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Conserving Resources and Building

ProductivityA Case for Fertilizer BMPsBy Mike Stewart

Bestmanagement practices (BMPs) are a hot topic these days. Farmers in the GreatPlains have implemented soil conservation practices that rival any other resourceconservation activity in the world. The resulting reduction in soil erosion by windand wateralong with moisture conservation practices have improved soils whileincreasing crop yields and improving whole-farm economics.

Fertilizer nutrients are essential inmeeting the crop yield and qualitygoals of modern agriculture. With

reduced tillage systems, many semiaridregions have been able to intensify crop-ping, thus reducing the use of fallow forsoil moisture accumulation, and increasingthe need to replace the nutrients removedby the increased cropping intensity. Howwe handle fertilizer inputs (e.g., rate, tim-ing, and placement) provides the founda-tion for fertilizer BMPs and the potentialfor maximum positive economic returnsfrom fertilizer use.

Matching Nutrient Supply with CrMatching Nutrient Supply with CrMatching Nutrient Supply with CrMatching Nutrient Supply with CrMatching Nutrient Supply with Cropopopopop

RequirRequirRequirRequirRequirementsementsementsementsementsThis involves using all available infor-

mation to establish the soil nutrient sta-tus and crop requirements prior to mak-

ing fertilizer application decisions. SpecificBMPs include soil testing, plant analyses,setting realistic yield goals, and balancingnutrient inputs with crop removal at opti-mum soil test levels.

Soil TSoil TSoil TSoil TSoil Testingestingestingestingesting The main science-basedtool we have to estimate a soils capacityto supply nutrients on agricultural land issoil testing. The soil testing process is basedon soil samples being taken from represen-tative areas in a field, analyzed using anappropriate chemical extraction method,and either correlated with plant nutrientuptake or calibrated with crop yield re-sponse. Resulting fertilizer recommenda-

tions are based on how a particular cropresponded to a nutrient, using the averageresponse from a multi-year and multi-sitedata set. If nutrient levels in a soil are al-lowed to decline to the point of limitingyield potential, substantial economic lossescan be expected. This was shown clearlywith phosphorus (P) in a long-term corn-soybean study in Kansas (Gordon, 2003).

FigurFigurFigurFigurFigure 1e 1e 1e 1e 1 shows that annual applicationof 30 lb P

2O

5/A over 42 years maintained

soil test P at near the initial (1960) leveluntil about 1985. Since then, soil P levelshave declined. Corn grain yields were 11%greater for the period 1985-2002 than for1960-1984. This indicates that the 30 lbP

2O

5/A rate was not keeping pace with the

crop removal rate. Where no P fertilizerwas applied, soil test P declined to half of theoriginal value.

Plant AnalysisPlant AnalysisPlant AnalysisPlant AnalysisPlant Analysis The term plant analysisrefers to the total or quantitative analysisof nutrients in plant tissue. Plant analysis

The DusThe DusThe DusThe DusThe Dust Bot Bot Bot Bot Bowwwwwlllll of the 1930s in the U.S. was a lesson to

the world concerning the importance of conserving

natural resources. That is one of the goals of BMPs.

SOUTHERN AND CENTRAL

G R E A T P L A I N S

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Better Crops/Vol. 90 (2006, No. 2) 5

works with soil testing to evaluate soil fertil-ity and overall nutrient availability. Plantanalysis is used in-season to help evaluate nu-trient deficiencies and take corrective actionon the current crop or future crops.

Establishing Realistic YEstablishing Realistic YEstablishing Realistic YEstablishing Realistic YEstablishing Realistic Yield Goalsield Goalsield Goalsield Goalsield GoalsSuggested recommended application ratesare often tied to yield goals for several nu-trients. Yield records should be used to setindividual realistic, but progressive, yield

goals for each field. Appropriate yield goalsfor a specific field should be high enoughto take advantage of high production yearswhen they occur, but not so high as to jeop-ardize environmental stewardship and/orprofitability when environmental condi-tions are not as favorable. Appropriateyield goals fall between the average yieldobtained in a field over the past 3 to 5 yearsand the highest yield ever obtained in a

particular field (Leikam et al., 2003).Nutrient BudgetsNutrient BudgetsNutrient BudgetsNutrient BudgetsNutrient Budgets There are a num-

ber of situations where crop advisers andfarmers find that they can make fairly goodestimates of crop nutrient requirementsbased on what was grown and what wasapplied in a specific field. Information suchas crop yield, grain protein concentration,and straw management can all be used toestablish the status of a nutrient such as

nitrogen (N). For P and potassium (K), theyear-to-year variation in plant-availablesupply is minor, and annual applicationbased on a balance between soil test levelsand crop requirements can avoid depletion

or over-application.

FerFerFerFerFertilizer Applicationtilizer Applicationtilizer Applicationtilizer Applicationtilizer ApplicationRight Rate and Balance of NutrientsRight Rate and Balance of NutrientsRight Rate and Balance of NutrientsRight Rate and Balance of NutrientsRight Rate and Balance of Nutrients

Most agronomists have heard aboutLiebigs Law of the Minimum, whichstates that the yield of a crop will be de-termined by the element present in mostlimiting quantity. In other words, the de-ficiency of one nutrient cannot be over-come by the excess of another. Use all avail-able tools to ensure that the crop receives com-plete and balanced nutrition.

Right FerRight FerRight FerRight FerRight Fertilizer Fortilizer Fortilizer Fortilizer Fortilizer Formmmmm Plants take upthe bulk of their nutrients from the soil inspecific forms. Fertilizers are formulated to be

either in the plant-available forms, or to beeasily converted to these forms after applica-tion to the soil. In some instances, this conver-sion limits immediate use by the plant, requir-ing specific application management for effi-cient use.

Right PlacementRight PlacementRight PlacementRight PlacementRight Placement An important partof optimizing crop response to a fertilizernutrient is placing the nutrient in such away that it provides rapid uptake by the

crop and reduces potential losses. The mo-bility of a nutrient in the soil is a majorconsideration in its placement. For ex-ample, low mobility of P in calcareous soilsmeans that short-term crop utilization ofthe P is improved considerably when it isplaced close to the germinating seed.

Right TRight TRight TRight TRight Timingimingimingimingiming The demand for a nu-trient by a growing crop generally variesthrough the growing season, with the high-

est uptake associated with the period ofmost rapid growth. Timing fertilizer ap-plications so that they provide a plant-available supply of nutrients when the cropneeds them is a desirable goal. Plants sub-ject to a deficiency during specific periodsof growth may not recover to achieve fullyield potential.

Site-Specific Nutrient ManagementSite-Specific Nutrient ManagementSite-Specific Nutrient ManagementSite-Specific Nutrient ManagementSite-Specific Nutrient ManagementFertilizing soils rather than fields is an

emerging BMP that continues to gain inpopularity with technology development. Thisinvolves using some form of field diagnostic,such as intensive soil sampling, soil sensing,aerial imagery, or yield mapping...some or all

78

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818182828180

4044

4650

52

30

40

50

60

70

80

90

1960

1965

1970

1975

1980

1985

1990

1995

2000

BrayP-1,ppm

160 N, 30 P2O5

160 N, 0 P2O5

FFFFFigurigurigurigurigure 1.e 1.e 1.e 1.e 1. Neglecting soil fertility severely depletedreserves of soil P in a long-term corn-soybean rotation study (Kansas, Gordon,2003).

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of these measurements can be used to dividefields into management zones or units that canbe fertilized independently (Koch et al., 2004).Site-specific fertility management increasesthe odds that nutrient needs are properly iden-tified and appropriate corrective fertilizer ap-

plications are made only where required. Thismanagement practice can take into accountthe natural variation in soil fertility and nutri-ent supply.

Minimizing Nutrient LossMinimizing Nutrient LossMinimizing Nutrient LossMinimizing Nutrient LossMinimizing Nutrient LossFrom an environmental impact per-

spective, a major goal of land managers shouldbe to retain soil and associated nutrients withinthe boundaries of a field and the rooting zone

of the crops grown. Fertilizer application basedon soil testing and realistic yield goals helpsto ensure that proper rates are recommendedand applied. This improves plant nutrient useefficiency, and lessens the potential for re-sidual nutrients to accumulate to excessivelevels that may pose an environmental threat.

Nutrient LeachingNutrient LeachingNutrient LeachingNutrient LeachingNutrient Leaching Retention of solublenutrients in the rooting zone helps ensure effi-cient recovery and effective use in crop pro-

duction systems. Leaching occurs when exces-sive residual nutrients are left in the soil pro-file and moved below the rooting zone by pre-cipitation or irrigation. While leaching is nota common problem in most semiarid regions,historic use of fallow may result in NO

3-N

accumulation below the rooting zone of crops.While there are no reported incidences of Pleaching through fertilizer use at soil test rec-ommended rates, leaching of P can occur with

the application of livestock manure at ratesgrossly in excess of crop requirements.

Conservation Tillage, Soil Erosion,Conservation Tillage, Soil Erosion,Conservation Tillage, Soil Erosion,Conservation Tillage, Soil Erosion,Conservation Tillage, Soil Erosion,and Carbon Sequestrationand Carbon Sequestrationand Carbon Sequestrationand Carbon Sequestrationand Carbon Sequestration The retentionof crop residues on the soil surface signifi-cantly reduces the loss of soil by wind andwater erosion, while at the same time improvesmoisture conservation and crop yields.

Crops grown with proper nutrition play amajor role in building soil organic matter. A

good fertility program results in more biomasand helps sequester atmospheric carbon diox-ide (CO

2), thus ultimately resulting in the re-

turn of more organic carbon (C) to the soil forstorage as soil organic matter.

Field Buffer StripsField Buffer StripsField Buffer StripsField Buffer StripsField Buffer Strips The movement ofN and P into surface waters with eroded soilposes a serious threat to aquatic ecosystems.While some nutrients are required for ecosys-tem function, too much can lead to a decline inproductivity. Eliminating soil erosion from ag-

ricultural lands has been a high priority for allfarmers. Eroded soil means loss of nutrients,organic matter, and future crop productivity.The adoption of conservation practices suchas reduced tillage and buffer strips adjacent tosurface water has been shown to reduce unde-sirable movement of nutrients. BC

Dr. Stewart is PPI Southern and Central Great PlainsRegion Director, located at San Antonio, Texas;

e-mail: mstewart@ppi-far.org.

To view an expanded version of this article and achart listing fertilizer BMPs for this region, plusadditional information and references, visit the PPI

website: >www>www>www>www>www.ppi-ppic.or.ppi-ppic.or.ppi-ppic.or.ppi-ppic.or.ppi-ppic.org