Fertilizers and Fertilization (Chapter 10) PDF

Fertilizers and Fertilization Chapter 10 Before scientists identified plant nutrients, farmers understood what materials they need to...

Fertilizers and Fertilization Chapter 10 Before scientists identified plant nutrients, farmers understood what materials they need to provide for their cropsin order to prosper. Farmersused lime, ashes, dead fish, ground bones,and guano to make better crops and yield. Inaddition, in some cultures, growersmay use night soil to improvetheir soils. Inthe United States, most farmers use fertilizers to supplement the nutrients to fulfill the demand of the crops, because after cropping for so many years, the native soil fertility cannot supply sufficient nutrients anymore for crop production. Scientifically,native soil fertility most of the time is not enough to provide all the nutrients needed for optimal cropgrowth,assuming temperature and water arenot limiting factors. Fertilization is used to supple ment the nutrients that native soil fertility provides to the soil solutionfrom the slowly and readily available nutrient pools. This chapter will explore the types of fertilizers, the methods of fertilization,the impact of fertilizers on soil,and the fertilizer management. A. Types of Fertilizers The four categories of fertilizers are: mineral, inorganic, organic, and synthetic organic. Mineralfertilizers are natural rocks and minerals that are rich in essential nutrients. Dolomite is a rich sourceof Ca and Mg but is primarily used to increase soil pH in acidic soils. Langbeinite is a sourceof three essential nutrients called, K, Mg. and S. Apatite or rock phosphate is a source of P. The negative issue with mineral fertilizers is that the nutrients within them areslowly available unless the material has fine size. Most of the time, these materials are difficultto dissolve. Inorganic fertilizers are mined or manufactured. Potassium nitrate or saltpeter, for example, is mined from desert Anhydrous ammonia is manufactured via the Haber-Bosch T deposits in countries like Chile. in atmosphericN, is chemically reduced at high temperatures and pressures. The combination s method of fertilizers such asammonium phosphate, ammonium nitrate, and ammonium sulfate are produced by mix ing anhydrous ammonium with phosphoric, nitric,and sulfuric acids, respectively. Organic fertilizers have some types of organic C associated with them. This includes urea, which is excreted in the wastes of mammals, including people. When people mention organic fertilizers, they usually mean manures,but composts,food by-products, crop residues, and cover crops can all serve as organic fertil izers by returning essential elements to the soil. Synthetic organic fertilizers are manufactured industrially but are chemically organic because they contain C and H. The best example of this is the chemical synthesis of urea. 213 214 Chapter 10 Fertilizers and Fertilization B. Source and Method of Delivery There are four groups of fertilizers that can be categorized on the basis ofmethod of delivery or form: fluid fertilizers, pressure liquids, dry fertilizers, and slow-release fertilizers. Fluid Fertilizers Fluid fertilizers are liquids that are either dissolved chemicals or suspensions of chemicals attached to very Liquid animal wastes from lagoon storages,for example, are a combination ofdissolved and fine clays. sus pended materials that are either sprayed onto the soil or injected beneath the soil surface (for odor control). For example, foliar fertilizers are liquid fertilizers. Golf courses that are managed by fertigation (mixing fertilizer with irrigation water) also employ the equivalent of liquid fertilizers. Many of the commercial solutions for houseplants are concentrated solutions of nutrients that are diluted to appropriate strength before use. Pressure Liquids Pressure liquids by anhydrous ammonia.Ammonia would normally be a gas at room tem are best typified perature falls -33°C(-28°F). The liquid is held in a pressurized tank until it can be injected into the below soil where the ammonia rapidlyconverts to ammonium. If the injection technique leaves cracks in the soil, the ammonia can be readily lost through volatilization. Dry Fertilizers Dry fertilizers are commonly broadcast on the soil surface and rely on a combination of tillage and rainfall to ensure that they mix with the soil. Some dry fertilizers are banded next to the growing crops. Dry fertilizers are typically pulverized, granulated,or prilled. Pulverized fertilizer is made by crushing the fertilizer to a fine powder. rials The advantage granules to the field is greater reactivity, and during application but the disadvantage Granulated fertilizer is easier to handle, but doesn't react as quickly.I that cakes, and because the size of granules is not perfeeth can cause an uneven wg is greaterdust and caking. distribution Can still settling of produce fine mate during transport of the particles, which leads to uneven fertilizer application rates. Prills are smooth, round, and dust-free. Some prills are coated to prevent caking. In terms of application, they spread easier than granules and are also free of fine material. However, ing makes them more expensive fertilizers to use. the extra processing Bulk-blend fertilizers are mixes of solid fertilizers added as individual components to the specifications of the producer prior to field application. Although this is a flexible way to produce custom blends for a spe cific ficld, it suffers from the disadvantage that even with premixing for uniformity, the different sizes of the starting materials willcause them to segregate en route. Slow-Release Fertilizers The use of slow-releasefertilizers is very popular in horticulture. This form of fertilizer is considered a slower rate than other fertilizers. These are also dry fertilizers. the featurethat makesslow-release fertil Typically. izers work is a coating of some relatively insoluble material around the fertilizer, such as with S-coated urea. Urea is very soluble in water but elemental S is not. By coating the urea with a thin film of elemental S, it is more difficult for the urea to come into contact with water and go into solution. Plastics -is or polymers are also uscd to coat urea and other soluble nutrients. Osmocote an eof this material. Sulfur r and an additional coating. exabined 1o such as wax, plastic, or a polymer, have also been increase the longevity ofthe fertilizer. The second coat surrounds the S coating.These products have been marketed by Scotts Co.and Purcell Industries. Nitrification and urease inhibitors have been added to some fertilizers to slow the transformation of N. Urcase inhibitors such as NBPT (N-{n-butyl]thiophosphorictriamide, trade name Agrotain) inhibit urcase  Chapter 10 Fertilizers and Fertilization 215 activity in soil. Nitrification will be inhibited by DCD (dyaciandiamide). The combination of the two with urea fertilizers will slow down the rate at which urea is hydrolyzed and thus ammonium (NH,)subsequently nitrified. These kinds of products are often referred to as "stabilized N fertilizers. C. Fertilizer Materials Few fertilizers are pure elements. Most fertilizers consist of compounds that can release nutrients in forms that are useful for plants. These compounds are known as nutrient carriers. Nitrogen Fertilizers Organic nitrogen has several forms. Decay changes the organic nitrogen to ammonia,which, in turn, changes to nitrates. Many organic sources are too expensive for farmers and are used mainly by home gardeners, how ever some sources, such as manure, continue to be used widely. The first commercial was organic seabird droppings, or guano,harvested in Peru. nitrogen fertilizer Later,deposits of sodium nitrate were mined in Chile. Today, fertilizer companies manufacture most nitrogen carriers from ammonia produced by the Haver-Bosch process. The Haber-Bosch process fixes nitrogen from the air by attaching hydrogen atomsfrom natural gas.This reaction,in the presence of heat, pressure, and an iron catalyst, produces ammonia.This reaction is costly in terms of an energy standpoint, and the price of natural gas accounts for 70%-90% of manufacturing costs. Ammonia can be used as fertilizer or changed to other forms. Table 10. summarizesthe characteristics of various N forms, anotherN carriers. set of - Anhydrousammonia has 82% N and is the cheapest and with water to relcase ammonium ions. Ifnot injected strongest of deep enough, N. In the particularly soil, ammonia reacts in sandy soil, it may evaporate and be lost from the soil. Ammonia destroys lung tissue ifinhaled, socare should be taken during transport and application to ensure that none escapes. - Aqua ammonia (24%N) comes from dissolving ammonia in water to form a low-pressure The use ofaqua ammonia has declined as growers switch to other fluid fertilizers. solution. Ammonium nitrate (33%N) is half ammonium nitrogen and half nitrate. This is a good general purpose dry material that is easy to handle and apply. It absorbs moisture from the air, causing it to cake. Ammonium sulfate (21% N), contains sulfur. It is a dry fertilizer. Ammonium sulfate is very acid-forming and is ideal for acid-loving plants. Abny ammonium fertilizer, including ammonium nitrate or sulfate, can lose nitrogen by volatilization when spread on recently limed or calcareous soils. To prevent loss of nitrogen, it is best mixed into the soil. - Nitrate of it soda(sodium nitrate16% N) is typically used for tobacco. Unlike most other N sources, raises plH. Calcium and potassium nitrates are similar but less salty materials,commonly used in greenhouse fertilizers. Both materials are dry but water soluble. -Urea (16%N) is a synthetic organic material. In soil, urea rapidly breaks down into ammonium nitrogen. Ifurea is left on the top of the soil surface, some ammonia may escape into the air. A popu lar dry fertilizer, perhaps the most widely used N fertilizer. It can be produced less expensive than ammonium nitrate. It is also used to make fluid fertilizer. -Urea-ammonium nitrate (UAN) is a nitrogen solution.UAN is made by liquid urea and ammonium nitrate to make cither a 28% N solution or a 32% N solution. - Urea-formaldehyde (UF),isobutyldiene urea (IBDU),and sulfur-coated urea (SCU)are slow-release synthetic organic materials.These are used mainly to fertilize turfgrass and potted plants. They are too costly for general agriculture use. Phosphorus Fertilizers Phosphorus fertilizers are obtaincd from surface mining of rock phosphate. Rock phosphatewas deposited during geologic time under shallow seas. It various apatite minerals, which are calcium phosphate contains minerals. The ground rockcan be applied directly to the soil but is usually treated with acid to break down the simplercompounds of higher water solubility. Table 10.2 shows some organic sources of phos apatite into phorus and Table 10.3 summarizes the common phosphatefertilizers. - Rock phosphateis a rock powder containing 25-35% of P,0, and calcium. It may be spread on the soilbut will dissolve slowly. It is an example of mineral fertilizer. Rock phosphateworks best when finely ground and used on acid soils,and is commonly used in organic systems. - Superphosphatewith 20% of P,0, is made from the reaction of rockphosphatewith sulfuric acid. This material is half gypsum (calciumsulfate) and half calcium phosphate. It is no longer used by growers because it has a low content of phosphatecompared to others. Growers usually would like to have the sulfur and the calcium is lacking in the soil. - Triple superphosphatehas higher phosphorus level than regular superphosphateand contains less sulfur or calcium.This is a popular fertilizer. Both superphosphatescontain smallamounts of fluo rine, making them unsuitable for the many potted foliage plants highly sensitive to fluorine. Neither is watersoluble enough to make up a solution. - Ammonium phosphate is made by mixing phosphoric acid with ammonia. Two similar products are monoammonium phosphate and diammonium phosphate, known as MAP and DAP, respectively. Both are more watersoluble than superphosphate compounds and arethe products of choice in applyingphosphorusin fluid form.They arealso used in dry fertilizers. - Bone meal and manure areboth organic sources of phosphate. Bone meal is made by grinding bones thatarea by-product of the meat-packing industry. Homeowners usually use bone meal as phosphate and calcium fertilizer.  Phosphoricacid 53 23 Source:Adapted by Handayani from Plaster, EJ. (2014). Potassium Fertilizers In the world of fertilizers, the term potash is commonly used when we speak about potassium. Potash is domi nantly mined in deep mines from deposits laid down in geologic time by evaporating shallow seas. Some are harvested in gigantic evaporation pans from evaporating water. Canada is the greatest supplier of the world's d purify the potash. These deposits are mixtures of various salts, which are processed to separate and potassium. Table 10.4 describes the primarycarriers of potassium and Table 10.2 lists some organic potassium sources. Muriateof potash (Potassiumchloride) has 60% potash and accounts for 97% of all potassium fertil ization. It costs less expensive than other carriers and readily dissolves in water. It is considered highly saline and contains chlorine. Potassium sulfate or sulfate of potash has 49% potash and is minor in dry fertilizer for crops that are sensitive to chlorine. - Potassium nitrate or nitrate pots/containers. It of potash (13% N and 44% potash) is a common can be used both as dry form and soil solution. fertilizer for plants in Sulfate of potash-magnesia (22% potash, 229% sulfur, 11% magnesium) is useful for chlorine-sensitive crops and for soil that has salinity problems. It is good for soil that has limited amounts of magnesium and sulfur. - Wood ashesand manure are also good potash sources. 218 Chapter 10 Fertilizers and Fertilization -Granite meal is a rock powder used by some farmers who are not using chemical fertilizers. It is a finely ground, gritty waste product of the monument and building-stone industry. - Greensand is another rock by organic farmers. powder derived from certain sedimentary deposits. It is commonly used Table 10.4 Common potash carriers. Potash Carrier Available K,o (%) Nitrogen (%) Muriate of potash 60 Sultate of potash Potassium nitrate 44 13 Sultate potash magnesia 22 Source: Adapted by Handayani from Plaster, EJ. (2014). Calcium, Magnesium, and Sulfur Fertilizers Mineral fertilizers mostofthe calcium, magnesium,and sulfur needed by plants. Thecommon fertil provide izers for these nutrients are lime, dolomitic lime,gypsum, and sulfur. The finer the grind, the faster these materials act. Except for gypsum, all the minerals influence soil pH. Table 10.5 describes several sources of secondary nutrients (calcium, magnesium,and sulfur). Table 10.5 Common sources of calcium, magnesium,and sulftur. Material Ca Mg S Effect on Soil pH Calcitic lime 31.7 Basic Dolomitic lime 21.5 114 Basic Gypsum 22.5 12.0 Neutral Hydrated lime 46.1 Basic Burned lime 60.3 Basic Magnesia 55.0 Basic Magnesium sulfate (Epsonsalts) 110 14.5 Neutral Potassium magnesium sultate 110 22.0 Neutral Flowers of sulfur 30-100 Acidic Source: Adapted by Handayani from Plaster, EJ. (2014). Trace Elements Fertilizers Traceelements are separately available in different chemical forms. Most trace clements arecommonly used in the following forms(Table10.6). - Sulfate saltsare low in price and casily dissolve in water. Trace elements are also available in various formsof salts They can be used asdry or fluid fertilizers. and oxides. - Fritted trace elements (FTEs)area safe way to apply trace elements. These materials canbe made by adding salts to molten glass, which is poured into cold running water. The glass cools and shat ters; the picces are then ground into fine powder. Frits are dry fertilizers that act as a slow-release source ofmicronutrients. Chelates are popular, beneficial,and water-soluble trace element fertilizers. They are used in fluid form. They arecommercially available as a fertilizer. Source: Adapted by Handayani from Plaster, E.J.(2014). D. Content of Fertilizers Mixed Fertilizers During fertilization,mixed fertilizerscan be used,meaning a fertilizationoperation can use a single nutrient or fertilizersthat have several nutrients. These fertilizers aremade by mixing several ofthe carriers/fertilizers previously discussed. Fertilizer Formulation All fertilizers areanalyzedto determinethe fertilizer grade. This grade is represented by three numbers that correspond to the percentageby weight nitrogen (N),phosphate (P,0,), and potash (K,0). It is often sim of plified as nitrogen, phosphorus,and potassium(N-P-K),however, it is really based on the percentage weight of N-P,O, -K,0. Fertility recommendations for the primary macronutrients are given in pounds per acre of plant-available nitrogen (N).phosphorus(P,0,), and potassium(K,0).This correspondsto the way fertilizer compositionis defined. For example, a fertilizer that is a 10-20-20means it contains 10% N, 20% P,O,,and 20% K,0. A bag of fertilizer has content that gives information about fertilizergrade and fertilizer analysis. Some times, the bag has lists of the fertilizerelements in the bag and their percent content as well as the percent of nitrogen as nitrate andammonium. The N and K are water-soluble, and the P is citrate acid soluble, reflecting the fertilizer's content of plant-available nutrients. Inaddition to the primary macronutrients, fertilizers may contain other nutrients such as sulfur (S), boron (B),and zinc (Zn). Ifa significant source of a particular nutrient, other than N, P, or Kis present in the it is typically labeled asa fourth value in the fertilizer grade. For example,ammonium fertilizer, sulfate is described as21-0-0-20S. Anotherexample is calcium nitrate may carry the grade 15-0-0-39Ca,mcan ing the material is 39% calcium. Similarly, sulfur or magnesium is found as a fourth number.Fertilizer contain ers typically also list the carriers used to formulate the product, sortof an ingredient statement. For example, a 100-pound bag of 10-10-10 fertilizercontains 10% or 10 pounds of nitrogen (N), 10% or 10 pounds of phosphate(P,0,), and 10% or 10 pounds of potash (K,0).Of the 10 pounds of phosphate, only 4.4 pounds are phosphorus (multiply the weight of the phosphate by 0.44). Of the 10 pounds of potash, 8.3 pounds is potassium (multiply the weightofthe potash by 0.83). So only 22.7 pounds of the 100pounds is made ofnitrogen, phosphorus,and potassiumelements. A fertilizer with the grade 0-0-60 is 60% potash with no nitrogen or phosphate. To decide how much po is in the bag or a ton of fertilizer, multiply the weighttimes the percentage. Thus, one ton of muriate notash contains the following amountof potash: Potash (2,000x 60%Y100 = 1,200pounds 220 Chapter 10 Fertilizers and Fertilization One might call this a fertilizer formulafordetermining the amount of active ingredient in any quantity of fertilizer. A fertilizer containing only one element/nutrient is called a single-grade fertilizer. Many fertilizers contain two or three nutrients and are known as mixed fertilizers. Based on the presenceof nutrients in fertilizer bags,there are several categories including complete fertilizers,balanced fertilizers,and incom plete fertilizers.Complete fertilizers are fertilizers that contain all three of the major plant nutrients (nitrogen, phosphorus,and potassium)such as 10-10-10, 5-10-10, or 16-4-8. Note that "complete fertilizer" does not mean that all 14 mineralnutrients are included. Soils requiring only one or two of the three major plant nutrients do not require a complete fertilizer. Balanced fertilizers arethose fertilizers which contain equal amountsof nitrogen, phosphate, and potash. For example, 10-10-10 and 17-17-17 arebalanced fertil izers. Incomplete fertilizers are fertilizer that lacks one or two of the three major plant nutrients. Multiple incompletefertilizers can be combined ed to create complete fertilizers to meet the neceds ofa specific soil as determined by a soil test. To determine the amount of cach nutrient in a complete fertilizer, the percentage of the nutrient is multi plied by the weightof the fertilizer. For example,in 50pounds bag of 20-10-20: Nitrogen=(50pounds x 20%/100 = 10 pounds Phosphate= (50 pounds 10%/i00 = 5 pounds x Potash =(50pounds x 10%/100 = 10pounds Additional information may also be found in the analysis, such as the percentage of nitrogen that is ammoniacal and the percentage that is nitrate. Some fertilizers, particularly those blendedfor turf, have nitro gen sources that dissolve slowly. These will be identified as water-insoluble nitrogen (WIN) or slow-release nitrogen (SRN). Fertilizers may contain other substances as well. They can contain conditioners to keep the fertilizer in granular form for easy spreading. They contain absorbents to slow the release of nutrients into the soil. In some cases, a filler may be added to dilute the concentration of nutrients to yielda low-analysis fertilizer. Premium fertilizers can also contain compounds like limestone, which act both asa filler and to neutralize acidic soils. Fertilizer Filler Fertilizer gradesnever total 100%.For example, 10-10-10 fertilizer is 30% nutrient and 70% other ingredients. remainderof the Primarily, the fertilizer is the weight ofthe other elementsthat are part of the carrier, such as hydrogen and oxygen. d A small percentage is fertilizer filler and conditioner. Fillers may be sand,clay, granules, ground lime ground corncobs. They are used to bring a load of bulk stone, or fertilizer toa weight a ton. Conditioners of improve the quality of the fertilizer and make it casier to use. Fertilizer Ratio The fertilizer ratio refers to the relative amount of nitrogen, phosphate, and potash in fertilizers. Ratios are useful when comparing two fertilizers. The details can be seen below: Grade Ratio  Chapter 10 Fertilizers and Fertilization 221 (a) and (b) have similar ratios, meaningthat one fertilizer can be used in place ofthe other. Applying one ton of 10-10-10 is the same asapplying 0.5 ton of 20-20-20. Inthis case, farmers can select fertilizer with the ratio suggested by soil test reports. For example, ifthe report suggested 100poundsofN,50poundsof phos phate and 75 pounds of potash per acre, then a single fertilizer application with a ratio 4:2:3 would be ideal. of Oxide Forms in Fertilizers In general, people think that grade is NPK. Actually, N is listed as the element but the other two fertilizer nutrients are listed in their oxide forms. The true gradeis listed as N-P,0,-K,0,which is read as nitrogen, phosphoric acid, and potash. Therefore, farmers need to do a conversion related to this fact. The following formulas s for the conversion are: Px 2.29 = P.0, P,0, x 0.44 =P Kx =K,0 1.2 K,0 x 0.83 K E. Fertilizer Application Method The form of a fertilizer affects how andwhereit is applicd.Fertilizers can be applied by injction, surfacebroadcast, broadcast incorporated, band application, fertigation, foliar application, sidedress,topdress,and seed placement. Injection is used to place liquid or gaseous fertilizer below the soil near plant roots. This method can reduce losses through the precise application of nutrients, but it is slow and expensive because it requires specialized equipment. Surfacebroadcast is a methodby which is applied on the surface across an entire field. High fertilizer capacity fertilizer spreaders are often used which dry fertilizer or spray liquid fertilizer on the soil sur spin face or ona growingcrop. This method is fastand economical, but it may create high nutrient losses and low uniformity, and the P efficiency is only 1/3 to that of thebanding method. Broadcast incorporated improves the efficiency of surface application by incorporating fertilizer through plowing or disking. Plowing is considercd better in terms of nutrient availability, as it creates a nutrient-richzone a few inches below the soil surface (wheredeveloping plant roots canabsorb it). The advan tages ofthis method are reduces losses compared to broadcast, improves plant uptake, but the disadvantages are slow, non-uniform application, erosion risk. Band application isknown asstarterapplication. Fertilizer isapplied in bands near where develop also ing roots will casily reach it; either to the side and below the seed rows, slightly below the seeds, or in between rows. A common practice is to band fertilizer 2 inches to the side and 2 inches deeper than the seeds or plants. This provides the plants with a concentrated zone of nutrients and can improvenutrient use effi ciency. The process can be done before or simultaneously with planting or seeddrilling. Liquid or dry fertil izers can be used. The advantages include high nutrient use efficiency and jump-starts carly growth. Many ficlds are deficient in P, due to soil binding and cold temperatures. Banding P makes it casier for plants to 1 grow.It also slows NH, conversion to NO, (nitrification), reducing the risk of leaching. But, the disadvan tages arecostly,slow, risk of salt burn to plants. Fertigation is the distribution of water-soluble fertilizers and chemicals through an irrigation system. The advantage is high nutrient use efficiency,but the disadvantages are irrigation equipment is needed (injec tion pump. etc) and the risk of uneven application in windy situations. p. Foliar application is the application of a small amount of fertilizer or mineral through direct spraying onto the leaves. It facilitates rapid uptake, but phytotoxicity, high expense, and limited to smalland/or repeated application may occur. 222 Chapter10 Fertilizers and Fertilization Sidedressingis when fertilizer applied between rowsofyoung plants toprovide a boostduring periods is of rapid and nutrient The most common use is sidedressing N on corn plants. The application amountgAenendent is thee results of a Pre-Sidedress Nitrate Test (PSNT) done when corn plants are on 12-24 inches tall. There are three methods of sidedressing: UAN applied with a pesticide sprayer fitted with drip nozzles; UAN injected between corn rows with disc openers; anhydrous ammonia injected into the soil. This placement is high nutrient use efficiency, but the timing often falls during the wet and busy seasonand slow process. Topdressing is when fertilizer or manure is spread on established fields (grasses, legumes). It is consid ered high nutrient use efficiency but sometimes losses may occur. Seed placement is also known as pop-up application. A small amount of fertilizer is placed with corn seeds during planting, sometimes in conjunction with banding. Both liquid and dry can be used. The advantages include lowerequipmentcosts, starter effect greater than just meeting nutrient requirements. Disadvantages are it can be phytotoxic if too much fertilizer is applied and retro-ftting planters can be expensive. In this method, urea and DAP cannotbe used, and to prevent salt burn the total rate must be kept below 10 Ibs ofN + K,0. F. Time to Apply and the Method of Fertilization Before Planting Fertilizing before planting brings soil fertility to a good level to launch the crop, and can supply much of the P and potash for the season.Preplant fertilization is particularly necessary for perennial crops. Common methods include broadcasting and injection. Fertilizing before planting could also include the incorporation of fertilizer into potting mixes before planting. At Planting The most common method of fertilizing at planting is banding. Banding is the most efficient way to apply phosphate, and sometimes potash. Banding can also be done more deeply to provide P and K throughout the growing season. Overall, banding provides more growth response with less fertilizer. After Planting Fertilizing after planting allows for split application, and is the only way to fertilize perennial crops in the years after planting crops such as turf, orchards, or forages. Most postplant fertilizers pay more attention to renewing N during the growing season,which is the most eficient time to apply N to rapidly growing crops. Several common methods of fertilizationare topdressing, sidedressing, fertigation, and foliar seeding G.The Impacts of Fertilizers on Soils Fertilizers affect the chemical properties of soil in addition to increasing nutrients in the soil. The effects are related to the change in soil pH and salinity. Most N fertilizers change the soil pH. Ammonium fertilizers depress soil pH due to the chemicalprocesses in soil that release hydrogenwhen ammonium nitrogen is added to the soil. In addition, nitrification of ammonium ions releases hydrogen into the soil, thus increasing soil acidity level. On the other hand, nitrate fertilizers increase soil pH. Thisoccurs becauseroots release basic hydroxideor bicarbonate ions into the rhizosphere when they absorb a nitrate ion. Fertilizers are salts, therefore they raise soil salinity. In humid areas where natural salinity is low, small increases in soil salts areof little concernexcept when fertilizersare placed near seeds,as in banding. In arid  Chapter 10 Fertilizers andFertilization 223 arcas, where natural salinity is high, and often increased by irrigation, less-saline materials may be recom mended on golf courses. Misapplying or spilling fertilizer may damage roots and cause fertilizer burn, as when a homeowner spills fertilizer on the lawn. H. Fertilizer Management There are several processes involved in fertilizer management,including: - determining what the fertility status of the soil is; - deciding what plants will be grown; -evaluating how much fertilizer they need; - calculating the economically optimum fertilizer rate; - determining when and where to apply the fertilizer; - makingany subsequent problems. fertilizer amendmentsthat can be used to address nutrient deficiency Soil Tests Most recommendations for fertility requirements are based on a soil test. Thus, the most important part of ensuring a good fertility recommendationis collecting a representative soil sample. As a general rule, most soil samples are taken from the first 15-20cm (6-8 in.), which represents a common depth to which many fertilizers are incorporated by tllage. In pastures or minimally tilled ficlds, the sampledepth is shallower, only 8-10 cm (3-4 in.), because fertilizer and nutrient movement is not as deep. Each sampletypically represents a composite of 10-20soil cores per 8 ha (about 20 acres) taken by auger, soil probe, or shovel and mixed to be asI as possible. If there is known variability in the ficld, or there will be known variability in the managementofthe field, then separate soil samples should be taken for each management area. Plant Nutrient Requirements Different plants have differentnutrient requirements. For any given state, this information is usually available from the state's Cooperative Extension Service, which publishes recommendations for fertilization based on soil test levels. For example, in Kentucky, the KentuckyCooperative Extension Services use a conservative approach to fertilizer recommendationssthat is based on assumed climatic conditions, management conditions, and average that is yields. The recommendations are not designed to increase soil test values for nutrients such as P and K. Other Cooperative Extension Services use different approaches to fertilizer fertilizations appropriate to their states. Nitrogen fertilization recommendationsare complicated by several factors, such as drainage that causes N loss due to leaching anddenitrifcation. denitrifi mineralization, N, fxation by legumes, inorganic N variability across the landscape, and tillage systems. Therefore, it is often recommended to performasoil test in the fall in preparation forspring fertilization, but the soil test values for nitrate and ammonium in this period. Residual Fertility Native fertility is the level of available plant nutrientsa soil provides without additional fertilization. Resid ual fertility is this inherent native fertility as measuredby a soil test) plus some additional amount that becomes available during the year as the organic and mineral formsof nutrients decompose and weather. 224 Chapter10 Fertilizers and Fertilization Residual fertility decreases with prolonged cropping and weathering,and it increases as a result of pro longed application of inorganic and organic nutrients. Beyond a certain level residual fertility increases to such an extent that no benefit to plant growth is obtained, and for some micronutrient toxicity is observed. Residual fertility also affects the placementof additional fertilizer in the soil. If the residual fertility levels are low, banding is more effective than broadcasting fertilizer in terms of affecting yield. At low residual fertility levels, more ofthe applicd nutricents are fixed into less available forms.By concentrating the fertilizer in a specific band,the effect is to saturate the capacity of the soil to immobilizethe nutrients in a limited zone, and more nutrients will be available for plant uptake. At high residual soil fertility levels, it generally does not matter whetherthe fertilizer is banded or broadcast. Fertilizer Rates Crops will respond differently to fertilizers depending on whether the initial fertility levels are high or low. Fertilizer application ratesdepend on the soil test values, desired yield, and cost of fertilizer. When fertility levels are low, plants aremost li likely to respond toadditional fertilizer. When fertility levels arevery low more fertilizer may have to be added than anticipated to seethe desired result because of fertilizer immobilization in the soil environment. At very high fertility levels, there is generally little benefit from adding fertilizer. Timing for Fertilization Timing of fertilization depends on the typeof fertilizer that is added. Fertilizers can be added before, during. or after crop planting. It also can be added to existing pastures. Some fertilizer nutrients, such as P, are rela tively immobile after addition to soil,so theycan be added at any time without great fear of loss. Other nutri ents, such asN,have great potential to be lost through, volatilization, denitrification, and leaching. Therefore, -applied N, would be expected to show fall-a considerable losses before planting in spring. Ifthe mobility of nutrients is expected to be great, then application closest to planting is recommended. Mobile plant nutrients should be applied close to the period of plant growth.