Explaining a little about fertilisers; What N:P:K means; the benefits of Organic Matter, mulches & green manure – and making your own compost & manure brews. Some steps towards Organic Gardening


I had quite a few ideas about gardening that I believed wholeheartedly but quite without foundation – that is before I started my formal horticultural training. One of these truths was that there was no such thing as the over-use of fertilisers. A little more would surely provide extra benefit. Well, no, it turned out.

So I have put together some notes on the place of fertilisers in the garden, sorting out some of the jargon and explaining the technical bits on the back of the various bottles that struggle for a place on garden centre shelves and highlighting the use of my favourites. Also what the home gardener can do and putting into the spotlight the benefits of organic matter, mulching, green manures, and home made fertiliser teas.

It’s a text dense piece, so settle down with a cup of (preferably not manure) tea and read on. Your soil will love you back as in turn will your plants.

Armchair touring

And while I am here, if you are going stir crazy in these difficult times, I’ve scheduled timely posts throughout April, with tours from years’ past at Kew Gardens and RHS Wisley, Pashley Manor in Kent, Great Dixter and more, so you can get out and about from the safety and comfort of your armchair. Further investigation of past posts will take you on a journey through many more gardens across Britain and a few treats abroad. Might help pass the time, or provide a temporary break from life’s commitments.

Fertilisers, mulches, manures and teas

We may not have a plot the size of Sissinghurst (above) but we may have one of the 15 million gardens in the UK, or a piece of an allotment – or a single solitary pot. We may however, dream. Then just as we want to get started – confusion strikes. We can all be bamboozled by the myriad bottles, tubs and packets of fertilisers on sale – small print upon even smaller print – and wonder whether we are doing our plants, or the wider environment much good by using them.

Fear not, with a little chemistry, biology, soil science, composting and home brewing – we can really make a difference to the living soil, creating happier plants and healthier, tastier crops. Understand a little about your soil and what your plants need. Embrace these steps towards organic gardening in your own piece of Eden.

The starting point for any of this is the headline – ‘Consider Your Soil’ – what it is, what it is made up of, why it is alive and how to care for it. I will talk use and benefits of Organic Matter and I’ll use the abbreviation OM throughout these notes, where I remember.

A background note to tall of this –  the level of demand we might be placing on the soil depending on what we are growing and how intensively. This falls more or less down the line between the ornamental flower/shrub garden and the market garden, growing fruit and vegetables throughout the year.

Specifically relating to fertilisers, soil improvers and amendments, we shall look at the difference between organic and chemical (non-organic) fertilisers, straight fertilisers, compound fertilisers, what plant invigorators are and more. Mycorrhizal fungi will come into the conversation too.

Importantly, we shall look at the major macronutrients that all plants need, and some of the micronutrients. What they are, what plants need them for and what deficiencies look like. Issues relating to time of release of different nutrients in different forms, the importance of water and watering, acidity and alkalinity (pH), the concentration of different formulas and the effect of your soil type. Why having too much of one can lead to a shortage of something else and why too much is never a good thing. Specific mention is made on growing in containers too, as these can present particular issues for all manner of ornamental and fruit/vegetable growing needs.

In addition to the organic and inorganic fertilisers that you can buy (and having a greater understanding of what all of the small print on the back of these packets and bottles actually now means) we shall cover home-made fertilisers and soil improvers, including compost tea, manure teas and comfrey tea, amongst others. This would naturally lead us neatly on to the next natural topic which is focused on composting at home, including hot and cold methods, bokashi, wormeries and more – but that is for another time.

First off, we’ll look at the different nutrients that your plants need – and need more of in different soils or circumstances. A lot of what I discuss will refer to these chemical elements so it is best to have a handle of them, what they are called, what they do, and what deficiencies may look like – and what we can do to put them back into the ground. Organic matter you will see comes around time and time again but we will get back to the soil after a little chemistry lesson….



One of the major macronutrients – there are six in total, N, P and K being top flight, Nitrogen, Phosphorous and Potassium. The second tier are Ca, Mg and S – Calcium, Magnesium and Sulphur. There are a whole host of micronutrients required in tiny amounts.

Nitrogen is very important!

  • Associated with leaf and stem growth
  • in every protein in plants; helps in defence against disease and insects
  • part of the chlorophyll molecule and therefore photosynthesis

It is a very common element – almost 80% of our atmosphere is composed of nitrogen but this cannot be used by plants.

Nitrogen may enter the soil through rainfall, plant residues, animal manures. It needs to be fixed or converted so that the plants can use it. Biological processes, where microbes break down organic matter, turn it into the kind of Nitrogen that plants can use. Rhizobia bacteria in the root nodules of peas, beans and clover for example, can fix atmospheric nitrogen in the soil into Nitrogen again for the plant to use. N

Chemical processes involve ammonia and the Haber process which made inorganic nitrogen fertilisers available cheaply for the first time in the early 20th Century and in part took agriculture – and gardening – away from more sustainable methods using animal manures and green manures.

Nitrogen is very water soluble therefore as a liquid fertiliser it can be very quick acting. If it is being supplied in compounds which are not soluble in water, it will need microbial activity to break it down and is therefore slow.

Use it in spring and over the growing season, or as a boost if deficiencies are showing – and then use a soluble feed.

Plants can’t discriminate between organic and inorganic nitrogen but too much will hurt the plant, burning the leaves or roots.

Excessive use of Nitrogen fertiliser will result in too-fast growth, spindly growth, the cell structure will be weaker and more susceptible to disease and insect attack. Lush, leafy growth will be a haven for insect pests especially Aphids. Too much can also decrease carbohydrate production – so not at all good for potatoes and carrots.

Recommendations – Make compost if you can – as a fertiliser it is slow, biodiverse, builds soil, helps microbes and encourages different soil organisms.

Remember that pellets or granules are slower to work but containers may need nutrients fast, either as a foliar feed or as a drench.

Chemical ratios for the the macronutrients are mostly signified as a three part ration N:P:K, three numbers separated by a colon (:) in the small print. It signifies the balance of each chemical – and you can see if it is equal for each is something like 5:5:5, or skewed towards one or another – more Nitrogen in 12:5:5, more Potassium in 2:5:9 (I’m making these up but you see the pint.

Whatever fertiliser you use, keep the N:P:K ratio to less than 12 – i.e 12:5:5 or 2:5:3 or 6:2:0

Green manures can act as nitrogen-fixing ground cover – specifically to improve soil structure, add organic matter and nutrition. They are quick-growing, weed smothering and their deep roots aerate the soil

Sweet pea seedlings


The second of the major macronutrients – N – P – K

Why is it important?

  • associated most with storage and transfer of energy in the plants
  • promotes strong root growth
  • helps in flower and fruit production
  • helps in plant and fruit maturity
  • cell division and tissue growth
  • how plant use sugars and starches
  • essential in early stages of growth and during seed formation
  • high levels of Phosphorous decline as plant reaches maturity
  • most soils have enough phosphorous unless organic matter is low, or soils are acidic.

How is it made –

Rock phosphate is the raw material. It takes a long time to break down and become available to plants.

The KEY is how much is available to your plants immediately and how much further down the line.

Sources of organic phosphate include Bonemeal (6:9:0) – it may take 3-6 months for the initial part of the P to become available, though it may continue to contribute to the soil fertility for several years thereafter.

In manures and composts, the P element may take between 3-6 months to break down for the plants us (other benefits aside)

pH is an important factor – how acid or alkaline the soil is – higher alkalinity locks ups the available phosphorous in other elements, so even though the soil may contain the vital element, it is locked away and the plants can’t use it at all.

If plants are struggling, bonemeal is not really quick enough remedy.

Phosphorous deficiency

  • stunted plant
  • overly dark coloured leaves
  • when you plants are out in the cold, root development is obviously slow – with phosphorous deficiency, establishment and root growth is even slower.
  • bottom leaves turn yellow, oldest leaves first, symptoms working their way UP the plant
  • purple stems or purple veins
  • flowers and fruit delayed and small


  • Check pH – too acidic and P absorbed by Iron and Aluminium oxides
  • too alkaline, and P absorbed by Calcium carbonate – the element is there but locked away
  • If using straight fertilisers, allow and plan for up to six months ahead. Perhaps consider an autumn application for spring planting. An alternative way to think of this is to count backwards from planting, to work out when to apply a particular feed. Allow at least 4 weeks.
  • use a synthetic fertiliser in spring if needed
  • if in doubt, use a balanced organic/soluble fertiliser in the spring – look for 10:10:10 or 7:7:7

Bonemeal has Calcium and Nitrogen too – and also feeds the soil and its colony of microbes.

Chemical fertilisers feed the plant only.

Chemical fertilisers are often salts – can harm the plant and kill the life in the soil if over used.

Remember you can always dilute the feeds from the recommended dose, or split the application over the season with two or three smaller doses!



The third of the six macronutrients with the letter in the elementary table K. Requirements for this element range from high to very high, depending on species. Potassium comes from clay particles so as clay is broken down, potassium is released – therefore sandy soils often have issues.

Why is it important?

  • in plant protein production – affects the whole of the plant
  • flow of nutrients up and down the system
  • regulates water or turgor pressure in each cell
  • opening and closing of stomata cells and hence gas exchange
  • cellulose/cell wall structure
  • ensures plants grow to size and produce healthy yields – important in flower and fruit production
  • helps fight pest and disease by keeping cells under pressure – strong cells harder for diseases to penetrate.
  • builds strong membranes between cells – affects hardiness

Potassium deficiency

  • curled leaf tips
  • brown/yellow leaf edges, working into the centre of the leaf
  • yellow leaves start to brown from the leaf edges
  • chlorosis – yellowing between the veins
  • purple spots on the underside of leaves
  • blue, yellow or purple tints with brown blotches on upper surface
  • Leaves bronzed and rolled inwards and downwards
  • upper leaves usually look good, bottom leaves first affected then works way up the plant.
  • shortages more often in light sandy soils, as more readily washed away by winter rains than in clay soils, where clay particles hold on to it.

There are inorganic potassium sources

Potassium sulphate 0:0:80

Potassium nitrate 18:0:44

Monopotassium phosphate 0:62:34

NEVER use these – far to strong!

Organic sources of potassium include Greensand, dried banana peel, and potash (literally a Pot of Ash)

Burned hardwood ash – which provides micronutrients too.

If in doubt, use a balanced feed – tomato food is 2:5:3 generally


  • Don’t by chemical forms of potassium
  • Greensand, rockdust, ash, banana peel
  • Mix hardwood ash with water and add small quantities to the compost heap
  • Banana peels can be dried in the oven and crumbled (or use coffee grinder) – add one tablespoon to the planting hole of tomatoes, chillies, aubergines etc., Otherwise add peel under mulch by roses for example or add to the compost bin.
Tomato Indigo Rose


Generally, once you have been gardening intensively you may need to add more calcium. Plants need calcium in substantial quantities but have to watch soil pH – Calcium is the controlling element controlling soil pH so low levels of Ca in the soil will inhibit plant growth via increased acidity long before Ca becomes limiting as a nutrient in its own right.

Why is it important

  • carrier molecule – transports essential nutrients in and out of cells – carbohydrates, enzymes etc.,
  • strengthens cell walls
  • helps with the uptake of other nutrients including NPK from the soil – therefore necessary to make sure garden soil gives up its nutrients.

Calcium deficiency

  • stunted growth
  • young leaves distorted and yellow
  • inward curling of leaves
  • blossom end rot in tomatoes (especially in containers)
  • black heart in celery
  • cavity spot in carrots
  • bitter pit in apples

The problem is that is absorbed in water therefore you have to have regular levels of water. If there are periods of drought when the water dries up – and this is a common issue in containers, here the calcium take-up slows or stops. Therefore very important to maintain levels – and not over water either – too much or too little water affects how other nutrients are taken up from the soil.

A note that too much calcium will not change the soil pH – Calcium carbonate will change the soil pH.

Calcium is the building block of good fruit size and flavour therefore problems are common in containers where there are limited resources and where watering can leach out nutrients. Similarly in sandy soils, rain will leach out nutrients too fast.

Sources of Calcium

  • Calcium nitrate – fast acting NPK 2:5:3 and Ca:Mg: S 7:5:4 Liquid feed
  • Gypsum – may lower pH but will not raise it. Slow take up
  • Eggshells – slow – crush after baking to finer particles. Also contains N, P and trace minerals – Boron, Copper, Iron, Manganese, Molybdenum, Sulfur, silicon and Zinc
  • Garden Lime – a mix release fertiliser, best added in winter. Contains Calcium carbonate, Calcium oxide, Calcium, Magnesium, Magnesium oxide and Magnesium carbonate. Will lower pH.
  • Bonemeal – 11% P, 2%N and 22% Calcium


  • Most soils have calcium – but as more claims are made on the soil, may need to supplement
  • Add compost and water regularly
  • Ca element doesn’t change pH – only Calcium carbonate will.
  • Lime raises pH
  • Containers may need more
  • Plan ahead generally and watch the watering, especially in containers.


Soils that contain a ready supply of calcium are not good for lime-hating plants such as Camellias, Azaleas and Rhododendrons, and in the productive garden, crops such as blueberries. You might use an ericaceous compost and a mulch of pine needles to maintain soil acidity. The problem is that while your soil may actually be fertile, many of the required minerals are captured and help by the calcium and these plants’ roots cannot access it. They starve of nutrition.

Using a fertiliser that contains Iron (Fe) will help the situation immeasurably. Sometimes referred to simply is Iron (such as Seaweed with Iron), or Sequestered Iron (sometimes as a wettable soluble powder), it knocks the required minerals free from their lock with the Calcium making them available to the lime-hating plant. In the first place, check your soil pH, plant appropriately to suit your soil – the right plant in the right place is always a good mantra to follow. That way you are not fighting nature in the first instance. But to help the situation along, if you need to fertiliser, or mulch, or plant in a growing medium, choose one to suit ericaceous plants.

Rhododendron Persil


95% of sulphur comes from Organic Matter – Compost!

Sulphur reacts with oxygen – soil bacteria change its form through mineralisation and your garden soil and the living ecosystem will do this for you – it is slow but ongoing.

It can be leached out of sandy soils, therefore another reason to maintain OM levels.

Deficiencies include yellow leaves and poor growth (again).

Three main sources of sulphur – organic matter, minerals and Acid Rain.

What it does –

  • aids root growth
  • helps with general growth and nutrition
  • helps plant take up other nutrients
  • involved in synthesis of amino acids, vitamins and proteins
  • legumes that fix Nitrogen need Sulphur in the process
  • adds heat and flavour to vegetables…


  • Compost and Organic Matter are Key
  • Even watering and good draining essential
  • Epsom salts provides a fast does – MgSO4 (Magnesium sulphate)
  • Gypsum provides a slow release
  • Containers once again can suffer shortages more rapidly.


It can become unavailable in plants, if too much Potassium or NH4 is present. Like Potassium it is present in clay soils, so may be low in sandy soils.

What is it important?

  • required to give green leaves their colour
  • production of chlorophyll – 20% of Mg is found in chlorophyll – and correct functioning of photosynthesis
  • strength of cell walls
  • enzyme production
  • seed germination
  • helps plant take up nutrition
  • It is there in most soils, so no need to panic!


  • interveinal chlorosis, again
  • leaf curl
  • leaf tips curl upwards
  • purple or red colour to leaves
  • early leaf fall
  • stunted growth
  • death

You can see the duplication of symptoms here! It is important to look at the particular plant stress, what else you are doing or have done, to determine deficiency. Diaries and garden logs are important. But it is very hard for soils not to have enough Magnesium.

But if you have to add – MgO – garden lime is useful

Epsom Salts – fast acting magnesium oxide – 10% MG 22%S

Add when plants are flowering and setting fruit – they often need a boost and as before containers often need more.

It’s there again in compost and organic matter and this will help with a whole range of nutrients.


Many fertilisers are actually salts and we know what happens if we spend too long in a bubble-bath – the tips of our fingers start to shrivel like prunes. Instead of water being absorbed by the skin, the higher concentration of chemical salts in the water actively draws water out of our skin – a reverse osmosis. It is exactly the same with plants – instead of the roots taking up water and nutrients from the soil, they begin to loose water and vital nutrients. If nothing else, over-use of chemicals or using them at the wrong time (in the growing cycle, in the year) will simply be a waste of money, not benefitting the plants or the soil.

This all brings us back to the soil and our options if we want to avoid chemical and inorganic feeds whilst still having a happy healthy garden and a productive vegetable plot.

Consider you soil

Your soil is a living, breathing thing, albeit that actually only a very small percentage is actually alive but this tiny proportion is an engine that feeds the entire thing we call soil.

Roughly made up of 45% minerals, 25% air and 25% water, and approximately 5% organic material, things which were once living and are now dead, sometimes a very long time dead. Of this 5%, 0.5% is actually still alive – all of the micro-flora and fauna, the earthworms (the heros of our soil) and bacteria and fungi and protozoa that do so much to manage the remaining 99.5% and which can make it a vital productive living soil.

I’m not going to look in detail at soil structure and texture, except to mention the difference between sandy soils and clay soils. Sandy soils are free-draining and hold less water, there is less OM (organic matter) as a whole and pans – thin hard layers in the soil profile – are common. Clay soils are characterised by being fertile, moist (though baking hard under a drying sun) and prone to water logging. Slow to warm up in the spring and more difficult to work. Surface crusts are common on bare soil and crusts can form through overworking at deeper levels.

If we think of sand particles as being the size of a baseball, then clay particles are the size of marbles – so you can already think of some the main differences in terms of these common traits as a factor of the main particle sizes.

By adding Organic Matter to both kinds of soils, miracles can be worked.

To recap, organic material was once alive – organic matter (OM) is what is left after the organic material has decomposed in humus, a stable organic component of the soil.

  • It can loosen and aerate clay soils
  • bind sandy soils together
  • improve nutrient and water holding capacity of sandy soils
  • makes clay soils easier to work
  • attracts a whole universe of micro-organisms – providing a varied habitat – for beneficial fungi, worms, bacteria and other creatures into the soil
  • these breakdown minerals, form aggregates with sticky excretions, as they move around, creating pathways for air and water in the root zone, bringing nutrients up from lower levels
  • aids in reducing compaction and surface crusting
  • can be a major source of Nitrogen, Phosphorous and chemical nutrients like Sulfur
  • a carbon supply for many microbes that perform other beneficial functions, for example the nitrogen-fixing bacteria (we’ll come to that)
  • humus buffers the soil against rapid changes in acidity, alkalinity and salinity, and damage by pesticides and toxic heavy metals.

The disadvantages of OM are few but it needs to be added regularly, it may contain weed seeds if not cured in a hot enough environment, some forms liked chipped bark and leaves can rob Nitrogen from the soil, it can encourage some pests if laid too deeply and can remain too wet on already clay soils.

We can add organic matter into the soil in several ways, by digging it in or by adding it to the soil surface as a mulch. Plenty of materials can be used as an organic mulch and the benefits of one over another will vary but these generally apply –

  • cover the soil
  • reduce soil erosion
  • reduce water runoff
  • reduce rainfall impact
  • prevents crusts from forming
  • regulates temperature
  • reduces drying out effects of the sun
  • conserves moisture
  • all more rainfall to be retained in the soil
  • provides nutrients, slowly leaching into the soil over time
  • if a no-dig system, prevents pans forming
  • smothers weeds and reduces time subsequently weeding
  • builds OM in the soil
  • feeds soil organisms, provides a varied habitat
  • helps improve sandy and clay soils
  • it can be decorative

Organic Mulches should be between 2″-4″ deep, no more as this can cause other problems, and can be –

  • straw
  • strulch
  • homemade compost
  • chipped bark
  • aged manure
  • mushroom compost
  • cocoa shell
  • chopped bracken (acid plants)
  • green manures
  • intercropping

Green manures are plants grown on bare soil, when it would otherwise be empty of crops, used specifically to improve soil structure and nutrition, particularly relating to Nitrogen (N) – it is also a living mulch, protecting bare soil and to outcompete weeds.On sandy or chalky soils leaching of nutrients is reduced.

Phacelia and roses

If ground is free for six weeks or more, and especially over the winter months, green manure is an ideal.

There are a whole host of plants which can perform the job of a green manure and most are inexpensive and with a number of different benefits.

  • Field Lupins (Lupinus angustifolius) – nitrogen-fixing (we’ll come to that), deep rooting, and hence aerating the soil. Sow in Spring (March to June) and cut back/dig in the autumn. Thrives on light and acid soils.
  • Mustards – (various Brassica species) Sow March to September, treat as brassicas in rotation. Fast, bulky and smothers weeds.
  • Alfalfa (Medicago sativa)  Rich in calcium and major elements. Deep rooting, bring trace elements up from deeper levels. Sow April to July – cut/turn in autumn or let stand through the winter. Avoid acid soils
  • Italian rye grass (Lolium multiflorum) sow March/April or August/September. Cut back three weeks before planting either in the autumn or following spring, as can inhibit germination. Fast winter cover if used, smothers weeds.
  • Buckwheat (Fagopyrum esculentum) – deep roots again, provides some calcium. Attracts hoverflies.Fast growth in poor soils,
  • All kinds of Clover – Red Clover is especially good – Sow between April and August. Cut back after flowering for second regrowth. Prefers lighter soils.
  • Field Beans (Vinca fabia)  Sow September-November for overwintering. Deep Nitrogen fixing roots. Grow to 45cm then dig in Spring. Happy in heavier soils though more limited N-fixing throughout winter.
  • Phacelia tanacetifolia – Bushy plants with bright blue flowers that the bees love. An annual, sow between March and September. Dig in either before or after flowering. Plant in patches throughout the garden plot as it attracts pollinators and looks good.
  • Winter Tares (Vicia sativa)Winters cover, summer nitrogen fixer –  a spring sowing is excellent preparation for winter brassicas – did in before the crop is planted out. Will sulk on acid soils!
  • Tubingen Mix – a specific green manure used a Bee forage for summer and autumn, contains Phacelia, Buckwheat, Mustard, Coriander, Calendula, Black Cumin, Oil Radish, Cornflower, Wild Mallow, Dill and Borage.
  • Consider intercropping as a valuable mulch between crops that take longer to mature, or use a different soil profile (radish utilises the top few centimetres of soil, carrots and turnips use the deeper soil profiles).
  • Borage and comfrey

But getting back to fertilisers and to explain some of the terms used above – like Nitrogen-fixing bacteria.


Fertilisers come in a number of forms

Single or straight fertilisers supply on major nutrient – Nitrogen, Potassium or Phosphorous, or Magnesium

Artificial Fertilisers

Manufactured chemicals, mined or processed minerals. Usually supplying one element.

  • Sulphate of Ammonia 20% Nitrogen
  • Urea 46% Nitrogen
  • Nitrate of Soda 16% Nitrogen
  • Sulphate of Potash 50% Potassium
  • Superphosphate 18% Phosphorous

Compound fertilisers – artificial fertilisers I mean specifically – might provide a range of chemicals, a more balanced nutrient mix though they are still inorganic.

The relationship between Nitrogen, Phosphorous and Potassium is always shown on the packaging for these kinds of fertilisers (as it will be too for the single element feeds supplying just one chemical).

This is known as the N:P:K ratio – the percentage by weight of each of the three main macronutrients being supplied. N for Nitrogen, P for Phosphorous and K for Potassium (after the Latin for this element, Kalium)

The N:P:K for these three common compound fertilisers is

Growmore         7:7:7

Vitax Q               5.3:7.5:10

Chempak           6:8:10

These feeds, whether they are straight or compound, can provide the nutritional needs of the plants but they do nothing to improve the soil or the living ecosystem in your garden or allotment. The doses are often very high, especially the single element feeds and caution must be exercised not to harm the plants – or the soil – with excessive use.

In most soils, Nitrogen is readily leached from the soil so more will be needed especially in intensive production

Phosphorous stays in the soil for several years

Potassium is usually there in sufficient quantities but more may be needed

Some fertiliser formulations have a coating which is biodegradeable at a particular known rate, usually by the action of water – so nutrition is released at a given rate for a given period. Such formulations are very useful in containers, especially for summer bedding, hanging baskets etc., where the plant combinations are often hungry things!

By contrast, Natural, Organic Fertilisers have several benefits in addition to supplying the basic nutrient needs of the plant – and the plant cannot distinguish between chemical and organic N, P or K.

Natural fertilisers include

  • Bonemeal – N:P is 3.5% and 18% – though the Phosphorous may take several years to break down and become available to the plants. Very useful on acid soils.
  • Chicken manure pellets – N:P:K is 4.2:5:2.3 with some OM and trace elements
  • Fish, Blood and Bone is 6:6:6 – you should use a face mask and gloves when handling
  • Seaweed – is a tonic and conditioner, with minerals and trace elements (one of my favourite fertilisers by a country mile)
  • Dried blood is about 12% nitrogen. It is very expensive but great value in glasshouse crops where it is quick acting and is also an organic nitrogen fertiliser

Note that organic refers to  their source from animal and plant sources – not as in ‘organic farming’ – so the product may be organic but the animals may not have been farmed organically.

The benefits of Organic fertilisers over their inorganic counterparts include

  • often recycled plant and animal byproducts
  • lower nutrient concentrations
  • longer release times
  • other benefits that straight fertilisers do not – having soil building qualities
  • generally contain balance of top flight macronutrients – N P K, but also the second tier of macronutrients, Calcium (Ca), Magnesium (Mg) and Sulphur (S)
  • often trace elements too
  • generally sustainable
  • increasing the quantity of one chemical through application of a single straight fertiliser will increase the demand for other fertilisers  – the addition of one nutrient may decrease the uptake of another
  • organic fertilisers can be applied at a heavier rate without risk of injury to roots
  • stimulate microbe activity in the soil


Benefits of Inorganic fertilisers – soluble (and some insoluble), cheaper often than organic counterparts with precise application rates

Application of fertilisers – these can be as base dressings (in with the plants at the bottom of the planting hole, beneath the root ball), top dressings (fresh compost replacing a couple of inches of the old, or granules mixed in with the top soil), liquid feeds (quick), foliar feeds (on the leaves) and green manures.

Why Organic?

The Soil Association has the following objectives for Organic Production –

The operation of a sustainable relationship between the health of the soil, plants, animals, people and the biosphere to produce healthy food whilst protecting and enhancing the environment.

With regards to plant nutrition the organic aims are

  • to work with natural systems and cycles
  • to maintain and increase long-term fertility
  • as far as possible to use renewable resources in preference to non-renewable one
  • to use other specific materials when extreme measures arise

In practice this means

  • optimising nutrient recycling by using manures to best effect
  • balancing nutrients within rotations
  • feeding the soil rather than the plant

By adopting such organic principles and methods it is understood that

  • production methods affect quality
  • more antioxidants in organic crops
  • fewer pesticides are needed

Given there are 15 million gardens in the UK and a goodly number of allotments, we can all make a difference….


Which leads us neatly to a few home brews…

COMPOST TEA, MANURE TEA , COMFREY & WEED TEAS & other things to consider

With compost teas, the aim is to prevent foliage diseases and increase nutrients to the plant while shutting down toxins in the soil. By adding good bacteria from home grown compost – a little like adding Yakult to our own systems – these multiply and outcompete the bad bacteria that might proliferate in badly drained, poorer soils. It is a good way of multiplying the benefits of home grown compost and applying it to a much larger area than you could with just the solid mass, or where it is impractical to use compost itself.

– to transfer microbial biomass, fine particulate organic matter and soluble chemical components of compost into an aqueous phase that can be applied to plant surfaces and soil in a way that is not possible or economically feasible with solid compost.

This can be something simple like placing a bag of home-made compost into a barrel of water and steeping it for a day or two before removing the bag – something like a hessian sack, pair of tights or an old pillow case – and using the liquor to water your plants.

Alternatively, if you are using a bucket rather than a watering can with a fine rose, drop everything into the barrel, loose and leave to macerate.

Manure tea is made in much the same way, with aged animal manure – though not for carrots, radishes, turnips, potatoes or beets – manure tea provides Nitrogen but not enough of the other nutrients they need for food root growth. Both compost and manure tea are nitrogen rich, the latter more so.

Garden Organic have little sachets of manures – and weeds – which are packaged in old coffee sack material – called Old Muckers Plant Food Bags, costing £5.95 each. The bags are macerated in a water barrel and reused daily for up to a month when the bag and contents can be thrown onto the compost heap.

Note that different animal manures have different NPK values and that any manure should be properly aged.

  • Horse 0.5:3.5:12
  • Sheep 6:5.5:27
  • Rabbit 3.5:6:11
  • Poultry 15:35 (N:K)
  • Cow 3:3.5:13

Compost tea for the connoisseur is a little more involved, needing the liquor to be aerated over 24-48 hours with a marine pump and several air tubes held at different levels within the tank. Garden Organic have a tank kit for £60.00 though this include pelleted feed to start the process. The kit was put together by Laverstock Park around a 20lt brewer, but a trip to the aquatic centre might save you some money.

You can use activator pellets, home grown compost, worm compost or worm manure. The process uses unsulphured molasses as a food stuff for the explosion of microfauna in the brew and the aerobic respiration keeps in fresh.

A half-way house affair can be produced without an aerator, as long as you are prepared to vigorously stir a large dustbin full of liquid with a piece of 2″ x 2″ wood, creating a vortex which aerates the solution right down to the bottom of the drum. Do this every 10 minutes for two hours and then use as a drench. Youtube is the place to go for all manner of advice and home-spun solutions here.

Of course, the higher quality the compost the better the tea. Even bagged manure from the garden centre, though it is a little higher in salts and lower in good bacteria, can be made to work harder for you.

Rainwater would be best, but otherwise allow the tap water to rest for a couple of hours before using, so that some of the chlorine will evaporate off.

Wormcasts either collected from the landscape or worm compost from wormeries is particularly effective when used to create this explosion of organisms and to create a significantly larger volume of a solution from just one or two litres of the original organic matter. Lovely lovely stuff!


COMFREY, Borage – and other lesser weeds

Comfrey is a wonder plant, growing in place where many would sulk, with exceptionally high levels of nutrients and trace minerals.

There are several varieties with blue or white flowers with distinctive hairy leaves and stems – gloves are recommended when handling either comfrey or its near-relative, Borage. Both are prolific self-seeders which you might not consider to be a problem, particularly if you are careful about using them before they flower.

There is a variety called Bocking 14 that is sterile, so will not seed everywhere and is usually sold as root cuttings in the spring. 10 root cuttings will set you back up to £10 on ebay.

It can be planted near your compost heap to take advantage of the extra nutrients that naturally seep out of the heap – and it makes an excellent accelerator so can be added to the green/brown mix to help things along nicely.

It has deep roots which aerate the soil to a good depth and the root, stem and leaf can all be used. Having planted your new plants out in position, refrain from cutting back in the first season, to get them settled and established in their new home (with Bocking 14 you don’t need to worry about them flowering or setting viable seed). In the second and subsequent years you can cut them hard three or four times in the growing season, and they will bounce back to give you another go!

They are naturally rich in Nitrogen, Phosphorous and Potassium, Calcium and all manner of trace elements – yet very little fibre so they mulch down/rot down to almost nothing. Importantly it can be made into an excellent fertiliser to be used as a drench for vegetables, leafy herbs, fruit and near-flowering plants. Possibly more nutritious than kelp.

It can be mixed up with borage – of the same family – which some may also consider a weed.

There are two ways of treating the comfrey to release its goodness and in both instances, wear gloves.

The first is to gather the plants and cut them up roughly, enough to fill any large container you may have – bucket, trug, something like that – with a loose fitting lid and something that will weigh the whole thing down – rocks or a brick or two. You might add a little water – or urine – to moisten the leaves before you clamp on the lid.

After a few weeks – 2-3-4- pour off the liquid that has pooled in the bottom of the vessel and use, diluted 20:1 with rainwater, as a drench. Bottle and label carefully but aim to use within the month.

You can use two containers, the top one containing the leaves with a hole in it, so that the liquor seeps into the lower container ready to be poured off.

You can also use a 4′ or 6′ drainpipe to streamline the whole process and attach this with clips onto to your shed. Fix the cut end of a fizzy drinks bottle with tape onto the bottom of the drainpipe, with the screw bottle top still attached. Fill another drinks bottle with sand or cement, making sure that it fits and slides down within the drainpipe. Connected by wires and a piece of cord, this can be pulled out so that the comfrey can be added to the top of the drainpipe – and using the weighted bottle, this will act as a plunger, compressing the leaves inside. As this rots down, the liquid collects in the bottom and can be drawn off by unscrewing the bottle top there, with a bucket underneath to catch the elixir.

The spent leaves can periodically be removed from the container or drainpipe and added to the compost heap, or used as a mulch on the rows between vegetables.

This process is more or less aerobic – at least you are not excluding air completely – so it is less of a dance with death than the next method, where any close connection with the resulting liquid will surely haunt you and your clothes for several days.

This second method involves collecting the comfrey as before and filling a barrel, trug or bucket at least 2/3rds with the greenery and topping the whole thing up with water, covering tightly and leaving it to macerate for 2-3 weeks. Dilute 1:10, or to the colour of pale straw and use as a drench.

The smell will be abominable, horrendous and a spectre of the Black Death, since the process is anaerobic, without oxygen.

You can make the process a little tidier by stuffing the comfrey leaves into a hessian sack, pair of tights or a pillow case before adding the water, and it will act much as a teabag does, though I would hate to have to stir this concoction over much as it matures.

You may leave the mixture for just two weeks in warm weather, and up to 4 weeks in colder periods.

The relative values of comfrey over manure are illustrated in the following comparison

% Water: Nitrogen: Phosphorous: Potassium    Carbon: Nitrogen ratio

Farmyard Manure   76          .64                   .23              .32                          14:1

Comfrey                  75          .74                   .24              1.19                        9.8:1


The process works with a mixture of comfrey and borage – and also with nettles. You might simply have a weedy barrel in your garden or on the allotment, where all weeds are (if they haven’t flowered or set seed) dropped into a barrel to macerate – being topped up regularly as weeds are picked and cleared. The strength of the tea will be weaker but it will be an effective way of breaking down the weeds for the compost heap, while maximising the goodness you can stew out of them!

We can thank Henry Doubleday (1813-1902) for the original work on comfrey, though it was Lawrence D Hills (1911-1991) who set up the foundation in Doubleday name in 1954, for the further research – an organisation that ultimately became Garden Organic. While Doubleday was interested in making glue for stamps, he found a strain of comfrey (from the Imperial Palace in St. Petersburg) that had incredible hybrid vigour and like the best F1 strains nowadays, sterile seeds. Bocking 14 gives the best results for fertilisers though the garden varieties will do nicely too – just watch it doesn’t take over!



Milk, used half and half with water – and perhaps mixed with Epsom Salts – can be used straight as an anti-fungal foliar spray which is useful for alleviating powdery mildew on cucumbers and courgettes. With the Epsom Salts it is a useful tonic for tomatoes.


6% Potassium, with some calcium and magnesium

Not for use on already alkaline soils

Best to use hardwood and young wood at that

Small quantities can be added to the compost heap. Can be used, in solution, as a drench but not for acid-loving raspberries or potatoes (alkalinity encourages potato scab).


Small bucket, chopped, steeped in 5 gallons of water, loosely covered for 2-3 weeks – used as a drench (diluted to weak tea colour) – provides lots of micronutrients. Maxicrop seaweed is a natural plant stimulant (5:2:5) though owned by Monsanto. Take that where you will.  Shropshire Seaweed for me is the Gold Standard but does not state an N:P:K ration as it is variable, a wholly natural product. But very useful for promoting a larger rooting area, increased photosynthesis and microbial activity in the soil. Some anti-fungal quality if used as a foliar spray on roses – and aphids rather dislike such an application too! I use it on my roses in bloom – on leaf and flower – and believe it promotes healthy foliage and if repeat-flowering, a quicker response time to come back in to flower.


A mulch to deter slugs and snails, contains approximately 2% N. Not as acidic as thought – much of that is leached out in the coffee making and drink-making process but still best not to use too much at once.


One part of the soil flora family and in nature, a relationship that plants form naturally over time, with soil fungi, enabling them to take up more water and nutrients, often by a significant order of magnitude – helping them withstand drought and resist soil disease (like rose replant disease). Creates a much larger secondary root system, linking plants together in larger more responsive networks.

Sufficient quantities of OM will help create the right conditions for these fungi to thrive but sometimes a helping hand is required – not to be combined with base dressings on planting like bonemeal. In tests plants that are treated and those that are not are pretty much at level pegging after a few years, but those with the treatment get off to a better start, particularly if conditions in the soil are not perfect. Very useful in combatting rose replant disease.


Wood burned without oxygen locks in carbon – an ancient technique done rather well in the Amazon basin and appreciating the fact that charcol acts as a magnificent sponge, with a huge surface area which can hold water and air, helping to retain Nitrogen too. Biochar includes mycorrhizal fungi, seaweed and wormcasts.

The founder (Craig Sams) is Green and Black’s founder, also former Chair of the Soil Association.


As foliar feed, is a quick acting and purely straight fertiliser for greening of ailing leafy plants, such as box. The chemical formula  is MgSO4. 7H2O. Or it is also known as Magnesium Sulphate heptahydrate. For leafy plants that are yellowing or bleaching, it can have a rapid effect in bringing the green back, important in chlorophyll production amongst other benefits. Quite a useful, if short term fix. Better to work with the soil.

General Rules for using any fertiliser

  • don’t use it all at once
  • don’t add liberally – too much can do more harm than good – salts form in the soil which pull moisture from plant roots and the solution can burn roots and leaves. Large doses of nitrogen cause soft leafy growth which is just delicious to aphids. If used too late in the year, N fertiliser can cause soft growth even on hardy plants that will be caught by first frosts.
  • use small amounts when flowering or fruiting rather than a single large application
  • watch watering
  • for containers, feeding may be more important as plants can quickly exhaust nutrients in growing medium but watch watering again
  • watch release times for quick and slow fertilisers
  • watch watering generally
  • consider pH
  • consider timing of applications, plan for when fertiliser applications will be available to the plants you are planning
  • compost if you can
  • mulch if you can
  • consider your soil – sandy soils will leach nutrients faster and start off with fewer than clay. With sandy soils, split the addition of any fertiliser into 2 or 3 doses during the growing season. Your soil is a living –  thing, so consider the nutrition and habitat of the billions of microflora and fauna in your soil. Remember earthworms are the stars!
  • consider biostimulants such as Maxicrop liquid seaweed or SB plant invigorator
  • make your own home grown garden fertilisers and soil improvers
  • nutrient deficiencies share several symptoms and the cause of any trouble isn’t always so straightforward. Keep a diary of any fertiliser applications you have made – and of your successes and failures so you might learn from past behaviours.
  • Don’t tip over the bucket of Comfrey Tea….


I’ve referred to recently the 1920’s book The Gardeners Enquire Within, produced by the good folk of Amateur Gardening – at a time before much of the inorganic chemistry boom of the 20th Century.

For their entry of Fertilisers it simply states: See Manures. Fair enough!

They talk about manure teabags too, though their recipe for Tonk’s Formula has more to do with Mrs Beeton’s recipe I saw once, where the instructions started with ‘fling twelve chickens into a pan’… which asked even more questions I think.

Tonk’s Formula for Roses

  • 12lbs Superphosphate of Lime
  • 10lbs Sulfate of Potash
  • 2lbs Sullfate of Magnesium
  • 1lbs Sulfate of Iron
  • 8lbs Gypsum

Mix thoroughly, crush as fine as possible, and apply at rate of 4 oz/square yard in April.

If all else fails….

2 thoughts on “Explaining a little about fertilisers; What N:P:K means; the benefits of Organic Matter, mulches & green manure – and making your own compost & manure brews. Some steps towards Organic Gardening

  1. With five acres of old cow pastures now English arts and crafts gardens all made by me in rural Latvia as Expat.Built house and now retired work all day in landscaping hedge and room making with drives and old brick columns and 9 foot high gates angles in stone each side as only entrance and postern gate Hidcote copy into what is now a rough field of one acre just over. Here I plan a Japanese lake on half acre alongside a full length pergoda walk viewing fish in safety. Borrow small digger for large stones and lots of bushes and trees. At 70 this year I think it keeps me fit.I make all composts and use farm manures over 12 year old. Costs me only the trailer to fetch back as farm is deserted and ruined. No one wants this black gold as gardens here are washing line and hens nothing else save wall pots of summer show. They think me mad but who cares they all laughed at Major Johnny Johnston with Hidcote manor gardens but what a man what a garden he left us. Few can achieve that but my God Ill try. Muck is the thing your spot on with that

    1. Thank you so much for taking the time to write – your project sounds amazing and a true labour of love. I wish you every success though the journey is the fun part not just the destination. I hope that you might continue to update me on your progress and if you have the time for photographs, that would be the icing on this particular cake. My warmest wishes, Martin

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