After Manure in all its forms, let's stay in the field of soil amendments.
Le BRF mon meilleur ennemi
A few weeks ago, we wrote a Blog proudly explaining that we used our hedge trimmers to make CRW and spread it around the vegetable garden around existing crops (spinach and salads).
Very bad idea!
The Carbon/Nitrogen ratio
To understand why this is a bad idea, let's keep in mind that all organic matter is made of carbon. It is the source of all life. We ourselves are mainly made up of carbon. Around these carbon atoms, there exist, in varying proportions, atoms of oxygen, hydrogen and nitrogen.
It's this chemical cocktail that is life.
What interests us as gardeners is the proportion of carbon to nitrogen in the organic matter that we bring to the vegetable garden. We talk about the C/N ratio. If a material has a C/N ratio of 25/1, it means it contains 25 times more carbon than nitrogen..
And here we come to the first “trap” – a material that has a C/N ratio of less than 25 is considered to be rich in nitrogen. A good biological balance of the soil is to contain 25 times more carbon than nitrogen. Overall, we can say that a hard, complex, dry, woody material is rich in carbon while a moist, green, flexible material is rich in nitrogen.
Here are some examples of C/N ratios sorted from the most nitrogenous to the most carbonic.
Matter | C/N ratio |
Urea | 0.7 |
Manure | 10 |
Lawn mowing | 10 |
Coffee grounds | 25 |
Compost | 30-35 |
Cereal straws | 50-150 |
CRW | 60-150 |
Sawdust | 200-500 |
So urine is strongly nitrogenous while sawdust is strongly carbonaceous..
NPK
If you remember the Blog on manure, a plant feeds mainly on 3 mineral elements, the famous NPK which allows you to be smart during dinners😉
A little reminder of what is hidden behind the acronym NPK: N for Nitrogen, P for Phosphorus and K for Potassium.
You will have noticed that in plant nutrients, we do not talk about carbon. However, we want to bring carbon to the vegetable garden. But in what interest then?
We will talk about it in more detail in another Blog but to keep it simple, remember that carbon elements nourish the soil while nitrogen elements directly nourish the plants. Nitrogen is directly assimilated by plants while carbon elements must first be broken down by biological life in the soil before they can be assimilated by plants. The carbon elements will therefore "boost" biological life which will, in the long term, improve our soil - humus, consistency, drainage.
A question of balance
So, to be effective, we could say that it is enough to give plants rich in nitrogen so that they grow quickly and well - this is the basis of chemical nitrogen fertilizers. It's true, but it's also true that we just need to eat fast sugars to perform well. We agree that they give a boost that allows us to surpass ourselves but in the long term we need a varied diet and not only based on quick sugars. Same for plants.
And the nitrogen hunger in all this?
We now understand that we need a good balance between short term and long term and therefore between Carbon and Nitrogen.
When I add a significant quantity of BRF (Fragmented Raméal Wood) to my soil, I add a lot of carbon (C/N ratio of 60 to 150). The organic life in the soil will get to work and decompose all this carbon to release the nitrogen - this is mineralization. This process requires a lot of energy from these organisms which will pump this energy directly into the nitrogen present in the soil. As a result, there is no more nitrogen available for the plants. !
To accomplish their task, the bacteria will multiply, and therefore consume even more nitrogen from the soil, to continue to decompose the carbon. At the end of the process, once all the carbon has been decomposed, they will have nothing left to eat and will therefore die... and return the nitrogen to the soil. Nitrogen which will be available again for plants.
But by waiting for the cycle to end, your plants will have had a nitrogen deficiency - nitrogen hunger. They are deficient, grow poorly and turn yellow.
Conclusion
It's all about balance. Keep a C/N ratio of 25.
Nitrogen hunger is not a problem in itself and is beneficial in the long term - we're talking months and even years. It builds better living soil that accumulates large reserves of nitrogen sufficient to decompose future carbon inputs. But when starting the vegetable garden and the first years you must be careful not to cause nitrogen hunger in the cultivated areas..
Make your carbon contributions during the rest periods of your vegetable garden, typically in autumn and winter.
And if you add too much carbon, compensate with an addition of nitrogen such as lawn clippings, manure or a little urine.
Like us, our vegetable garden likes diversity and not eating the same thing every day..