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Use of Phosphogypsum to Increase Yield and Quality of Annual Forages


Field studies were conducted for three years to evaluate the effects of phosphogypsum on the yield and quality of wheat cut for forage. Yield responses to added fertilizer sulfur are expected on deep sandy soils such as those found in the coastal plain region of the southeastern United States. These soils have a low capacity for retaining reserves of plant available sulfur. Past research has shown that sulfur fertilization of forages can increase livestock performance and the livestock carrying capacity of pastures. Pastures fertilized with sulfur tend to produce a more palatable and digestible forage. In Alabama and Florida there are more than eight million acres of forage crops (improved permanent pastures and annual forages) and phosphogypsum should be an excellent source of sulfur.

The studies were conducted on a Dothan fine sandy loam and a Benndale fine sandy loam located in south Alabama. Treatments included two methods of tillage, two nitrogen rates, five rates of sulfur and two times of sulfur application. Tillage treatments were 1) Conventional-turn and disk prior to planting and 2) Reduced-disk only prior to planting. The soil was turned with a moldboard plow at a depth of 20 to 25 cm. Disking was with an offset disk to a depth of 8 to 13 cm. Nitrogen rates were 134 and 202 kg per ha. Sulfur rates were 0, 11, 22, 45 and 90 kg per ha or 0, 1, 2, 4 and 8 phosphogypsum units per acre. The rates of added sulfur corresponded to phosphogypsum rates of 0, 30, 59, 119 and 293 kg per ha, respectively. Times of sulfur application were 1) prior to planting (fall) and 2) top dressing in early February (spring). Sulfur was applied as phosphogypsum which contained 15.3% sulfur. Additional treatments were included where agricultural grade gypsum was applied at a single rate of 45 kg per ha.

Wheat was planted in October or November of each cropping year and harvested as needed for forage yields. Forage was subsampled by plot as needed for nitrogen, sulfur, forage quality and percentage leaves determinations. Soil samples from selected treatments were collected in the spring of each year after the final wheat harvest.

Wheat forage yields were affected by tillage during the third year of the study on the Benndale soil and during all three years of the study on the Dothan soil. During the third year of the study on the Benndale soil, average forage yields were 38% higher under conventional tillage as compared to the reduced tillage system. On the Dothan soil the conventional tillage system produced an average of 33.6% more forage as compared to the reduced tillage system. Root density and soil penetrometer measurements showed that lower yields under reduced tillage resulted from soil compaction.

As expected, forage yields were increased with increasing rates of nitrogen. On the Benndale soil average forage yields were increased by an average of 13.3% by increasing the nitrogen rate from 134 to 202 kg per ha. When averaged over three years, the 180 pounds nitrogen per acre rate increased forage yields 18.6% as compared to the 134 kg nitrogen rate on the Dothan soil.

Wheat responded to the application of sulfur at both locations, but the greatest response was obtained on the Dothan soil. On the Benndale soil, three-year average yields were increased by 5.4% by the application of sulfur. Wheat forage yields on the Dothan soil were increased by an average of 9.3% by the application of sulfur. The greatest yield response was obtained at the second cutting during the second year of the study on the Dothan soil. Yields at this harvest were increased 34% by the applied sulfur. Response to the time of sulfur application was inconsistent and varied among sites. During the first two years of the study an average of 6% higher yields were obtained by applying sulfur in the spring to the Benndale soil. In contrast, an average of 8% higher yields were obtained during the second and third years of the study by applying sulfur in the fall. Results from both locations show that maximum forage yields were obtained when 22 to 45 kg of sulfur were applied per acre as phosphogypsum. Statistical analysis showed that there were no differences in forage yields between phosphogypsum and agricultural grade gypsum when applied at a rate of 45 kg of sulfur per acre. Yield responses to added sulfur were generally greater under reduced tillage as compared to the conventional tillage system, however, tillage effects were not completely eliminated by applying higher rates of sulfur.

Nitrogen concentration in harvested wheat was increased by the rate of nitrogen and the concentration of sulfur was increased by added sulfur. Nitrogen:sulfur ratios tended to decrease with increasing rates of phosphogypsum indicating that a higher quality forage was being produced. Based on nitrogen:sulfur ratios, in some instances a higher quality forage was being produced at the 45 and 90 kg per ha sulfur rates although wheat yields leveled off at 22 to 45 kg of sulfur per ha. During the second year of the study, crude and digestible protein were increased in forage from the second cutting on the Dothan soil by the application of sulfur. Percentage of leaves and stems in harvested wheat were not affected by any fertility treatments.

The phosphogypsum contained 21 pCi Radium-226 per gram and 8.1 pCi Polonium-210 per kilogram. Analysis of selected soil and forage samples showed that there were little if any effects of phosphogypsum on the levels of these isotopes in either the soil or harvested forage.

The application of phosphogypsum increased the level of extractable SO4 in the subsoil at each location. Relative increases in extractable subsoil SO4 suggest that most of the applied sulfur may have leached out of the rooting zone. The application of phosphogypsum resulted in a displacement of exchangeable magnesium in the surface 0 to 25 cm of soil.

Results of this three year field study show that yield increases in wheat forage can be expected by applying sulfur as phosphogypsum to deep sandy soils located in the coastal plain region of Alabama. Phopsphogypsum was shown to be an acceptable source of fertilizer sulfur for annual forage production. Maximum yields were obtained at approximately 22 kg of sulfur per ha.