The Need of Basic Research in Phosphate Fertilization Problems of Sugar Beets

Transcription

1 The Need of Basic Research in Phosphate Fertilization Problems of Sugar Beets STERLING R. OLSEN 1 Many practical problems related to the phosphate fertilization of sugar beets have been answered reasonably well as a result of field, greenhouse, and laboratory experiments. Few basic relationships have been discovered, however, which are applicable to all soils. Usually the results obtained in one field experiment have a limited application for predicting fertilizer response on a different soil. Repetition of the usual field experiment is not providing the answers for the solution of this problem. There still exists a primary concern to develop a suitable method to measure plant available phosphorus in the soil applicable to a wide range of conditions. Soils are known to react in various ways with fertilizer phosphate to change its availability to plants. The nature of the soil properties which cause these differences and some of the properties of the forms of phosphorus resulting from these interactions have been studied extensively (2, 3, 5, 7, 11) 2 but the evaluation of these soil properties in terms of observed differences in fertilizer response is very limited. With newly developed methods for measuring plant available phosphorus and the nature of the reaction products of the fertilizer phosphorus with the soil, however, it would appear that more thorough studies can now be made relating soil properties to fertilizer response. In addition, more information is needed on the effect of environment (1), the relative properties of soil and root surfaces as they affect the relationships of yield to phosphorus uptake and supply, and the differences between crops in relation to their need and absorbing capacity for phosphorus. This paper reviews a few studies which have contributed toward the solution of these problems and points out some lines of approach which appear most promising. Nature of Reaction Products of Fertilizer Phosphate with Soil The purpose of this section is to emphasize the known properties of these reaction products and to show how the information may be applied to fertilizer problems. Specific properties of the various forms of phosphorus are not known in detail, nor is there a sharp distinction between them, but their over-all contribution in supplying phosphorus to the soil solution is the most important factor to measure. More complete discussions of the various forms of soil phosphorus have been described previously (3, 5, 7, 10, 11, 12). In general, the fertilizer phosphate reacts with the soil in two different ways to form (a) a fairly reactive fraction of high availability, and (b) an inactive fraction of low availability. The reactive fraction is probably dissolved fertilizer phosphate, phosphate on the surface of soil particles and finely divided phosphate precipitates, which readily exchanges with anions in the soil solution, and

2 358 AMERICAN SOCIETY OF SUGAR BEET TECHNOLOGISTS phosphate held in the double layer surrounding clay particles. The inactive traction is the phosphate bound inside precipitates formed from ions in the soil solution, and phosphate bound on the surfaces of clay minerals, hydrous iron and aluminum oxides. The fraction exchanges very slowly with radio-phosphate ions in the soil solution. The relationship between the effectiveness of a phosphate fertilizer and the manner in which it reacts with the soil involves two important considerations, i.e. (a) the rate of conversion to the active and inactive fractions, and (b) the relative amounts of each fraction which may be formed. Assuming a water-soluble type fertilizer such as concentrated superphosphate, the rate of conversion to the active fractions, for that portion of the phosphate which diffuses away from the fertilizer particle to contact the soil, is probably complete within a few hours. The secondary reactions converting the active fraction to the inactive fraction proceed slowly and with varying intensity in different soils- The evidence indicates that these secondary reactions require several months or more for completion (6, 9, 13). The rate of these secondary reactions is increased by increasing temperature, soil moisture, clay content, number of wetting and drying cycles and by a decrease in ph or exchangeable calcium. The relative amounts of the active and inactive fractions formed change with time and cropping (10, 13). Initially, the reaction products appear to exist as minute pockets of phosphate-rich soil, their availability decreasing slowly over 3 to 4 months. After cultivation, cropping, biological transformations, wetting and drying, and plowing, these phosphate-rich pockets become more thoroughly mixed with the soil and less available to plants. As a result of these various reactions, the fertilizer phosphorus is held by the soil with varying bonding energies. The rate of renewal and capacity to replenish the soil solution phosphorus will depend upon the relative amounts of the active and inactive fractions present, their solubility, and surface area. The availability to plants of the soil solution phosphorus depends also upon the nature and surface area of the roots, the competition of other anions in the soil solution for adsorbing sites on the root surface, competition by microorganisms, metabolic processes in the plants, soluble salts, ph, and others. A practical objective in making use of the above information is the development of methods to measure plant available phosphorus and more rapid chemical methods which satisfactorily estimate the available phosphorus. This problem is difficult to solve in view of the complexity of factors involved. If predictions of fertilizer response can be made with 85 to 90 percent accuracy, such methods have great practical value in fertilizer usage. Measurements of Plant Available Phosphorus "A" value A measurement of plant available phosphorus more rapid than field tests has long been needed as a standard by which to judge the reliability of plant and soil analyses. Such a measurement can be obtained through the use of radioactive phosphorus in greenhouse pot tests (4). The available soil phosphorus is measured in relation to the available phosphorus from a phosphate fertilizer, usually concentrated superphosphate. The effect

3 PROCEEDINGS EIGHTH GENERAL MEETING 359 of the subsoil is not determined, although this factor may be quite important on some soils. If a survey were needed of the phosphate fertility status of a large number of soils, or the residual effect of applied phosphate fertilizers or soil management practices, a measurement of "A" values would probably provide the most reliable index. The relationship of "A" values to yield response in the field and the greenhouse is shown in Table 1. These samples were described previously (8), but one additional modification was used for the data in Table 1. If the yield of the check plot was less than 50 percent of the fertilized plot in the greenhouse experiment, the soil was considered likely to respond to phosphate applications in the field. Fifteen soils fell in this category and a significant response in the field experiment was not observed, possibly as a result of other limiting growth factors. Surface phosphorus A measurement of "A" value may not be feasible in all cases. More rapid determinations of available phosphorus closely related to "A" values are obtained through the use of radio-phosphorus in the laboratory based on essentially the same principles as the "A" value measurement (7). The surface phosphorus represents that portion of the various forms of phosphorus in the soil which will readily exchange with radiophosphorus in the soil solution, or the portion of the solid pha'e phosphorus in near equilibrium with the solution phosphorus. No dissolving action on the various forms of phosphorus is involved in this measurement. Approximately 30 to 50 percent of the residual phosphorus which accumulated in calcareous or neutral soils occurs as surface phosphorus (7). NaHCO 3 soluble phosphorus The amount of soil phosphorus soluble in 0.5 molar solutions of NaHCO 3 at ph 8.5 has been shown to be highly correlated with "A" values and yield response for a wide variety of soils (8). This method should prove useful in making surveys of the phosphate fertility status of a wide range of soil types, as well as provide reliable information useful in making fertilizer recommendations. The relationship of NaHCO 3 soluble phosphorus to yield response in field and greenhouse experiments is shown in Table 2. The soils are the same as those described for the relationship with "A" value.

4 360 AMERICAN SOCIETY OF SUGAR BEET TECHNOLOGISTS Recommendations for Future Studies on Phosphate Fertility Problems of Sugar Beets This section is meant to be a guide only, with emphasis placed on the application of newly developed methods to soil management and fertilizer usage problems directly related to phosphate fertility. A major problem is to develop an accurate soil test to estimate available phosphorus for sugar beets. Such a test will indicate the need for phosphate on a given field. It should reflect the contribution of residual phosphate from fertilizers and manure and the approximate rate of renewal of the soil solution phosphate so that subsequent needs for the following crops can be estimated. This test would not be intended to show crop differences in need of phosphate, so the levels of soluble phosphate indicating a low or adequate supply of available phosphate may be different depending on the crop grown. The contribution of the subsoil to the available phosphate would not be measured by the test. Differences in soluble phosphate between soils as affected by the energy with which the phosphate is held by the soil due to chemical combinations with iron, aluminum, and calcium should be measured by the soil test, but differences in solubility of the phosphate as a result of soil texture may need adjustment by employing different levels of soluble phosphate in relation to texture. For example, a clay loam and a sandy loam may contain the same amount of available phosphorus, but the clay soil will release less phosphorus to the extracting solution because of its higher adsorption capacity for phosphate. The clay loam, however, may contain twice as much soil solution as the sandy loam. The relationship between texture and level of soluble phosphate indicating low or adequate supplies needs further study. Final calibration of the soil test should be made with soils from sugar beet fields of known phosphate response. The field experiment should be carefully conducted and observed to note if any growth factors limit yields in addition to available phosphate levels. If the soil test performs as indicated in the above conditions with corrections for soil texture and crop requirement, high correlations between soluble phosphate and yield response would likely be found. Other factors affect the available phosphorus supply, however, to modify this relationship. These factors are (a) the supply of inorganic phosphorus released from biological transformations of organic phosphorus, and from the action on the soil phosphorus of the microorganisms themselves or their decomposition products, and of the plant roots, (b) the effect of other anions, such as hydroxide, silicate, bicarbonate, nitrate, and soluble salts, in the soil solution on the rate of phosphate absorption by the root, (c) environmental factors of temperature, rainfall, etc., and (d) the effect of soil structure, moisture supply, soil depth, and aeration on the development of roots in the soil. Some of these factors such as (b) above may be studied in the greenhouse and available phosphorus measured by the "A" value technique. The effect of (a) is difficult to assess, but for practical purposes may be

5 PROCEEDINGS EIGHTH GENERAL MEETING 361 considered to be essentially similar for soils with similar organic matter contents and soil temperatures, and for a given crop. Field experiments are necessary to evaluate the effects of (c) and (d). For a given soil and season, these factors would be evaluated by the yield response relationship to soluble phosphate. If these factors cause large effects between different soils, then the number of field experiments required would depend on the soil variation encountered. When methods are worked out to evaluate these factors, then the number of field experiments necessary for accurate calibrations can be decreased. The measurement of surface phosphorus referred to previously appears to be the best laboratory method at present for estimating the available phosphorus in soils. The "A" value measurement is longer but more accurate since plants are employed to measure phosphorus uptake. A close approximation of either the "A" value or the surface phosphate is provided by the NaHCO 3 extraction method. It is believed that the application of these methods should facilitate the solution of some of the phosphate fertility problems in sugar beets. Literature Cited (1) CLEMENTS, H. F Environmental influences on the growth of sugar cane. Mineral Nutrition of Plants, pp University of Wisconsin Press, Madison. (2) COLE, C. V., OLSEN, S. R., and SCOTT, C. O The nature of phosphate sorption by calcium carbonate. Proc. Soil Sci. Soc. Amer. 17, (3) DEAN, L. A Fixation of soil phosphorus. Advances in Agonomy, Vol. 1, Academic Press, Inc., New York. (4) FRIED, M., and DEAN, L. A A concept concerning the measurement of available soil nutri ents. Soil Sci. 73: (5) KURTZ, L. T Inorganic phosphorus in acid and neutral soils. Agronomy IV, Soil and Fertilizer Phosphorus in Crop Nutrition, pp Academic Press, Inc., New York. (6) MCAULIFFE, C, STANFORD, G., and BRADFIELD, R Residual effects of phosphorus in soil at different ph levels as measured by yield and phosphorus uptake by oats. Soil Sci. 72: (7) OLSEN, S. R Inorganic phosphorus in alkaline and calcareous soils. Agronomy IV, Soil and Fertilizer Phosphorus in Crop Nutrition, pp , Academic Press, Inc., New York.

6 362 AMERICAN SOCIETY OF SUGAR BEET TECHNOLOGISTS (8) OLSEN, S. R., COLE, C. V., WATANABE, F. S., and DEAN. L. A Estimation of available phosphorus in soils by extraction with sodium bicarbonate. U.S.D.A. Circular 939. (9) TOLMAN, B Time and method of fertilizer application. Amer. Soc. Sugar Beet Tech. Proc. 6, (10) WlKLANDER, L Kinetics of phosphate exchange in soils. Ann. Roy. Agr. Coll. Sweden 17, (11) WILD, A The retention of phosphate by soil. J. Soil Sci. 1, ' (12) WILLIAMS, E. G Phosphate fixation. N.A.A.S. Quarterly Review 1, (13) WILLIAMS, E. G Phosphate fixation and availability. J. Sci. Food and Agric. 1,