Hillingdon Allotment News 16: Plants eat soil - don't they?

Well, no of course they don’t but that’s what Aristotle the Greek philosopher though as at that time, 384 – 322 BC, as that seemed the most logical explanation. Science has moved on a bit since then and while there is still some mystery about the movement of chemical elements within the plant, the process of the manufacture of food is now well known. It may come as a surprise to some that about 96% of a plants weight is made up of the compounds of Oxygen, Hydrogen and Carbon, i.e. sugars manufactured in the leaves, while the other 4% or so consists of other chemicals derived from the soil.

So what is all this fuss about manures and composts and fertilizers if they only make up a small amount of the plants requirements? We have all heard about the three major chemical elements, Nitrogen, Phosphate and Potash, the N P K of fertilizer packets, and the importance of a balanced diet for plants depending upon their requirements. The answer of course is that these are required for the other major function that of the production of sugars and starches via the leaves, (photosynthesis), to take place. Out of all the restrictions placed upon plants in their natural environment the most restricting of these is the lack of Nitrogen. 

Therefore plants don’t eat soil, but derive the nutrients that they require from the very complex chemistry that goes on within it. It is clear that plants can only take up chemicals in certain forms, for example, nitrogen is a major element for plants and also one of the major constituents of air, but it cannot be taken into the plant in that form and has to be combined either with oxygen in the form of nitrates NO3-, or hydrogen as ammonium NH4 both of these can be found from organic sources, manures and composts, or from inorganic fertilizers like sulphate of ammonia. The principle differences between these two sources is that decomposing organic matter slowly releases small quantities of unspecified nutrients into the soil, whereas inorganic fertilizers are concentrates of selected elements prepared for immediate and clearly defined use. What is important to recognise is that mineral nutrition required by plants is the same whether they are derived from organic or inorganic forms.

This is interesting as it takes away one of the main planks of those who consider that only organic practices are the correct ones as in terms of the plants requirements there is no difference whatsoever. That is not to say that composts and manures are not required quite the opposite, it is these in their decomposition that maintain the soil flora and fauna including bacteria some of which are capable of ‘fixing’ nitrogen for plant use, and when fully decomposed provide along with water and air, the necessary elements to break down concentrated fertilizers ready for plant use, and so both are complimentary to each other.

The analogy that I like to use in this respect is that of a person who can only eat soup. Whatever sort of soup it is, you would not be able to detect whether the carrots or parsnips or chicken were organically grown or not, such is the combination of flavours, and so it is with plants.

So plants don’t eat soil, which is a good thing, – or there would be some very large holes in the ground by now!

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