A Beginner’s Guide to Organic Matter
When I started growing food in the earth as opposed to container gardening, one of my first conundrums was how to treat the soil. I remember gazing around, thinking that if I’m taking plants out, surely I must have to put something back in. But I didn’t know how much or what. Everything I read pointed to adding fertilisers, (chicken pellets, blood, fish and bone etc.,) but they fed the plants: add this for extra nitrogen for your cabbages or that for potassium for your tomatoes. What about the soil? I knew that worms were great, but what should I be adding to keep them happy? Alongside that, magazines were talking about organic matter, manure, soil conditioners and compost as if I knew what they were talking about. I didn’t.
Hoping to learn more about soil was a primary reason that I enrolled as a mature student to study horticulture. As it transpired, soil science was the module I had to work the hardest to get to grips with given its ions, cations and anions. Chemistry was a subject I’d barely looked at 30 plus years ago, never mind one I’d be tested on in middle age. I almost quit on the first day.
Thanks to technology, we’re learning more than ever about the complex world that lives below our feet. We’re finding that it’s the millions of microbes, fungi, nematodes and their associations within the soil that are so beneficial, how they communicate, live and get along with one another. Feed the soil and the soil will feed the plants. As a result, no dig and no till methods of soil care are becoming popular as they cause the least upheaval to this microscopic world. Matthew Wallenstein, associate professor and director of the Innovation Center for Sustainable Agriculture at Colorado State University, wrote a piece for The Conversation about feeding the microbes which is worth a look at.
If, like me, you’ve ever wondered what the common terms are in relation to organic matter (OM), you might find the following guide helpful. In no particular order and with links and tips to some interesting videos and slideshows I’ll be looking at:
- Organic matter
- Soil conditioners or improvers
- Garden compost
- Well rotted manure
- Leaf mould
- Green manure and cover crops
- Top soil, sub soil and soil horizons
- and a brief foray into soil structure and texture.
I hope this helps to dispel some of the confusion. Adding well-rotted organic matter to soil is a more holistic and sustainable approach to gardening and one of the underlying principles of ‘organic’ growing methods.
Soil most definitely matters! https://t.co/UcbFBEn3y8
— Organic Trust (@organictrust) February 21, 2020
One of the simplest definitions for OM is that it’s something that was once alive. Organic matter is derived from a living thing. Whether that’s us, farmyard manure, twigs or leaves, over time the materials will rot down to become organic matter. When gardeners talk about adding organic matter, they can mean anything from garden compost, animal manures and leaf mould, to the remains of plants that have been planted as cover crops (green manures), as well as some soil conditioners.
OM adds nutrients to the soil that will feed the plants, and organic materials that will feed the soil microorganisms. It’s great for soil structure. No matter what soil you have, clay, loam, peat or sand, organic matter helps to break it up, increase drainage or improve porosity, allows oxygen to move around and plant roots to find water and nutrients. Organic matter also prevents the erosion of top soil, protecting it from the elements.
Soil Conditioners or Improvers
Soil conditioners or improvers can be made from organic material that is added to the soil to improve plant growth and soil health such as organic matter above, or fertilisers. Examples include compost, manure, coir, green manures and peat. Soil conditioners can also include inorganic minerals such as clay, sand, lime or silt and some can adjust the soil pH.
Just like ourselves who need proteins, carbs and vitamins to keep our bodies functioning healthily, several nutrients are necessary for plants to grow and fight off pests and diseases. The major elements they need in various quantities depending upon the plant include Nitrogen (N), Phosphorous (P), Potash(K), Sulphur (S), Calcium (Ca) and Magnesium (Mg) and the trace or minor elements essential for plant growth but in much smaller quantities include Baron (B), Manganese (Mn), Molybdenum (Mo), Copper (Cu), Zinc (Zn), Iron (Fe), Chlorine (Cl) and Nickel (Ni).
Seaweed is one example of a soil improver that contains all of these elements in abundance. It can be dug in or added as a mulch. Stephen Alexander from Teagasc lists in detail the nutrients required for all common vegetables in his publication A Guide to Vegetable Growing.
Organic growers apply regular applications of organic matter, toping up with organic fertilisers to feed the soil when necessary.
You can find a more detailed explanation about the differences in this slide share from Dr Radhey Shyam below:
Compost is a synthetic manure that can be made from various amounts of decomposing organic matter, fertilisers and soil. Usually sold in bags, some composts contain top soil, most contain peat, despite the knowledge that the later is not a sustainable source of organic matter. Peat bogs take hundreds of years to create and are fantastic carbon sinks. Its extraction releases carbon dioxide into the atmosphere, a major greenhouse gas.
If you’re planning to garden under strict organic guidelines, avoid using store bought compost unless it carries a symbol to say that it’s organically certified. Some manufacturers are misguiding shoppers by printing ‘organic compost’ on their bags. It may have been derived from an organic base, but unless certified, compost is not ‘organic’ as we think of it and you’re wasting money buying organic seeds to plant into it. Research your source carefully.
Composting your own waste materials is a great way of creating organic matter that will add nutrients to your soil and help with soil structure. If you’re planning to start composting this year, or you’d like some tips on how to do it better, head over to Stop Food Waste for more information. The image in the top photo is of our own home made garden compost made from uncooked kitchen scraps, coffee grounds, grass clippings, animal bedding, twigs and garden waste. It took over a year to make but felt great to be making our own and the soil benefits immensely from its addition.
Is what it says on the tin. Manure primarily derived from herbivores: cattle, horse and poultry that’s been left to rot until it no longer smells or resembles it’s original form. Do not use manure from meat eaters (dogs etc) as it can contain harmful bacteria. It takes three months to a year or more for manure to rot down sufficiently for garden use depending upon the type and heat of the pile. If the manure is too fresh when you add it, it can harm the roots and microorganisms within the soil. Be careful where you source the manure from too. The chemical aminopyralid hit the headlines in recent years when it was found that residues could pass through animals in sufficient quantities to cause damage to many crops.
Composting toilets are gaining in popularity but it’s advised not to use the waste on edible plants. More information can be found here.
Leaf mould is made by collecting leaves in the autumn, placing them in a container separate from the normal garden compost (they take longer to rot down), and patiently waiting. Different leaves can provide more nutrition or less and some take longer than others to deteriorate (anything from one to three years), but leaves are a great soil conditioner and you can make your own potting compost with them as a base. The RHS have a handy guide to leaf mould here.
Green Manure and Cover Crops
Cover crops or green manures are plants that have been grown specifically to protect the soil by covering it (nature tends not to leave soil bare) between crops. Usually before they flower, green manures are cut and dug in to the soil which helps with soil structure and provides food for bacteria, worms and microorganisms. They can also be cut and left on the top of the soil to act as a mulch. Cover crops are a great way of adding organic matter to soil if you don’t have ready access to compost or manures. A PDF containing some of the more popular green manures can be found here. Green manures include plants like Alfalfa, Phacelia, field beans and Hungarian grazing rye. Seeds can be brought online or from garden centres.
Humus is more than the organic matter that’s added to soil, humus includes decaying insects, animals, microbial bodies and fungus. It’s a dark organic material that builds up over time. Think of the soil beneath leaf litter in a forest to envisage humus. It’s rich, dark and earthy. It can occur naturally or is the result of a well managed compost pile.
Top Soil, Sub Soil or Soil Horizons
Soil is divided into layers known as horizons. These include: O – organic matter, A – topsoil, mostly minerals with organic matter incorporated, E – eluviated, missing in some soils, B – subsoil, where the minerals that have leached down from the higher horizons settle, C – parent material from which the soil developed, R – bedrock such as granite, limestone or sandstone that forms the parent material for some soils. If you stick a spade into the soil and dig down, you can see the different colours and textures of the horizons.
The importance of soil
It takes around 100 years to creat 2.5cm of topsoil and the majority of the world’s food is grown in this horizon. In 2015, the Food and Agriculture Organisation of the United Nations published a technical summary of the World’s Soil Resources. They summarised that ” the overwhelming conclusion from the regional assessments is that the majority of the world’s soil resources are in only fair, poor or very poor condition. The most significant threats to soil function at the global scale are soil erosion, loss of SOC [soil organic carbon] and nutrient imbalance. The current outlook is for this situation to worsen unless concerted actions are taken by individuals, the private sector, governments and international organizations”. Using sustainable soil management techniques which includes adding organic matter to soil will help to reverse this trend.
This is a layer of organic or inorganic material that sits on the top of the soil. Made from straw, compost, wood chips, dried leaves or pine needles, mulch can also be an aggregate such as pebbles, slate or stones. Mulching adjusts the temperate of soil, insulating the ground against cold or heat, helps with moisture retention, and the spread of plant disease can be reduced by its application. More information about garden mulches can be found in this archive guest post from Jerry Day.
Soil structure refers to the architecture of the soil, or the arrangement of all the particles (clay, sand, silt etc) within it. In heavily compacted soil there will be little drainage or oxygen available to plants and soil biodiversity. A more detailed explanation of the A, B, C’s of soil structure can be found in this Teagasc guide.
Soil texture is the type of soil you have, sand, clay, peat etc. Knowing your soil texture can help you to determine what plants to grow. An example is carrots that prefer a looser, sandier soil if they’re to develop the long roots of say, an Autumn King variety. If you’re gardening in a clay soil, choose shorter or round varieties of carrot seed such as the Chantenay. Work with your soil where possible rather than fighting against it. I’ve shared a simple experiment to learn your soil texture in an archive post here.
How much Organic Matter Should I Add?
Now we’ve established what the definititons are, how much organic matter should you add? Unfortunatrely that’s a million dollar question as it depends on what gardening method you’re following and what soil you have. As a rule of thumb I use two parts top soil to one part organic matter when creating raised beds. No Dig guru Charles Dowding recommends a layer of around 15cm or 6″ of well rotted organic matter mulched on top of the soil to create a new No Dig bed. An early organic, Gardeners World presenter Geoff Hamilton used to recommend a bucket full per square yard. I aim to add around 10kg per square metre in our clay soil.
For a closer look below the soil surface, here’s a lovely short video ‘The Living Soil Beneath Our Feet’ from the California Academy of Sciences. We need to stop treating soil like dirt and look after it. Our lives may depend upon it.
If you really dig your soil or would like more information on regenerative agriculture, some great resources that include videos, podcast and peer reviewed papers can be found: