NRES 201 Lecture 40 Outline of Last Lecture I Some fundamentals II Nutrient deficiency symptoms III Plant analysis IV Basic aspects of soil testing Outline of Current Lecture V Practical aspects of soil testing VI Yield based nitrogen management VII Soil based nitrogen management Current Lecture Field Sampling Fundamental Assumption Samples for soil testing accurately represent the entire field Depth of Sampling A crucial factor has been overlooked 7 inches for pH P and K are there to represent the plow layer and simplify sampling Method of sampling o A 1 inch soil probe is recommended Automated soil samplers can also be used Time of sampling o Every 4 years in late summer or fall to reduce seasonal variability in K tests o Sample before applying lime or fertilizer otherwise you ll need to wait a few months Extraction Significance It is the crucial step in any soil test procedure that determines selectivity in estimating the plant available forms of a nutrient A potential source of problems o Extraction procedures modified for the sake of speed and convenience o It can lead to invalid fertilizer recommendations o Soil testing labs should be certified Chemical Analysis Requirements Accuracy Speed Convenience Implications of new methodology o The modern trend is toward multi nutrient testing with inductively coupled plasma ICP spectroscopy These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o This has changed what is being measured and requires recalibration with crop response Calibration Factors that affect soil test calibrations Soil factors Cropping factors Fertilizer factors and Tillage The usual tests are for pH P and K Other tests can be done but calibration may be unreliable This is especially a problem for micronutrients N recommendations Corn 1 2 x yield goal with adjustments for a previous legume manure or late planting Soybean N is not recommended Late planting o Reduce the N rate by 20 lb A for each week delay in planting after the optimum date Apr 10 15 in Southern IL Apr 20 May 1 in Central IL May 1 May 10 in Northern IL o Later planting lowers the yield potential so less N will be needed and a rate adjustment should be made if N will be applied to the growing crop sidedressed There is more crop N uptake from fertilizer than from soil The Illinois Soil N Test ISNT A simple diffusion method that estimates soil N supplying power by measuring Alkali labile organic N and NH4 but not NO3 Valuable insights about the ISNT o Management affects the critical range o Calibration shifts upward With low soil pH P or K With low subsoil fertility With higher plant populations With accumulation of carbonaceous residues Potential applications for the ISNT o Estimation of soil N supplying power o Site specific management of N fertilization and planting rate o Land valuation
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