Research has demonstrated that animal manure may increase soil test P (STP) differently than inorganic fertilizer when applied at the same total P rate. The exact mechanisms controlling the increase in STP after manure application are not well understood. In this study, 40 manure samples [beef and dairy (Bos taurus), goat (Capra aegagrus hircus), sheep (Ovis aries), horse (Equus caballus), turkey (Melleagris galloopavo), chicken (Gallus domesticus), and swine (Sus scrofa domesticus)] were applied to 23 agricultural soils at a total P rate of 40 mg kg-1 and incubated for 70 d along with an untreated control. The total P in the manures ranged between 2.8 to 48.7 mg P kg-1 of which 28 to 96% was inorganic P (Pit) and 4 to 28% was enzymatically hydrolyzable P (Pet). The initial soil Bray-1 P ranged from 14 to 69 mg kg-1. For 19 soils, the STP after manure application increased linearly as either the applied manure Pit increased or the applied manure Pit + Pet increased. Hydrolysis of Pet was related to soil clay content. Soils with clay content greater than 121 g kg-1 seemed to provide physical protection against Pet hydrolysis. In contrast, soils with clay content lower than 71 g kg-1 appeared to provide conditions that were suitable for manure Pet hydrolysis. The role that soil clay content may play a role in influencing how manure changes STP has not previously been reported.
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