How to break down organic matter after harvest
Post-harvest management of organic matter, such as straw and stubble, is an important aspect of sustainable agriculture.
The decay of organic matter returns nutrients directly to the soil. This improves soil health and fertility by adding vital nutrients and increasing the carbon content of soils.
There are various methods of natural decomposition, such as spreading and incorporating, which work to break down straw and stubble post-harvest.
The choice between spreading and incorporating straw depend on various factors, such as:
- Specific farming system
- Equipment available
- Labour constraints
- The desired outcome
Straw spreading
After harvest is complete, chopped straw can be spread evenly across the field using a straw spreader to start the process of decomposition and return nutrients back into the soil.
Spread straw takes longer to break down compared to incorporated straw, as a result of reduced soil contact.
This can pose problems during the subsequent seeding operation if the straw layer is too thick. This is because it minimises the exposure of the straw to soil microbes, which play a crucial role in breaking down organic matter.
Benefits of straw spreading
One of the primary benefits of straw spreading is erosion control; the straw helps prevent soil erosion by shielding the soil surface from both water and wind.
The practice can also help to regulate soil temperature through moisture retention reducing the rate of evaporation from the soil’s surface.
Methods for straw spreading
Most modern combine harvesters come equipped with straw choppers and spreaders. As the combine harvests the grain, the chopper cuts up the straw residues and spreads them evenly behind the machine.
When spreading, it is essential to create a consistent and even layer of straw, as uneven distribution can lead to problems such as irregular germination, uneven soil moisture retention, and challenges during subsequent field operations such as drilling and planting.
Although finely chopped straw settles more easily between rows than coarsely chopped straw, uniform spreading is more easily achieved when the chop length is longer – this is important for sunlight and water penetration.
When spreading straw, aim for an even thickness of 1-2 inches for optimal results.
Factors to consider when spreading straw
Equipment, such as seed drills, might have trouble working through a thick layer of straw. This can result in non-uniform seed placement, reduced seeding efficiency and even equipment damage.
Also, for seeds to germinate effectively, they need good contact with the soil. A thick layer of straw can prevent this, leading to poor germination rates or uneven crop development.
As straw decomposes, it can temporarily tie up certain nutrients, like nitrogen, making them unavailable to crops. If the decomposition is slow this can potentially lead to nutrient deficiencies in the early stages of crop growth.
Incorporation into the soil
Post-harvest, incorporating straw into the soil can provide a valuable source of crop nutrients by increasing organic matter content for improved soil health and enhanced water retention, while supporting a diverse microbial population.
Benefits of straw stubble incorporation
Straw incorporation can return significant amounts of potash (salts that contain potassium in water-soluble form), and some phosphate, magnesium, and sulphur to the ground.
Soil microbes, including beneficial fungi and bacteria, feed on the straw and aid in decomposition. The process of incorporation enhances microbial activity and results in boosted soil health.
Studies from the AHDB have shown that straw incorporation in the medium to long term (around 8-10 years) can make marginal improvements to soil organic matter and soil physical properties.
However, there is little evidence of short-term impacts on soil quality, workability of the topsoil or improvements to crop yields.
Methods for straw incorporation
A rotary tiller is recommended to mix straw into the topsoil, as the rotating blades chop up the straw and mix it into the soil. This incorporates the organic matter into the top few inches of the soil.
Ploughing is the most traditional method of incorporating straw. Here, a plough is used to turn over the top layer of soil, burying the straw residues underneath. The overturned straw decomposes in the soil increasing organic matter.
Despite a long-standing history, ploughing is now seen as much less preferable option when considering conservation or regenerative farming principles, as it can lead to soil erosion and reduced soil moisture.
Farmers should ensure the straw is short for incorporation to be effective. Shorter straw is beneficial for several reasons:
- Uniform incorporation: Short straw mixes more uniformly with the soil. This promotes even decomposition and ensures benefits such as increased organic matter levels are consistent across the field.
- Faster decomposition: Chopping straw into smaller pieces increases its surface area and therefore increases microbial activity, leading to faster decomposition compared to longer pieces.
- Better seed-soil contact: In subsequent planting, short, well-incorporated straw is less likely to disrupt seed placement.
Stubble incorporation
After harvest, properly managed stubble can provide numerous benefits to the soil, future crops, and the environment.
Just like straw, as stubble decomposes, it releases nutrients that can be absorbed by future crops. This decomposition also enhances soil structure, promotes microbial activity and improves water retention.
One of the most effective ways to manage stubble is to incorporate it back into the soil.
Benefits of stubble incorporation
Incorporated stubble can provide a protective barrier against soil erosion caused by wind and water. It acts as a binder, holding soil particles together and preventing them from being washed or blown away.
Turning stubble into the soil helps in capturing carbon, which might otherwise be released into the atmosphere if the stubble was burned, a less common but still practised method.
Methods of stubble incorporation
- Ploughing:
Ploughing is the most traditional method of turning the soil over the remaining stubble. This effectively buries the stubble, allowing it to decompose in the soil as it is in direct contact with soil microbes.
Turning the soil also disrupts the life cycle of many weeds, reducing their reproduction into the next crop cycle, and potentially cutting down the need for pesticides.
While ploughing can be effective, it should be noted that it can also be disruptive to soil structure, especially when carried out excessively.
- Disking or harrowing:
Disking and harrowing are tillage methods that use circular blades (disks) to break down stubble and mix it with the soil’s upper layers. These cultivations are less invasive than ploughing, as they break down the stubble into smaller pieces and mix it more accurately into only the soil surface.
Since they are less invasive, these methods maintain more of the soil’s natural structure and beneficial organisms, therefore providing a more sustainable approach to stubble incorporation.
Conclusion
The choice to spread or incorporate straw or stubble after harvest impacts soil health, seeding operations and crop yields.
While straw spreading aids erosion control, its slower decomposition can inhibit seeding. Incorporating straw or stubble helps nourish the soil and benefits microbes but requires proper chopping for effectiveness.
Ultimately, the goal of breaking down straw and stubble after harvest is to enhance soil health, increase yields and uphold sustainability.