trunk and compress the tree's vascular system. Eventually the tree begins to weaken and die. More dramatically, the tree may snap off at the soil line in high winds.
Stem-girdling roots can be avoided by planting trees at the proper depth. Established trees may be inspected for the condition--often visible by a flattened side on a tree trunk--and girdling roots removed.
Transplanting. The key to successful transplanting is rapid root growth. After all, for plants dug out of a field nursery, up to 98 percent of the root system is lost. Even containerized stock must grow new roots quickly, because it can't survive on just the soil mass it was planted with. How do landscapers ensure that the roots of newly planted trees and shrubs will grow quickly?
Research shows that plastic mulch, commonly used in shrub beds to keep out weeds, restricts root growth by keeping oxygen out of the soil, as pictured in figure 1-10. Organic mulches, like wood chips, keep down weeds and conserve moisture without robbing the soil of oxygen. Below the wood chips, one could place landscape fabric rather than plastic sheeting. This woven material is porous, allowing movement of air and moisture but restricting weed growth.
There is disagreement over the need for fertilization of newly planted trees and shrubs. Some experts claim little need; that trees come already supplied from the nursery with all the nutrition needed at first. The author, however, has observed that plants being held for sale often appear depleted by frequent watering without fertilization. However, high fertilization can actually retard new root development. The author supports light fertilization with a complete slow-release fertilizer at the time of planting if the soil tests low in nutrients.
There has been increasing interest in inoculating plants with mycorrhizae before planting them. This may improve transplant success and later plant growth. Preparations of inoculum are now available.
A recommended procedure for transplanting is as follows:
1. Check the drainage of the site. If it is poor, drainage systems may need to be installed. If that is impractical, then create planting berms--attractive, rounded, raised beds that will get the plants above the wet soil.
2. In a balled and burlapped tree, use a probe like a screwdriver to find the uppermost root in the soil ball. This is necessary because trees may come from the nursery with roots buried deep in the ball. When planted, this root should be at the soil surface or even slightly above in heavy soils. Remove soil above that root.
3. Prepare a planting hole much wider than the tree roots but never deeper. If dug deeper and then partially filled back in, the plant will settle and end up too deep. To improve air movement to roots and to lessen the chance of soil remaining water-soaked around the roots, make the hole even shallower in heavy, less well-drained soil. A large hole will provide loose soil for the roots to grow into.
4. Carefully place the root system of the plant in the hole. If a tree, the top major root should be at the soil surface.
5. Backfill the hole with the same, unamended soil that came out of the hole. Research indicates that amending the soil is not helpful and may create more problems with sharp interfaces. If the tree was planted high in heavy soil, create a gentle mound to cover the whole root system. As low-release fertilizer may be included in the backfill if it seems called for.
6. Water the plant carefully to remove air pockets and thoroughly soak the soil.
7. Mulch around the tree with about three inches of coarse organic mulch (figure 17-14). Avoid fine mulches or more depth to avoid retarding aeration. Do not create a volcano-looking pile, and it is best if the mulch not actually touch the trunk.
Amending Soil pH. All too often, the native pH of a soil is not proper for every plant one wants to use. Obviously, this calls for designers and others to check for pH, an easy enough task with a soil test or even a piece of pH paper. Once the pH is known, a designer can make informed plant selections.
Nevertheless, there are times when soil pH should be altered. Because many landscape plants prefer acid soil, acidification is common. One common suggestion for acid-loving plants is to dig an especially large planting hole and then put sphagnum peat moss (half by volume) into the backfill. This helps temporarily, but as the peat decays, the pH returns to normal.
Three chemicals are used to acidify soils more permanently: sulfur, aluminum sulfate, and iron sulfate. There are disagreements about which is best, but the author prefers sulfur as being inexpensive, easy to use, and because of concern with aluminum toxicity. Figure 11-19 suggests amounts to be used. Mix the sulfur into the soil. The pH of calcareous or very alkaline soils may be very difficult to change even by this means.
Landscapers should be aware of the many sources of calcium that can raise pH in a landscape site, including lime in a concrete foundation, limestone rock mulches, and alkaline irrigation water. Annual use of acid-forming fertilizers like ammonium sulfate or special acid preparations may maintain acidity. Sulfur-coated urea, a slow-release fertilizer, will also enhance soil acidity.
Trees that do begin to suffer chlorosis can be treated by sulfur spread over the soil surface. While this changes the pH of only the top inch or so, there may be enough roots there to provide adequate iron for the whole tree. However, this and other treatments do not always work well. Foliar feeding with chelated trace elements can provide a temporary green-up in some cases. One may also use devices that inject trace elements into the trunk or root flare, but may be risky to tree health drilling holes in the trunk.
Fertilizing Established Trees. Experts disagree on the value of fertilizing landscape trees and shrubs, because there is so much variation between different trees and sites that experimental evidence is mixed. It is generally agreed that young, growing trees benefit from fertilization. Some feel that older established trees benefit little except to relieve an actual deficiency. Stressed trees should only be fertilized lightly because fertilizers may add to stress by increasing soil salinity or by supplying nitrogen the tree cannot handle. Nevertheless, fertilizing trees remains a common practice.
Timing strongly affects landscape plant response to fertilization. The best time to fertilize woody plants is when they go dormant. Nutrients absorbed in the fall will promote rapid growth the following spring. Early spring is also a good time to fertilize landscape plants. In cold climates, woody plants should not be fertilized from midsummer to early fall because it could spur late growth and keep the plant from hardening off for winter.
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Nitrogen is the most important element for trees. To understand how to fertilize trees, one must first know where tree roots are (figure 17-15). Trees may root deeply, but 80 percent of a tree's feeder roots are in the top foot of soil. These feeder roots reach far from the trunk of the tree. One misconception is that tree roots all grow within the dripline of the tree. In fact, they often extend triple that distance from the tree trunk.
Shade trees may be fertilized by broadcasting over the whole root system of the tree. To be most accurate, dig a few holes to find how far the root system reaches. If this is not practical, assume the root system extends 100 percent beyond the dripline. Then spread a high-nitrogen fertilizer (2-1-1, etc.) over the entire root system at the rate of 3 1/2 pounds of nitrogen per 1,000 square feet. Nitrogen will, of course, leach into the root zone. If the shade tree grows in a lawn, split the fertilizer into two applications to avoid overfeeding the turf. One application should be in the late fall, the other in spring.
Another method of feeding shade trees is called perforation. This method is more difficult but is better if the tree lacks phosphorus or potassium. The amount of nitrogen to be applied can be calculated by the method described previously. Another method, simpler but less accurate, is based on the trunk diameter (caliper) measured twelve inches above the ground. If the caliper is less than six inches in diameter, one-quarter pound of nitrogen is added per inch of caliper. For trees more than six inches in diameter, a half pound of nitrogen per inch is applied. A complete fertilizer high in nitrogen, such as a 2-1-1 ratio fertilizer, should be used. This fertilizer is applied by drilling holes into the ground in a grid pattern, on twenty-four-inch centers, under the tree, and dividing the fertilizer amongst the holes. An eighteen-inch depth is often recommended, but this places the fertilizer below most tree roots, where it will be less available and subject to leaching. An eight- to twelve-inch depth is preferred. Figure 17-16 shows how this is done.
[FIGURE 17-15 OMITTED]
[FIGURE 17-16 OMITTED]
Some workers feed trees by injecting a solution through a root feeder. In this system, a solution is pumped out of a tank under pressure and forced through a tube inserted into the soil. The pattern for injection would be the same as in figure 17-16, except that it should be on thirty-inch centers. Inexpensive versions are sold in most garden centers.
Turf. Except in arid regions, turf is the most common element of a landscape. Turf not only looks nice, it also protects the ground fromerosion. Turf may be planted by seed, sod, or in some cases, sprigs and plugs. Whatever method, good soil preparation is important to success.
Many turf specialists no longer recommend covering the existing soil with "black dirt" before planting turf, because the interface is a problem. If any is added, it should be tilled into the existing soil to avoid the sharp interface. It is beneficial to till a two-inch deep layer of compost into the soil.
Chemical amendments should be tilled into the soil before planting, based on good soil tests. Phosphorus should be added now because of its immobility and importance for good root growth and turf spread. Potassium, which promotes good wear tolerance and resistance to disease, drought, and cold, also would be best incorporated now. Lime or sulfur may also be incorporated to adjust pH to 6 to 7.
After the soil is tilled to incorporate amendments and to loosen the soil, it is carefully raked or harrowed to make a fine seedbed free of bumps or pits. Turf may then be planted by seed, sod, or sprigs, according to local practice. Frequent irrigation is needed while the turf is being established.
Turf is fed by broadcasting a high-nitrogen fertilizer. Often a fertilizer with a ratio of 20-1-3 is suggested, but seek out local recommendations. Fertilizers with a high water-insoluble nitrogen (WIN) content are best. In areas where soil salinity is a problem, turf fertilizers should be of low salinity, and care must be taken to avoid overfertilization.
Fertilizer recommendations vary widely depending on the region of the country, the type of turfgrass, the level of maintenance, and the use of the turf. The annual rate of nitrogen application varies from as little as one pound nitrogen per thousand square feet for low-input, low-maintenance turf with tolerant turf types to as much as six pounds. The larger amounts are split into several applications--often spring, early fall, and late fall. Fall applications are the most beneficial.
There has been concern that nitrates may leach into the ground water below turf, or that phosphates will run off into surface waters and induce eutrophication. There is good evidence that turf roots are very efficient at scavenging nitrates out of the soil, and phosphates tend to tie-up in the thatch. Thus, properly applied fertilizers at moderate rates, following soil testing, should not create serious problems. One should take care to sweep up fertilizers and clippings that fall on surfaces like sidewalks.
Core aeration is a most useful turf soil-management practice. To aerate, a machine, preferably one with hollow tines, creates small pits in the lawn. This breaks up compaction and increases aeration. Aerate only when the soil is moist; wet soil will compact worse, while turf in dry soil is under stress. Schedule aeration when the turf is growing actively. This means spring and early fall for cool-season grasses and late spring and early summer for warm-season grasses.
For season-long green color, irrigation of turf is usually needed. One expert suggests adding about an inch of water when 30 to 50 percent of the lawn shows signs of water stress--a slight rolling of the older leaf blades. Some grasses can be allowed to go dormant in a dry season, but excessive dryness can damage turf.
Xeriscaping. Landscapers--and their customers--of the more arid regions of the United States face the problem of water shortages. The problem has been dramatically compounded by the planting of landscapes adapted to the more moist eastern and northern states. An answer to excess water use is xeriscaping (from the Greek word xeros, or dry). Xeriscaping is landscaping adapted to dry climates.
A wide range of plants are available that thrive under low-moisture conditions. The most dramatic of these include cacti, succulents, and yuccas (figure 17-17). However, xeriscaping need not mean only a mixture of sand and cacti. A number of shrubs, trees, and flowers can tolerate dryness. Examples of nonthirsty plants include palo verde (Cercidium sp. ), rose periwinkle (Catharanthus roseum) and several salvias (Salvia sp.). Some short-grass prairie grasses can be grown for lawn with minimal watering, such as blue gramma grass, the state grass of Colorado.
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For even lower water usage, some plantings can be replaced by "hard" features. Turf areas may be replaced by paving, such as brick patios, or by mulching with pebbles. Some shrubs can be replaced by boulders, pole tips set upright in the ground, or similar features.
Lastly, the xeric landscape strives to conserve water. Plants can be installed into beds recessed below grade several inches to catch water, then mulched to slow moisture loss. Also, plants tend to be planted less densely, reducing competition for water. Drip irrigation greatly reduces water consumption. Irrigation systems should be well designed, zoned for different plant needs, and be well maintained and audited.
Xeriscapes may not always function as planned. In sixteen months of monitoring water use in Phoenix, Arizona, one study found that homeowner study subjects applied more water to xeriscapes than to conventional landscapes. (1) These results suggest there may be some challenge to making xeriscapes successful.
Arid regions that might practice xeriscaping contend with soil problems other than dryness. These can include alkaline soil pH, and saline or sodic soils. These problems call for selection of tolerant plants, or treatment as described in chapter 11. Incorporation of peat and sulfur into beds is helpful. These soils often suffer from the presence of a lime-cemented hardpan called caliche. If severe, the caliche may need to be broken up before planting.
This chapter discussed how soils are managed to grow fruits, vegetables, and ornamental plants. An irrigated, coarse soil allows early production of vegetables, while a medium soil gives the best production. Vegetable soils must be very well drained. Many growers make good use of animal and green manures to make up for the small amount of organic matter most vegetable crops produce.
Fruits and field nurseries need a deep, well-drained soil. Clean-cultivated fruits and nurseries need level land and close attention to erosion control. Because the crops stay in the ground for many years, the land must be carefully prepared before planting, including drainage, terracing, irrigation installation, leveling, and plowing lime and nutrients into the soil. In the years after planting, nitrogen is the most important nutrient. Maintenance of soil organic matter is a challenge for these growers.
Soil in pots is poorly drained because the soil column ends at the bottom of the pot. To overcome this, potting mixes are very porous and consist of coarse aggregates such as sand, an organic material like peat moss, and sometimes soil. Soil-based mixes must be sterilized by heat or chemicals to kill weed seeds and soil pathogens.
For landscape plantings to grow well, the soil must be handled properly. Plants are chosen that will thrive in the soil on a landscape site. Landscapers must be aware of the effects that textural interfaces have on the movement of water and the growth of roots. Proper soil preparation for planting turf and good transplant practices are needed for healthy plants. Fertilizing established turf and trees also keeps them growing well. Xeriscaping is landscaping adapted to dry climates.
Chapter 19 of this text contains information related to this chapter, particularly material on structured soils in urban landscapes.
1. Why is preliminary soil testing and preparation more important for fruit crops than for most other crops? What are examples of preparations?
2. Why are vegetables often grown on sandier soils in northern states? Provide a full explanation.
3. Why is nursery culture hard on soil? What practices do nurseries do to compensate?
4. Gardeners often put a layer of rocks in the bottom of a pot to improve drainage. Does it? Explain your answer.
5. Why is it critical for health of plants growing in containers that there be drain holes in the pots?
6. We plant a tree in heavy clay soil. To make sure it does not fall over, we bury a few inches of the stem. To make it easier to water, we leave a deep depression under the tree. What are possible consequences of such actions?
7. As a landscape designer, you like balsam fir (Abies balsamea).What kind of soils should be avoided? What soil tests might you ask for to evaluate a yard for suitability for balsam fir? See appendix 6.
8. Describe a soil we would consider good for both a home landscape and a nursery. See appendix 5.
9. What are elements of a xeriscape that help save water?
10. A case study: In 2002 the author's home state passed a law restricting phosphorus in lawn fertilizers in its largest city to preserve water quality in city lakes. The city is sometimes known as the City of Lakes. Describe the problem of phosphorus in surface water. Knowing what you have learned in your soils class, and following an Internet search on "lawn fertilizer and phosphorus," write a paragraph about your reaction to this ordinance.
1. This little experiment will demonstrate the effect of the depth of a soil column on drainage. Soak a common kitchen sponge until it is saturated. Now hold it up horizontally (but not flat).When it stops dripping, turn to a vertical position. What happens now that the column has become deeper?
2. Check out numerous sites about xeriscaping on the Internet. An example is: <http://www.pan-tex.net/usr/a/aggie/ag05055.htm>. Compare several sites and judge them on reliability. Are they from a knowledgeable source? What is the authority or credentials of the source? Do they seem to have a biased viewpoint? How old is the site?
3. Study the Web site on stem-girdling roots by the man who did most of the research, Dr. Gary Johnson, at: <http://www.extension.umn.edu/distribution/naturalresources/DD7501.html>. If you type "stem-girdling roots" into an Internet search engine, you will also find other pages on the topic.
(1) Peterson, K., Stabler, L, & Martin, C. 2000. Irrigation application volumes in urban residential landscapes. 2000 Symposium, Central Arizona--Phoenix Long Term Ecological Research.
FIGURE 17-7 Sample guidelines for fertilizing nursery stock. The best practice is to follow local guidelines. Crop Lb N/1000 [Ft.sup.2] Lb N/Acre Deciduous plants 5 225 Narrowleaf evergreens 4 175 Broadleaf evergreens 3 125 FIGURE 17-10 For these common potting mixes, the numbers represent parts. For example, Peat-lite A consists of one part peat and one part vermiculite. These mixes also contain some combination of lime, gypsum, fritted trace elements, superphosphate, and other fertilizers. These mixes are only a sample of the large number of mixes in common use. Common Potting Mixes UC Mix Cornell John Innes D Peat-Lite A Loam 7 -- -- Sand 2 1 -- Peat 3 3 1 Perlite -- -- -- Vermiculite -- -- 1 Composted bark -- -- -- Cornell Peat-Lite B Bark Mix Loam -- -- Sand -- 1 Peat 1 -- Perlite 1 -- Vermiculite -- -- Composted bark -- 2