Alternative fertilization strategies are needed to reduce costly mineral P inputs while maintaining yields and soil fertility. Increasing the use of composts and cover crops could help reduce mineral P inputs because composts recycle P found in wastes whereas cover crops reduce soil P losses and could help mobilize soil P “fixed” from previous P applications. We studied the effects of compost and cover crops on soil P dynamics in two agricultural systems of California. We used the first system (Davis) to measure the effects of a single cover crop mixture (oats and legume) and composted poultry manure across treatments varying in cropping intensity, water availability, and fertilization levels. The second system (Salinas) allowed us to study how multiple cover crops (rye, mustard, or rye and legumes) and yard compost affected soil P dynamics in intensive organic vegetable production. We analyzed soils for microbial P (chloroform fumigation), sorption capacity and P saturation (Langmuir isotherms) and several organic and inorganic P pools (Hedley fractions) to determine changes in soil P dynamics after 18 (Davis) or 8 (Salinas) years. Cover crops, regardless of mixture composition, increased microbial and organic P, but had no effect on sorption capacity, P saturation, or inorganic P pools. Yard compost additions increased labile inorganic, microbial and organic P pools but had no impact on other pools, sorption capacity, and P saturation. Composted chicken manure increased all pools (microbial, organic and inorganic), did not affect sorption capacity, and increased P saturation by about 3 fold. Our results suggest a limited, moderate, and strong role of cover crops, yard compost, and composted chicken manure, respectively, in affecting soil P dynamics here. Overall, composts appear to have more potential as an alternative P fertilization strategy than cover crops in these California systems.