My sources tell that compost weighs between 38 and 70 pounds per cubic foot. The Mustang has a 13 cu ft trunk so at the low end (40 lb per cu ft) theres between 520 and 900 pounds of weight sitting above and behind the rear wheels. The 2011 Mustang is rated for a maximum load of 700 pounds, the average woman weighs 165 while the average male weighs 194 so with 520 pounds of compost the vehicle is already at the weight limits recomended by the manufacturer. Then theres the location of the added weight, having it well behind the rear wheels adds leverage so the 520 pound load would be magnified by how far behind the wheels its sitting. Then theres the problem with an unrestrained cargo in the case of an emergency stop, that extra weight moving forwards could smash through the opening between the rear seat and trunk possibly smothering the driver. And one final comment, if the compost was worth shoveling into the car wouldn`t it be worth unloading so it could be used for the purpose you got it to begin with?