Why fertilise wheat on stubble after harvest

• nourishing the soil system after grain removal
• enhancing crop residues as a resource for soil fertility

This is not about “fertilising a crop that is no longer there”, but rather about preparing the soil to host the next crop, improving stubble decomposition and increasing organic carbon content over time.

It is therefore a practice whose value is even more evident in the current context, where cereal farming is dealing with soils poor in organic matter and degraded, environmental stress due to climate change, rising input costs and regulatory constraints.

The harvest of wheat, both soft and durum, leaves behind straw, stubble, roots and crop residues with a generally high Carbon/Nitrogen ratio. This material represents a significant carbon input, but its transformation into stable humus is not automatic: it depends on nitrogen availability, soil moisture, temperature, mechanical management of residues and microbial activity. In this context, the application of organic or nitrogen-based organo-mineral fertilisers can favour the balance between mineralisation, immobilisation in microbial biomass and the formation of stable organic matter.

The objective is not to directly feed the harvested wheat plant, but to guide the biological processes of the soil that act on the residues. Stubbles thus become the physical and chemical substrate on which bacteria, fungi and actinomycetes initiate their degradation activity.

Field experience confirms that crop residues are one of the main sources of carbon in cereal agroecosystems. The addition of nitrogen can accelerate straw decomposition, stimulating microbial communities and enzymatic activity involved in the breakdown of structural carbohydrates.

The intervention is more effective when the soil retains some moisture or when rainfall is expected, allowing gradual fertiliser incorporation and activation of the microflora. Under extreme dry conditions, surface application may remain inactive for weeks.

Effective management starts with uniform straw chopping and even distribution of residues behind the combine harvester. Any shallow tillage, such as harrowing, light discing or minimum tillage, can improve contact between stubble, soil and nitrogen-based organic fertiliser, without necessarily resorting to deep ploughing. In conservation or regenerative agriculture systems, the goal is to maintain soil cover and biological activity while reducing mechanical disturbance.

The timing of the intervention must also consider the subsequent crop and its nitrogen requirements. In nitrate-vulnerable zones or areas subject to regional regulations, application periods and quantities must comply with current legislation.

To decompose carbon-rich residues, microbial biomass requires nitrogen: if it is not available in the residue, it is taken from the soil solution. This leads to the phenomenon of immobilisation, i.e. a temporary reduction in mineral nitrogen availability for crops.

Immobilisation is not negative in itself, as it is part of the natural organic matter cycle, but it can create deficiencies in the following crop if not properly managed.

A nitrogen-based organic or organo-mineral fertiliser applied on stubble can mitigate this imbalance by supplying nitrogen in an organically bound form that is progressively mineralised.

• The first concerns organic carbon. Returning stubble and straw, rather than consistently removing them, increases carbon inputs into the system and, over time, the soil organic matter content. The addition of nitrogen-based organic fertilisers further supports this balance, although the final result depends on carbon stabilisation within the organo-mineral fraction and soil aggregates. Not all added carbon becomes stable humus: part is respired as CO₂, part feeds soil biomass and some may be physically and chemically protected.
• The second concerns microbial life. High-quality organic fertilisers provide energy and nutrients to bacterial and fungal communities. A biologically active soil transforms residues more efficiently, buffers stress, improves nitrogen cycling and supports fertility less dependent on corrective inputs.
• The third effect is nutritional. Post-harvest fertilisation does not always replace basal or top-dressing fertilisation of the following crop, but it can reduce system imbalances. In soils low in organic matter, a multi-year programme based on residue return, cover crops and organic fertilisers can improve the soil’s ability to retain and release nutrients. This is particularly important for exhaustive crops such as wheat.

The benefit should not be measured only in yield of the subsequent crop, but also in:

• production stability
• reduced compaction issues
• improved water management
• increased microbial biodiversity

An additional advantage concerns straw management. Where straw is chopped and left in the field, post-harvest fertilisation facilitates residue transformation and reduces the risk that large amounts of undecomposed material interfere with the next sowing.

Where straw is removed for livestock or energy use, organic inputs can partially compensate for carbon loss, although systematic residue removal should be carefully evaluated in soils low in organic matter.

Pelleted or granular products are often easier to distribute with standard farm spreaders; powdery or very light materials require greater care to avoid uneven distribution.

As always, products must be formulated from high-quality organic matrices. Organic fertilisers with unclear composition, high salinity, unstable moisture or irregular particle size may create more problems than benefits.

The type of matrix influences the outcome: well-matured manure acts mainly as a soil amendment and stable carbon source, while protein-rich matrices with high nitrogen content may have a stronger nutritional effect.

In this context, the post-harvest phase becomes a valuable technical window to nourish the soil, rebalance the Carbon/Nitrogen ratio of residues and plan fertility for the next cycle.

In conventional systems, the practice can help reduce dependence on synthetic fertilisers, improve nitrogen efficiency and increase organic carbon. In organic systems, it is often essential to maintain acceptable yields and grain quality over the long term.

From a practical standpoint, fertilisation on stubble should be seen as a soil fertility management intervention rather than merely nutrient application. If properly designed, it helps prevent straw residues from behaving as “cold” material, slow to decompose and capable of temporarily immobilising available nitrogen.

If instead it is carried out without knowledge of soil characteristics, without considering the following crop or under unfavourable climatic conditions, it may lead to low efficiency, nutrient losses or limited agronomic benefit.

When correctly applied, post-harvest fertilisation on stubble allows conventional cereal farming to improve efficiency and organic cereal farming to enhance production stability. In both cases, the real goal is to transform crop residues into agronomic capital for the future, also within a circular economy perspective.