Nutrent efficiency
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The document discusses nutrient use efficiency and provides context around commonly used terms and metrics. It notes that while interest in improving nutrient use efficiency is high, the terminology is easily misunderstood without proper context. It then reviews four common indices used to describe nutrient use efficiency and provides an example showing how the same data can be used to calculate very different efficiency rates depending on the metric and context. The document suggests optimizing efficiency through fertilizer best management practices like applying nutrients at the right rate, time and place to balance optimal efficiency with optimal productivity.
![split N applications in rice and now maize production in
Asia (Witt et al., 2005). Precision farming technologies
have introduced, and now commercialized, on-the-go N
sensors that can be coupled with variable rate fertilizer
applicators to automatically correct crop N deficiencies on
a site-specific basis.
Another approach to synchronize release of N from
fertilizers with crop need is the use of N stabilizers and
controlled release fertilizers. Nitrogen stabilizers (e.g.,
nitrapyrin, DCD [dicyandiamide], NBPT [n-butyl-
thiophosphoric triamide]) inhibit nitrification or urease
activity, thereby slowing the conversion of the fertilizer
to nitrate (Havlin et al., 2005). When soil and
environmental conditions are favorable for nitrate losses,
treatment with a stabilizer will often increase fertilizer N
efficiency. Controlled-release fertilizers can be grouped
into compounds of low solubility and coated water-
soluble fertilizers.
Most slow-release fertilizers are more expensive than
water-soluble N fertilizers and have traditionally been
used for high-value horticulture crops and turf grass.
However, technology improvements have reduced
manufacturing costs where controlled-release fertilizers
are available for use in corn, wheat, and other commodity
grains (Blaylock et al., 2005). The most promising for
widespread agricultural use are polymer-coated products,
which can be designed to release nutrients in a controlled
manner. Nutrient release rates are controlled by
manipulating the properties of the polymer coating and
are generally predictable when average temperature and
moisture conditions can be estimated.
Right place: Application method has always been
critical in ensuring fertilizer nutrients are used efficiently.
Determining the right placement is as important as
determining the right application rate. Numerous
placements are available, but most generally involve
surface or sub-surface applications before or after
planting. Prior to planting, nutrients can be broadcast
(i.e. applied uniformly on the soil surface and may or may
not be incorporated), applied as a band on the surface, or
applied as a subsurface band, usually 5 to 20 cm deep.
Applied at planting, nutrients can be banded with the
seed, below the seed, or below and to the side of the
seed. After planting, application is usually restricted to N
and placement can be as a topdress or a subsurface
sidedress. In general, nutrient recovery efficiency tends to
be higher with banded applications because less contact
with the soil lessens the opportunity for nutrient loss due
to leaching or fixation reactions. Placement decisions
depend on the crop and soil conditions, which interact to
influence nutrient uptake and availability.
Plant nutrients rarely work in isolation. Interactions
among nutrients are important because a deficiency of
one restricts the uptake and use of another. Numerous
studies have demonstrated that interactions between N
and other nutrients, primarily P and K, impact crop yields
and N efficiency. For example, data from a large number
of multi-location on-farm field experiments conducted in
India show the importance of balanced fertilization in
increasing crop yield and improving N efficiency (Table 3).
Adequate and balanced application of fertilizer
nutrients is one of the most common practices for
improving the efficiency of N fertilizer and is equally
effective in both developing and developed countries. In a
recent review based on 241 site-years of experiments in
China, India, and North America, balanced fertilization
with N, P, and K increased first-year recoveries an
average of 54% compared to recoveries of only 21%
where N was applied alone (Fixen et al., 2005).
Efficient Does Not Necessarily Mean Effective
Improving nutrient efficiency is an appropriate goal
for all involved in agriculture, and the fertilizer industry,
with the help of scientists and agronomists, is helping
farmers work towards that end. However, effectiveness
cannot be sacrificed for the sake of efficiency. Much
higher nutrient efficiencies could be achieved simply by
sacrificing yield, but that would not be economically
effective or viable for the farmer, or the environment.
This relationship between yield, nutrient efficiency, and
the environment was ably described by Dibb (2000) using
a theoretical example. For a typical yield response curve,
the lower part of the curve is characterized by very low
yields, because few nutrients are available or applied, but
very high efficiency (Figure 1). Nutrient use efficiency is
high at a low yield level, because any small amount of
nutrient applied could give a large yield response. If
nutrient use efficiency were the only goal, it would be
achieved here in the lower part of the yield curve.
However, environmental concerns would be significant
because poor crop growth means less surface residues to
protect the land from wind and water erosion and less
root growth to build soil organic matter. As you move up
the response curve, yields continue to increase, albeit at a
slower rate, and nutrient use efficiency typically declines.
Improving Nutrient Use Efficiency
180](https://image.slidesharecdn.com/nutrentefficiency-130418090311-phpapp01/85/Nutrent-efficiency-4-320.jpg)
Nutrent efficiency
- 1. Introduction Awareness of and interest in improved nutrient use efficiency has never been greater. Driven by a growing public belief that crop nutrients are excessive in the environment and farmer concerns about rising fertilizer prices and stagnant crop prices, the fertilizer industry is under increasing pressure to improve nutrient use efficiency. However, efficiency can be defined in many ways and is easily misunderstood and misrepresented. Definitions differ, depending on the perspective. Environmental nutrient use efficiency can be quite different than agronomic or economic efficiency and maximizing efficiency may not always be advisable or effective. Agronomic efficiency may be defined as the nutrients accumulated in the above-ground part of the plant or the nutrients recovered within the entire soil-crop-root system. Economic efficiency occurs when farm income is maximized from proper use of nutrient inputs, but it is not easily predicted or always achieved because future yield increases, nutrient costs, and crop prices are not known in advance of the growing season. Environmental efficiency is site-specific and can only be determined by studying local targets vulnerable to nutrient impact. Nutrients not used by the crop are at risk of loss to the environment, but the susceptibility of loss varies with the nutrient, soil and climatic conditions, and landscape. In general, nutrient loss to the environment is only a concern when fertilizers or manures are applied at rates above agronomic need. Though perspectives vary, agronomic nutrient use efficiency is the basis for economic and environmental efficiency. As agronomic efficiency improves, economic and environmental efficiency will also benefit. Nutrient Use Efficiency Terminology Nutrient use efficiency can be expressed several ways. Mosier et al. (2004) described 4 agronomic indices commonly used to describe nutrient use efficiency: partial factor productivity (PFP, kg crop yield per kg nutrient applied); agronomic efficiency (AE, kg crop yield increase per kg nutrient applied); apparent recovery efficiency (RE, kg nutrient taken up per kg nutrient applied); and physiological efficiency (PE, kg yield increase per kg nutrient taken up). Crop removal efficiency (removal of nutrient in harvested crop as % of nutrient applied) is also commonly used to explain nutrient efficiency. Available data and objectives determine which term best describes nutrient use efficiency. Fixen (2005) provides a good overview of these different terms with examples of how they might be applied. Turk J Agric For 32 (2008) 177-182 © TÜB‹TAK 177 Improving Nutrient Use Efficiency Terry L. ROBERTS* International Plant Nutrition Institute, 3500 Parkway Lane, Suite 550 Norcross, Georgia 30092 USA Received: 09.01.2008 Abstract: Public interest and awareness of the need for improving nutrient use efficiency is great, but nutrient use efficiency is easily misunderstood. Four indices of nutrient use efficiency are reviewed and an example of different applications of the terminology show that the same data set might be used to calculate a fertilizer N efficiency of 21% or 100%. Fertilizer N recovery efficiencies from researcher managed experiments for major grain crops range from 46% to 65%, compared to on-farm N recovery efficiencies of 20% to 40%. Fertilizer use efficiency can be optimized by fertilizer best management practices that apply nutrients at the right rate, time, and place. The highest nutrient use efficiency always occurs at the lower parts of the yield response curve, where fertilizer inputs are lowest, but effectiveness of fertilizers in increasing crop yields and optimizing farmer profitability should not be sacrificed for the sake of efficiency alone. There must be a balance between optimal nutrient use efficiency and optimal crop productivity. Key Words: Fertilizer best management practices, BMPs, balanced fertilization, nitrogen efficiency, right rate, right time, right place * Correspondence to: troberts@ipni.net
- 2. Understanding the terminology and the context in which it is used is critical to prevent misinterpretation and misunderstanding. For example, Table 1 shows the same maize data from the north central U.S. can be used to estimate crop recovery efficiency of nitrogen (N) at 37% (i.e. crop recovered 37% of added N) or crop removal efficiency at 100% (N removed in the grain was 100% of applied N; Bruulsema et al., 2004). Which estimate of nutrient use efficiency is correct? Recovery of 37% in the above-ground biomass of applied N is disturbingly low and suggests that N may pose an environmental risk. Assuming the grain contains 56% of the above-ground N, a typical N harvest index; only 21% of the fertilizer N applied is removed in the grain. Such low recovery efficiency prompts the question … where is the rest of the fertilizer going and what does a recovery efficiency of 37% really mean? In the above data, application of N at the optimum rate of 103 kg ha -1 increased above-ground N uptake by 38 kg ha -1 (37% of 103). Total N uptake by the fertilized maize was 184 kg ha -1 ; 146 from the soil and 38 from the fertilizer. The N in the grain would be 56% of 184, or 103 kg ha -1 : equal to the amount of N applied. Which is correct — a recovery of 21% as estimated from a single-year response recovery in the grain or 100% as estimated from the total uptake (soil N + fertilizer N) of N, assuming the soil can continue to supply N long-term? The answer cannot be known unless the long-term dynamics of N cycling are understood. Fertilizer nutrients applied, but not taken up by the crop, are vulnerable to losses from leaching, erosion, and denitrification or volatilization in the case of N, or they could be temporarily immobilized in soil organic matter to be released at a later time, all of which impact apparent use efficiency. Dobermann et al. (2005) introduced the term system level efficiency to account for contributions of added nutrients to both crop uptake and soil nutrient supply. Current Status of Nutrient Use Efficiency A recent review of worldwide data on N use efficiency for cereal crops from researcher-managed experimental plots reported that single-year fertilizer N recovery efficiencies averaged 65% for corn, 57% for wheat, and 46% for rice (Ladha et al., 2005). However, experimental plots do not accurately reflect the efficiencies obtainable on-farm. Differences in the scale of farming operations and management practices (i.e. tillage, seeding, weed and pest control, irrigation, harvesting) usually result in lower nutrient use efficiency. Nitrogen recovery in crops grown by farmers rarely exceeds 50% and is often much lower. A review of best available information suggests average N recovery efficiency for fields managed by farmers ranges from about 20% to 30% under rainfed conditions and 30% to 40% under irrigated conditions. Cassman et al. (2002) looked at N fertilizer recovery under different cropping systems and reported 37% recovery for corn grown in the north central U.S. (Table 2). They found N recovery averaged 31% for irrigated rice grown by Asian farmers and 40% for rice under field specific management. In India, N recovery averaged 18% for wheat grown under poor weather conditions, but 49% when grown under good weather conditions. Fertilizer recovery is impacted by management, which can be controlled, but also by weather, which cannot be controlled. Improving Nutrient Use Efficiency 178 Table 1. Fertilizer N efficiency of maize from 56 on-farm studies in north central U.S. (Cassman et al., 2002, source of data, Bruulsema et al., 2004, source of calculations). Average optimum N fertilizer rate, kg ha–1 103 Fertilizer N recovered in the crop, kg ha–1 38 Total N taken up by crop, kg ha–1 184 N removed in the harvested grain*, kg ha-–1 103 N returned to field in crop residue, kg ha–1 81 Crop recovery efficiency (38 kg N recovered/103 kg N applied), % 37 Crop removal efficiency (103 kg N applied/103 kg N in grain), % 100 * assumes a typical N harvest index of 56%
- 3. The above data illustrate that there is room to improve nutrient use efficiency at the farm level, especially for N. While most of the focus on nutrient efficiency is on N, phosphorus (P) efficiency is also of interest because it is one of the least available and least mobile mineral nutrients. First year recovery of applied fertilizer P ranges from less than 10% to as high as 30%. However, because fertilizer P is considered immobile in the soil and reaction (fixation and/or precipitation) with other soil minerals is relatively slow, long-term recovery of P by subsequent crops can be much higher. There is little information available about potassium (K) use efficiency. However, it is generally considered to have a higher use efficiency than N and P because it is immobile in most soils and is not subject to the gaseous losses that N is or the fixation reactions that affect P. First year recovery of applied K can range from 20% to 60%. Optimizing Nutrient Use Efficiency The fertilizer industry supports applying nutrients at the right rate, right time, and in the right place as a best management practice (BMP) for achieving optimum nutrient efficiency. Right rate: Most crops are location and season specific — depending on cultivar, management practices, climate, etc., and so it is critical that realistic yield goals are established and that nutrients are applied to meet the target yield. Over- or under-application will result in reduced nutrient use efficiency or losses in yield and crop quality. Soil testing remains one of the most powerful tools available for determining the nutrient supplying capacity of the soil, but to be useful for making appropriate fertilizer recommendations good calibration data is also necessary. Unfortunately, soil testing is not available in all regions of the world because reliable laboratories using methodology appropriate to local soils and crops are inaccessible or calibration data relevant to current cropping systems and yields are lacking. Other techniques, such as omission plots, are proving useful in determining the amount of fertilizer required for attaining a yield target (Witt and Doberman, 2002). In this method, N, P, and K are applied at sufficiently high rates to ensure that yield is not limited by an insufficient supply of the added nutrients. Target yield can be determined from plots with unlimited NPK. One nutrient is omitted from the plots to determine a nutrient-limited yield. For example, an N omission plot receives no N, but sufficient P and K fertilizer to ensure that those nutrients are not limiting yield. The difference in grain yield between a fully fertilized plot and an N omission plot is the deficit between the crop demand for N and indigenous supply of N, which must be met by fertilizers. Nutrients removed in crops are also an important consideration. Unless nutrients removed in harvested grain and crop residues are replaced, soil fertility will be depleted. Right time: Greater synchrony between crop demand and nutrient supply is necessary to improve nutrient use efficiency, especially for N. Split applications of N during the growing season, rather than a single, large application prior to planting, are known to be effective in increasing N use efficiency (Cassman et al., 2002). Tissue testing is a well known method used to assess N status of growing crops, but other diagnostic tools are also available. Chlorophyll meters have proven useful in fine-tuning in- season N management (Francis and Piekielek, 1999) and leaf color charts have been highly successful in guiding T. L. ROBERTS 179 Table 2. Nitrogen fertilizer recovery efficiency by maize, rice, and wheat from on-farm measurements (Cassman et al., 2002). Number Average N recovery*, Crop Region of farms N rate, kg ha –1 % Maize North Central U.S. 56 103 37 Rice Asia-farmer practice 179 117 31 Asia- field specific management 179 112 40 Wheat India – unfavorable weather 23 145 18 India – favorable weather 21 123 49 * recovery is the proportion of applied N fertilizer taken up the crop, calculated as the difference in total N uptake in above-ground biomass at physiological maturity between fertilized plots and an unfertilized control.
- 4. split N applications in rice and now maize production in Asia (Witt et al., 2005). Precision farming technologies have introduced, and now commercialized, on-the-go N sensors that can be coupled with variable rate fertilizer applicators to automatically correct crop N deficiencies on a site-specific basis. Another approach to synchronize release of N from fertilizers with crop need is the use of N stabilizers and controlled release fertilizers. Nitrogen stabilizers (e.g., nitrapyrin, DCD [dicyandiamide], NBPT [n-butyl- thiophosphoric triamide]) inhibit nitrification or urease activity, thereby slowing the conversion of the fertilizer to nitrate (Havlin et al., 2005). When soil and environmental conditions are favorable for nitrate losses, treatment with a stabilizer will often increase fertilizer N efficiency. Controlled-release fertilizers can be grouped into compounds of low solubility and coated water- soluble fertilizers. Most slow-release fertilizers are more expensive than water-soluble N fertilizers and have traditionally been used for high-value horticulture crops and turf grass. However, technology improvements have reduced manufacturing costs where controlled-release fertilizers are available for use in corn, wheat, and other commodity grains (Blaylock et al., 2005). The most promising for widespread agricultural use are polymer-coated products, which can be designed to release nutrients in a controlled manner. Nutrient release rates are controlled by manipulating the properties of the polymer coating and are generally predictable when average temperature and moisture conditions can be estimated. Right place: Application method has always been critical in ensuring fertilizer nutrients are used efficiently. Determining the right placement is as important as determining the right application rate. Numerous placements are available, but most generally involve surface or sub-surface applications before or after planting. Prior to planting, nutrients can be broadcast (i.e. applied uniformly on the soil surface and may or may not be incorporated), applied as a band on the surface, or applied as a subsurface band, usually 5 to 20 cm deep. Applied at planting, nutrients can be banded with the seed, below the seed, or below and to the side of the seed. After planting, application is usually restricted to N and placement can be as a topdress or a subsurface sidedress. In general, nutrient recovery efficiency tends to be higher with banded applications because less contact with the soil lessens the opportunity for nutrient loss due to leaching or fixation reactions. Placement decisions depend on the crop and soil conditions, which interact to influence nutrient uptake and availability. Plant nutrients rarely work in isolation. Interactions among nutrients are important because a deficiency of one restricts the uptake and use of another. Numerous studies have demonstrated that interactions between N and other nutrients, primarily P and K, impact crop yields and N efficiency. For example, data from a large number of multi-location on-farm field experiments conducted in India show the importance of balanced fertilization in increasing crop yield and improving N efficiency (Table 3). Adequate and balanced application of fertilizer nutrients is one of the most common practices for improving the efficiency of N fertilizer and is equally effective in both developing and developed countries. In a recent review based on 241 site-years of experiments in China, India, and North America, balanced fertilization with N, P, and K increased first-year recoveries an average of 54% compared to recoveries of only 21% where N was applied alone (Fixen et al., 2005). Efficient Does Not Necessarily Mean Effective Improving nutrient efficiency is an appropriate goal for all involved in agriculture, and the fertilizer industry, with the help of scientists and agronomists, is helping farmers work towards that end. However, effectiveness cannot be sacrificed for the sake of efficiency. Much higher nutrient efficiencies could be achieved simply by sacrificing yield, but that would not be economically effective or viable for the farmer, or the environment. This relationship between yield, nutrient efficiency, and the environment was ably described by Dibb (2000) using a theoretical example. For a typical yield response curve, the lower part of the curve is characterized by very low yields, because few nutrients are available or applied, but very high efficiency (Figure 1). Nutrient use efficiency is high at a low yield level, because any small amount of nutrient applied could give a large yield response. If nutrient use efficiency were the only goal, it would be achieved here in the lower part of the yield curve. However, environmental concerns would be significant because poor crop growth means less surface residues to protect the land from wind and water erosion and less root growth to build soil organic matter. As you move up the response curve, yields continue to increase, albeit at a slower rate, and nutrient use efficiency typically declines. Improving Nutrient Use Efficiency 180
- 5. However, the extent of the decline will be dictated by the BMPs employed (i.e. right rate, right time, right place, improved balance in nutrient inputs, etc.) as well as soil and climatic conditions. The relationship between efficiency and effective was further explained when Fixen (2006) suggested that the value of improving nutrient use efficiency is dependent on the effectiveness in meeting the objectives of nutrient use, objectives such as providing economical optimum nourishment to the crop, minimizing nutrient losses from the field, and contributions to system sustainability through soil fertility or other soil quality components. He cited 2 examples. Figure 2 shows Saskatchewan data from a long-term wheat study where 3 initial soil test levels were established with initial P applications followed by annual additions of seed-placed P. Fertilizer P recovery efficiency, at the lowest P rate and at the lowest soil test level, was 30% … an extremely high single-year efficiency. However, this practice would be ineffective because wheat yield was sacrificed. The second example is from a maize study in Ohio that included a range of soil test K levels and N fertilizer rates (Figure 3). N recovery efficiency can be greatly increased T. L. ROBERTS 181 Table. 3. Effect of balanced fertilization on yield and N agronomic efficiency in India (Prasad, 1996). Yield, t ha –1 Agronomic efficiency, kg grain kg N –1 Crop Control N alone* +PK N alone +PK Increase Rice (wet season) 2.74 3.28 3.82 13.5 27.0 13.5 Rice (summer) 3.03 3.45 6.27 10.5 81.0 69.5 Wheat 1.45 1.88 2.25 10.8 20.0 9.2 Pearl Millet 1.05 1.24 1.65 4.7 15.0 10.3 Maize 1.67 2.45 3.23 19.5 39.0 19.5 Sorghum 1.27 1.48 1.75 5.3 12.0 6.7 Sugarcane 47.2 59.0 81.4 78.7 227.7 150.0 * 40 kg N ha –1 applied on cereal crops and 150 kg N ha –1 applied on sugarcane } } Highest yields and lowest nutrient use efficiency Low yield and highest nutrient use efficiency Increasing nutrient inpit and decreasing nutrient use efficiency 100 75 50 25 0 Yieldpotential,% Figure 1. Relationship between yield response and nutrient use efficiency (adapted from Dibb, 2000). 2800 2600 2400 2200 2000 1800 Wheatyield,kgha-1 0 5 10 15 20 Seed-placed P, kg ha-1 Highest P efficiency (~30%)... but not effective Initial broadcast kgP ha-1 Soil test P after 5- yr, mg kg-1 0 80 160 5 8 15 Figure 2. Relationship between P efficiency and wheat yield (adapted from Wagner et al., 1986).
- 6. by reducing N rates below optimum … yield is sacrificed. Alternatively, yield and efficiency can be improved by applying an optimum N rate at an optimum soil test K level. Nitrogen efficiency was improved with both approaches, but the latter option was most effective in meeting the yield objectives. Conclusion Improving nutrient efficiency is a worthy goal and fundamental challenge facing the fertilizer industry, and agriculture in general. The opportunities are there and tools are available to accomplish the task of improving the efficiency of applied nutrients. However, we must be cautious that improvements in efficiency do not come at the expense of the farmers’ economic viability or the environment. Judicious application of fertilizer BMPs … right rate, right time, right place … targeting both high yields and nutrient efficiency will benefit farmers, society, and the environment alike. Improving Nutrient Use Efficiency 182 110 100 90 80 70 60 50 40 30 20 10 0 Nrecoveryinbiomass,% Four year average 13.1 t ha-1 213 kg ha-1 Soil K, mg kg-1 80 100 116 135 139 90 180 270 360 Fertilizer N, kg ha-1 Grain yield at optimum N 10.5 t ha-1 308 kg ha-1 Figure 3. Adequate K improves N efficiency in an Ohio maize study (adapted from Johnson et al., 1997). References Blaylock, A.D., J. Kaufmann and R.D. Dowbenko. 2005. Nitrogen fertilizer technologies. In: Proceedings of the Western Nutrient Management Conference, Salt Lake City, Utah, March 3-4, 2005. Vol. 6, pp. 8-13. Bruulsema, T.W., P.E. Fixen and C.S. Snyder. 2004. Fertilizer nutrient recovery in sustainable cropping systems. Better Crops. 88: 15- 17. Cassman, K.G., A. Dobermann and D.T. Walters. 2002. Agroecosystems, nitrogen use efficiency, and nitrogen management. Ambio. 31: 132-140. Dibb, D.W. 2000. The mysteries (myths) of nutrient use efficiency. Better Crops. 84: 3-5. Dobermann, A., K.G. Cassman, D.T. Waters and C. Witt. 2005. Balancing short- and long-term goals in nutrient management. In: Proceedings of the XV International Plant Nutrient Colloquium, Sep. 14-16, 2005. Beijing, China. Fixen, P.E. 2005. Understanding and improving nutrient use efficiency as an application of information technology. In: Proceedings of the Symposium on Information Technology in Soil Fertility and Fertilizer Management, a satellite symposium at the XV International Plant Nutrient Colloquium, Sep. 14-16, 2005. Beijing, China. Fixen, P.E. 2006. Turning challenges into opportunities. In: Proceedings of the Fluid Forum, Fluids: Balancing Fertility and Economics. Fluid Fertilizer Foundation, February 12-14, 2006, Scottsdale, Arizona. Fixen, P.E., J. Jin, K.N. Tiwari, and M.D. Stauffer. 2005. Capitalizing on multi-element interactions through balanced nutrition — a pathway to improve nitrogen use efficiency in China, India and North America. Sci. in China Ser. C Life Sci. 48: 1-11. Francis, D.D. and W.P. Piekielek. 1999. Assessing Crop Nitrogen Needs with Chlorophyll Meters. Site-Specific Management Guidelines, Potash & Phosphate Institute. SSMG-12. Reference 99082/ Item#10-1012. Havlin, J.L., J.D. Beaton, S.L. Tisdale and W.L. Nelson. 2005. Soil Fertility and Fertilizers. An Introduction to Nutrient Management. Pearson Education, Inc., Upper Saddle River, New Jersey. Johnson, J.W., T.S. Murrell and H.F. Reetz. 1997. Balanced fertility management: a key to nutrient use efficiency. Better Crops. 81: 3-5. Ladha, J.K., H. Pathak, T.J. Krupnik, J. Six and C. van Kessel. 2005. Efficiency of fertilizer nitrogen in cereal production: retrospects and prospects. Adv. Agron. 87: 85-156. Mosier, A.R., J.K. Syers and J.R. Freney. 2004. Agriculture and the Nitrogen Cycle. Assessing the Impacts of Fertilizer Use on Food Production and the Environment. Scope-65. Island Press, London. Prasad, R. 1996. Management of fertilizer nitrogen for higher recovery. In: Nitrogen Research and Crop Production. (Ed. H.L.S. Tandon), FDCO, New Delhi, India. pp. 104-115. Solie, J.B., M.L. Stone, W.R. Raun, G.V. Johnson, K. Freeman, R. Mullen, D.E. Needham, S. Reed and C.N. Washmon. 2002. Real- time sensing and N fertilization with a field scale Greenseeker tm applicator. Accessed on-line on 1/26/2006 at: http://nue.okstate.edu/Papers/Minnesota_2002_Solie.htm Wagner, B. I., J.W.B. Stewart and J.L. Henry. 1986. Comparison of single large broadcast and small annual seed-placed phosphorus treatments on yield and phosphorus and zinc contents of wheat on Chernozemic soils. Can. J. Soil Sci. 66: 237-248. Witt, C. and A. Dobermann. 2002. A site-specific nutrient management approach for irrigated, lowland rice in Asia. Better Crops International. 16: 20-24. Witt, C., J.M.C.A Pasuquin, R. Mutters and R.J. Buresh. 2005. New leaf color chart for effective nitrogen management in rice. Better Crops. 89: 36-39.