Residual Effects of Phosphorus on Dryland, Grain Sorghum (Milo)

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Residual Effects of Phosphoru s on Dryland
Grain Sorghum (Milo) 1 Kenneth W. Kelley and Larry Murphy 2 Southeast Kansas Branch Experiment Station When phosphorus fertilizer is broadcast, it stays within 2 or 3 inches of the topsoil unless mixed deeper with tillage implements.So phosphorus can be stored in the soil without being leached from the root zone by rainfall.Residual phosphate, therefore, is still available for succeeding crops, although a certain portion becomes fixed in unavailable form over time so it can not be used by plants.
Residual P compared with continued applications of P were studied at the Southeast Kansas Branch Experiment Station (Parsons Field) from 1974 through 1977 to see if heavy, first-year applications (200 pounds P205 acre) would be as effective for grain sorghum as 100 pounds PzOs per acre applied every other year, or as effective as annual applications of SO pounds P205 per acre.
After 4 years all plots had received the same total amount of P. Phosphorus was broadcast on the soil surface before planting then incorporated with a springtooth.
The two P sources used were diammonium orthophosphate (AOP, 18-46-0) and solid ammonium polyphosphate (APP, 15-62-0).APP was obtained from the Tennessee Valley Authority and contained 80% of the P in the APP form, with the remainder as AOP.
Initial soil P levels were very low (6 pounds of available P per acre).Soil series was a Parsons silt loam with 2.3% organic matter.

Results and Discussion
Results of the 4-year investigation indicated good P response despite moisture limitations in 1974 and 1976.As expected, first-year yields were highest from 100 or 200 pound~ of P per acre.
The second year, yields were good from the initial 200-pound application and the annual SO-pound applications.The 100-pound application, only residual in the second year (1975), gave a significantly lower yield than either of the other two.
Third-year data (1976) indicated that the initial 200-pound P application was starting to become fixed in the soil.The SO-pound annual treatments and the 100-pound treatments (applied in 1974 and 1976) were both significantly more effective than the firstyear 200-pound application.
The fourth year showed annual SO-pound ap~ plications slightly superior to 100-pounds every othe year, and 13 bushels per acre superior to 200 pounds the first year only.
Means for the 4 years indicate no significant differences because of first-year (1974) yields.
Yield differences between P sources were nonsignificant throughout the 4-year study.
Soil test values at the end of the study indicated that the P treatments had .essentially doubled the available soil P during the study.Soil pH at the end of the study was 6.1.

Conclusion
Results indicate that heavy, one-time P applications for grain sorghum are not so effective as annual applications.Biennial applications were more effective than applications once in four years.
For maximum efficiency, phosphorus should be applied annually because phosphorus fixation is quite likely in the acid soils of southeastern Kansas.
Comparisons of orthoand polyphosphate materials indicate no significant difference.
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Table 1 .
Residual effects of phosphorus: on dryland grain sorghum yields, 1974-1977.This publication from Kansas State University Agricultural Experiment Station and Cooperative Extension Service has been archived.Current information: http://www.ksre.ksu.edu.