Impact of Manure and Compost Management on Soil Test Phosphorus Buildup and Drawdown in a Corn-Alfalfa Rotation.

Multi-year repeated dairy manure application based on nitrogen (N) needs of corn (Zea mays L.) results in accumulation of soil test phosphorus (STP) which increases risk of P runoff. A P-removal based application of manure adds less P to the soil but could impact yield. A corn field trial was implemented in 2001 to compare the impact of N- vs. P-based manure (tillage incorporation) and compost management on yield, forage quality and soil parameters. Treatments were compared to zero N (starter only) and 112 kg ha^-1 side-dress N as controls and continued for five years. In 2006, the trial was rotated to alfalfa (Medicago sativa L.). Our objectives with this rotation were to assess alfalfa yield and quality (2006-2011) and document dynamics in STP and organic matter (SOM) for the 10-year rotation. No further nutrient addition took place under the alfalfa years with the exception of 90 kg ha^-1 0-25-25 (N-P₂O₅-K₂O) at establishment. Alfalfa was harvested in a 2, 3, or 4 cut system depending on the weather. Alfalfa yield was not impacted by fertility treatments during the first three years while N-based manure and compost management treatments out-yielded alfalfa in P-based manure and the control plots. Crude protein was not impacted by fertility treatments averaged 182 g kg^-1 across all treatments. In the corn years, STP remained unchanged in control treatments but increased 4- and 6-fold and 2- and 4-fold for N- and P-based manure and compost management, respectively. After 5 years of alfalfa, STP levels in the controls remained unchanged while in the P-based manure treatment, STP had decreased to its initial level in 2001. In N-based compost, P-based compost, and the N-based manure treatments, STP levels decreased during the alfalfa years but were still 3-, 2-, and 3-fold higher than the initial level in 2001 after the 5^th year of alfalfa, suggesting buildup over the rotation. Soil organic matter increased in N-based compost in corn years by 0.4 g kg^-1, remained stable in N-based manure and P-based compost, and decreased in P-based manure (tillage-driven), zero N control, and inorganic N side-dress treatments. In the alfalfa years, SOM remained unchanged in N-based compost and manure and increased by 0.5, 0.8, 0.5, and 0.4 g kg^-1 in P-based compost, P-based manure, zero N control, and inorganic side-dress N treatments. We conclude that manure and compost application in corn years benefit alfalfa yields and that alfalfa in the rotation increased or retained organic matter levels. Soil test P increases during corn years with N- and P-based manure or compost management but with P-based manure management, STP can be managed sustainably over the entire rotation, while for P-based compost and N-based management STP will increase over rotations.