The nutrients that plants need to grow are available to them in a healthy soil, but from time to time, these nutrients need to be replenished. What, how much and when should we add amendments and fertilizers to our soil? Here are some recommendations. What to add to the soil Amendments such as compost and worm castings are added to our rose beds to improve the nutritional content, texture, structure, aeration, pH, water retention, drainage and microbial activity of our soil. Organic mulches that we apply to the surface of our rose beds amend the soil as they break down.