Soil Nutrient management
Nitrogen
Forms of Nitrogen Taken up by Plants
Plant roots take up nitrogen from the soil principally from ammonium (NH₄⁺) and nitrate (NO₃¯), sometimes called mineral nitrogen. Ammonium is held on the soil particles and can be exchanged with other cations in order for plants to take it up, but it does not leach easily from the soil. Nitrate, on the other hand, is found in the soil solution and can be leached from the profile.
Nitrogen Deficiency in Plants
Plants deficient in nitrogen tend to exhibit chlorosis (yellowish or pale green leaf colors), a stunted appearance, and thin, spindly stems (Figure 10.2). In all plants, slow growth and stunting are the most obvious signs of nitrogen shortage. Because nitrogen is highly mobile in the plant, under conditions of low nitrogen it is translocated from older to younger leaves to continue growth. This means that older leaves of nitrogen-starved plants are therefore the first to turn yellowish, typically becoming prematurely senescent and dropping off.
Nitrogen Toxicity in Plants
When too much nitrogen is available, excessive vegetative growth occurs; the cells of the plant stems become enlarged but relatively weak, and the top-heavy plants are prone to falling over (lodging) with heavy rain or wind. High nitrogen applications may delay plant maturity and cause the plants to be more susceptible to disease (particularly fungal disease) and to insect pests. These problems are especially noticeable if other nutrients, such as potassium, are in relatively low supply.
Mobility of Soil Nitrogen
The three most commonly applied nutrients are nitrogen, phosphorus, and potassium. Of these nutrients, nitrogen is the most likely to be leached from the soil. This is because a portion of nitrogen applied as fertilizer is converted by microorganisms in the soil to nitrate (NO3¯) via mineralization.
Management of Soil Nitrogen
Nitrogen is the most difficult nutrient to manage in crop production systems. Management factors, such as timing, the rate, and method of application, source of nitrogen, irrigation management, residue management, and the type of crops grown all have an affect in how efficiently nitrogen is used by crops and the amount of nitrogen loss.
Timing of Nitrogen Applications
Timing has a major effect on the efficiency of nitrogen management systems. Nitrogen applications should be timed to coincide as closely as possible to the period of maximum crop uptake. This application strategy results in a better match between nitrogen supply and crop demand, and reduces the risk of early-season nitrogen losses. Most crops use relatively little nitrogen in the early growth stages, but require increasing amounts as they grow larger. More can be applied later as crop needs increase. For example, wheat takes up most of its nitrogen in the spring and early summer, and corn absorbs most nitrogen in midsummer, so ample availability of nitrogen at these times is critical.
Nitrogen Application Rates
One of the most difficult decisions around nitrogen fertilizer management is the correct rate. This difficulty arises out of the myriad of factors that affect fertilizer availability: soil texture, soil mineralogy, organic matter content, season, and crop, to name a few. The variation among even these few variables makes choosing a correct fertilizer rate difficult, although ranges have been developed for certain crops. To calibrate the field-specific “right rate,” it is helpful to utilize a combination of rate trials, pre-plant soil tests, and in-season plant tissue analyses (e.g., petiole tests, remote sensing, etc.). New technologies for sensing plant nitrogen status, combined with global positioning systems (GPS) and variable rate controllers, allow for the added benefit of changing nitrogen application rates within a field to account for field variability (Section 11.4).
Minimizing Nitrogen Leaching
Fertilizers that contain nitrate such as urea ammonium nitrate (UAN) solution, ammonium nitrate, calcium nitrate are susceptible to nitrogen losses through leaching if substantial rainfall occurs soon after application.
Minimizing Volatilization Losses
All ammonium and ammonia-based fertilizers (e.g., urea, urea-ammonium nitrate, anhydrous ammonia), including manure, have the potential for ammonia volatilization—the loss of nitrogen to the air as ammonia gas (NH3). The amount of nitrogen loss varies greatly depending on placement of the fertilizer, soil pH, soil texture, and climatic conditions after application. A number of factors influence the occurrence and/or rate of ammonia volatilization.
Organic Nitrogen Sources
Management of nitrogen from organic sources is challenging. Some organic matter sources are inputs (e.g., manure or compost), while some are recycled in place (e.g., crop residues and soil organic matter). Manures, composts, and other organic materials vary in the amount and timing of nitrogen release via nitrogen mineralization. For example, composted organic materials are more resistant to decomposition, and they release plant-available nitrogen more slowly than fresh organic materials.
Legume Cover Crops
Plant species in the legume family, however, have a unique ability to collaborate with a specific species of bacteria (Rhizobium) that can transform this atmospheric nitrogen into forms that plants can utilize. In return, microbes trade the plants for sugars that only the plants can create from atmospheric carbon dioxide and sunlight energy. This unique partnership makes nitrogen unique among the major plant nutrients in that nitrogen becomes a renewable resource, which is different than phosphorus, potassium, calcium, etc.
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