modeling of soil nutrients – an introduction to logical spreadsheeting russell yost department of...
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Modeling of Soil Nutrients – An Introduction to Logical Spreadsheeting
Russell Yost
Department of Tropical Plant and Soil Science, University of Hawai`i at Manoa
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Facilitating calculations with a simple logical tool
• Undergraduate students usually have difficulty with quantitative calculations necessary in soil science– Unit conversion:
• Conversion of mg kg-1 to cmolc kg-1
– Converting laboratory results to field quantities• Nutrients: ppm or mg kg-1 to kg ha-1
• Adjusting soil pH
– Calculating estimates of fertilizer requirements• Amount of N fertilizer of various qualities• Amount of P fertilizer of various types• Amounts of lime to neutralize soil acidity and reduce Al toxicity
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Facilitating calculations with a simple logical tool
• Graduate students in fields other than soil science may not know the calculations
• Students in Plant and Soil Science may have detailed calulations for their research
• Some conceptual models need actual calculation to assess value.
• Useful for researchers to quantify thoughts and explore options in a quanitative way.
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Logical Spreadsheeting – Buckmaster’s notes:
• Use a readily available calculator – Excel spreadsheets
• Organize the calculation or problem into 3 logical units:– Data Entry– Calculations to perform– Results to obtain
• Document equations, units, values, descriptions• Enable quick, accurate changes
Buckmaster. 2006. Notes for Agricultural Engineers. Purdue University.
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Some nutrient calculations can become detailed:
• Fertilizing Lettuce • Suppose you have the following information on the soil of your
garden. For a sample 0-20 cm you know the bulk density is 1.25, the organic C is 10 g kg-1,soil solution P is 2.5 mg kg-1, soil solution K is 10 mg kg-1, water content is 30%. Please answer the following questions:– a) What is the amount of organic N in the surface 20 cm of soil
assuming a C/N ratio of 12?– Calculate the quantity of inorganic N assuming a rate of
mineralization of 3% per year– Assuming the lettuce uses the surface 20 cm of soil, calculate the
quantity of soil solution K available to the crop– Calculate the amount of soil solution P available to the lettuce.– Calculate the quantity of 20-10-10 needed to supply the needs of
lettuce assuming it needs 150 kg N ha-1. • While conceptually simple and routine, these calculations can trip up
just about everyone.
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Logical Spreadsheeting – 3 sections, 5 columns:
Data Entry Values Units Equation Description
Desired N 100 kg N ha-1 Required NFertilizer available 46 % Fert.AnalysisField length 300 meter Field lengthField width 100 meters Field widthCalculations
Area (m2) 30000 m2 =D5*D6 Field, meters2
Area (ha) 3 ha =D8/10000 Field, ha1
N analysis 0.46 fraction =D4/100 Urea AnalysisResultsAmt of fertilizer 652 kg fertilzer ha-1 =D3*D9/D10 Fert. Amount
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Solution to the lettuce fertilization question
Data Value Units NotesBulk density 1.25 tonnes GivenOrganic C 19 g kg-1 GivenTotal P 420 mg kg-1 GivenSoil Solution P 2.5 mg kg-1 GivenSoil Solution K 10 mg kg-1 GivenAmount of H2O 0.3 decimal GivenC-N Ratio 12 ratio AssumptionMineralization 3% % yr GivenCrop N absorption 150 kg ha-1 Given
Calculation1 hectare 10000 m2 Conversion20 cm soil 0.2 m ConversionArea of 20cm depth ha-1 2000 m3 CalculationOrganic C 47500 g kg-1 CalculationAmount of N mineralized 1.1875 g kg-1 Calculation
ResultsOrganic N 3958.33 g kg-1 =(B25/B13) Part AInorganic N 118.75 kg ha-1 =(B32*B14) Part BSoil Solution K 6 kg ha-1 =(B23*B10*B26)/B27 Part CSoil Solution P 1.5 kg ha-1 =(B9*B23*B26)/B27 Part DFert quantity 20-10-10 156.25 kg =(B15-B33)/0.2 Part E
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Logical Spreadsheeting – student comments
• Results with students:– Appreciated the powerful way to solve word
problems– Empowered to do more quanitative analyses
in the course of their research and learning.– Now they have a convenient way to test
concepts and ideas by writing the Excel code to do the actual calculation
– Introduction to the potential of modeling as a tool to enhance and improve learning.
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Logical Spreadsheeting – extensions
• Extensions to other problems– Optimization
• Lime selection: Given:– Lime quality(CCE, fineness), lime quantity, liming depth,
lime cost– Soil Ca content and target amount, soil Mg content and
target– Distance of lime transport, transportation cost
• Calculate the amount of lime of each of five types suitable for a particular location. (Li et al. Liming Material Selection by Computer Spreadsheet. JNRLSE 25:26-30.
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Logical Spreadsheeting – use in other courses
• Useful in advanced coursework:– Write out the actual equation and conversions
to learn how mathematical calculations and analysis aids learning.
• Used in implementing concepts of nutrient management in the following areas:– Estimating crop needs of Nitrogen,
Phosphorus, Potassium and Limestone needs for specific crops.
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Logical Spreadsheeting – use in other courses and applications:
• Implementing Stanford’s N balance ideas:
Stanford J.Environ. Qual. 2:159-165)
Prediction – case of P
• Where: Preq=Predicted amount of P fertilizer• bc = Critical level of P for specified crop• b0 = Measured extractable P in the field• a2 = P buffer coefficient (PBC, increase in extractable P
per unit added P)• a1 = slow reaction coefficient• d = depth of incorporation(value of 10 to 20cm typical)• BD = bulk density• placement = function of the fraction of row width fertilized
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factorplacementBDdaa
bbc***1*
2
0 Preq
Prediction – case of P
• Where: Preq=Predicted amount of P fertilizer• bc = Critical level of P for specified crop• b0 = Measured extractable P in the field• a2 = P buffer coefficient (PBC, increase in extractable P
per unit added P)• a1 = slow reaction coefficient• d = depth of incorporation• BD = bulk density• placement = function of the fraction of row width fertilized
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factorplacementBDdaa
bbc***1*
2
0 Preq
Crop property
Prediction – case of P
• Where: Preq=Predicted amount of P fertilizer• bc = Critical level of P for specified crop• b0 = Measured extractable P in the field• a2 = P buffer coefficient (PBC, increase in extractable P
per unit added P)• a1 = slow reaction coefficient• d = depth of incorporation• BD = bulk density• placement = function of the fraction of row width fertilized
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factorplacementBDdaa
bbc***1*
2
0 Preq
Soil factors
Prediction – case of P
• Where: Preq=Predicted amount of P fertilizer• bc = Critical level of P for specified crop• b0 = Measured extractable P in the field• a2 = P buffer coefficient (PBC, increase in extractable P
per unit added P)• a1 = slow reaction coefficient• d = depth of incorporation• BD = bulk density• placement = function of the fraction of row width fertilized
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factorplacementBDdaa
bbc***1*
2
0 Preq
Soil management factors
2. Environmental Health
• Two of the most environmentally challenging nutrients must also be considered:– Nitrogen
• Contamination of surface and groundwater
– Phosphorus• Primarily contamination of surface waters
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2. Environmental Health
• Our approach to evaluating the adverse effects of excessive nutrients for Hawai`i conditions is similar for both nutrients:– 1. Evaluate the amount of nutrient– 2. Evaluate the potential transport– 3. Evaluate the vulnerability of the associated
water bodies (surface or groundwater)
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2. Environmental Health - Nitrogen
• 1. The amount of nutrient – N availability– Identify soil series of the field– Measures of nitrate / ammonium– Crop N removal– Fertilizer applied– Result: Estimate of risk due to N availability
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2. Environmental Health - Nitrogen
• 2. N transport potential– Evaluate runoff potential– Evaluate N attenuation– Evaluate N leaching– Evaluate N irrigation– Result: Estimate of risk due to transport
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2. Environmental Health - Nitrogen
• 3. N vulnerability of associated waterbodies– Evaluate vulnerability to N in runoff– Evaluate vulnerability to N leaching– Evaluate waterbody current condition– Result: Estimate of risk due to vulnerability of the
waterbody
• Combined evaluation of risk of N impairment: (Availability, Transport, Vulnerability)
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2. Environmental Health - Phosphorus
• 1. Availability of P, measured by extractable P• 2. Assess the potential transport of P
desorbed into runoff (same as for N, except without leaching hazard)
• 3. Identify and assess risk of P impairment of associated waterbodies (same as for N)
• Combined evaluation of risk of P impairment due to Availability, Transport, and Vulnerability
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Summary• Logical spreadsheeting permits rigorous application
and understanding of how calculations are made• Useful as a preliminary step towards logically
structuring a problem that will be modeled• Useful for estimates of fertilizer need as well as
potential impairment due to excessive N and P. (Not illustrated, spreadsheets for limestone and Potassium needs)
• Considerations of nutrient needs both for Productivity and Environmental Quality can be modeled and quantified.
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