Update on organic waste products

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As part of the “Optimise nitrogen resources” initiative, agricultural players in Ile-de-France are offering to take stock of organic waste products through an educational sheet.

Organic waste products (Pro) have varied and variable compositions which will influence their behavior. (©Arvalis-Plant Institute)

The soil organic matter group together all the organic compounds that are or have been alive. They represent only a few percent of the volume of the soil but are essential for crop production.

As they decompose, these organic materials will act on the physical characteristics of the soil (density, stability, porosity), on its chemical composition by releasing mineral elements, but also on the biological activity through the micro-organisms which will degrade them.

Roles and actions of soil organic matterRoles and actions of soil organic matter (©Arvalis-Institut du Végétal)

A nitrogen supply that depends on the C/N ratio

The release ofnitrogen available to plants will depend on the ratio of the carbon (C) and nitrogen (N) contents of the organic matter considered (humus, crop residues, cover crops, organic products). More the C/N ratio is low, the faster the release of nitrogen for the crop will be during the degradation of this organic matter. For example, the legumes (rather low C/N ratios) release nitrogen for the next crop while the degradation of the straws (higher C/N) will consume nitrogen.

The quantities of nitrogen supplied by soil mineralization and by the degradation of organic matter are accessible for a given crop through dynamic models or via reference values ​​such as Mh (humus), Mr (residues), MrCi (residues of cover crops) of the balance sheet method. These values ​​are available in the brochure on the reasoning of the nitrogen fertilizationpublished every year by the Ile-de-France Region Chamber of Agriculture.

What is the effect of organic waste products?

The organic waste products (Pro) have varied and variable compositions that will influence their behavior. They can have a fertilizing effect in the short and medium term and an amending effect in the longer term.

When Pro is added, part of the nitrogen contained in the product is directly assimilated by plants : it is the mineral fraction of nitrogen. A second part, in an organic form, must mineralize to be available in the year of the contribution or the following one. Finally, a last fraction will be transformed with the humus of the soil and will be mineralized at the same rate as the organic matter of the soil (2 to 6% per year).

The most stable products, such as compost, have a more significant action over the long term whereas the most labile products, often the richest in mineral nitrogen, will participate more quickly in plant nutrition the following year. Note the exception of digestates which provide both stable organic matter and largely mineral nitrogen.

For example, 50% of the nitrogen provided by cattle manure will be stored in a humified organic form in the soil after one year. This value can vary from 33 to 53% depending on the cropping situation.

Forms and availability of nitrogen from ProForms and availability of nitrogen from Pro (©Arvalis-Institut du Végétal)

Contribution of a Pro: evaluating the elements available for the culture

Of the equivalence coefficients (Keq) exist in order to estimate the quantities of NPK available for cultivation. Thus, the nitrogen equivalence coefficient (KeqN) corresponds to the quantity of nitrogen of a mineral fertilizer (ammonitrate) which has the same effect on the nitrogen supply of plants as 1 kg of nitrogen provided by the organic product.

  • The nitrogen contained in the Pro has an ammonium nitrate equivalence between 0 and 80%.
  • The potassium of the Pro is available quickly, its Keq with potassium chloride is equal to 1.
  • As for the phosphorus of the Pro, it has an equivalence with the super45 between 60 and 95% depending on the product. The supplement will feed the available phosphorus store in the soil.

Several factors influence the efficiency of Pro, in particular the ability of the crop or the next to capture the nitrogen released, the climatic conditions at the time of application and those that will follow.

Note that the decrease in nitrogen volatilization risks in ammoniacal form contributes to improving the efficiency of the intake. Certain practices at the time of and after spreading make it possible to limit these losses:

  • prepare the soil before spreading to facilitate the infiltration of Pro into the soil;
  • avoid conditions that are too dry, too hot and windy to spread liquid products;
  • use equipment allowing the Pro to be buried when spreading (drift for liquid Pro);
  • work the soil at least 5 cm deep immediately after spreading to bury the Pros.

Finally, to reason nitrogen fertilization, it is necessary to know the composition and behavior of Pro (nitrogen content, rate of nitrogen release), to adapt the doses according to the needs of the following crop, to reason the spreading dates according to plant absorption periods and favoring climatic conditions that limit losses by volatilization.

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