What is the "nutrient cycle?

What is the meaning of "nutrient cycle"?

• Answer:

  Nutrient Cycling & Maintaining Soil Fertility Essential plant nutrients. There are at least 16 essential chemical elements for plant growth. Carbon (C), hydrogen (H), and oxygen (O), obtained from air and water, frequently are not included as ‘nutrient’ elements. Nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), and chlorine (Cl) are obtained from the soil and required by all plants. Sodium (Na), silicon (Si), and nickel (Ni) are essential elements for some plant species and, although not required, have positive or beneficial effects on the growth of other species. Cobalt (Co) is essential for nitrogen fixation by legumes. Additional elements, such as selenium (Se), arsenic (As), and iodine (I) are not required by plants, but can be important in plant nutrition since they are essential nutrients for humans and other animals that consume plants. All essential nutrients are equally important for healthy plant growth, but there are large differences in the amounts required. N, P, and K are primary macronutrients with crop requirements generally in the range of 50-150 lb. per acre. Ca, Mg, and S are secondary macronutrients, required in amounts of 10-50 lb. per acre. Micronutrient requirements (Fe, Mn, Zn, Cu, B, Mo, and Cl) are generally less than 1 lb. per acre. Nutrient Cycling Sources of plant nutrients in the soil. Mineral nutrients are obtained by plants through root uptake from the soil solution. Sources of these soluble nutrients in the soil include: 1) weathering of soil minerals, 2) decomposition of plant residues, animal remains, and soil microorganisms, 3) application of fertilizers and liming materials, 4) application of manures, composts, biosolids (sewage sludge) and other organic amendments, 5) N-fixation by legumes, 6) ground rock powders or dusts including greensand, basalt, and rock phosphate, 7) inorganic industrial byproducts, 8) atmospheric deposition, such as N and S from acid rain or N-fixation by lightning discharges, and 9) deposition of nutrient-rich sediment from erosion and flooding. Losses of plant nutrients from the soil. Mineral nutrients also can be lost from the soil system and become unavailable for plant uptake. Nutrient losses are not only costly and wasteful, but they can be a source of environmental contamination when they reach lakes, rivers, and groundwater. Nutrient losses occur through: 1) Runoff - loss of dissolved nutrients in water moving across the soil surface, 2) Erosion - loss of nutrients in or attached to soil particles that are removed from fields by wind or water movement, 3) Leaching - loss of dissolved nutrients in water that moves down through the soil to groundwater or out of the field through drain lines, 4) Gaseous losses to the atmosphere - primarily losses of different N forms through volatilization and denitrification, and 5) Crop removal - plant uptake and removal of nutrients from the field in harvested products. Nutrient pools in the soil. In addition to the variety of inputs and outputs, plant nutrients exist in many different forms or ‘nutrient pools’ within the soil. These pools range from soluble, readily available forms, to weakly bound forms that are in rapid equilibrium with soluble pools, to strongly bound or precipitated forms that are very insoluble and become available only over long time periods. Nutrients in solution can be taken up immediately by plant roots, but they also move with water and can easily leach below the plant root zone or be lost from farm fields. The ideal fertile soil has high nutrient concentrations in the soil solution when crop growth rates are high, but a large storage capacity to retain nutrients when crop needs are low or there is no growing crop. Exchangeable cations are a short-term storage pool that can rapidly replenish nutrient ions in the soil solution. Soil organic matter releases nutrients slowly as it decomposes, but is an important supply of N, P, S, and micronutrients. Soil minerals and precipitates vary from fairly soluble types (carbonates, sulfates, chlorides) in equilibrium with the soil solution to rather insoluble forms (feldspars, apatite, mica) that release nutrients through reactions with chemical agents such as organic acids. Adsorbed anions, such as phosphate and iron oxides bound to clay and organic matter surfaces, are held strongly and released very slowly, but can contribute to the long-term supply of plant-available nutrients. Basic Plant Nutrient Cycle Nitrogen Cycle. The Nitrogen Cycle is the most complex nutrient cycle. N exists in many forms, both chemical and physical, so transformations between these forms make the N-cycle resemble a maze rather than a simple, circular cycle. Chemical transformations of N, such as nitrification, denitrification, mineralization, and N-fixation are performed by a variety of soil-inhabiting organisms. Physical transformations of N include several forms that are gases which move freely between soil and the atmosphere. Although the N-cycle is very complex, it is probably the most important nutrient cycle to understand. There are two reasons for this: 1) N is usually the most limiting factor for plant growth in terrestrial ecosystems, so there is often a very large crop yield response to additional N, and 2) N in the nitrate form is very soluble and one of the most mobile plant nutrients in the soil, so it can easily be lost from farm fields and become a contaminant in surface waters or groundwater. Managing N is a critical part of soil fertility management. Nutrient balance & nutrient budgets. Nutrient cycling is not 100% efficient. There are always some losses or ‘leaks’ from the cycle, even for natural ecosystems. In farming systems, where products are bought and sold, the balance between nutrient inputs and outputs is easily shifted in one direction or the other. When the balance between inputs and outputs is quantified, a nutrient budget can be calculated. Nutrient budgets can be determined at different scales, from single fields to whole farms to landscapes and even broader regional areas. Strictly speaking, a cycle is a circular, closed-loop pattern, so the nutrient cycles diagramed above are not true cycles. They describe a larger picture where there is movement, or ‘flows’, of nutrients into and out of smaller systems such as farm fields. Nutrient balances or budgets look at these nutrient flows between different systems. Whole-Farm Nutrient Budgets. Different types of farms have different patterns of nutrient flow. They vary in patterns of internal movement within the farm as well as in the amounts of external transfers both on-to- and off-of- the farm. Cash-grain and concentrated-livestock farms represent two extremes in nutrient-flow patterns, with mixed crop and livestock farms in an intermediate position. Looking at these three farm types outlines the consequences and challenges faced by a range of different farm types in maintaining soil fertility and using plant nutrients efficiently. Cash Grain. Cash-grain farms export large amounts of plant nutrients in off-farm grain sales. A 150-bu./acre corn crop, for example, contains about 135-lb. of N, 25-lb. of P, and 30-lb. of K in the grain. When stover or small grain straw is also sold, nutrient losses are even larger, especially for K. To maintain high yields, these nutrients must be replaced. Biologically-fixed N from soybeans in the rotation supplies some N, but large N inputs from forage legumes are not usually a part of systems without livestock to consume the forage. Some soils may have enough residual P, K, or other nutrients to meet crop needs over the short term, but over the long term large amounts of off-farm fertilizer inputs are required to maintain nutrient balance and crop yields in cash-grain systems. In this age of globalization, international grain sales have become an important market for U.S. farmers. One consequence of global trade is the associated, world-wide transfer of plant nutrients. Mixed Crop & Livestock. Farms with both crops and livestock have the potential to recycle a large portion of the nutrients used by crops back to the soil, because about 70% of the nutrients consumed in animal feed are excreted in manure or urine. Efficient recycling depends upon storage, handling, and application methods that minimize losses, and an effective nutrient management plan that applies manure to fields in amounts matching crop needs with the nutrient content of the manure. Within a farm, manure can be a method of transferring nutrients between fields. Depending upon the balance between crop and livestock enterprises, whole-farm nutrient budgets on mixed farms include different amounts of nutrient losses in milk, meat, or eggs, and different levels of nutrient inputs from purchased feed and fertilizer. Concentrated Livestock. Concentrated animal-feeding operations import large amounts of plant nutrients in purchased grain, forage, and bedding. They are generally net nutrient importers, because purchased inputs exceed nutrient losses from milk, meat, or egg sales. These excess nutrients accumulate in animal wastes that often create disposal or storage problems. High-density livestock operations often have an inadequate land base to efficiently use all the manure they generate, so there is increased risk of water contamination. As livestock operations have become larger, they have also tended to concentrate regionally, resulting in increased geographic separation between feed-grain producers and consumers. Manures are bulky products that are difficult and costly to apply and transport long distances. In many locations it is not economical to recycle the nutrients in animal waste, so long-term storage rather than reuse has become the solution to the waste problem. The net result is increasing transfer of nutrients from one part of the country to another and increased dependence on purchased fertilizer inputs in grain production areas. Maintaining Soil Fertility Management practices to maximize nutrient cycling & nutrient-use efficiency. Nutrient management is defined as the efficient use of all nutrient sources and the primary challenges in sustaining soil fertility are to: 1) Reduce nutrient losses, 2) Maintain or increase nutrient storage capacity, and 3) Promote the recycling of plant nutrients. In addition, cultural practices that support the development of healthy, vigorous root systems result in efficient uptake and use of available nutrients. Many management practices help accomplish these goals, including establishing diverse crop rotations, growing cover crops, reducing tillage, managing & maintaining crop residue, handling manure as a valuable nutrient source, composting & using all available wastes, liming to maintain soil pH, applying supplemental fertilizers, and routine soil testing. These beneficial cultural practices have multiple effects on the soil fertility factors described above, which makes it important to integrate their use and examine their effects on the complete soil-crop system rather than just a single component of that system. Crop rotations. Growing a variety of crops in sequence has many positive effects. In a diverse rotation, deep-rooted crops alternate with shallower, fibrous-rooted species to bring up nutrients from deeper in the soil. This captures nutrients that might otherwise be lost from the system. Including sod crops in rotation with row crops decreases nutrient losses from runoff and erosion and increases soil organic matter. Growing legumes to fix atmospheric N reduces the need for purchased fertilizer and increases the supply of N stored in soil organic matter for future crops. Biologically-fixed N is used most efficiently in rotations where legumes are followed by crops with high N requirements. Rotating crops also increases soil biodiversity by supplying different residue types and food sources, reduces the buildup and carryover of soil-borne disease organisms, and creates growing conditions for healthy, well-developed crop root systems. Cover crops. Growing cover crops can be viewed as an extension of crop rotation and provides many of the same benefits. Growing legume cover crops adds biologically-fixed N. The additional plant diversity with cover crops stimulates a greater variety of soil microorganisms, enhances carbon and nutrient cycling, and promotes root health. The soil surface is covered for a longer period of time during the year, so nutrient losses from runoff and erosion are reduced. This longer period of plant growth substantially increases the capture of solar energy and the amount of plant biomass produced, which in turn increases organic matter additions to the soil. This organic matter is a pool of stored energy in the soil, in addition to a nutrient storage pool, and is the food and energy source for soil organisms. If you look at a farming system as an ‘ecosystem’, and measure the health or productivity of that ecosystem by its harvest of solar energy, then cover crops increase the health of farming systems by increasing the flow of energy and productive capacity through them. The extended growth period obtained with cover crops also extends the duration of root activity and the ability of root-exuded compounds to release insoluble soil nutrients. A winter cover crop traps excess soluble nutrients not used by the previous crop, prevents them from leaching, and stores them for release during the next growing season. Cover crops can also suppress weeds which otherwise would compete with crops for nutrients. Soil & water conservation practices. Soil erosion removes topsoil, which is the richest layer of soil in both organic matter and nutrient value. Implementing soil & water conservation measures that restrict runoff and erosion reduces nutrient losses and sustains soil productivity. Tillage practices and crop residue cover, along with soil topography, structure, and drainage are major factors in soil erosion. Surface residue reduces erosion by restricting water movement across the soil and tillage practices determine the amount of crop residue left on the surface. Reduced tillage or no-till maximize residue coverage. Water moves rapidly and is more erosive on steep slopes, so reducing tillage, maintaining surface residue, and planting on countour strips across the slope are recommended conservation practices. As discussed above, rotations and cover crops also reduce erosion. Soils with stable aggregates are less erosive than those with poor structure and organic matter helps bind soil particles together into aggregates. Tillage breaks down soil aggregates and also increases soil aeration, which accelerates organic matter decomposition. Well-drained soils with rapid water infiltration are less subject to erosion, because water moves rapidly through them and does not build up to the point where it moves across the surface. Drainage improvements on poorly drained soils reduce erosion. Improving drainage also decreases N losses from denitrification, which can be substantial on waterlogged soils, by increasing aeration. Manure management. Returning manure to crop fields recycles a large portion of the plant nutrients removed in harvested crops. On farms where livestock are fed large amounts of off-farm purchased feeds, manure applied to crop fields is a substantial source of nutrient inputs to the whole farming system. However, just as nutrients can be lost from the soil, nutrient losses from manure during storage, handling, and application are both economically wasteful and a potential environmental problem. Soluble nutrients readily leach from manure, especially when it is unprotected from rainfall during storage. Nitrogen is also readily lost through volatilization of ammonia, both during storage and when manure is not incorporated soon after field application. Nutrient losses from manure also occur when it is applied at excessive rates. Analyze manure for its nutrient content and adjust application rates based on crop needs and soil tests. Following heavy manure applications with crops that have high nutrient requirements, especially for N and P, reduces losses and increases nutrient-use efficiency. In addition to nutrient value, manure adds organic matter to the soil and provides benefits such as increased CEC for nutrient retention. Compost and other soil amendments. In addition to manure, organic amendments such as biosolids (sewage sludge), food processing wastes, animal byproducts, yard wastes, seaweed, and many types of composted materials are nutrient sources for farm fields. Biosolids contain most plant nutrients, and are much ‘cleaner’ than they were twenty years ago, but regulations for farm application must be followed to prevent excessive trace metal accumulation. Composting is a decomposition process similar to the natural organic matter breakdown that occurs in soil. Composting stabilizes organic wastes and the nutrients they contain, reduces their bulk, and makes transportation and field application of many waste products more feasible. On-farm composting of manure and other wastes also facilitates their handling. Most organic materials can be composted, nearly all organic materials contain plant nutrient elements, and recycling all available wastes through soil-crop systems by either composting or direct field application should be encouraged. These practices build up soil organic matter and provide a long-term, slow-release nutrient source. Inorganic byproducts also can be recycled through the soil and supply plant nutrients. Available materials vary by region, but rock powder from quarries, gypsum from high-sulfur coal scrubbers, and waste lime from water treatment plants are among the waste products that have been beneficially used. When considering the agricultural use of any byproduct, a thorough chemical analysis and review of possible regulations should be done to avoid soil contamination problems. Even seemingly benign byproducts should be analyzed and field-tested on a trial basis before using them on a large acreage. Healthy, vigorous root systems. Vigorous root systems tap nutrient supplies from a larger volume of soil, so management practices that stimulate root growth increase nutrient uptake. Uptake efficiency by extensive, well-distributed root systems results from increases in the amount of root surface area in contact with the soil. The extent of root-soil contact is only about 1-2% of total soil volume, even in the surface 6-inch layer of soil where root density is greatest. For immobile nutrients like phosphorus, root growth to the nutrient is very important. In most soils, phosphate will only move a few millimeters toward a root over the entire growing season. Root-soil contact is determined by root length, root branching, and root hairs. Root hairs are located just behind the root tip and have a relatively short life span of a few days to a few weeks. Actively growing feeder roots are necessary to continually renew these important locations for nutrient uptake. Nutrient absorbing capacity is also increased by symbiotic associations between soil fungi and plant roots. These fungi, called mycorrhizae, function as an extension of plant root systems. Mycorrhizae obtain food from plant roots and in return increase the nutrient absorbing surface for the plant through their extensive network of fungal strands. Mycorrhizae are particularly important for phosphorus uptake and can increase zinc and copper uptake as well. Root activity also has direct effects on nutrient availability in the soil. Insoluble nutrients are released and maintained in solution by the action of organic acids and other compounds produced by roots. Nutrients are also released because the soil immediately adjacent to roots, the rhizosphere, often has a lower pH than the bulk soil around it as a consequence of nutrient uptake. The rhizosphere stimulates microbial activity and microbes also release organic acids and other compounds that solubilize nutrients. A number of soil factors and management practices affect root growth, distribution, and health. Compacted soil layers restrict root penetration, low pH in the subsoil can restrict rooting depth, water saturation and poor aeration inhibit root growth, and roots will not grow into dry zones in the soil. Alleviating these conditions through some of the management practices described above can increase nutrient uptake. Cultural practices that maintain soil biodiversity promote healthy root systems, since an active and diverse microbial population competes with root pathogens and reduces root disease. Soil acidity and liming. Soil pH has strong effects on the availability of most nutrients. This is because pH affects both the chemical forms and solubility of nutrient elements. Trace metals such as Fe, Zn, and Mn are more available at lower pH than most nutrients, whereas Mg and Mo are more available at higher pH than many other nutrients. The ideal soil pH for most crops is slightly acid, about 6.3-6.8, because in that range there is well-balanced availability for all nutrients. This pH range is also optimum for an active and diverse soil microbial population. Some crops grow better at higher or lower soil pH than 6.3-6.8, usually because of specific nutrient requirements. Blueberries grow best around pH 4.5-4.8 and are Fe deficient when the pH is much over 5. Most crops suffer from Al, Fe, or Mn toxicity when soil pH is that low. Legumes do best at a higher pH than most other crops, due to the high requirement for Mo by N-fixing bacteria. Limestone is the most commonly used material to increase soil pH. Liming also supplies Ca and dolomitic lime supplies Mg as well. Liming rates depend upon the buffering capacity of a soil in addition to the measured pH. Buffering capacity, or ability to maintain pH within a given range, is related to CEC and increases as clay and/or organic matter content of the soil increases. The lime requirement to raise soil pH the same amount is much larger for fine-textured, high organic matter soils than for coarse-textured, sandier soils. Low soil pH is a more common problem than a pH that is too high, but reducing pH may be necessary for acid-loving plants. Elemental S is the most commonly used material to lower soil pH. Fertilizer applications. Many materials can be applied to soil as sources of plant nutrients, but the term ‘fertilizer’ is usually used to refer to relatively soluble nutrient sources with a high-analysis or concentration. Commercially available fertilizers supply essential elements in a variety of chemical forms, but most are relatively simple inorganic salts. Advantages of commercial fertilizers are their high water solubility, immediate availability to plants, and the accuracy with which specific nutrient amounts can be applied. Because they are relatively homogeneous compounds of fixed and known composition, it is very easy to calculate precise application rates. This is in contrast to organic nutrient sources which have variable composition, variable nutrient availability, and patterns of nutrient release that are greatly affected by temperature, moisture, and other conditions that alter biological activity. The solubility of commercial fertilizers can also be a problem, because soluble nutrients leach when applied in excess or when large rains occur soon after fertilizer application. Increasing soil cation exchange capacity by increasing organic matter reduces the leaching potential of some nutrients. Management practices that synchronize nutrient availability with crop demand and uptake also minimize leaching. Both application timing and the amount of fertilizer are important. Splitting fertilizer applications into several smaller applications rather than a single, large application is especially important on sandy, well-drained soils. Excess nutrient applications can be eliminated or at least significantly reduced by soil testing on a regular basis, setting realistic yield goals and fertilizing accordingly, accounting for all nutrient sources such as manure, legumes, and other amendments, and using plant tissue analysis as a monitoring tool for the fertilizer program. Soil testing. Soil testing is a very important soil management tool and serves as the basis for nutrient recommendations and liming requirements. General guidelines for soil testing include: 1. Soil test each field every 1-3 years, depending upon crop rotation and field history 2. Test for organic matter every 3-5 years to follow trends and make sure levels are increasing or being maintained 3. Have soil tested at a reliable, experienced laboratory: the OSU Research-Extension Analytical Laboratory or one of the private soil testing labs 4. If you use a private lab, especially from out-of-state, make sure results are interpreted correctly and recommendations are suitable for Ohio conditions 5. Test at the same time every year, so you can make uniform comparisons over time Soil sampling. Collecting a representative soil sample is often the weakest link in a soil testing program. Important guidelines for sample collection include: 1. Divide fields for sampling based on differences in soil type, slope, crop history, tillage, and previous lime, fertilizer, or manure applications 2. If you can think of any reason why one part of a field may be different from another, then sample it separately 3. A single sample should represent no more than 20-25 acres; divide larger fields even if they appear uniform 4. Collect 20-25 soil cores or subsamples from the selected area, mix the soil well, take about a pint of this composite sample and submit it for analysis 5. If the soil is wet, spread in a thin layer and air dry or oven-dry at less than 120 degrees F 6. Alternatively, samples can be collected on a grid system; this requires many more samples, but helps map field variability and is necessary for ‘precision’ or variable-rate nutrient applications 7. Samples should be taken from the 0-8 inch depth in tilled fields 8. No-till fields require two samples: one from 0-4 inches for pH & lime requirement and a separate 0-8 inch sample for nutrient tests 9. Ridge-till fields are sampled at 0-8 inches, but sample position relative to the ridge (row) is important; either sample between the rows after ridging or one-half the way up the ridge shoulder before ridging 10. Pastures should be sampled at the 0-4 inch depth Soil nitrate testing. Nitrogen is a common limiting factor in crop growth, despite the abundance of N in both the soil and atmosphere. Plant roots grow through soil with about 1,000-lb. of N per acre for every 1% soil organic matter. Plant leaves grow in an atmosphere that is 78% N, so there are about 35,000 tons of N in the column of air above every acre of land. Organic N must be broken down to ammonium and nitrate before it is available to a growing crop. Atmospheric N can be captured by legumes and eventually used by other crops in a rotation, but this also requires cycling through an organic phase. Organic N from soil organic matter, crop residue, manure, compost, biosolids, and elsewhere is a large source of available N for crops. The use of soil organic N in soil testing is usually unreliable, however, because the rate of N release is variable and unpredictable. It is a biological process that varies with temperature, moisture, aeration, the type of organic compounds being decomposed, and the relative abundance of different types of soil organisms. For this reason, N fertilizer recommendations are commonly based on average requirements for N by the crop being grown, and at a specific, anticipated yield level for that crop. Downward adjustments to this N requirement are made for manure applications, preceding legume crops, soil organic matter content, and other N sources, but these adjustments are based on ‘average’ conditions rather than resulting from reliable and reproducible chemical tests on the specific soil involved. Pre-Sidedress Nitrate Test. The majority of the N taken up by most crops is in the nitrate form. Testing for soil nitrate is easily and accurately done, but until recently nitrate testing was only used for fertilizer recommendations in regions with low rainfall and limited leaching. In more humid areas like Ohio, nitrate testing during the normal spring or fall soil-testing periods is not an accurate measure of nitrate availability during the growing season because it is too mobile. The usefulness of soil nitrate testing in many parts of the humid, eastern U.S. has changed, however, with the development of the Pre-Sidedress Nitrate Test (PSNT) for corn. In Iowa, this same soil nitrate test is called the Late-Spring Nitrate Test (LSNT). Soil samples for the PSNT are collected from the upper foot of soil when corn plants are 6-12 inches tall. This is late enough in the season to measure the effect of spring weather conditions on the amount of organic N released, but still early enough to apply additional N if necessary. In wet springs, nitrate testing at this time also gives information about N losses from pre-plant or starter fertilizer due to leaching or denitrification. The PSNT is a tool for efficient N management that can reduce excessive N applications, especially on manured fields and for corn following legumes. Successful research calibrating the test for Ohio has not been accomplished, although it is used to make N fertilizer recommendations for corn in several nearby states. Research efforts are also underway to extend the use of soil nitrate testing by calibrating it for other crops. End-of-Season Stalk Test. This is a nitrate test on corn stalks done at the end of the growing season to evaluate the outcome of N-management decisions made that year. Nitrate concentrations in the section of stalk 8-16 inches from the base are used to determine whether N availability was too low, too high, or in the adequate range for high yields. The value of the test is due to the fact that nitrate is removed from the stalk during grain-filling, so residual levels are an indication of N availability during the season. The stalk test is a good example of using plant tissue analysis to give feedback on soil fertility. This type of information permits producers to monitor crop responses on their own farms to different soil test levels and standard fertilizer recommendations. Over time, they can use the accumulated results to adjust these ‘average’ recommendations to the unique conditions of their farms and soil management practices. Biological & chemical approaches to soil fertility and nutrient management. The goals of effective nutrient management are to provide adequate plant nutrients for optimum growth and high-quality harvested products, while at the same time restricting nutrient movement out of the plant root zone and into the off-farm environment. Biological processes in the soil control nutrient cycling and influence many other aspects of soil fertility. Knowledge of these important processes helps farmers make informed management decisions about their crop and livestock systems. How these decisions affect soil biology, especially microbial activity, root growth, and soil organic matter are key factors in efficient nutrient management. Managing soil organic matter and biological nutrient flows is complex because crop residues, manures, composts, and other organic nutrient sources are variable in composition, release nutrients in different ways, and their nutrient cycling is strongly affected by environmental conditions. Chemical processes in the soil, to a large extent, control mineral solubility, cation exchange, solution pH, and binding to soil particle surfaces. Knowledge of soil chemistry makes it possible to formulate fertilizers that supply readily-available plant nutrients. Management of inorganic nutrient sources is simpler than organic nutrient sources, because of their known and uniform composition and the predictability of their chemical reactions. Chemical and biological processes and their effects on plant nutrients cannot be clearly separated, however, since inorganic nutrients can quickly be incorporated into biological cycles. Chemical fertilizers should also be used only after accounting for all organic nutrient sources to avoid overloading the system and losing soluble nutrients. When used to supplement biological nutrient sources, they can help make more efficient use of other available plant-growth resources, such as water and sunlight, and work together with biological processes for a productive agriculture and healthy environment.