The second principle in CA is that the farmer aims to have plants growing in the fields all year round or at least for as long as possible. Unless the field is going to be established with a winter crop, a cover crop is established after harvest. If you have harvested very early, you may sow a catch crop before sowing the winter crop.
THE PURPOSES OF COVER CROPS & CATCH CROPS
• Cover and catch crops retain nutrients in the upper soil layers, avoiding leaching and thus benefiting the next crop with the nutrients.
• Cover crops make an important contribution to the soil’s carbon content and the life in the soil is best kept going when there is plant growth and living roots in the field. This also applies to fungi that live in the soil and are part of symbiosis with living plants. A vigorous cover crop may also provide competition against weeds.
• Different types of cover crops have different characteristics which can further help to ensure and improve soil fertility.
Cover crops with strong and deep roots such as oil radish can take up nutrients from deeper soil layers and bring them to the upper layers. The roots of the oil radish thus create root canals, which crops can later use as access to deeper soil layers.
Cover crops with legumes can fix nitrogen from the atmosphere and thus a pool of plant-available nitrogen can be built up for the subsequent crop. Species such as white lupine and buckwheat are also thought to be able to help increase the availability of the soil’s immobile phosphorus pool.
In order to achieve as many of the positive effects as possible, CA often works well with a mix of cover crop species (for example, both legumes and strong deep-rooted species).
The establishment of cover crops is a large topic in itself as it is often a challenge to get satisfactory germination and development before the temperatures start to decline in the autumn.
The most common method is to sow the cover crops with a seed drill after harvest. The cover crop can also be established prior to harvest if the right conditions are present for good germination - sufficient moisture in the soil and a suitable soil temperature.
After harvest, the amount of moisture in the soil will regularly be limited due to a high evaporation rate. If August is cold and wet, the challenge may be that development is slow due to low temperatures.
Therefore, an early establishment of cover crops is also being experimented with either by undersowing cover crops in the cash crop or by overseeding right before harvest. The latter is carried out by broadcasting seeds before harvest.
Both of these methods have their advantages but also disadvantages. An undersown cover crop provides high security for a good establishment but limits the possibilities for crop herbicide usage. At the same time, there may be a risk that the undersown cover crop competes with the cash crop for water and nutrients. Finally, a vigorous undersown cover crop can result in harvest challenges.
Overseeding a cover crop shortly before harvest can be a good solution as it gives the cover crop a head start of 1-2 weeks compared to sowing after harvest. In addition, it can be beneficial to overseed the cover crop prior to the busy harvest season. However, the method only gives a satisfactory result if there is enough moisture for the cover crop to germinate. If the weather is very dry, the seeds will merely lie dormant on the surface and there is a risk of the seeds being eaten by insects and birds before emergence. If the cover crop is underseeded early and the weather takes a turn, there is a risk of the cover crop becoming ragged and will be damaged during harvest.
The ’right’ method for establishing cover crops, therefore, depends a lot on the conditions in the individual year.