1. Phosphorus and Potassium

2. 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.

4. 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).

5. 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.

6. Mehlich A more recently developed procedure (1980’s) 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.

7. 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

8. 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)

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

10. P Fertilization Sufficiency: Fertilize the Crop Maintenance: Fertilize the Soil Maintenance – Replace what the crop removes – Often used with Build-up model. – Build up the soil then maintain

11. P Buildup Since soluble fertilizer forms of P react with the soil to form less soluble compounds soon after they are added, plant uptake efficiency, or fertilizer recovery, for soil incorporated fertilizer is usually only about 15 percent for most growing seasons (crops) As a result of this, about 85 percent of the fertilizer- P remains in the surface soil in forms that are only slightly soluble, but which do contribute a small amount of plant available-P.

12. P Build-UP Soil test-P associated with net P 2 O 5 input. (Lahoma- 502, 1971-1997).

13. 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.

14. 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

15. Correcting P Deficiencies Although the relationship varies somewhat for different soils, one can use the relationship of 15 lb P2O5/ STP (unit of soil test-P) to estimate the amount of fertilizer, and cost, required to correct a deficient soil to a fertile soil. A soil that tested 15 would require about 750 lb P2O5 in excess of harvested removal to raise the soil test to 65 (65 STP-15 STP = 50 STP; 15 lbs P2O5/STP x 50 STP = 750 lb P2O5). At $0.40/ lb P2O5 (a realistic price) it would cost about $300/acre to build the soil test from 15 to 65. Estimates such as this are useful in comparing the relative value of lands that have widely differing P fertility levels. Calculating the amount of P2O5 required to change a deficient soil to a fertile soil is also useful when it is desirable to make a “long-term” adjustment prior to starting a small-scale perennial crop or planting that will not be cultivated.

16. Correcting P Deficiencies In a home landscape, trees and bushes may be grown more successfully if a single large application of lb P 2 O 5 is incorporated into the intended rooting area prior to planting (calculations must convert lb/acre rates to lb/1000 ft2 basis or smaller). When possible, straight phosphate fertilizer (0-46-0) should be used instead of ammonium phosphates to avoid excess N applications, and the fertilizer should be applied a few weeks or months before planting to allow some “aging” (water soluble P reacting to form insoluble P) to avoid exposing the new plants to abnormally high levels of plant available P (H 2 PO 4 - and HPO 4 2- ).

17. KSU Fertility Rec.