Effect of Phosphorus Fertilization on the Selenium Concentration in Alfalfa

A study was conducted to determine the effect of P fertilization on the Se concentration in alfalfa. Adding P to the soil increased the Se concentration in alfalfa grown in the greenhouse on six of 14 soils from the northwestern United States. The Se concentration increase in alfalfa resulting from P addition was noted on some alkaline and some acid soils. Phosphorus addition increased the availability to alfalfa of both native and applied Se in the Portneuf silt loam. Applying 160 kg P/ha either as H3PO4 or concentrated superphosphate to Gooding sandy loam in the field increased the Se concentration in alfalfa from a level marginal for animal requirements to an adequate level. Additional Index Words: white muscle disease (WMD), P response, P concentration in alfalfa, P x Se interaction. 1111-1E DISCOVERY that Se is required to prevent white muscle 1 disease (WMD) in calves and lambs (15) stimulated Se research activity. One area of research has been concerned with the minimal Se concentration required in the diet to prevent WMD (1, 9, 10, 11, 18). Results from studies in this research area indicate that the critical deficiency level in the diet is actually a range from 0.03 to 0.07 ppm, depending upon the vitamin E level and other factors. Some investigators have suggested that 0.10 ppm may be a reasonable practical level. A second area of research has been concerned with determining the Se concentration in forage and hay crops throughout the United States, and has provided maps (5, 7, 13) delineating areas of different Se concentrations in forage and hay. Unfortunately, unexplainable farm-to-farm variability required labelling some areas on these maps as variable in Se concentration. The available information on the Se concentration in soils (14), the different Se forms and their reactions in soils, and the Se uptake by plants (4, 6, 8, 12, 17) indicated that some farm-to-farm variability in the Se concen'Contribution from the Northwest Branch, Soil & Water Conservation Research Division, ARS, USDA; Idaho Agr. Exp. Sta. cooperating. Received Jan. 7, 1972. Approved March 1, 1972. 2 Soil Scientists, Snake River Conservation Research Center, Kimberly, Idaho 83341. tration in forage resulted from different management practices. Crop rotations and fertilizer practices used in the variable areas indicated that some of the variability may be related to P fertilization. We investigated the Se concentration in P fertilizer materials and the associated uptake by plants (17), and results established that some P fertilizers contain sufficient Se to enhance concentrations in plants. However, the proportion of the P fertilizers containing significant amounts of Se appeared too small to explain much of the observed variability in the field. This paper reports the effects of P fertilization on the uptake of indigenous and applied Se, and the resulting Se concentrations in alfalfa (Medicago sativa L. Ranger). MATERIALS AND METHODS Five separate investigations, four in the greenhouse and one in the field, were conducted. These will be referred to as Studies 1, 2, 3, 4, and 5 for identification purposes. Procedures and materials applicable to all studies will be discussed first, followed by a discussion of treatments and experimental procedures associated with specific studies. General Procedures and Materials Portneuf silt loam was used exclusively in Studies 1, 2, and 3, and was one of 14 soils used in Study 5. The soil for Study 1 was collected from one site, and that for Studies 2 and 3 from another, but all measured properties were the same. The available P determined by NaHCO3 extraction (16, 19) was 7.5 ppm on a scale where values above 10 ppm are probably adequate. Soil from still a different site was used in Study 5, and it will be discussed later. The available P levels in the Gooding sandy loam at the field study site (Study 4) were 8.8 and 6.0 ppm for the 0= to 15and 15to 30-cm depths, respectively. All soils used in Studies 1, 2, 3, and 4 were P deficient. Both the Portneuf and Gooding soils have pH values from 7.8 to 8.2 and contain free CaCO3, particularly below the 15-cm depth. Pots used in the four greenhouse studies each contained 3 kg of soil. All pots were placed in a water bath table where the water temperature was maintained at about 21C. Selenium-free water was added according to pot weight to maintain between 20 and 100% of available water. Alfalfa (Medicago sativa L. Ranger) was grown in all studies, and it was the principal crop sampled in mapping studies. Rooted cuttings were transplanted to all greenhouse pots, four per pot in Studies 1 and 5, and three per pot in Studies 2 and 3. Alfalfa crops were harvested at approximately 1/10 bloom, both in the greenhouse and the field. All samples were dried at 50C, ground to pass a 2-mm sieve, and analyzed for Se (2).