Modeling of Soil Nutrients – An Introduction to Logical Spreadsheeting Russell Yost Department of Tropical Plant and Soil Science, University of Hawai`i at Manoa 1

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 cmol c 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 2

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. 3

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 Notes for Agricultural Engineers. Purdue University. 4

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 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. 5

Logical Spreadsheeting – 3 sections, 5 columns: 6

Solution to the lettuce fertilization question 7

Logical Spreadsheeting – student comments 8 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.

Logical Spreadsheeting – extensions 9 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.

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. 10

Logical Spreadsheeting – use in other courses and applications: 11 Implementing Stanford’s N balance ideas: Stanford J.Environ. Qual. 2: )

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 12

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 13 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 14 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 15 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 16

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) 17

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 18

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 19

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) 20

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 21

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. 22