Mikkelsen N Use Efficiency Pg2

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NUTRIENT USE EFFICIENCY: USING NUTRIENT BUDGETS

Robert Mikkelsen

Potash & Phosphate Institute

ABSTRACTThe use of nutrient budgets has become increasingly popular in recent years.Three types of budgets are described with examples: Soil Surface Balance, Farm

Gate Balance, and Soil System Balance. Nutrient budgets are sometimes used to

get an estimate of nutrient use efficiency. This efficiency term can be definedin many ways and is subject to misinterpretation. Nutrient efficiency can be

defined in agronomic, economic, or environmental terms with widely varying

results. It is not always advisable to achieve the highest efficiency possible.

INTRODUCTION

There are very few ideal soils in the world- that is, soils that contain all of the essentialnutrients in the proper balance required by crops. Overcoming these pre-existing deficiencies isthe goal of the fertilizer industry. While animal manures are excellent at providing many of the

essential nutrients for crops, their composition is rarely in balance with what the soil requires to

adequately supply the plants needs. Similarly, legume cover crops are good as a N source forsubsequent crops, but provide no other additional nutrients that were not already in the soil.

It is in everyones best interest to utilize nutrients as efficiently as possible. However,

accomplishing this goal- or even defining it- is difficult to achieve. In general, getting as much ofa nutrient as possible into the harvested portion of a crop is the concept of efficient nutrient use.

Tracking the recovery of applied nutrients is a key component to measuring nutrient efficiency.

NUTRIENT BUDGETSThe generally accepted approach to nutrient balance measures the difference between

nutrient inputs and outputs in an agricultural system. Nutrient or mineral balances establish a

link between agricultural nutrient use, changes in environmental quality, and the sustainable useof soil nutrient resources. Depending on the data input, these budgets can be used at a variety of

scales.

Nutrient budgets are becoming increasingly common as a tool to describe nutrient flowswithin farming systems and to assist in the planning of the complex spatial and temporal

management within rotational cropping and mixed farming systems.

Budgets are the outcome of a nutrient accounting process, ranging from simple to complex,

which details all the inputs and outputs to a given system over a fixed period of time. The

underlying assumption of a nutrient budget is that of mass balance (i.e. nutrient inputs to thesystem minus any nutrient exports equal the change in storage within the system (Meisinger and

Randall, 1991).Many approaches have been used to estimate nutrient balances, depending on the intended

use. For example, the technique for developing national, regional, or global estimates ofefficiency may be much different from a field-scale or micro-plot approach. Additionally, a

nutrient deficit or surplus over the short term does is not immediately indicative of undesirable

consequences, but in fact may be beneficial and desirable for building overall soil fertility.

Western Nutrient Management Conference. 2005. Vol. 6. Salt Lake City, UT. Page 2


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Several basic techniques are used to measure nutrient balances- all of which have various

limitations depending on the level of measurement and the availability of data. The usefulnessand reliability of any type of budget depends on its completeness. The three main approaches

are:

Soil Surface Balance: This approach measures the difference between the inputs (or theapplication) of nutrients and the output (or removal of nutrients) from the soil (Figure 1). While

this budget provides the most detail for nutrient management planning, there is usually

uncertainty associated with the data inputs and the partitioning of the components of the nutrientbalance between air, soil, and water. An example:

Sheldrick et al (2002) conducted a nutrient balance for 197 countries using the

soil surface balance technique. Working at a national level allowed them to use theFAO data base, which contains detailed information related to crop and livestock

production, as well as fertilizer consumption statistics.

They reported that nutrient efficiency is approximately 50% for N, 40% for P,and 75% for K. In a few countries (Western Europe, Japan, and Rep. of Korea) there

is a surplus of these primary nutrients. However, in almost all other countries, foodproduction is currently depending on depleting large quantities of nutrients from soil

reserves and this unsustainable trend is likely to continue into the future. The worldaverage soil depletion of nutrients was estimated to be 10 lb N/A, 9 lb P2O5/A, and 21

lb K2O/A. They concluded that the current depletion of K is particularly severe and

could ultimately lead to a serious loss of crop production in several countries.

Farm Gate Balance: This type of balance simply measures the difference between the nutrient

content of farm inputs and the nutrient content of farm outputs. This balance has beensuccessfully used for P and K, but it ignores many of the complex on-farm transformations that

N is subject to (e.g. NH3volatilization, denitrification, volatile losses during crop senescence,etc.). This method quantifies nutrients supplied to and removed from the farm, but does not

quantify the nutrients circulating within the farm enterprise. This type of budget is easy to

construct and requires relatively little data, it is consequently used widely for policy analysis. Anexample:

Nelson and Mikkelsen (2005) constructed a P budget for a typical swine farm in

North Carolina to examine the potential nutrient accumulation patterns and make

predictions of future trends. They measured the nutrient content of all feed andpiglets entering the farm. They subtracted the P in the mature hogs, animal

mortalities, and crops leaving the farm. The difference between imports (30,664 lb

P/yr) and exports (13,633 lb P/yr) indicates an average accumulation of 17,030 lbP/yr on this particular farm (Figure 2). This type of analysis can be used for making

farm-level nutrient management plans and regional estimates of nutrient use.



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Inorganic

Fertilizer

Animal

Manure

Nitrogen

Fixation

Aerial

Deposition

Organic

FertilizerSeeds

Harvested

Crops

Grass &

Forage

Nutrient BalanceSurplus or Deficit into:

AirSoil

Water

Agr icul tural Land

Figure 1. Example of a soil surface balance showing various inputs and outputs from a farm,

region, or country.

______________________________________________________________________________________

Imported Swine: 2545 lb P/yr Exported Swine and Crops:13,633 lb P/yr

Inputs Outputs

Imported Feed: 27,749 lb P/yr Remaining on Farm: 17,030 lb P/yrFigure 2. Example of farm gate phosphorus budget for a typical swine farm in North Carolina

(Nelson and Mikkelsen, 2005).

Another example of a farm gate-type budget applied on a state scale was recently

conducted to examine average nutrient balances (PPI, 2002). This budget was basedon crop production statistics, average nutrient content of harvest crops, recoverable

animal manure, and fertilizer consumption (Figure 3). The contribution of legumes to

the overall nutrient budget is an important N input in many states. For simplicity,legume-derived N is included as both an N source and a harvested removal. It is

important to remember that these state-wide budgets reflect average conditions andshould not be used to make specific nutrient recommendations.

The degree of soil K depletion reflects how the removal of K in harvested crops

greatly exceeds its replacement through fertilizer or manure. Phosphorus removal in

harvested crops is generally less than that applied with fertilizer and manure.However, this result masks the areas in proximity to animal production facilities that



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frequently receive more P than is agronomically required- and areas further from

animal facilities that frequently receive inadequate additions of fertilizer P tomaintain appropriate soil fertility levels.

0

150

300

450

600

N N P P K K

Removal

Manure

Legume

Fertilizer

Washington: Nutrient Inputs and Removal

82%

131%

265%

NRemoval

PRemoval

KRemoval

Millionlb

K2OP2O5N

0

300

600

900

1,200

1,500

1,800

N N P P K K

Removal

Manure

Legume

Fertilizer

California: Nutrient Inputs and Removal

58%

62%

202%

NRemoval

PRemoval

KRemoval

Millionlb

K2OP2O5N

0

100

200

300

400

500

600

700

N N P P K K

Removal

Manure

Legume

Fertilizer

Idaho: Nutrient Inputs and Removal

81%

87%

431%

NRemoval

PRemoval

KRemoval

Millio

nlb

K2OP2O5N

Figure 3. Example of nutrient balances in Washington, California, and Idaho based on a farm-gate nutrient budget of inputs and outputs (PPI, 2002).

Soil System Balance: This approach is commonly used where detailed information on inputs,

outputs, and internal transformations is available for all the important components. This type of

balance requires much larger data inputs than the previous approaches, but the use of relevantcomputer models can help with parameter estimates when field observations are not available.

A number of excellent mechanistic models have been developed to trace the fate of

nutrients. The use of isotopes (e.g. 15N) to trace the behavior of applied fertilizer has also beenvery useful in understanding the complex physical/chemical/and microbial transformations thatoccur after nutrients are added to soil. The commonly used models operate at different scales

(from global to micro-plot scale) and this scale issue must be considered when choosing the most

appropriate model for a specific nutrient balance.



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Figure 4. Example of inputs required for a soil system balance based on mechanistic nutrient

transformations (Brown and Johnson, 1996).

NUTRIENT USE EFFICIENCY

Efficient use of nutrients in agriculture may be defined differently when viewed from

agronomic, economic, or environmental perspectives. Proper definition for the intended use is

essential to understand published values and have meaningful discussion. For example,efficiency is frequently defined as the nutrient accumulated in the aboveground part of the plant.

For N, this value frequently varies between 40 and 60%. Another common definition of

efficiency is the nutrient recovered within the entire soil-crop-root system. For N, this value may

be in the range of 65 to 85%, and even higher for P and K additions.

It is a fallacy that the highest possible nutrient efficiencies should be the ultimate goal offertilizer users. The highest efficiency occurs when small amounts of nutrients are applied on

deficient soils. (Figure 5- Area 1). While efficiency may be very high in this condition, cropgrowth in this region is generally stunted, profitability is low, water use efficiency is sub-

optimal, and the potential for nitrate leaching is enhanced- compared with the situation where

balanced and appropriate nutrition is provided.Another example of inadequate understanding of efficiency is when an insufficient

quantity of nutrients is regularly added to meet crop needs. In this condition, soil productivity

will gradually decline as crop production continues to be increasingly reliant on nutrient stocksfrom soil reserves. Nominal nutrient efficiency may be very high under these circumstances, but

it is clearly a non-sustainable scenario.

Economic efficiency occurs when farm income is maximized as a result of nutrient inputs.This can be complex to predict when factors such as future yield responses, the cost of nutrientinputs, and crop prices may not be known in advance of the growing season. Bock (1984)

provides a good overview of difficulties associated with achieving high economic efficiency.

Environmental nutrient efficiency is important since nutrients not used by the crop are atpotential risk for loss. All agro-ecosystems cause a disruption of native nutrient cycles an

inevitable consequence of all modern food production systems. The susceptibility of loss varies

among the essential plant nutrients, and their loss mechanisms are each unique. This measure of



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environmental efficiency must be made on a case-by-case basis by looking at the local

environmental sensitivity and the vulnerable targets for nutrient impacts. Considerable researchhas shown that nutrient loss is greatly enhanced when fertilizers or manures are added at rates

beyond their agronomic need (e.g. Nitrogen: Broadbent and Carlton; 1979; Phosphorus:

Tarkalson and Mikkelsen; 2004). The local conditions, such as rainfall, frozen snow,

denitrification, leaching, and runoff potential all need to be assessed to determine the level ofacceptable loss and environmental efficiency.

The concept of plant nutrient efficiency is certainly not a new one- but it is still not

adequately understood and practiced. It is time to move beyond the concept of managing singlenutrients, but instead consider providing balanced crop nutrition for producing foods of high

nutritional quality with sustainable economic and environmental yield levels.

0

25

50

75

100

0 0.5 1 1.5 2 2.5

Increased nutr ient additions

YieldPo

tential,%

Area 1

Area 2

Area 4

Area 3

Figure 5. Crop yields respond favorably to nutrient additions, resulting in decreasing efficiencyas yields and economic sustainability increase beyond their optimum level.

REFERENCESBock, B.R. 1984. Efficient use of nitrogen in cropping systems. p. 273-294. InR.D. Hauck et al

(ed.). Nitrogen in crop production. ASA, CSSA, and SSSA. Madison, WI.

Broadbent, F.E., and A.B. Carlton. 1978. Field trials with isotopically labeled nitrogen fertilizer.

p. 1-41. InD.R. Nielsen (ed.) Nitrogen in the environment. Academic Press.

Brown, L.C. and J.W. Johnson. 1996. Nitrogen and the hydrologic cycle. Ohio State Univ. Ext.Fact Sheet AEX-463-96. Columbus, OH.

Meisinger, J.J., and G.W. Randall. 1991. Estimating nitrogen budgets for soil-crop systems. p.

85124. In R.F. Follett et al. (ed.) Managing nitrogen for groundwater quality and farmprofitability. ASA, CSSA, and SSSA, Madison, WI.

Nelson, N.O., and R.L. Mikkelsen. 2005. Balancing the phosphorus budget of a swine farm: A

case study. J. Natural Resources and Life Sci. Educ. (in press).Potash & Phosphate Institute. 2002. Plant nutrient use in North America. Tech Bull. 2002-1.

Norcross, GA.

Sheldrick, W.F., J.K. Syers, and J. Lingard. 2002. A conceptual model for conducting nutrientaudits at national, regional, and global scales. Nutrient Cycling Agroecosystems. 62:61-72.

Tarkalson, D.D., and R.L. Mikkelsen. 2004. Runoff phosphorus losses as related to phosphorus

source, application method and application rate on a Piedmont soil. J. Environ. Qual.

33:1424-1430.


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Mikkelsen N Use Efficiency Pg2