P, K, S and Micronutrients

1. P, K, S and Micronutrients Bill Raun Oklahoma State UniversityWheat Technology MeetingJuly 31, 2007

2. How much and what forms of P are found in soils? • Total P content in soil ranges from about 0.03 to 0.3 % P, and is not related well to plant available P because much of the total P is found in very insoluble primary minerals and precipitated secondary minerals. • Availability of H2PO4- and HPO42- at the root surface is strongly influenced by temperature. Cold temperatures decrease solubility of the compounds supplying H2PO4- and HPO42- to the soil solution, and cold temperatures also decrease their movement by diffusion from the soil solid surface to root surfaces.

3. Distribution of soil-P between solid and solution • The connecting tube represents dissolution/precipitation reactions. • Solid forms of soil-P may be differentiated further by considering those forms that may readily (labile-P) move into the soil solution from those that will not (fixed-P).

4. Characteristics of solution-P • P in the soil solution is primarily inorganic. • Concentration of inorganic P in the soil solution is very small in natural systems • Also small in fertilized soils after the added fertilizer has reached a ‘near equilibrium’ condition. • A solution concentration of 0.05 ppm is believed to characterize soils with adequate P for plant growth and development • Weak concentration only supplies about 1% of the total P required for plants by mass flow transport. • Most P reaches the root surface by diffusion and root interception, and that the amount of P removed from the solution by a growing plant (e.g. corn) may be replenished two to three times each day during the growing season, as solid forms dissolve.

5. P Dissolution • When solid forms of P dissolve in the soil solution or when fertilizer-P is added, the ionic form of P present in the soil solution is pH dependent. • Stepwise dissociation of phosphoric acid (H3PO4) and the appropriate equilibrium or dissociation constants (Keq or Ka).

6. Orthophosphate P Ionic forms of P taken up by plants (H2PO4- and HPO42) exist in equal amounts at about pH 7.2. Plants do not appear to have a preference for one form over the other, thus there is little justification for trying to lime a soil to a pH where ‘P is most available’.

7. Characteristics of solid-P? • Since the phosphate ion may exist in the tri-valent form (PO43-), it is capable of forming highly insoluble compounds with di-valent and tri-valent cations, if such cations are present in the soil solution. • The relationship of soil pH and percentage base saturation, and the lyotropic series, characteristic of all soils, provides evidence that phosphates will react with Fe 3+ and/or Al 3+ in acid soils and Ca 2+ in near neutral and basic soils. • Throughout the soil pH range where plants will normally grow, one or more of these cations will be present to react with phosphate ions. • As a result of these reactions, surface applied phosphate does not leach through soils, but is instead retained near the surface in these solid forms.

8. Precipitation/Dissolution pH 4.5 Event Precipitate Formed 1. add fertilizer soluble P added - 2. 1 - 2 soluble P decreases DCP 3. 2-3 DCP dissolves FA 4. 3-4 FA dissolves Variscite

9. Since phosphate precipitates from solution to form solid iron phosphates, aluminum phosphates, and calcium phosphates, it follows that the concentration of plant-available inorganic P is governed by the solubility of these compounds. Minerals present in acid soils are of the general type

10. Calcium Phosphates * Times in italics are the approximate time required for monocalcium phosphate to revert to the indicated, less soluble, forms.

11. Factors influencing P retention • Factors responsible for plant available-P being retained in the soil surface, are those characteristics that have been identified in the retention and fixation processes. • Soil pH is important, and in near neutral to basic soils the amount of naturally occurring lime present increases the reaction and formation of insoluble calcium phosphates. • In acid soils the level of acidity (e.g. pH < 5.5) and, high clay content, and dominance of 1:1 over 2:1 clay types all increase the retention and fixation of phosphates. • 1:1 clay types offer more Al reactions sites than 2:1 types. • So where would we expect increased P fixation? What type of environments?

12. Organic soil P • Like other plant nutrients that form strong bonds in organic compounds (e.g. N and S), P in soil organic matter may be a significant source of plant available P in virgin soils. • Mineralization of organic-P is an important source of plant available-P for several years as virgin soils are brought under cultivation. • Eventually, P availability in cultivated soils is governed mostly by the inorganic reactions already described. In soils where P fixation is high, organic-P fertilizers (animal waste, etc.) can be effective ‘slow release’ P sources.

13. How is P managed? • Key to managing soil and fertilizer P: Knowledge of whether or not the level of soil solution P is adequate (about 0.05 ppm) to meet the needs for plant growth. • When the level of solution P is not adequate, it is important to know how much P fertilizer should be added, and/or how much yield loss will occur if the P deficiency is not corrected. Phosphorus soil tests have been developed to help provide this information. • The concentration of plant available soil-P is extremely low and does not represent the total amount that may become available during a growing season. • Effective soil tests extract P that is immediately available (intensity factor) and a representative portion of the P that will become available during the growing season. • The latter fraction represents aluminum and iron phosphates in acid soils and calcium phosphates in near neutral and basic soils. Because the tests do not exactly simulate plant root extraction of P from the soil, relationships must be developed (correlation) between what the soil test extracts and what plants extract.

14. Extracting Ca-P in acid soil?

15. P soil tests • In the early period of soil test development, many chemical solutions and extraction procedures were used. • Over time, similarities have been recognized that allow reliable extraction and analysis to be made using only one procedure, with consideration for soil pH. • A common P soil test for acid soils is the Bray P1 procedure, developed by Bray and Kurtz at the University of Illinois. • The procedure is designed to dissolve Al-phosphates by precipitating Al with fluoride (F).

16. Olsen • For neutral and basic soils a bicarbonate solution developed by Olsen at Colorado State University, has proven effective in dissolving Ca phosphates by precipitating Ca with carbonate.

17. Mehlich • A more recently developed procedure developed by Adolph Mehlich, working at the North Carolina Department of Agriculture lab uses a solution of acetic acid, ammonium nitrate, ammonium fluoride, and EDTA to extract a portion of plant available P from either acid or basic soils. • This procedure, identified as the Mehlich-3, is becoming widely used and is replacing regionally specific procedures like the Bray P1 and Olsen’s bicarbonate.

18. Correlation • For any P soil test procedure to be beneficial, the extracted P must relate to crop response or growth and development in the field. • The extent to which this relationship is found can be identified by a statistical procedure called correlation • When there is a good general relationship between the soil test extraction values (usually expressed in ppm-P or lb/acre-P) and the percentage of maximum yield obtained (% sufficiency), then the procedure has promise as an effective tool to help manage fertilizer-P inputs. Generalized correlation of soil test-P and crop response

19. Calibration • Calibration is a process that involves continuation of the research to identify the amount of fertilizer-P that must be added by a conventional method (usually preplant incorporated) to correct an existing deficiency. • An important aspect of the calibration process is to identify the “critical level”, or soil test level that corresponds to a soil-P fertility conditions above which plant response does not occur when fertilizer-P is added (this may also have been identified in the correlation process) . • For the Mehlich-3 procedure this corresponds to about 33 ppm P (65 lb/acre). (see next slide)

20. Calibration ppm * 2 = pp2m ppm * 2 = lb/acre 2,000,000 lbs /afs (0-6”)

21. P Build-UP Soil test-P associated with net P2O5 input. (Lahoma-502, 1971-1997).

22. Build-UP • With continued annual fertilization a gradual build-up of P results in developing a soil-P condition that will provide adequate P to meet crop needs. • This development can be monitored by annual soil testing, and while it varies depending on the soil and the soil test procedure used, for the Bray P1 and the Mehlich-3, the build up is about 1 soil test unit (lb P/acre or pp2m) for every 15 lb P2O5 fertilizer P added in excess of crop removal.

23. P Build Up • Build up of soil-P (soil test-P) that will become available to plants during a growing season can also be envisioned using the reservoir diagram • The small reservoir represents soil test-P and the large reservoir to which it is connected represents the amount of slowly available soil-P. • When fertilizer additions exceed crop removal the large reservoir eventually “fills up” to the point where the soil test reaches 65 and fertilizer may be unnecessary for several years. Soil test-P in relationship to soil capacity to adsorb and precipitate P

24. Methods of P fertilization • Most common application of P fertilizers: Broadcast fertilizer over the soil surface and then incorporate it with a tillage operation. • Alternative: Band with the seed, or two inches below and to the side of the seed at planting. • Broadcast-incorporation is less time consuming and is popular when large acreages must be fertilized and planted in a short period of time, or labor is scarce. • Banding: more of the applied fertilizer is positionally available (placed where the developing root will be) and rates required to correct deficiencies for the season may be one-half to one-third that needed for the broadcast-incorporated method. • Soil build-up and associated increase in STP levels will be less with annual banding than broadcast-incorporated fertilization.

26. Foliar applied P • Although foliar fertilization is usually restricted to the correction of micronutrient plant deficiencies, there is reason to believe foliar P fertilization could be effective in select situations. • Interest in this approach results from the recognition that soil applied P fertilizers, while effective in correcting plant deficiencies, contribute only a small amount of P to the crop. • When 30 lbs P2O5/acre is broadcast-incorporated only about 15 % (4.5 lb P2O5) is absorbed by the crop. • Foliar absorption would be in the range of 50 to 80 % efficient and a rate of only 10 to 12 lb P2O5 , or less, would be as effective as the soil applied method. • This approach has special appeal in countries where the soil has a high P-fixing capacity and labor is inexpensive to allow hand spraying of small-scale production systems (e.g., developing counties in tropical environments).

27. Inorganic fertilizers • All mineral fertilizers originate from mined geologic formations of the mineral apatite (rock phosphate). • Rock Phosphate (0-20-0). Finely ground rock phosphate was one of the first inorganic P fertilizer used. • Its low P2O5 analysis and low solubility were associated with high rates and costs when it was used. • Although very little rock phosphate is currently used, it can be an important source of P on soils that have a high P fixing capacity or a single application is desired to correct a severe soil deficiency in a small area such as a home landscape. • Application to highly acid soils?

28. OSP • Ordinary Super Phosphate (0-20-0). Reacting rock phosphate with sulfuric acid to form more soluble monocalcium phosphate plus gypsum produced one of the first processed P fertilizers. Common in early use of fertilizers, it is still important in developing countries and also supplies sulfur (from gypsum). Hydroxy apatite

29. Concentrated Super Phosphate (0-46-0). • Reacting rock phosphate with phosphoric acid results in a higher concentration fertilizer because gypsum is not a product of the process. • For both ordinary and concentrated super phosphate (also referred to as triple super phosphate or TSP) the phosphate compound is monocalcium phosphate, a highly water soluble compound.

30. Diammonium Phosphate (18-46-0). • With time, cultivated soils became increasingly deficient in N and the fertilizer industry recognized the increased value of fertilizer materials containing both N and P. • Reacting phosphoric acid with ammonia produces ammonium phosphates, which have become the most popular form of P fertilizers in use today. • Diammonium phosphate, or DAP as it is commonly referred to, is the most popular. • Monoammonium phosphate (11-52-0, MAP) differs from DAP only in its more concentrated grade and that dissolves to form a slightly acidic solution instead of the basic solution formed from DAP. • Both are solid granular materials that can be easily blended with other solid fertilizers.

31. Ammonium Polyphosphate (10-34-0, APP) • This fertilizer is a liquid, and although it is usually considerably more expensive on a cost/lb P2O5 basis, it is gaining in popularity because of the convenience in handling liquid compared to solid materials. • When DAP, MAP, APP, and TSP have been compared in research trials at the same application rate of P2O5, effectiveness in correcting deficiencies has been equal. Selection of one P fertilizer over another should be made based on availability, convenience, and cost/lb P2O5.

32. Soil Potassium • Total K in soils averages about 40,000 lb/acre • Soil potassium is present in four categorical forms • occluded (within soil minerals such as feldspar, mica, etc), 98% of total • fixed (trapped within the lattice of 2:1 expanding clay minerals), 1% of total • exchangeable. 1% of total (100-1000ppm) • solution, 0.1% of total (1-10 ppm)

33. An equilibrium exists between each • Soil K Available K. Solution and exchangeable K normally represent "available" K for plants during a growing season

34. Available soil K Plant uptake is by diffusion (90%) and mass flow (10%) • K is immobile in soil (on a scale of 1 to 100, with 100 being most mobile, NO3- is 99, K+ is 33, and HPO42- is 1) • Factors affecting amount of available K to plants • soil mineralogy and climate • CEC • clay and organic matter content • K fixation and/or release • wetting and drying • freezing and thawing • subsoil and rooting depth • soil pH • competing exchangeable ions

35. Factors affecting plant uptake • Any condition that affects root growth effects uptake (plant response) of available K, all other things being equal. • compacted soil wet soil • acid soil • shallow soil • herbicide injury • K leaching (only a concern on permeable, low CEC soils) • K Soil testing • Exchangeable plus solution K (any extraction solution that will provide a strongly held cation, or a weakly held cation in high concentration) • Must be correlated and calibrated • calibrated on % sufficiency basis like P,

36. Fertilizer K • Muriate of potash (KCI), 0-0-62 • most common • mined in Canada and New Mexico • solid, 100% soluble • Application methods are similar to that for P because it is relatively immobile in soil. • exception: for high yielding forage crops, where forage is removed (bermudagrass or alfalfa, or turf such as putting greens) if soil is sandy, K management should be more like that for N, where amount required is more closely related to yield. • When both P and K are deficient, the yield loss will be a product of the % sufficiency’s for P and K. For example, if P is 80 % sufficient and K is 70 % sufficient, if neither deficiency is corrected by fertilizing, then the expected yield will be 80 % X 70 % (.80 X .70), or 56 % (0.56 X potential yield). • Salt Effect: Salt Rate N + K20Corn: <10 lbs Salt/ac with the seedWheat: < 30 lbs Salt/ac with the seed

39. K Management • Nutrient availability for a soil changes with time in relation to management. • Continued harvest removal of nutrients may result in deficiencies of those that are generally present in high concentrations in plants and for which the soil may have limited capacity to provide in plant-available form (e.g. N and K). • Continued fertilizer input of some nutrients may result in a “build-up” of the nutrient to the point that a previous deficiency no longer exists (e.g. P fertilization of low yielding crops)

40. K Management • Approaches to nutrient (fertilizer) management • Ask the fertilizer dealer “what are farmers using this year?” • Find out what the neighbor is using and fertilize like the neighbor • Soil test one or two fields and fertilize the rest of the farm based upon the average • Soil test each field, every year, until you have developed a confidence in your knowledge of what the field should test, knowing that soil test pH, P and K (immobile chemical properties) should not change much from year-to-year under normal practices.

41. What are the Primary Nutrientsneeded by all crops *Range of total amount in soil. From Chemical Equilibria in Soils. W.L.Lindsay, 1979. Wiley & Sons. **Calculated for 2 ton crop yield (67 bushels/ac, wheat).

42. Secondary Nutrients Neededby all Crops * Range of total in soil. From Chemical Equilibria in Soils. W.L.Lindsay, 1979. Wiley & Sons. **Calculated for 2 ton crop yield (67 bushel wheat).

43. Micronutrients Needed by all Crops *Range of total in soils. From Chemical Equilibria in Soils. W.L.Lindsay, 1979. Wiley & Sons. **Calculated for 2 ton crop yield (67 bushel wheat).

44. Review: Nutrients Needed by all Crops

45. Nutrients are grouped according to crop removal. • Primary (N, P, K). • Removed in largest amount by crop. • Most commonly deficient. • Secondary. • Removed in moderate amount by crop. • Micro. • Removed in minute amount by crop.

46. Nutrients not found deficient in Oklahoma crops. • Calcium. • Liming prevents Ca deficiency. • Manganese. • Copper. • Molybdenum.

47. Nutrients seldom found deficient in Oklahoma crops. • Magnesium. • Sulfur. • Iron. • Zinc. • Boron. • Chlorine.

48. Nutrients often Deficient in Oklahoma crops. • Nitrogen (N). • Legumes like soybeans and alfalfa get their N from microorganisms (rhizobium) that fix N from the atmosphere. • Phosphorus (P). • Potassium (K).