Why Natural, Organic Soil Improvement is Best
In a spade full of soil, there are many billions of bacteria, hundreds of metres of fungal hyphae, and millions of amoeba and protozoa.
Plants need a mix of core nutrients to grow: the 'Big 3' are Nitrogen, Phosphorous and Potassium (the N:P:K you see on fertiliser packets), but there are also a list of minor or macro nutrients which play a part (more or less so depending on what you want to grow).
All these elements occur naturally in rotted organic material like manure or compost, but they are also available in a much more concentrated form in processed man made fertilisers.
If we stop to consider the size of a large head of cabbage (taking up nearly a square metre of soil) or a tomato plant producing 50 or 60 ripe tomatoes we can see that all that growth needs to come from somewhere, a low nutrient soil or compost simply can't support that level of growth.
Crops growing in low nutrient soil can be affected directly and show poor, stunted growth or indirectly because weak plants are more prone to attack by pests or disease.
Feed Your Soil, Not Your Plants
The science behind the organic growing mantra, 'Feed your soil, not your plants' is that it is better to add raw organic material to your soil and allow the soil (micro)organisms to process it into its key elements.
In the natural cycle which takes place in the soil someone has to do the processing and that someone (or someones) are an army of bacteria, protozoa, fungi, nematodes, arthropods, earthworms and other soil micro-life who process dead plant or animal material (organic matter). The waste products of all this processing are the nutrients required to feed your plants.
Organic Soil Improvers vs Synthetic Plant Feed: The Advantages
In general a plant doesn't care where it gets its nutrients from. Artificially produced fertilizers feed the plant directly: the nutrients are pre-processed like a supermarket ready meal and can be absorbed directly by the plant.
However - much like the aforementioned ready meal - this is a short-term measure that you don't want to rely on, and isn't going to nourish you, in the long-term.
Regular use of synthetic fertilizers can reduce soil microbial diversity and disrupt natural nutrient cycles. Over time, this can degrade soil health, making crops more dependent on chemical inputs.
This is one of the conundrums we have to deal with in modern intensive agriculture, which is increasingly dependent on chemical fertilizers as farm land is degraded over time. It may be the case that completely replacing the use of chemical fertilisers with organic agriculture isn't a viable option as a global farming solution - but organic and sustainable growing at home will make your little part of the world more fertile and more biodiverse.
Adding organic soil improver to your garden soil ensures that it will become more fertile with every passing year.
The reason that supplying key NPK nutrients indirectly using bulky organic material is better than adding chemical fertilisers directly is that organic material doesn't only feed your plants. Rotted compost or manure is worked into the soil by earthworms and other soil life to make it more free draining, better able to hold moisture in dry periods and reduces compaction making it easier for plant roots to spread and draw up nutrients.
Another advantage of using bulky material is that it takes time to break down so is a naturally slow release fertiliser. We mentioned that man made feeds are processed but organic feeds also need to be refined before the key elements are released; the difference with organic growing is that the organisms in your soil (from microscopic bacteria up to earthworms) does the processing work for you which results in a slow, drip feed of nutrients. Bulky manure or compost therefore needs only to be added to the garden once a year (provided it is in sufficient quantities) and will slowly release it's nutrients over the growing season.
What is Soil Organic Matter?
Organic matter encompasses dead or decaying plant, animal or microorganism residues already present in the soil. These residues contain carbon as well as essential plant nutrients.
It also includes anything which is added to the soil by us, such as:
- compost
- manure
- cover crops
- mulches
- leaves
This added material is important not just because it (indirectly) feeds the plants, but it also builds the soil itself. Decomposed organic material (compost) helps loosen and aerate heavy soil as well as improving nutrient and water-holding capacity.
When compost has completely broken down it can eventually form 'humus', a dark, carbon-rich, soil-like substance which acts as a storage and transfer site for plant nutrients. This process of organic matter accumulation is critical to the formation of topsoil and the growth of healthy crops.
What is the Best Way of Adding Organic Matter to Soil?
It is better to add organic matter to the surface of the soil instead of digging it in. Digging (or worse, rotavating) soil is a lot of work, but it also breaks up beneficial fungal pathways which transport nutrients to plant roots, and destroys worm tunnels or pores which keep the soil aerated.
Sourcing manure and compost
I am very aware that not everyone can get their hands on enough organic material to feed a hungry vegetable garden so we may need to make some compromises but in the round, the concept of feeding the soil should apply.
The first key discipline to master is making your own compost both from garden and kitchen waste as this material is a very valuable source on nutrients. Unfortunately, unless we bring in a lot of material from outside the garden, there will never be enough (partly because we don't generally compost our own waste so nutrients from the food we eat don't make it back to the garden) but good homemade kitchen and garden compost is a superb plant feed.