Nitrogen Availability for Agriculture - Essay Example

1.0 Introduction to Nitrogen budget In order for the farmer to optimize the yield and protein content of crops like whet that is being cultivated under rain fed conditions, it is important for the farmer to enough but not excess nitrogen to the crop. It is important for one to be aware that what may work in one year may not work the year that follow. This could be attributed to the fact that the amount of rainfall and its distribution results into varied responses to a set rate of N-fertilizer. The nitrogen available for the crops depends on many factors for its availability. In these paper several factors the nitrogen availability to crops. Mineralization is one of the important process that avails nitrogen to the crops. While water is important in assimilation of the nitrogen by crops when water become excess this will result in the loss of nitrogen as illustrated. The soil type plays a big role in nitrogen processing and this has also been discussed. 2.0 Mineralization In the cases where organic farming is practiced there is gradual accumulation of N. In organic system of farming, the crop residues, composts and manure are the major means by which N is introduced into the soil. A bigger proportion of the N that is introduced in to the soul as organic material and this is not readily available to the crops uptake. In a soil where there is 3 percent organic matter in the top one foot depth there will be approximately 5,000 lb N per acre (Bundy, 2010) where N is only a small fraction of the N that is released in the form that is available for the plants in the period of plant growth in the process of mineralization. A process described as crop N uptake is used in the estimation of mineralization from the organic matter in soil (Camberato, 2001). The process in which the crops absorb N from the soil is unique to each location when it is being used in the investigation of N available to a crop. The measurement of crop absorption of N is an integration of many factors affecting the amount of N availed through mineralization. These factors include the percentage of soil organic matter, cropping history, biological activity, management and previous soil amendments. In organic farming, the N uptake by crops is seen to be the best tools that are used in the determination of appropriate N rates inputs for current season. One is able to use data on soil mineralization, in conjunction with site information so as to have what is termed customized N management program. The challenge is that it is required to collect and process crop N uptake data during the part of the year that is busiest. The increase in the yields and quality of crops like potato in combination with a reduction in N input costs, often results into profitability in contracting a consultant to perform and make interpretation of the measurements. 2.1 Mineralization, and calculation of fertilizer rate The level of plant-available nitrogen that is in the soil will often be lower than 10 percent of the N available and needed for the crop. Additional N is sourced from organic matter in the soil through mineralization process with the advance in the growing season (Schmidt et al ,2001). The quantity of available N sourced from materials like fish fertilizer, and broiler litter that is necessary in supporting of the growth and development of crop is dependant on level of N that is generated through mineralization. The amount of rapidly available N is found by subtracting N mineralized from the crop N requirement. The sad thing is that those engaged in growing of crops are not capable of to accurately make an estimation of N mineralization amounts through soil tests. In a laboratory set up it is possible to quantify both soil organic N and the N that is available to the plants that is found in the soil at sampling time but it is not possible to make a prediction of organic N which will undergo mineralization in the season that the plants will be growing. At this point rule of thumb becomes applicable in the estimation of typical amounts of N that will have been supplied through mineralization. In some documents the estimation made is about 2 percent of soil N that undergo mineralization yearly and thus being transformed into forms that are readily available to the plants. In that case if a soil containing 5,000 lb organic N in one acre is taken, the amount of N readily available to the plants as a result of mineralization would approximate to 100 lb of N per acre (Mackie-Dawson , 1999). However, there is a substantial variation of N mineralization from the estimates made from rule of the thumb. 2.2 Zero-N plot Planning Location An area is marked out in the years crop field where it is possible to eliminate current season fertilization. In order for the field to be represented adequately more than one zero-N plot is created in one field. The zero-N plots are to be located in areas that well represents the whole field with wet and low fertility areas being avoided. Size of plot The best size is where the plot four rows widthwise with the two rows in the outer sides acting as buffer rows protecting them from application of fertilizers accidentally. It is recommended that the row length to be at least 25 feet but having longer plots gives one higher opportunity of collecting representative plant samples. Plot management When it comes to cultural practices such as planting, irrigation and cultivation, these are to be done in the same manner for the rest of the field. Utmost care is to be taken to ensure that there is no application of dry or liquid fertilizer on the plot in the entire season. Effect of system The zero-N plot is a reflection of N mineralization for a specified crop rotation. The N readily available to plants whose supply is from newly incorporated crop residues, composites or winter cover crops make a contribution to the plants crop N uptake. Nitrogen which is supplied in the irrigation water is utilized by the plants with need to test the irrigation water so as to quantify nitrate-N content. In order for the quantity of N obtained from one acre inch of water to be determined, the water N analysis (mgN/litre or ppm) is multiplied by 0.227. Thus 1 acre-inch of water having 10 ppm N supplies 2.3 lb N (10 ppm x 0.227 = 2.3 lb N). 3.0 Evaluating Nitrogen Losses In areas where large amounts of rainfall are received for several weeks there is always concerns about losses in N from cropland with questions being raised whether there is need for having additional or supplemental application of N. Leaching of available N below root zones of the plants and losses of readily available N in gaseous form by dinitrification process in places where the soils are saturated or flooded. The extent at which N is lost through leaching and/or denitrification subsequent to rains of high intensity is not usually known. The two processes of N loss involve N in nitrate form, with chances for significant loss being determined from quantity of the crop N supply available in form of nitrates following the occurrence of the high intensity rainfall. The extend of the losses is dependant on many factors such as the time of application of N, forms of application of N, or form of N that is expected to be the source of N to the plants, characteristics of the soil and to what extend the soil was wet. Generally losses associated with leaching are most to be experienced in soils that are sandy where water movement across the profile is fast. On the other hand dinitrification is mostly experienced in medium and fine textured soils whose drainage is poor. The soils have a tendency of becoming saturated and/or flooded areas being retained for a number of days after the rains. In the cases where there is supply of N fertilizer before crop planting, fertilizer application timing and the N form used are of importance in the determination of risk loss. Putting into consideration occurrence of losses of N in the form of nitrate, timing in the formation of nitrate is a vital consideration in the evaluation of chances of N losses. N fertilizer applied during the fall is likely to be lost following high level of rainfall a high proportion of N would be likely to be in the form of nitrate by mid of May. From the case of preplant applications in the spring, ammonium forms of N ( unhydrous ammonia and urea) are transformed to nitrates in approximately 4 to 6 weeks. The conversion of urea to nitrate is very rapid in comparison to the conversion of anhydrous ammonia. Nitrogen solutions which contain 50% of the N in the form of urea with the remainder being in the form of ammonia nitrate. This fertilizer will essentially contain 75% of N being in the form of ammonia and 25% being as nitrate during application. The fertilizers with urea will be transformed into ammonium-N in a period of 3 to 5 days following their application with the transformation taking place regardless of level of water saturation level. Losses due denitrification most likely occur after a few days suppose the soil is maintained in a saturated state or flooded in the presence of N. If there is temperatures that are warm and the soils being saturated for long periods the soils will be liable for high losses of N. Suppose relatively high proportion of the crop N requirement is to be sourced organically from manure or from leguminous crops previously planted in the plot, losses as a result of leaching or dinitrification will be quite low at such point of growing season. The reason being that most of the N sourced organically will have not been changed to nitrate. Production of nitrates at high rates from manure and leguminous crops will most likely commence in mid-June and would continue for overall weeks of the growing season. Checking of availability of N from organic sources can be through preplant soil nitrate test as described in next section. 3.1 Determining the need for additional N There is a great need for the farmers to determine if there is need to have extra N applied so as to compensate for N losses that might have occurred. There are a number of decision aids which are helpful in determining the need to have supplemental N. Preplant soil nitrate test (PSNT)- at this point of crop growth the PSNT will offer a diagnostic method that can be used in evaluation of N supply for crops. This test is useful in cases where previous legumes or manure application are used to provided either part of N required by the crop. This can also be so as to confirm to what extent there has been fertilizer N loss in samples of the soil depth. Nitrogen loss score sheet – this method was developed by a Minnesota soil scientist Michael Schmitt and Gyles Randall who developed a worksheet that could help in deciding if more was required to apply for crop where N losses were being suspected 4.0 Soil characteristics and N mineralization The level of N mineralization is linked to the level of soil organic matter (SOM) available in the soil (Franzluebbers et al, 1994). The agronomic activities that are responsible building up of SOM through the addition of crop residues in the soils have a great impact on soil fertility level which has a bearing on increased N mineralization (Torbert, et al, 1999). Soil texture also has influence on N mineralization rate of the soil. Soil texture that is favorable in retention of soil organic carbon C and N is linked with increased soil aggregation (Beare et al, 1994). The soils that has high level of aggregates the clay sized particles are found to be bound on organic material and thus protection of organic matter from decaying In soils with high amounts of aggregates, the clay-sized particles are bound around organic material, thereby protecting organic matter from decay (Jastow 1996). The organic matter will be prone to microbial attack if soil aggregates are destroyed. The cycles of wetting and drying also affect the microbial activities in the soil and in the long run affecting decomposition of SOM (Beare et al, 1994). There will be an increase in organic substrate available for use in microbial attack if drying is rapidly followed by rewetting. The substrates are derived from death of a fraction of soil organism when they dry resulting into the microbes undergoing osmotic shock which in turn has potential of inducing microbial cell lysis or these may result into the release of intracellular solutes (Beare et al, 1994). The labile substrates that are then available undergo rapid mineralization by remaining soil microbes and this will cause pulse N and C mineralization (Jastow 1996). Wetting and drying cycles have also been associated breaking apart of soil aggregates, exposing physically protected organic matter to further degradation (Franzluebbers et al, 1994). Soil organic matter decomposition followed by release of inorganic N from the organic N pool will take place owing to the activity of soil microflora majorly compost of bacteria and fungi. Microflora players together with their actions are affect by environmental and soil mineralogy factors and this in turn determines the speed at which the process of N mineralization in the soil will occur and consequently the amount that will be mineralization overtime. There is also interaction of the climate and soil properties such as microflora such that it affects the size as well as chemical nature of the soils organic N pool. Soil temperature as well as the moisture content bears a lot of effect on N mineralization reactions. Microbial activities will seize at a certain soil temperature and will increase with increase in temperature. The maximum level of N mineralization will take place at the point when the temperature rises to 30-350 C (Cullivan D. M. et al, 2008). In soils which are dry, low level of oxygen supply hinders mineralization because it is only soil micro-organisms that can survive under anaerobic conditions that will remain active. N mineralization reaction is dependant on the amount and type of clay. The level of mineralization has been observed to be high coarse textured soils whose clay content is low and reduce further with increase in clay content. Finely textured soils which are high in clay have more micropores which offer organic matter some form of physical protection from attack by microbial decomposition. In comparison to soil texture, soil mineralogy effects on mineralization are not clearly seen. Soils which are dominant with clay minerals associated with shrinking and swelling as the moisture fluctuates, such as montmorillonite, will tend have higher rates of mineralization unlike those containing non –shrinking and non-swelling clays such as kaolinite. Volcanic ash soils which have high content of organic matter are associated with high rates of mineralization. There is an association between N mineralization and immobilization processes and carbon cycle because decomposition of micro-organisms get energy from carbon compounds that is found in soil organic matter. Carbon and N compounds found in soil organic matter can be placed into two pools namely; a labile (active) and stabilized (passive) pools. The labile pool is found to be composed of microbial biomass, particulate organic matter and compounds that can easily decompose through microbial activities. On the other hand the stable pool composition primarily has compounds which can easily decompose through the action of microorganisms. 5.0 Conclusion From this paper it has been seen that nitrogen available for the crops depends on many factors for its availability. The availability of N to was discussed by addressing three issues: mineralization, evaluating nitrogen losses and soil characteristics and mineralization. References Beare, M.H., Hendrix, P.F., and Coleman, D.C. (1994) Water-stable aggregates and organic matter fractions in conventional and no tillage soils. Soil Science Society of America Journal, 58: 777–786. Bundy L. G. (2010). Evaluating Nitrogen Losses Following Excessive Rainfall Camberato J. J. (2001). Nitrogen in soil and fertilizers. SC Turfgrass Foundation News, , volume 8, number 1,page 6-10. Franzluebbers, K., et al (1994) Carbon and nitrogen mineralization form cowpea plants part decomposing in moist and in repeatedly dried and wetted soil. Soil Biology and Biochemistry, 26: 1379–1387. Jastow, J.D. (1996) Soil aggregation formation and the accrual of particulate and mineral-associated organic matter. Soil Biology and Biochemistry, 28: 656–676. Mackie-Dawson L A (1999). Nitrogen uptake and root morphological responses of defoliated Lolium perenne (L.) to a heterogeneous nitrogen supply. Plant Soil 209, 111–118. Malpassi R N, et al 2000 Oat and rye root decomposition effects on nitrogen mineralization. Soil Sci. Soc. Am. J. 64, 208–215. McMichael B L and Burke J J (1996) Temperature effects on root growth. In Plant Roots: The Hidden Half, 2nd Ed. Mutch D R and Snapp S S (2003). Cover crop choices for Michigan. Michigan State University Extension Bulletin E2884. Puget P and Drinkwater L E (2001). Short-term dynamics of root and shoot-derived carbon from a leguminous green manure. Soil Sci. Soc. Am. J. 65, 771–779. Rasse P D, (1999). Modifications of soil nitrogen pools in response to Snapp S S, Koide R and Lynch J P 1995 Exploitation of localized P-patches by common bean roots. Plant Soil 177, 211–218. Sanchez J E, et al (2001). Enhancing the mineralizable nitrogen pool through substrate diversity in long term cropping systems. Soil Sci. Soc. Am. J. 65, 1442–1447. Schmidt W E, et al (2001). Value of Legumes for Plowdown Nitrogen. The Ohio State University Extension Factsheet. AGF111-01. Sullivan D. M. et al (2008). Estimating Nitrogen Mineralization in Organic Potato Production Torbert, H.A., et al (1999) Land management effects on nitrogen and carbon cycling in an Ultisol. Communications in Soil Science and Plant Analysis, 30: 1345–1359. Read More

The conversion of urea to nitrate is very rapid in comparison to the conversion of anhydrous ammonia. Nitrogen solutions which contain 50% of the N in the form of urea with the remainder being in the form of ammonia nitrate. This fertilizer will essentially contain 75% of N being in the form of ammonia and 25% being as nitrate during application. The fertilizers with urea will be transformed into ammonium-N in a period of 3 to 5 days following their application with the transformation taking place regardless of level of water saturation level.

Losses due denitrification most likely occur after a few days suppose the soil is maintained in a saturated state or flooded in the presence of N. If there is temperatures that are warm and the soils being saturated for long periods the soils will be liable for high losses of N. Suppose relatively high proportion of the crop N requirement is to be sourced organically from manure or from leguminous crops previously planted in the plot, losses as a result of leaching or dinitrification will be quite low at such point of growing season.

The reason being that most of the N sourced organically will have not been changed to nitrate. Production of nitrates at high rates from manure and leguminous crops will most likely commence in mid-June and would continue for overall weeks of the growing season. Checking of availability of N from organic sources can be through preplant soil nitrate test as described in next section. 3.1 Determining the need for additional N There is a great need for the farmers to determine if there is need to have extra N applied so as to compensate for N losses that might have occurred.

There are a number of decision aids which are helpful in determining the need to have supplemental N. Preplant soil nitrate test (PSNT)- at this point of crop growth the PSNT will offer a diagnostic method that can be used in evaluation of N supply for crops. This test is useful in cases where previous legumes or manure application are used to provided either part of N required by the crop. This can also be so as to confirm to what extent there has been fertilizer N loss in samples of the soil depth.

Nitrogen loss score sheet – this method was developed by a Minnesota soil scientist Michael Schmitt and Gyles Randall who developed a worksheet that could help in deciding if more was required to apply for crop where N losses were being suspected 4.0 Soil characteristics and N mineralization The level of N mineralization is linked to the level of soil organic matter (SOM) available in the soil (Franzluebbers et al, 1994). The agronomic activities that are responsible building up of SOM through the addition of crop residues in the soils have a great impact on soil fertility level which has a bearing on increased N mineralization (Torbert, et al, 1999).

Soil texture also has influence on N mineralization rate of the soil. Soil texture that is favorable in retention of soil organic carbon C and N is linked with increased soil aggregation (Beare et al, 1994). The soils that has high level of aggregates the clay sized particles are found to be bound on organic material and thus protection of organic matter from decaying In soils with high amounts of aggregates, the clay-sized particles are bound around organic material, thereby protecting organic matter from decay (Jastow 1996).

The organic matter will be prone to microbial attack if soil aggregates are destroyed. The cycles of wetting and drying also affect the microbial activities in the soil and in the long run affecting decomposition of SOM (Beare et al, 1994). There will be an increase in organic substrate available for use in microbial attack if drying is rapidly followed by rewetting. The substrates are derived from death of a fraction of soil organism when they dry resulting into the microbes undergoing osmotic shock which in turn has potential of inducing microbial cell lysis or these may result into the release of intracellular solutes (Beare et al, 1994).

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