Sign up for email alerts of new Fluid Journal issues!
Fluid Journal : Winter 2016
treatment than for either granular fertilizer. The low P mobility was most likely due to the finer-textured soil. For the Idaho soil (Figure 3) much of the P added remained in the first dish section for all treatments with no significant differences between them. There was P movement into the second dish ring for the 3 treatments with APP having slightly greater diffusion though there were not significant differences. The soil texture should not limit the P movement from the point of application in this soil. For the Australian soil, significantly more fertilizer P remained in the center section for MAP than for the DAP or APP treatment. In the second section , both APP and DAP had significantly more P fertilizer than MAP, with APP having enhanced movement over the DAP treatment (data not shown). A spectroscopic image of the inside of an incubated Australian soil DAP granule (Figure 4) reveals the possible inward movement of Ca from the surrounding soil, prompting the formation of Ca-P reaction products as indicated by the yellow-colored areas along the inside edge of the granule. This evidence lends additional support for the lack of P diffusion from the point of application for P granular fertilizer in the calcareous soils. Percent Resin P soil is used as an estimate of potential P available for plant uptake. The greater the percent resin P for a dish section, the greater the proportion of potentially plant available P for a particular treatment. In the Kansas soil (Figure 5), the APP treatment had significantly more resin extractable P as a percentage of total P for the first and second sections as compared to the other two P treatments. Surprisingly, percent resin P for the center section for DAP was very low, suggesting very limited dissolution of the granule and/or plant availability. All P treatments were very similar to the control treatment for the third and fourth sections. Results for the Idaho soil were very similar to the Kansas results (data not shown). For the highly calcareous Australian soil, there were no significant differences between the three P treatments, and MAP had slightly more percent resin P in the first section as compared to the two other treatments. However, for the second and third sections, APP had significantly Winter 2016 greater resin P extractability compared to the other P treatments (data not shown) XANES analysis. In the Kansas soil (Table 1), the control soil suggests the presence of Ca-P species (72.1 0/0) and alumina-adsorbed P (27.9 0 /0). These particular P solid phases are not uncommon to find in a calcareous soil, especially with a pH 8.7. The DAP treatment had a similar pH to the control but the percentage of Ca-P species (79.2 0 /0) was greater while alumina- absorbed P (20.8 0 /0) was less. We could expect to see the presence of a small amount of aluminum-bound P in these higher pH soils. The MAP treatment had a lower percentage of Ca-P species (54.8 0 /0) and more strengite-like Fe-P than the DAP treatment near the point of application. The soil pH of 8.0 for the MAP treatment was lower than the DAP treatment. The APP treatment had the lowest pH (7.7) of all the treatments and had considerably lower Ca-P species (47.5 0 /0), alumina-adsorbed (22.1 0 /0), and a small amount of strengite-like Fe-P (30.4 0 /0). The prevalence of both Fe and AI associated P species with APP may explain, at least in part, the greater plant- available P compared to the granular P treatments. For the Idaho soil (Table 2), the control treatment contained Ca-P species (68.6 0 /0) and ferrihydrite-adsorbed P (31.4 0 /0). These particular P solid phases would be common minerals found in a higher pH (8.5) calcareous soil. The MAP, and especially the DAP treatment, had higher percentages of Ca-P like species forms as compared to the APP treated soil. With APP having less Ca-P like species, there is an increase in Fe-P, either an adsorbed or precipitated species. Again, this gives an explanation of the increased percent resin P compared to the granular P treatments For the Australian soil (Table 3), the control treatment indicates the presence of Ca-P species (74.1 0/0) and alumina- adsorbed P (25.9 0 /0). At pH 8.7, it would be expected that much of the P found in the calcareous soil to be strongly associated with Ca-P species. The MAP treatment (pH 8.23) contained 67.0 0 /0 Ca-P species with small amounts of crandallite-like P (15.2 0 /0) and alumina- adsorbed P (17.8 0 /0). This would not be uncommon in a calcareous, higher pH soil. The DAP treatment had a slightly lower pH of 8.12, and a somewhat similar percentage of Ca-P species (70.3 0 /0) The Fluid Journal as compared to the control and MAP treatments, and had more alumina- adsorbed (29.8 0 /0). The APP treatment had the lowest pH of all the treatments and had much less Ca-P species (55.6 0 /0) and increased AI and Fe associated P (alumina-adsorbed P of 33.7 0 /0 and goethite-adsorbed P of 9.4 0 /0). The decrease in precipitated Ca-P species and increase in Fe-adsorbed species appear to be associated with greater P movement away from the point of application plus more P remained in a potentially plant- available form. It was useful to combine the results of percent resin P for the 3 soils with calcium-P speciation data (Figure 6). The fraction of the P that is resin extractable greatly decreases as the percentage of Ca-P increases. The relationship suggests that as added P fertilizer is converted to Ca-P species it may no longer be available for plant uptake. In addition, the P use efficiency would be low. Summing up The 3 soils in this study had unusually high pH and were calcareous. With the addition of P fertilizer, either granular or fluid, the pH decreased near the point of fertilizer application from the control soil pH, most likely due to nitrification. We saw that with the fluid P fertilizer the soil pH was significantly lower than either granular fertilizer added and that change in pH was directly or indirectly responsible for the increased P solubility. With increasing distance from the point of application, soil pH slowly increased toward the pH of the control soil. In terms of fertilizer P diffusion, there was evidence of somewhat greater movement of P near the point of application for the Idaho soil and Australian soil compared to the Kansas soil. This most likely was due to the Kansas SiL texture adsorbing soil water, thereby decreasing P mobility. For all 3 calcareous soils there was significantly more resin P as a percentage of the total P in the first and second dish sections for APP as compared to all other treatments. The P speciation results strongly suggest the addition of P fertilizer promoted the formation of both adsorbed and secondary P mineral solid phases. For both Kansas and Idaho soils, the lower pH was coupled with a decrease in Ca-P speciation and an increase in adsorbed-P species and may be the reason for the significant increase in 8