Adequate phosphorus (P) and potassium (K) nutrients are needed for optimum crop growth and development. Soil fertility levels for P and K are affected by their inherent availability in the soil, past crop removal amounts, and soil pH. Prior to the growing season, existing P and K fertilizer programs should be evaluated. If soil tests indicate adequate P and K levels, farmers should continue to apply fertilizer to compensate crop removal levels. If soil tests are below minimum levels, “buildup” and crop removal P and K fertilizer should be applied.
P is considered an “immobile nutrient” compared to nitrogen (N) as it has a relatively short range of movement within soil. Plants take up soluble P that is in soil solution; however, the amount of soluble P in solution at any one time is quite low. As the supply of soil solution P decreases from plant uptake, the “supply” is replenished from other sources of soil phosphate. Essentially, unavailable P is slowly released to available forms to replenish P in soil solution. This is a continuous process, as the “pool” of soluble P may be replenished up to 500 times during the growing season to meet crop needs.1 The rate of replenishment is influenced by soil pH, current phosphorus levels, soil P fixation, and the placement of added phosphate fertilizers. Optimum phosphorus availability is at a soil pH range of 6.0 to 7.0.1,3 When soil is very acidic, aluminum and iron can bind to P to form insoluble phosphate compounds. With alkaline soils, phosphorus reacts with excess calcium to form unavailable compounds.
Ability of corn plants to take up P is influenced by root distribution relative to P location in the soil. Soluble P only moves about 1/8 inch/year, therefore roots must grow through the soil to reach adequate P.1 Plant uptake of P is enhanced with adequate soil moisture; therefore, dry soil conditions can reduce P uptake by the root system. Even when soils have adequate P, plant seedlings may exhibit P deficiency symptoms as young root systems can be slow to grow due to cold, wet conditions that are common during the early part of the season. P deficiency associated with cool soils is usually more of a concern with corn than with soybean. Soybean is generally planted when soil temperatures are warmer and plants have a more vigorous rooting system than corn.3 Adequate phosphorus supplied to crops results in higher yield potential, improved crop quality, greater stalk strength, increase root growth, and earlier crop maturity.3
Symptoms of P deficiency in corn and soybean include stunted plants and initially dark green leaves. Older leaves on young corn plants may appear purple (Figure1), while soybean has dark green to bluish green leaves. Symptoms appear first on the lower leaf tips and extend down the margins toward the leaf base. When P deficiency is severe leaf edges may become brown and lower leaves often die, especially during hot, dry, and windy conditions. In addition, stalks may be thin and short and maturity can be delayed. Deficiency can be confirmed with soil testing for P level. Generally, crop removal and surface runoff erosion are the primary mechanisms that contribute to the reduction of soil P levels.
There are large amounts of K in the soil (frequently over 40,000 lbs/acre) but only a small fraction is available for plant uptake.4 Three forms of K exist in soils, unavailable, slowly available or fixed, and readily available or exchangeable. K that is dissolved in soil water and held on clay particles (exchangeable K) is considered readily available to plants. As soluble K concentration declines due to plant uptake, exchangeable K is released from clay particles to replenish the soluble K “pool”. Over time “slowly available K” is released replacing exchangeable K; however, plants can not use much of this form of K in a single growing season.4
Symptoms of K deficiency in corn and soybean are characterized by yellowing or browning of the leaf margins, beginning at the leaf tips, and can often be confused with N deficiency. Similar to N, K is mobile in the plant so older leaves show deficiency symptoms first (Figure 2).
When soil tests indicate P and K are in the optimum range, fertilizer application recommendations should be equal to the amount of nutrients removed at harvest. If soil tests are below optimum, “build-up” and crop removal P and K should be applied. Water solubility of phosphate fertilizers should not be of concern under typical soil and field conditions because common phosphate fertilizers all have water solubility above 80%.3 In order to verify soil P and K levels, it is recommended that soil test be preformed at least every 4 years.2
Each bushel of corn harvested per acre, removes the approximate equivalent of 0.35 lbs of P22O5and 0.25 lbs of K22O (Table 1).6 Soybean removes a greater amount of P22O5 and K22O per bushel compared to corn: 0.73 lbs of P22O5/bu and 1.2 lbs of K22O/bu. These values are important when calculating the amount of crop removal fertilizer to apply.
A yield example of 200 bu/acre corn, will remove about 70 lbs/acre of P22O5 (200 x 0.35 lb/bu) and 50 lbs/acre (200 x 0.25 lb/bu) of K22O. Also, 60 bu/acre soybean yield will remove about 44 lbs/acre of P22O5 (60 x 0.73 lb/bu) and 72 lbs/acre (60 x 1.2 lb/bu) of K22O. Crops cut for silage, corn stover, straw, or forage remove additional nutrients because the majority of the aboveground tissue is harvested as well.
1 Bengle, D.B. and Durst, P.T. 2002. Managing phosphorus for crop production. Agronomy Facts 13. Penn State Extension. http://extension.psu.edu/plants/nutrient-management/educational/soil-fertility/managing-phosphorus-for-crop-production.
2Fernandez, F.G. and Hoeft, R.G. 2009. Managing soil pH and crop nutrients. Chapter 8. Illinois Agronomy Handbook. http://extension.cropsci.illinois.edu/handbook/.
3 Hoeft, R.G., Nafziger, E.D., Johnson, R.R., and Aldrich, S.R. 2000. Modern Corn and Soybean Production. MCSP Publications.
4 Rehm,G. and Schmitt, M. 2002. Potassium for crop production. University of Minnesota Extension. http://www.extension.umn.edu/agriculture/nutrient-management/potassium/potassium-for-crop-production/.
5 Malvick, D. 1995. Corn stalk rots. RPD number 200. University of Illinois Extension. https://ipm.illinois.edu/.62014. Nutrient removal by selected crops, Table 4.5. International Plant Nutrition Institute. http://www.ipni.net/article/IPNI-3296. 7 Busman, L., Lamb, J., Randall, G., Rehm, G.., and Schmitt, M. 2009. The nature of phosphorus in soils. University of Minnesota Extension. http://www.extension.umn.edu/agriculture/nutrient-management/phosphorus/the-nature-of-phosphorus/.
Web sources verified 02/15/16. 150126142754