How should I go about restoring the totally depleted soil of my back garden? I have just moved into a new home and would like to have a beautiful garden. I have acquired a neglected backyard surrounded by old trees, some scraggly plants trying to survive in the heavy clay soil, and garden beds that have received no attention for decades that are mostly growing weeds.
Thank you for asking this interesting question. Restoring a garden’s soil is a wonderful project. Restored garden beds can grow flowers, vegetables, bushes and trees, and enhance the environment with pollinators for insects, habitat for wildlife, and food and beauty for people. Adding soil, compost and mulch enhances the humus content of the soil, introduces beneficial colonies of microbes to the soil, suppresses pathogens and weeds, helps with water retention, controls soil erosion, and fertilizes the vegetables and flowers you want to grow.
The following tries to answer your question by highlighting whether you should add soil, mulch, and/or compost, and how much. Plants do well with a combination of all three, and in summary, to help you with your immediate calculations and planning, about 8 cm (4″) seems to be a good layer to initially introduce into your garden and dig into the existing soil of your plant beds before you start planting.
What kind of topsoil should you use? Topsoil is very good for establishing the foundation for a good garden. It’s major advantage is that is holds moisture and its structure well, has abut 5-10% organic matter, and is foundational to a garden. However, topsoil usually comes from the top few inches of ground that has been cleared for future construction, and hence it is often sourced from land that is not necessarily local to where you live (so there are environmental implications as to the carbon footprint of the energy used to remove it, bag it, and deliver it). It is sold in small or very large bags (1 cubic yard) which makes it very convenient. Not all topsoil is equal and it is better to buy from a reputable company.
What kind of compost should you use? Compost is decomposed organic matter. It can be made from all kinds of organic materials from kitchen scraps, garden waste, livestock manure (usually sheep or cow, but also rabbit and goat manure, and manure from the zoo), or decomposed mushroom soil. Compost is very rich in the nutrients that your garden bed needs. The choice of compost often comes down to a question of availability and cost. The compost that you make in your own garden usually wins hand’s down especially if you have a fast-composting process. However, since you are just starting your garden and may not have started composting, purchasing composted manure may be an easier option.
Also keep in mind that each spring the City offers bagged compost for free at Community Environment Day events. This year, because of Covid-19, these events were cancelled, but they will start up again, and you can get updates here: City compost
What kind of mulch should you use? Mulch is usually organic matter but it can be wood or bark chips, coco shells, leaf-mold, grass clippings, peat moss, pine straw or even plastics. All of these work, so it is a question of availability and cost, and the speed with which you want the mulch to be composted and incorporated into the soil: grass clippings disintegrate very quickly, whereas wood or bark chips can take years to compost.
And now to the final question. How much topsoil/mulch/compost should you add to your back garden to prepare for a beautiful and productive garden.
In order to answer these questions, I sourced a number of reputable experts and sources. However, the answer is not as straight-forward as one would think, especially since most of the systematic research on restoring depleted soil has been done in agriculture and not gardening. A factsheet produced by the Ontario Ministry of Agriculture and Food and the Ontario Federation of Agriculture provides excellent basic knowledge on how to amend soil on a large scale: advice from Min of Agriculture
Similarly, from an agricultural perspective, Washington State University recommends that the top 2-3” of soil should be rich in organic matter. “The fix [to non-productive soil] according to [agricultural] research in several countries (Franzluebbers, 2013), is to manage the soil so that organic matter is concentrated in the top 2-3” of the soil. The benefits from doing this can be even greater than those from increased total soil profile organic matter. In one test, increased total organic matter improved water infiltration by 27%, but concentrating organic matter at the surface improved water infiltration by nearly 300%.” agricultural research
The Royal Horticulture Society in the UK suggest that gardeners should apply 5-10kg (10-20lbs) of organic matter (manure or compost) per square metre (yard): Advice from the RHS.
Advice from Charlie Nardozzi that is posted on the U.S. National Gardening Association website is to: “Apply a four- to six-inch-deep layer of grass clippings, hay or leaves or a one- to two-inch-deep layer of compost and till it into the soil.” advice from New England
Toronto Master Gardeners in collaboration with the City of Toronto have a guide sheet on How to Improve your Soil Organically. They say: “The only limitations [on how much organic matter to add] are how deep you can dig, how much you can acquire, or how strong your back is. It is almost impossible to add too much organic matter to soil, especially if you add well composted materials.” advice from TMG and City of Toronto
Likewise, Paul Zammitt, who was the Toronto Botanical Garden’s Director or Horticulture, advocates as much quantity as feasible: “Add as much composted organic matter to the soil as possible. I’m a big fan of composted manure – especially rabbit manure. It’s fantastic! If manure is not available, apply composted kitchen waste. Shredded leaves applied as a mulch in late fall can be an excellent source of organic matter.” advice from the TBG
Mark Cullen, the well-known Canadian gardening expert, says that there is no way to overestimate the importance of adding finished compost to your garden soil. He recommends spreading 20 Kg of composted sheep or cattle manure over a sq. meter of soil. advice from Mark Cullen
Linda Chalker-Scott at Washington State University, who writes a well-known blog on gardening myths, takes an opposing view. She says that gardens often have compost excessively added and that is having an impact on plant health and is nutrient loading adjacent natural waterways. However, the soil samples that she was basing her advice upon was the soil of a well-established, demonstration organic garden. This is not your situation. She says that purchased topsoil often has an organic matter content of 15% which is too high. “Ideal soils, from a fertility standpoint, are generally defined as containing no more than 5% OM by weight or 10% by volume.” advice from Linda Chalker-Scott
With the research still in its infancy, especially at the scale of gardening, it is difficult to find a clear direction. So I am adding to this research my gardening experience of many decades. My conclusion is that in Toronto, with our heavy clay soil, if a garden has been totally depleted, it is a good idea to add and dig in about 8 cm or 4″ of a combination of topsoil, compost and mulch.