modeling of soil nutrients

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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



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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bbc***1*

2

0 Preq

Crop property




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bbc***1*

2

0 Preq

Soil factors




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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. 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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• 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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modeling of soil nutrients – an introduction to logical spreadsheeting russell yost department of...

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