Thursday, 10 December 2015

Yet more myths and legends: misconceptions in gardening.

Really I shouldn't get upset when people like Dowding say they feed plants.  Indeed the definition of food is not all that clear even in science.  However, it does lead to some interesting misconceptions. 

Plants use the energy from sunlight to convert carbon dioxide and water into molecules that can provide them with energy and mass.  They make their own food.  The process is called photosynthesis and plants are referred to as photo autotrophs.  They also need relatively tiny amounts of elements like nitrogen, phosphorous and potassium.  They combine these elements with the molecules formed from carbon dioxide and water to make all the compounds they need to survive. 

Animals can't do this so they eat the chemicals that plants make.  They are called heterotrophs.  (They can also get these chemicals from other animals that have eaten plants.) These chemicals are usually called food. 

Fungi can't photosynthesise and need to get their energy and mass from chemicals produced by plants.  They are heterotrophs too.  Bacteria get their energy and mass from a number of different sources.  They can be heterotrophs, however some  are autotrophs.  Some can utilise energy from the Sun and others can utilise energy from chemicals.  These include the decomposers. 

Why is this important for gardeners?  Well understanding this means that you can appreciate what is happening when you add fertilisers and manures and it will help you to understand what is happening when you are making compost.

 So you can't feed plants in the same way as you can feed animals and fungi. 

You can supply chemicals that contain nutrients like phosphorus, nitrogen and potassium to the soil where they can break down into soluble salts which can be taken up by the roots of plants.  However, I find it difficult to describe this as, "feeding plants." 

Also there is a limit to the amount of these nutrients needed by plants and their rate of uptake.  Plants will need them when and where they are actively growing. 

Adding chemical fertilisers bypasses the normal pathways of decay and decomposition.  These soluble salts dissolve relatively quickly and  are readily taken up by plants to enhance growth.  However, this is not really sustainable because it relies a great deal on fossil fuels to generate these fertilisers.  Also it leads to soil degeneration.  Chemicals that stick soil particles together like humus are lost and organisms, like worms, fungi and bacteria that produce sticky mucilage do not have food to survive.  Soil structure begins to break down and eventually yields are considerably reduced.  Less water can be stored in the soil and it becomes very dusty and easily eroded by both wind and rain. 

Nutrients are stored in the soil by being attached to clay and organic material.  Both clay and organic material are light and easily removed by wind and water leaving the heavier sand and stone which store much less nutrient and water.  This we call soil degradation.

Compost and manure in bays made from pallets
Providing nutrients appropriately is achieved much more efficiently by adding organic material such as compost or manure.  Composts and manures add much more than nutrients to the soil.  They add material in different states of decomposition and a community of decomposer organisms and their predators.  Composts and manures also add soil particle binding chemicals that improve the soil structure, increase air and water filled porosity, reduce bulk density and produce a very friable soil. 

Manure and woody chippings are both valuable organic matter.
Some people suggest that there is a limit to the amount of organic matter that can be added to the soil.  If there is then I have not found it yet.  I am adding copious amounts of compost, manure and woody chippings throughout the year and there does not seem to be any depletion of yield. 

Both fungi and bacteria excrete enzymes that can be stabilized by clay and organic material and continue to break down composts and manure to produce plant nutrients.   Decomposers produce mucilage which is sticky and binds the soil particles together and prevents them from being blown away by the wind. 

Compost bins make good compost if you turn the compost regularly but they rarely heat up
very much.
It is true that there is much less nutrient in composts and manures than there is in the same volume of inorganic fertilisers.    However, plants need a tiny amount of these nutrient compared to that of carbon dioxide and water.  We are doing well providing plants with carbon dioxide from ancient carbon when we burn oil.  So really the limiting factor is water. 

Composts, manures and other organic matter also help to retain water in the soil.  They are sponges that soak up excess water and release it slowly to make it available to plants.  A covering mulch of organic material will reduce surface evaporation considerably.  It will also help to prevent wind and rain erosion. 

Really, in the UK we should not have to water plants outside, however if we do water then we should give the water when the plants are actively photosynthesising.  That's when they need it.  This will be when it is light and the best time to give it is in the morning so that it will be available to use during the daylight time of the day.  (A day is 24 hours.) 

It is true that anything that was once alive will decompose and form a good compost.  Wood rots very easily if it gets wet.  I put up to 15cm long and  5cm diameter cut branches in the compost.  First they change colour to a very dark brown.  Then they go very brittle and can be easily broken.  Then they become very friable and can be pulled apart by hand.  That's why I don't like to use wooden edging for the vegetable beds. 

As Dowding says, perennial weeds such as couch grass make very good compost. And I agree with him. They look very unpromising in the photographs below but they change colour to a very dark brown after a week and start to break apart after about ten days. Bits of plastic, metal, glass and stone fall out of it, and by three weeks you cannot tell what the compost has been made from.  At one month it is good enough to sieve and put onto the vegetable beds.  Although this looks very woody, there are still  nutrients locked up in it. 

Mixture of stinging nettle, couch grass and mare's tail being composted. I dried it very well
before composing it.

I added comfrey liquid and turned it every two days to produce a very good compost. I'm doing
the same with the perennial weeds we got off the new car park.
Cotton and wool (good sources of nitrogen) in the form of sweaters, jumpers, trousers and shirts all rot down.  In Victorian times farmers used a lot of rags as a fertiliser but they did not want them washed! 

Leather shoes, belts and bags decompose very slowly.  The gardeners at the Alternative Technology Centre in Wales told me that old horse saddles and tackle rots down particularly slowly. 

Cardboard rots down remarkably quickly and so too does paper shredded or not. 

When we had horse manure delivered free to the allotment site, some allotmenteers would separate out the horse hair, rejecting it and leaving it to one side.  Hair is about 90% keratin.  Keratin is a protein and proteins contain nitrogen.  I collected the hair up, thanking those that sorted it out for me, and added it to my soil. 

Dowding says correctly that both citrus peel and rhubarb leaves can be added to the compost heap.  They both rot down very quickly.  Citrus peel contains a natural pesticide called d-limonene, however the pesticide can only be made from very concentrated citrus oil.  It is an insecticide and does not affect the decomposer worms, fungi and bacteria in the compost heap.  They will rot it down just like all the other organic chemicals in the heap.  Rhubarb leaves contain oxalic acid which might cause us a problem if we eat too much of it but does not present a problem for worms, fungi and bacteria.  Its just another source of carbon for them. 

Woody chippings used as mulch.  They decompose slowly over a year and there is no
noticeable nitrogen immobilisation. 
The growing points and small leaves at the tips of stems are where most nutrients obtained from the soil are being used.   This is where the plants will use most energy and resources to grow.   

Where does most of the energy and resources come from?   

Well from leaves that are no longer growing.  The large moth eaten leaves near the base of the plant.  These are the photosynthesising powerhouses that fuel growth in the rest of the plant. 

Yet these are the leaves some people remove from their tomato plants. 

Leave your tomato leaves alone. You will get a lot more tomato fruit. 


  1. Fascinating and as useful to gardeners as usual Anthony. I think you are a bit hard on Charles! Most of us use terms like 'liquid feeding' and fully understand we are just talking about adding nutrients.
    I do not agree that fertilisers degrade soils although of course bad practices can lead to high salt concentration such as in greenhouse soils when there is insufficient irrigation to provide drainage.
    The MYTH that fertilisers degrade soil probably arises from where some farmers degrade soils by all those bad practices we both abhor but they continue getting good yields because of fertiliser. Until soil degradation by all those bad practices makes it all go pear shaped… It was not the fertiliser that did the damage, just by the complacency it gives the grower...

    1. Hi Roger. Absolutely right about artificial fertilisers. Still find them unnecessary though. See what you think of my next blog.