Dairy Manure Influence on Soil and Sediment Composition: Implications for Phosphorus Retention

Area of Science:

• Environmental Chemistry
• Soil Science
• Geochemistry

Background:

• Manure-derived phosphorus (P) poses a risk to surface water quality in sandy soil regions due to poor P retention.
• Dairy farm soils in the Lake Okeechobee Basin readily release P, despite high calcium and pH, indicating complex P dynamics.

Purpose of the Study:

• To identify inorganic components influencing phosphorus (P) stability in surface soil horizons and stream sediment.
• To understand why stream sediment releases less P than soil, despite similar total P content.

Main Methods:

• Separation of soil fractions (coarse fragments, sand, silt, clay) via sieving and centrifugation.
• Analysis of crystalline and noncrystalline components using microscopy, X-ray diffraction, SEM, EDS, electron microprobe, thermogravimetry, density separation, and selective dissolution.

Main Results:

• Ap horizons contained quartz and biogenic opal; clay fraction was primarily noncrystalline silica (opal A) with low P affinity.
• Manure components likely inhibited stable calcium-phosphate (Ca-P) mineral crystallization, leading to high P solubility.
• Stream sediment contained apatite and vivianite, suggesting different P stabilization mechanisms compared to soil.

Conclusions:

• Noncrystalline silica, derived from manure phytoliths, is the dominant P-retaining component in these soils, not stable Ca-P minerals.
• Eliminating the inhibition of Ca-P crystallization could significantly reduce phosphorus flux from agricultural lands to aquatic systems.