Organic matter self-sufficiency in market gardening

Rational use of inputs, MSV, soil fertility, etc...

Self-sufficiency in organic matter for market gardeningRational use of inputs, MSV, soil fertility, etc...

1. Market gardening
2. Organic matter self-sufficiency in market gardening

In agriculture soil organic matter is a complex mixture of animal and plant residues, fresh at all stages of decomposition, living and decomposing microbial tissues and heterotrophic biomass, as well as relatively stable humic substances.. It plays a key role in soil structure, fertility and chemical and biological activity, as well as providing a range of ecosystem services (carbon sequestration, biodiversity support, etc.).

At market gardeningwhere crop succession is more intensive, managing soil organic matter levels is an important issue in crop fertilisation.

This portal describes the main forms of organic matter management in market gardening, and outlines the major issues involved.

Rationalising inputs

In market gardening, the management of fertiliser is an agronomic and environmental issue, and must therefore be considered in the light of the specific characteristics of the farm. In organic market gardening, fertilisation is often expected to benefit the soil first and foremost, in order to encourage plant growth via natural mechanisms. In fact, organic matter inputs must be based on soil analyses (physical and chemical) and the needs of the season's crops. Knowing these values, it is possible to calculate the humic balance, to know the quantity of humus mineralised annually per hectare, and thus compensate for this degradation by inputs. Other management indicators can also be used, such as the nitrate test or the BRDA-Hérody method.

Organic inputs

Fertilisers

Organic fertilisers are substances of "natural" origin (animal or plant) that are applied to agricultural land to provide crops with the nutrients they need to grow. They can therefore be predominantly nitrogen, phosphate or potassium based. In market gardening, a distinction is made between soluble fertilisers and foliar fertilisers, which are mainly used to correct deficiencies. The main purpose of most foliar fertilisers is not to contribute to "soil nutrition".

Amendments

Conversely, organic soil improvers are inputs whose main function is toenrich the biological and physical quality of the soil..

There are several types of organic soil improver that are frequently used in market gardening :

• Compost made from green waste and grape marc, which is relatively inexpensive and can be spread in large quantities.
• Industrial compost, which is more expensive and can therefore be spread in large quantities.
• Livestock manures, available in the production areas concerned, but whose composition can vary widely.
• Calcareous soil improvers, to raise the pH of soils where analyses suggest this is necessary.
• Green manures, which are non-commercial intercrops that are mainly returned to the soil and 'trap' mineral elements in the soil until the next crop is planted. They also sometimes fulfil certain additional functions, depending on the species chosen (allelopathy, biofumigation, decompaction, fixation of atmosphericnitrogen, etc. ).
• To a certain extent mulches and mulchand inputs of Ramial Chipped Wood (RCW) constitute a form of soil amendment through their impact on soil structure and biological activity.

Miscanthus mulch (Wikipedia commons)

Market gardening on living soil (MSV)

Living soil market gardening can be defined as a set of farming practices that aim to " reconstitute the natural cycle of soil fertility, drawing inspiration from 'non-anthropic' ecosystems where plant biomass production is stable without the soil being worked or amended. It therefore borrows technical itineraries from permaculture, natural agriculture andconservation agriculture. " .

Its main principles are based on stopping working the soil (both superficially and at depth), establishing as much plant cover as possible over the course of the season (in particular through plant cover and green manures), intensive inputs of carbonaceous organic matter (mulch, straw, shredded wood, leaves, etc.) and so on.

The aim is to create favourable conditions for the accumulation of organic matter, which will lead to the proliferation of soil micro-fauna and flora. The challenge is to eventually compensate for mechanical porosity with biological porosity, so as to benefit from a good mineralisation dynamic, necessary for the nutrition of the plant population without intensive use of inputs and mechanised interventions.

Points to watch

Given that these practices are relatively recent and that the terminology used is still the subject of scientific debate (including the concepts of "living soil" and "dead soil"), it seems appropriate to take a step back and consider the known limitations of this method of management.

• Maintaining (almost ) permanent plant cover is essential : it provides structural and light protection for the surface horizons, as well as being a potential future source of organic matter. More specifically, given that the success of such a cropping system is based on mineralisation, it is crucial to be able to 'trap' mineral elements during the periods when this dynamic is most intense. If the plant population is not sufficiently removed, there is a risk of nutrients, particularly nitrates, being washed away, which poses problems of pollution of aquatic environments. The autumn peak in mineralisation is the case most frequently given as an example, via the Nitrate Directive, and often involves the introduction of Nitrate Trapping Intermediate Crops (CIPAN).
• The phenomenon of 'nitrogen starvation' is a criterion to be taken into account when designing a technical itinerary for living soil. This phenomenon is often the result of adding organic matter that is too fresh or too lignified, because in order to break down these components, which have a high C/N ratio, the soil micro-organisms need to mobilise the mineral nitrogen in the soil. In the short to medium term, this metabolism results in a deficit of mineral nitrogen available to crops, before gradually returning to equilibrium. This phenomenon is exacerbated if such inputs are made during periods or conditions of low mineralisation (soil too cold, hydromorphic, or pH too low etc.).. By its very nature, market gardening on living soil is relatively sensitive to this phenomenon, especially after the first applications..
• Controlling weed infestation is a predominant criterion when considering the system, since the amount of leverage available to reduce weed pressure is relatively limited by the absence of mechanical work. We therefore have to accept the presence of spontaneous vegetation up to a certain threshold, or 'smother' it with tarpaulins or mulches, or even experiment with steam weeding.
• The time needed to reach optimum production : market gardening on living soil requires large quantities of soil improvers, as thesoil degrades. In addition to the phenomenon of nitrogen starvation, it can take several years for the soil's biological activity to develop until an optimum level is reached, with a definite impact on yields.. This is therefore an economic aspect to be considered in the short term.

Ways of increasing organic matter self-sufficiency

In cropping systems that use intensive inputs of organic matter, it may be worthwhile, from an environmental point of view, to look for more localised sources of these inputs. To do this, we can work on various avenues for increasing organic matterautonomy, at farm or regional level :

• As mentioned above, the use of green manurescan be a solution for reducing dependence on external OM.
• The creation of collectives is a more territorial solution, for composting or exchanges between market gardeners and livestock farmers.
• Multi-activity farming is a difficult but powerful form of self-sufficiency, particularly for systems that include a livestock unit.
• Recycling green waste on the farm, using specialised practices or equipment (shredders, composting platforms, etc.). This waste can be produced by the farm or neighbouring farms, but also by activities such as catering.
• Develop the production of RCW on the farm or on the scale of the small agricultural region, by setting up hedges and agroforestry schemes.

Windrows of compost on a composting platform (Wikipedia commons)

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