Organic soil amendments

The concept of replacing soil nutrients with a range of organic soil amendments is a sound one. When I started gardening more than half a century ago, there was plenty of excitement surrounding the use of synthetic fertilisers and the various chemical treatments which promised to increase the fertility and manageability of the basic soil types. With the exception of a few traditionalists, there was minimal interest in evaluating the effects of natural organic materials on soil productivity and sustainability.

I was privileged to work with a group of dedicated gardeners who appreciated the importance of healthy, living soils. When I reflect back, they were quite often ridiculed for their resistance to the (so called) scientific treatments of the time. Their approaches to soil and gardening in general were largely based on the practical skills and knowledge which had been passed on to them from earlier generations of gardeners. Well, for a variety of reasons, there is an increasing tendency to revive many of these traditional approaches to soil management. From my perspective, the most exciting aspect of this revival is the integration of current findings from ecological and biosustainability research. In effect, many of the traditional approaches have been verified by these important branches of contemporary science.

What are soil amendments?

Soil amendments are also known as conditioners and include the familiar range of decayed organic materials like compost and leaf mould. Their role is to improve a soils structure and ultimately its ability to deliver water, air, and nutrients to plants. They encourage nutrient recycling by developing the innate structure of a soil. In comparison to fertiliser, soil amendments work over a long period and will benefit the entire garden. I’m always encouraging gardeners to concentrate primarily on their soil and growing conditions. At the risk of becoming a little technical, I hope to convince you that organic amendments are the safest and most effective means to promoting soil fertility.

The foundation of each soil’s ability to deliver effective plant nutrients can be described in terms of charged ions which must be gathered and held on particular locations within a soil’s structure. Depending on their electrical charge, individual ions will be attracted and loosely bound to the organic particles within a soil. For example, a zinc ion has a positive charge and is therefore attracted to organic particles with a negative charge. Any deficiency of organic soil particles may result in a nutrient supply which is wasted in terms of its sustained availability to plants. By increasing the overall quantity of organic material in your garden, the soil’s capacity to attract beneficial ions can be significantly enhanced.

Despite superficial similarities in outcome, the biochemical processes which underlie the distribution and availability of organic soil nutrients is distinct from those based on the inclusion of synthetic and chemical based fertilisers. The best example is nitrogen, which can be readily supplied to most gardens in the form of complex organic materials or as a synthetic ammonium compound or water soluble salt. In both instances the plant ultimately requires nitrogen to be supplied in the form of inorganic nitrates. With complex organic material, this process is dependent upon a range of soil micro-organisms which feed on the decomposing remains and break some of these into compounds of ammonium and ultimately nitrate which can be used by the plants. The process of decomposition is gradual, ensuring that organic materials provide a steady availability of absorbable nitrogen.

Chemical fertilizers

Chemical fertilisers remove many of the biochemical steps and provide a direct means for rapid nitrogen absorption. With their rapid efficiency, they are potentially toxic to soil organisms, contribute to soil nutrient imbalance, and create negative environmental impacts. Due to their high solubility, nitrate compounds cannot be secured within a soil structure. Whether derived from organic materials or chemical factories, nitrate compounds are readily washed away by the effects of rain, garden watering, and agricultural irrigation. While the quantity of nitrates leaking from chemically treated gardens and farms is limited only by the scale of application, those originating from organic compounds are lost in relatively small quantities. In terms of sustained nutrient availability, there really is no substitute for high quality organic soil amendments.

When I’m invited to speak about the benefits of effective soil conditioning, it often surprises me to discover so many seasoned gardeners who never bother to make their own compost or leafmould. Immediately following the Second World War, it appears there was a general neglect and movement away from what I would describe as garden self sufficiency. A few of my contacts have suggested this relates to increased levels of consumer affluence. Why bother building compost when you can purchase alternative products from garden centres and nurseries? Well, I could suggest any number of reasons, but I don’t necessarily agree with the affluence explanation. In recent times, I’ve encountered a new generation of enthusiastic gardeners keen to apply the old techniques of organic soil conditioning. They are motivated primarily by their passion for environmental sustainability and the desire to tread lightly on this vulnerable planet of ours.

Organic options

Most organic waste from the kitchen and garden can be recycled then returned to the soil. Composting is a process which enables the waste to decompose harmlessly. The final product is a crumbly, sweet smelling, fibrous solid that can be mixed directly into the soil. Raked up leaves are an ideal material for improving soil quality. Depending on their variety, they will take up to two years to transform to a dark crumbly leafmould. The decay process is initiated by fungi so occurs most readily in dark, moist conditions. Plastic rubbish bags containing damp leaves can be stored in the corner of a shed or garage. Once the material inside has begun to darken it can be used as a coarse mulch but the texture of any leafmould will improve with age. Finer grades can be used as the foundation for home made potting mixes.

Manure sourced from healthy, grass eating animals is a readily available soil conditioner. It has a high nitrogen and phosphate content so should be measured sparingly and properly dried before applying to the garden. Shredded bark and woodchip is economical when purchased in bulk quantities. It contains fewer nutrients than manure but is effective in improving the capacity to distribute moisture and pockets of air within a soil structure. Dependent on the timber species, there may be some influence on soil pH. Despite this drawback, bark and woodchip rates highly with professional gardeners and landscapers. They can both be used as a general purpose mulch and soil conditioner.