Evaluation and selection of soil amendments for field testing to reduce P losses

The effectiveness of chemical amendments in reducing P losses from manure impacted Florida soil was evaluated using a variety of protocols, including total elemental analysis, short-term laboratory equilibrations, column leaching experiments, and simulated rainfall studies. Amendments used included: two Fe-humates [a Fe-water treatment residual (WTR) and a titanium-mine waste], two Al-WTRs, one Ca-WTR, a coal combustion slag, a Si-rich material (Pro-Sil), a Leonardite material (dinoSoil), and two agricultural materials (lime and gypsum). In equilibration studies, Al-WTRs were the most effective at sorbing P, while Fe-humate sorbed the least P of all treatments. Other amendments effectively reduced soluble P, but increased suspension pH and electrical conductivity (EC) to an extent expected to adversely affect plant growth. Gypsum was the most effective amendment in reducing P leaching in small column studies. DinoSoil treatment significantly altered the soil hydraulic properties, yielding the greatest runoff volumes in a simulated rainfall study. Total P loss (runoff + leachate) was largely determined by the leaching component, and was essentially all soluble reactive phosphorus (SRP) for both bare and grassed soil surfaces. AD treatments reduced runoff SRP compared to the control. Amendment effects in the grassed rainfall study (dominated by leaching) were inconsistent with results from the small column leaching study, possibly due to the different hydraulic conditions and kinetic factors involved in the two studies. The Al-WTRs amendments were recommended for field evaluation.