Mixing cow manure with soil is a great way to fertilize your garden. The general guideline is 1 part manure with 3 part quality soil. If you have lower quality soil, you need to make up for the lack of nutrients in the poor soil with more compost that's nutrient-rich. You can have an equal 50/50 ratio of manure to soil in that situation. Mixing cow manure into your soil will help make sure that your plants receive everything they need to grow healthy and strong.