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Influence of Phosphogypsum on Forage Yield and Quality and on the Environment in Typical Florida Spodosol Soils. Volume I. Forage Yields and Quality of Bahiagrass and Annual Ryegrass Pastures Fertilized with Phosphogypsum as a Source of Sulfur and Calcium


Executive Summary

Phosphogypsum (PG), a by-product of the wet-process manufacture of phosphoric acid from phosphate rock, is primarily calcium sulfate or gypsum (CaSO4.2H2O) and, thus, is a potential source of sulfur (S) and calcium (Ca) for crops.

The agronomic objective of this project was to determine the effects of PG on bahiagrass (Paspalum notatum Flugge) and annual ryegrass (Lolium multiflorum Lam.) in terms of forage yield and quality. The environmental objective was to assess the environmental impacts of PG, at agronomic rates, on forage, soil, groundwater, and the immediate ambient atmosphere. The project was carried out by the University of Florida, Institute of Food and Agricultural Sciences (IFAS) at the Range Cattle Research and Education Center (RCREC) at Ona, Florida. The field experiments were conducted on a long-established bahiagrass pasture and on tilled land cropped to annual ryegrass from 1990 to 1993. The soils were typical Florida Spodosol soils.

In the agronomic experiments, the amendments used were PG alone, PG+limestone, PG+dolomitic limestone, elemental sulfur, and limestone. The PG rates in treatments containing PG were 0.2, 0.4, and 1.0 MT PG ha-1 (MT or metric ton = 106 grams = megagrams or Mg) applied annually for three years, and 2.0 and 4.0 MT PG ha-1 applied once at the start of the experiments.

In the environmental experiments, PG alone was applied annually at 0.4 MT ha-1 for 3 years and one time at the start of the experiments at 2.0 and 4.0 MT PG ha-1.

This report (Volume I) deals with the agronomic results of the agronomic and the environmental experiments. Volume II deals with the radiological and the agro-environmental results.

A. Phosphogypsum on Established Bahiagrass Pasture

1. Effect on Forage Yield and Quality

Forage yield. Based on the significant linear regression equations, the 3-year average results of the agronomic study showed that PG alone applied annually increased forage dry matter (DM) yield by 0.23 MT DM per MT PG ha-1. The one-time rates also increased forage yield by 0.56 MT DM at the highest rate of 4.0 MT PG ha-1. Also based on the linear regression equations, S applied annually at the highest rate of 65 kg S ha-1 could reduce forage yield by 0.17 MT DM ha-1. Limestone applied one-time up to 2.0 MT ha-1 could also reduce forage yield by 0.24 MT DM ha-1 at the highest application rate- Based on the actual yields for the individual rates, the agronomic study showed that PG alone applied annually at 0.4 MT PG ha increased forage DM yield over the control by 17% in the first year (1990) and 23% in the second year (1991). The 1992 yield at 0.4 MT PG ha-1 was also 8% higher than the control, but the difference did not attain a significant level. In the environmental study, PG applied annually at 0.4 MT ha-1 increased forage yield by 46% over a 3-year period. Thus, the results from the two studies indicate that it is reasonable to expect increases in forage yield with PG by 15 to 20%. These increases in forage yield with PG applied at 0.4 MT ha-1 annually are a good basis for evaluating the economics of PG use in bahiagrass pastures in Central Florida.

Nutrient content of forage. The 3-year mean S contents in bahiagrass forage from both the agronomic and the environmental studies increased by 0.02 to 0.08% per 0.4 MT PG ha-1 applied annually. Calcium increased only by 0.01% at the same annual rate in both studies. The results indicate that as a Ca source, PG rates higher than 0.4 MT ha-1 are needed to make a difference.

The N content of forage harvested monthly (regrowth) was not affected by the treatments in either experiment. The environmental study, however, showed apparent and consistently higher N contents in PG-fertilized hay forage (mature forage harvested once a year) than in the control forage. The 3-year mean crude protein (CP = %N x 6.25) contents in hay fertilized with PG were 7.7 to 9.8% higher than those of the control. The effects of the treatments were significant at P<0.10.
Both studies showed that PG alone had no effect on %P in bahiagrass forage, but PG may enhance %K in forage. The agronomic study indicated that limestone, PG+limestone, and PG+dolomitic llmestone, but not PG alone, may reduce %P in the forage particularly during the first year. In both studies, the one-time application of 4.0 MT PG ha-1 reduced %Mg in bahiagrass forage, probably due to the loss of Mg from the main root zone, but the effect was significant only during the first year.

The application of PG reduced the N:S ratio in forage in all three years in both studies. The 3-year mean N:S ratio in PGfertilized forage ranged from 6.7:1 to 7.6:1 in the environmental study. The control forage showed a N:S ratio of 10.2:1. Similar values were noted in the agronomic study. In both studies, the N:S ratios in the PG-fertilized and the control forages were well outside the ideal N:S ratio range of 12:1 to 15:1 for cattle. This should be remedied using higher rates of N application. Some of the PG-fertilized regrowth harvests in the environmental study had higher in vitro organic matter digestibility (IVOMD) than the control.
From both studies, the Ca:P ratios in PG-fertilized forage ranged from 1.6:1 to 2.6:1. The Ca:P ratio in the control forage ranged from 1.47 to 2.3:1. Feeds with Ca:P ratios below 1:1 or over 7:1 had been shown to decrease feed efficiency and reduce growth. To keep the Ca:P ratio in heavily P-fertilized pastures from dropping below 1:1, PG fertilization is highly recommended.

The PG used in this work contained 0.43% F. The 3-year mean F levels in PG-fertilized forage ranged from 7.7 to 8.6 mg kg-1. The control had 7.2 mg F kg-1. The statistics showed that rates up to 4.0 Mg PG ha-1 did not increase the F content of bahiagrass. This is a significant finding since long term feeding of diets containing more than 30 mg F kg-1 may lead to fluorosis in cattle within a period of 2 to 3 years.

2. Effect on Soil

By itself, PG had no effect on soil pH. Also, PG had no acidifying effect compared to elemental S or liming effect compared to limestone. Both studies showed that soil Ca levels were not different between treatments one year after PG application. Also, PG showed no effects, adverse or otherwise, on soil P in the top 15 cm. The agronomic experiment indicated that soil Mg and soil K decreased in the top 15 cm one year after PG application. The environmental experiment showed that PG had little or no effect on soil Cu, Fe, Mn, Na, Zn, and Cl.

B. Phosphogypsum on Annual Ryegrass on Tilled Land

1. Effects on Forage Yield and Quality

Forage yield. Both the agronomic and the environmental studies showed that the treatments had no effect on forage yield of ryegrass, but the agronomic study indicated that PG may increase the dry matter contents in the forage.

Nutrient content of forage. From both studies, PG applied annually increased the S content of ryegrass forage by 0.02 to 0.05% per MT PG ha-1. Since ryegrass has low S but high N contents, PG fertilization should improve the quality of ryegrass forage. The agronomic study, but not the environmental study, showed that PG applied annually increased Ca in ryegrass forage by 0.01, 0.04, and 0.05% per MT PG ha-1 in 1991, 1992, and 1993, respectively. In the latter study, the first PG application was made two months before seeding; in the former, PG was applied at seeding. Thus, the relative ineffectiveness of the PG treatments in the environmental study may have been due to differences in the time of applying the amendment.

Neither the environmental nor the agronomic experiments indicated any significant effects of PG on the N content of the forage. There were no consistent effects of PG on P contents in the forage over the three years, but compared with limestone, PG had less adverse effects on P than limestone. Both at the low annual and at the high one-time rates, PG could reduce the %K and %Mg in forage. Magnesium loss was rectified by the addition of dolomite.

For cattle the ideal dietary N:S ratio ranges from 12:1 to 5:1. In the agronomic study, PG applied annually at 0.2 to 0.4 MT PG ha-1 brought the N:S ratios in ryegrass forage within the ideal range. However, PG as applied in the environmental study was relatively ineffective compared to the application of PG at seeding in the agronomic study. Except for the 4.0 MT PG ha-1 rate in 1990-91 with 12:1 N:S ratio, the rest of the ratios ranged from 17:1 to 24:1 with the control having the highest. In both studies, the annual or the 3-year mean %IVOMD showed no significant effects of PG on regrowth forage digestibility.

The 3-year mean Ca:P ratios in PG-fertilized ryegrass forage in the environmental study ranged from 1.1:1 to 1.3:1. The control showed a Ca:P ratio of 1.1:1. Similar values of Ca:P ratio (1.5:1) with PG rates applied annually were obtained in the agronomic study. Thus, as with bahiagrass, PG fertilization is highly recommended for ryegrass pastures that are heavily fertilized with P to keep the Ca:P ratio from dropping below 1:1.

The F content of ryegrass forage was unaffected by the PG treatments. The 3-year mean F contents ranged from 8.4 mg kg-1 for the control to 10.5 mg kg-1 for the 4.0 MT PG ha-1 rate. As in the case of bahiagrass, this is an important finding on the use of PG on ryegrass pastures.

2. Effects on Soil.

The environmental study showed no significant effects of PG on soil pH, Ca, P, Mg, K, Cu, Fe, Mn, Na, Zn, and Cl. However, there were strong indications that soil K and Mg were reduced in the top 15 cm at the highest level of PG application. The agronomic study showed that soil Mg in the top 15 cm was significantly reduced with PG rates. Although not significant, the reduction in soil K with PG was also very apparent.

C. Conclusions

The two field studies on an established pasture and the two on tilled land have demonstrated that PG can serve as an effective source of S and/or Ca for pastures and for tilled crops. Because of its high solubility and its SO4-form of S, PG supplies S to crops at a much faster rate than elemental S. Phosphogypsum also supplies Ca to crops at a much faster rate than limestone, primarily because PG is much more soluble than limestone. Thus, when either S or Ca or both are limiting the yield potentials of forage crops, PG application should increase forage yield and improve forage quality as well. Unlike elemental S, PG up to 4.0 Mg PG ha-1 has shown no significant acidifying effect on the soil, nor does PG have the liming effect of limestone. Hence, the fertilizer value of the S and/or the Ca in PG should make PG a valuable resource for agriculture, in general, and for Florida agriculture in particular.

FIPR Publication No. 01-085-127, Volume 2

Influence of Phosphogypsum on Forage Yield and Quality and on the Environment in Typical Florida Spodosol Soils. Volume II. Environmental Aspects Associated with Phosphogypsum Applied as a Source of Sulfur and Calcium to Bahiagrass and Annual Ryegrass Pastures Growing on Florida Spododol Soils. University of Florida. July 1996.