Technical Abstract: The organic P composition of semi-arid arable soils is largely unknown, but such information is fundamental to understanding P dynamics in irrigated agriculture. We used phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy and phosphatase hydrolysis to characterize organic P in semi-arid arable soils from the western US (organic C 2.0-30.7 g C per kg soil, clay 2-48%, pH 5.2-8.2, lime <1-480 g per kg soil). Total P concentrations ranged from 220 to 1210 mg P per kg soil, of which between 12 and 45% was extracted with NaOH-EDTA. Inorganic orthophosphate was the dominant P compound, but concentrations determined by phosphorus-31 NMR spectroscopy were consistently greater than those determined by molybdate colorimetry. Concentrations of organic P were relatively small, and were dominated by orthophosphate monoesters (11-132 mg P per kg soil), with smaller concentrations of orthophosphate diesters (0-7 mg P per kg soil). Pyrophosphate was present in almost all soils at concentrations up to 14 mg P per kg soil. Bicarbonate-extractable organic P ranged from 1.7 to 22.8 mg P per kg soil, of which between 37 and 87% was hydrolyzed by phosphatase enzymes, suggesting its bioavailability. Soil organic P concentrations were positively correlated with mean annual precipitation, organic C, clay, and oxalate-extractable metals (Al, Fe, Mn), and negatively correlated with mean annual temperature and soil pH. However, lime concentrations were not significantly correlated with any soil property. These results indicate that equilibrium levels of organic P in semi-arid arable soils are controlled by a balance between the physical protection offered by the soil matrix and the suitability of the environment for biological productivity.