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Understanding the terminology and the context inwhich it is used is critical to prevent misinterpretation andmisunderstanding. For example, Table 1 shows the samemaize data from the north central U.S. can be used toestimate crop recovery efficiency of nitrogen (N) at 37%(i.e. crop recovered 37% of added N) or crop removalefficiency at 100% (N removed in the grain was 100% ofapplied N; Bruulsema et al., 2004). Which estimate ofnutrient use efficiency is correct?Recovery of 37% in the above-ground biomass ofapplied N is disturbingly low and suggests that N maypose an environmental risk. Assuming the grain contains56% of the above-ground N, a typical N harvest index;only 21% of the fertilizer N applied is removed in thegrain. Such low recovery efficiency prompts the question… where is the rest of the fertilizer going and what doesa recovery efficiency of 37% really mean?In the above data, application of N at the optimumrate of 103 kg ha-1increased above-ground N uptake by38 kg ha-1(37% of 103). Total N uptake by the fertilizedmaize was 184 kg ha-1; 146 from the soil and 38 fromthe fertilizer. The N in the grain would be 56% of 184,or 103 kg ha-1: equal to the amount of N applied. Whichis correct — a recovery of 21% as estimated from asingle-year response recovery in the grain or 100% asestimated from the total uptake (soil N + fertilizer N) ofN, assuming the soil can continue to supply N long-term?The answer cannot be known unless the long-termdynamics of N cycling are understood.Fertilizer nutrients applied, but not taken up by thecrop, are vulnerable to losses from leaching, erosion, anddenitrification or volatilization in the case of N, or theycould be temporarily immobilized in soil organic matter tobe released at a later time, all of which impact apparentuse efficiency. Dobermann et al. (2005) introduced theterm system level efficiency to account for contributionsof added nutrients to both crop uptake and soil nutrientsupply.Current Status of Nutrient Use EfficiencyA recent review of worldwide data on N use efficiencyfor cereal crops from researcher-managed experimentalplots reported that single-year fertilizer N recoveryefficiencies averaged 65% for corn, 57% for wheat, and46% for rice (Ladha et al., 2005). However,experimental plots do not accurately reflect theefficiencies obtainable on-farm. Differences in the scale offarming 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 rarelyexceeds 50% and is often much lower. A review of bestavailable information suggests average N recoveryefficiency for fields managed by farmers ranges fromabout 20% to 30% under rainfed conditions and 30% to40% under irrigated conditions.Cassman et al. (2002) looked at N fertilizer recoveryunder different cropping systems and reported 37%recovery for corn grown in the north central U.S. (Table2). They found N recovery averaged 31% for irrigatedrice grown by Asian farmers and 40% for rice under fieldspecific management. In India, N recovery averaged 18%for wheat grown under poor weather conditions, but49% when grown under good weather conditions.Fertilizer recovery is impacted by management, which canbe controlled, but also by weather, which cannot becontrolled.Improving Nutrient Use Efficiency178Table 1. Fertilizer N efficiency of maize from 56 on-farm studies in north central U.S. (Cassmanet al., 2002, source of data, Bruulsema et al., 2004, source of calculations).Average optimum N fertilizer rate, kg ha–1103Fertilizer N recovered in the crop, kg ha–138Total N taken up by crop, kg ha–1184N removed in the harvested grain*, kg ha-–1103N returned to field in crop residue, kg ha–181Crop recovery efficiency (38 kg N recovered/103 kg N applied), % 37Crop removal efficiency (103 kg N applied/103 kg N in grain), % 100* assumes a typical N harvest index of 56%
The above data illustrate that there is room toimprove nutrient use efficiency at the farm level,especially for N. While most of the focus on nutrientefficiency is on N, phosphorus (P) efficiency is also ofinterest because it is one of the least available and leastmobile mineral nutrients. First year recovery of appliedfertilizer P ranges from less than 10% to as high as 30%.However, because fertilizer P is considered immobile inthe soil and reaction (fixation and/or precipitation) withother soil minerals is relatively slow, long-term recoveryof P by subsequent crops can be much higher. There islittle information available about potassium (K) useefficiency. However, it is generally considered to have ahigher use efficiency than N and P because it is immobilein most soils and is not subject to the gaseous losses thatN is or the fixation reactions that affect P. First yearrecovery of applied K can range from 20% to 60%.Optimizing Nutrient Use EfficiencyThe fertilizer industry supports applying nutrients atthe right rate, right time, and in the right place as a bestmanagement practice (BMP) for achieving optimumnutrient efficiency.Right rate: Most crops are location and seasonspecific — depending on cultivar, management practices,climate, etc., and so it is critical that realistic yield goalsare established and that nutrients are applied to meet thetarget yield. Over- or under-application will result inreduced nutrient use efficiency or losses in yield and cropquality. Soil testing remains one of the most powerfultools available for determining the nutrient supplyingcapacity of the soil, but to be useful for makingappropriate fertilizer recommendations good calibrationdata is also necessary. Unfortunately, soil testing is notavailable in all regions of the world because reliablelaboratories using methodology appropriate to local soilsand crops are inaccessible or calibration data relevant tocurrent cropping systems and yields are lacking.Other techniques, such as omission plots, are provinguseful in determining the amount of fertilizer requiredfor attaining a yield target (Witt and Doberman, 2002).In this method, N, P, and K are applied at sufficiently highrates to ensure that yield is not limited by an insufficientsupply of the added nutrients. Target yield can bedetermined from plots with unlimited NPK. One nutrientis omitted from the plots to determine a nutrient-limitedyield. For example, an N omission plot receives no N, butsufficient P and K fertilizer to ensure that those nutrientsare not limiting yield. The difference in grain yieldbetween a fully fertilized plot and an N omission plot isthe deficit between the crop demand for N and indigenoussupply of N, which must be met by fertilizers.Nutrients removed in crops are also an importantconsideration. Unless nutrients removed in harvestedgrain and crop residues are replaced, soil fertility will bedepleted.Right time: Greater synchrony between crop demandand nutrient supply is necessary to improve nutrient useefficiency, especially for N. Split applications of N duringthe growing season, rather than a single, large applicationprior to planting, are known to be effective in increasingN use efficiency (Cassman et al., 2002). Tissue testing isa well known method used to assess N status of growingcrops, but other diagnostic tools are also available.Chlorophyll meters have proven useful in fine-tuning in-season N management (Francis and Piekielek, 1999) andleaf color charts have been highly successful in guidingT. L. ROBERTS179Table 2. Nitrogen fertilizer recovery efficiency by maize, rice, and wheat from on-farm measurements (Cassmanet al., 2002).Number Average N recovery*,Crop Region of farms N rate, kg ha–1%Maize North Central U.S. 56 103 37Rice Asia-farmer practice 179 117 31Asia- field specific management 179 112 40Wheat India – unfavorable weather 23 145 18India – favorable weather 21 123 49* recovery is the proportion of applied N fertilizer taken up the crop, calculated as the difference in total N uptakein above-ground biomass at physiological maturity between fertilized plots and an unfertilized control.
split N applications in rice and now maize production inAsia (Witt et al., 2005). Precision farming technologieshave introduced, and now commercialized, on-the-go Nsensors that can be coupled with variable rate fertilizerapplicators to automatically correct crop N deficiencies ona site-specific basis.Another approach to synchronize release of N fromfertilizers with crop need is the use of N stabilizers andcontrolled release fertilizers. Nitrogen stabilizers (e.g.,nitrapyrin, DCD [dicyandiamide], NBPT [n-butyl-thiophosphoric triamide]) inhibit nitrification or ureaseactivity, thereby slowing the conversion of the fertilizerto nitrate (Havlin et al., 2005). When soil andenvironmental conditions are favorable for nitrate losses,treatment with a stabilizer will often increase fertilizer Nefficiency. Controlled-release fertilizers can be groupedinto compounds of low solubility and coated water-soluble fertilizers.Most slow-release fertilizers are more expensive thanwater-soluble N fertilizers and have traditionally beenused for high-value horticulture crops and turf grass.However, technology improvements have reducedmanufacturing costs where controlled-release fertilizersare available for use in corn, wheat, and other commoditygrains (Blaylock et al., 2005). The most promising forwidespread agricultural use are polymer-coated products,which can be designed to release nutrients in a controlledmanner. Nutrient release rates are controlled bymanipulating the properties of the polymer coating andare generally predictable when average temperature andmoisture conditions can be estimated.Right place: Application method has always beencritical in ensuring fertilizer nutrients are used efficiently.Determining the right placement is as important asdetermining the right application rate. Numerousplacements are available, but most generally involvesurface or sub-surface applications before or afterplanting. Prior to planting, nutrients can be broadcast(i.e. applied uniformly on the soil surface and may or maynot be incorporated), applied as a band on the surface, orapplied as a subsurface band, usually 5 to 20 cm deep.Applied at planting, nutrients can be banded with theseed, below the seed, or below and to the side of theseed. After planting, application is usually restricted to Nand placement can be as a topdress or a subsurfacesidedress. In general, nutrient recovery efficiency tends tobe higher with banded applications because less contactwith the soil lessens the opportunity for nutrient loss dueto leaching or fixation reactions. Placement decisionsdepend on the crop and soil conditions, which interact toinfluence nutrient uptake and availability.Plant nutrients rarely work in isolation. Interactionsamong nutrients are important because a deficiency ofone restricts the uptake and use of another. Numerousstudies have demonstrated that interactions between Nand other nutrients, primarily P and K, impact crop yieldsand N efficiency. For example, data from a large numberof multi-location on-farm field experiments conducted inIndia show the importance of balanced fertilization inincreasing crop yield and improving N efficiency (Table 3).Adequate and balanced application of fertilizernutrients is one of the most common practices forimproving the efficiency of N fertilizer and is equallyeffective in both developing and developed countries. In arecent review based on 241 site-years of experiments inChina, India, and North America, balanced fertilizationwith N, P, and K increased first-year recoveries anaverage of 54% compared to recoveries of only 21%where N was applied alone (Fixen et al., 2005).Efficient Does Not Necessarily Mean EffectiveImproving nutrient efficiency is an appropriate goalfor all involved in agriculture, and the fertilizer industry,with the help of scientists and agronomists, is helpingfarmers work towards that end. However, effectivenesscannot be sacrificed for the sake of efficiency. Muchhigher nutrient efficiencies could be achieved simply bysacrificing yield, but that would not be economicallyeffective or viable for the farmer, or the environment.This relationship between yield, nutrient efficiency, andthe environment was ably described by Dibb (2000) usinga theoretical example. For a typical yield response curve,the lower part of the curve is characterized by very lowyields, because few nutrients are available or applied, butvery high efficiency (Figure 1). Nutrient use efficiency ishigh at a low yield level, because any small amount ofnutrient applied could give a large yield response. Ifnutrient use efficiency were the only goal, it would beachieved here in the lower part of the yield curve.However, environmental concerns would be significantbecause poor crop growth means less surface residues toprotect the land from wind and water erosion and lessroot growth to build soil organic matter. As you move upthe response curve, yields continue to increase, albeit at aslower rate, and nutrient use efficiency typically declines.Improving Nutrient Use Efficiency180
However, the extent of the decline will be dictated by theBMPs employed (i.e. right rate, right time, right place,improved balance in nutrient inputs, etc.) as well as soiland climatic conditions.The relationship between efficiency and effective wasfurther explained when Fixen (2006) suggested that thevalue of improving nutrient use efficiency is dependent onthe effectiveness in meeting the objectives of nutrientuse, objectives such as providing economical optimumnourishment to the crop, minimizing nutrient losses fromthe field, and contributions to system sustainabilitythrough soil fertility or other soil quality components. Hecited 2 examples. Figure 2 shows Saskatchewan datafrom a long-term wheat study where 3 initial soil testlevels were established with initial P applications followedby annual additions of seed-placed P. Fertilizer P recoveryefficiency, at the lowest P rate and at the lowest soil testlevel, was 30% … an extremely high single-yearefficiency. However, this practice would be ineffectivebecause wheat yield was sacrificed.The second example is from a maize study in Ohio thatincluded a range of soil test K levels and N fertilizer rates(Figure 3). N recovery efficiency can be greatly increasedT. L. ROBERTS181Table. 3. Effect of balanced fertilization on yield and N agronomic efficiency in India (Prasad, 1996).Yield, t ha–1Agronomic efficiency, kg grain kg N–1CropControl N alone* +PK N alone +PK IncreaseRice (wet season) 2.74 3.28 3.82 13.5 27.0 13.5Rice (summer) 3.03 3.45 6.27 10.5 81.0 69.5Wheat 1.45 1.88 2.25 10.8 20.0 9.2Pearl Millet 1.05 1.24 1.65 4.7 15.0 10.3Maize 1.67 2.45 3.23 19.5 39.0 19.5Sorghum 1.27 1.48 1.75 5.3 12.0 6.7Sugarcane 47.2 59.0 81.4 78.7 227.7 150.0* 40 kg N ha–1applied on cereal crops and 150 kg N ha–1applied on sugarcane}}Highest yields and lowestnutrient use efficiencyLow yield and highest nutrient useefficiencyIncreasing nutrient inpit anddecreasing nutrient use efficiency1007550250Yieldpotential,%Figure 1. Relationship between yield response and nutrient useefficiency (adapted from Dibb, 2000).280026002400220020001800Wheatyield,kgha-10 5 10 15 20Seed-placed P, kg ha-1Highest P efficiency (~30%)...but not effectiveInitialbroadcastkgP ha-1Soil test Pafter 5-yr, mg kg-10801605815Figure 2. Relationship between P efficiency and wheat yield (adapted from Wagner et al., 1986).
by reducing N rates below optimum … yield is sacrificed.Alternatively, yield and efficiency can be improved byapplying an optimum N rate at an optimum soil test Klevel. Nitrogen efficiency was improved with bothapproaches, but the latter option was most effective inmeeting the yield objectives.ConclusionImproving nutrient efficiency is a worthy goal andfundamental challenge facing the fertilizer industry, andagriculture in general. The opportunities are there andtools are available to accomplish the task of improving theefficiency of applied nutrients. However, we must becautious that improvements in efficiency do not come atthe expense of the farmers’ economic viability or theenvironment. Judicious application of fertilizer BMPs …right rate, right time, right place … targeting both highyields and nutrient efficiency will benefit farmers, society,and the environment alike.Improving Nutrient Use Efficiency1821101009080706050403020100Nrecoveryinbiomass,%Four year average13.1 t ha-1213 kg ha-1Soil K, mg kg-18010011613513990 180 270 360Fertilizer N, kg ha-1Grain yield atoptimum N 10.5 t ha-1308 kg ha-1Figure 3. Adequate K improves N efficiency in an Ohio maize study(adapted from Johnson et al., 1997).ReferencesBlaylock, A.D., J. Kaufmann and R.D. Dowbenko. 2005. Nitrogenfertilizer technologies. In: Proceedings of the Western NutrientManagement 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 nutrientrecovery in sustainable cropping systems. Better Crops. 88: 15-17.Cassman, K.G., A. Dobermann and D.T. Walters. 2002.Agroecosystems, nitrogen use efficiency, and nitrogenmanagement. 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 efficiencyas an application of information technology. In: Proceedings ofthe Symposium on Information Technology in Soil Fertility andFertilizer Management, a satellite symposium at the XVInternational Plant Nutrient Colloquium, Sep. 14-16, 2005.Beijing, China.Fixen, P.E. 2006. Turning challenges into opportunities. In: Proceedingsof 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. Capitalizingon multi-element interactions through balanced nutrition — apathway to improve nitrogen use efficiency in China, India andNorth America. Sci. in China Ser. C Life Sci. 48: 1-11.Francis, D.D. and W.P. Piekielek. 1999. Assessing Crop Nitrogen Needswith 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. SoilFertility 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 fertilitymanagement: 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: retrospectsand prospects. Adv. Agron. 87: 85-156.Mosier, A.R., J.K. Syers and J.R. Freney. 2004. Agriculture and theNitrogen Cycle. Assessing the Impacts of Fertilizer Use on FoodProduction and the Environment. Scope-65. Island Press, London.Prasad, R. 1996. Management of fertilizer nitrogen for higherrecovery. 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 scaleGreenseekertmapplicator. Accessed on-line on 1/26/2006 at:http://nue.okstate.edu/Papers/Minnesota_2002_Solie.htmWagner, B. I., J.W.B. Stewart and J.L. Henry. 1986. Comparison ofsingle large broadcast and small annual seed-placed phosphorustreatments on yield and phosphorus and zinc contents of wheaton Chernozemic soils. Can. J. Soil Sci. 66: 237-248.Witt, C. and A. Dobermann. 2002. A site-specific nutrient managementapproach for irrigated, lowland rice in Asia. Better CropsInternational. 16: 20-24.Witt, C., J.M.C.A Pasuquin, R. Mutters and R.J. Buresh. 2005. New leafcolor chart for effective nitrogen management in rice. BetterCrops. 89: 36-39.