Nutrient Movement • Phosphorus and potassium are typically considered relatively immobile – Due to their nature and behavior in soils Fertilizer Manure Plant uptake Adsorption Labile Adsorbed P Desorption Solution P H2PO4- Mineralization Organic Precipitation Secondary Matter Minerals Dissolution HPO42- Bound Fe/AlPO4 ImmobilizationNonlabile P CaHPO4 <0.3 ppm Leaching Total P in soil – 50-1500 ppm Primary Dissolution ~ 100-3000 lb/acre Minerals
Nutrient Movement • Potassium cycle Fertilizer Manure Adsorption Exchangeable K Desorption Plant uptakeFixation Solution K Release K+ 1-10 ppm Nonexchangeable Fixation (occluded) K Total K in soil – 5,000-25,000 ppm ~ 10,000-50,000 lb/acre Leaching Feldspars Micas Weathering
Nutrient Movement• Soil textural influence on K movement Claasen and Jungk, SSSAJ, 1982
Nutrient Movement• So, P and K do not move much, so what; how does that influence things like soil testing?• Before we go there, let’s see how these immobile nutrients are taken up.
Nutrient Movement and Uptake• Nutrient mobility and competition Root system sorption zone Root surface sorption zone
Nutrient Uptake• Since they are available from a relatively small volume of soil, is there much competition between plants for these nutrients?• There may be some competition, but not like for a mobile nutrient like nitrogen• This is important, due to a lack of competition between plants, the amount of nutrient required is not related to yield level• All we need to do is achieve a nutrient concentration to ensure adequate availability!
Soil Potassium• Exchangeable K – Remember that clays are 3-dimensional structures11
Soil Potassium• Occluded K is not necessarily unavailable to the plant, nor is supplied K necessarily plant available, why?12
Potassium• Adequate potassium nutrition increases water use efficiency and reduces drought stress• Opening and closing of stomatal pores in plant leaves is regulated by K concentration in the guard cells regulating transpiration Guard Cell
Critical Levels• Iowa State data – relative yield versus STP
Critical Levels• Iowa State data – relative yield versus STK
Critical Levels• Yield response as a function of soil test - P
Critical Levels• Yield response as a function of soil test - K
Critical Levels• It is not just the relative magnitude of the yield response, but also the probability of responseSTP (ppm) Probability of response (%) STK (ppm) Probability of response (%)<9 80 < 90 809-15 65 90-130 6515-20 25 130-170 2520-30 5 170-200 530 + <1 200 + <1
Declining Soil Test Levels• “A random survey from 2007 and ’08 found 45% of Illinois fields checked were below critical potassium levels needed for maximum yields.” – Fabian Fernandez, University of Illinois
Declining Soil Test Levels Soil test P Soil test KBoth P and K soil testlevels are trending down.
Summary of Soil Concepts• P and K are relatively immobile• Soil testing can be used as a management tool• Soil testing is not perfect – Spatial variability, error in sampling/analysis, and temporal variability in analysis (conditions at sampling time) – It is, by far, our best tool
Philosophy of Fertilization• What is the goal of fertilization…• To maximize net return on inputs each year?• To assure that fertility limitations do not exist within a production year/rotation?• To maximize short-term or long-term productivity?
Philosophy of Fertilization• Let’s revisit this data and put some economics to it
Philosophy of Fertilization• Net return to P application
Fertilizing Rotations• What if you are fertilizing rotations, do you have adequate P and K for a soybean crop after a good corn crop?• It depends…• Upon your starting soil test level• For every bushel of corn you harvest you are removing 0.37 pounds of P2O5 and 0.27 pounds of K2O – So a 200 bushel yield will remove 76 pounds of P2O5 and 54 pounds of K2O – A 250 bushel yield will remove 95 and 68 pounds of P2O5 and K2O, respectively
Fertilizing Rotations• If your starting soil test was near the critical, you will likely come up short on your P and K for the soybean crop• For every 18 pounds of P2O5 removed, soil test will change by 1 ppm• For every 8 pounds of K2O removed, soil test will change by 1 ppm
Summary• Soil testing is our best tool, it is not perfect• You (and your clients) need to figure out the approach that best fits their goals and economic desires• Applying same maintenance rates as practiced historically is likely not maintaining current soil test levels