The soil in my garden is composed of very heavy stiff clay. I want to plant a border and have the area dug. I’m going to dig in well rotted horse manure to try to improve the soil. I have a load of bark mulch that has been on a different part of the garden for therlast three years. Is there any reason not to dig this material nito the soil in the new bed. Will it improve the organic content of the soil?