In biological cultivation every opportunity to control soil-borne diseases is a bonus. The idea of providing compost components that can make the soil more resistant to pests and diseases is therefore logical. But its implementation is difficult. The material is complex.
Verticillium, root knot nematodes, Fusarium and Pyrenochaeta (corky root rot) cause problems during the organic cultivation of fruit vegetables, while Pythium and Sclerotinia cause problems in leafy crops. In 2014 Wageningen UR Greenhouse Horticulture, of Bleiswijk, the Netherlands, and knowledge institute, Louis Bolk Institute (LBI) together with Bionext and a number of growers therefore started to search for good composts and the possibility of making plants resilient to these pathogens by using compost.
A lot of information on the subject is already available but it mostly relates to experience in practise. There is no clear framework. In addition, compost is a heterogeneous product because it is made from different starting materials and there are different ways of composting.
2014 was mostly a year for stocktaking and design of methodologies. You need be able to measure the resilience of the soil and plant before you start the research. Tools were also developed to measure the resistance to nematodes. For this it is important to determine the pore volume of the ground, activity of the microbes, salts and calcium.
Incidentally it is difficult to point to one single mechanism that is directly responsible for the resistance. However, pore volume does affect drainage, distribution of moisture, oxygen permeability and the type of microorganisms present and thereby the degradation of organic matter. Fortunately not all the mechanisms need to be clear so long as the measurements give a good prediction of the resilience. Wageningen UR Greenhouse Horticulture has developed a model that incorporates this knowledge and can give advice about how to stimulate the soil resilience.
Researcher André van der Wurff: “Also, more and more we use measurements based on the DNA of plants. Eventually it is all about the plant, yield and the taste. On the basis of the DNA we can see when a plant is feeling well, and when the preventative defence mechanism is switched on. Substances in the root environment can make plants turn on or off certain genes, so that they may or may not get sick. The ability to predict this is important. Growers are looking for a safe and sure growing concept.”
Willemien Cuijpers and Leen Janmaat, of LBI, both focus on good ways of composting. Together with Van der Wurff and a large number of overseas colleagues they are working on a manual for growers and compost suppliers about how to deal with compost: For example, what do they need to pay attention to; what should the compost look like, smell and feel and when does it offer resistance?
“Compost is a result of manually controlled degradation and rebuilding process. In this way you build up stable humus,” explains Janmaat. “And humus is the slow biodegradable part of organic material in the soil.”
Compost gives structure to all soil types and plays an important role in resilience. It gives light sandy soil ‘body’, while heavy clay soil becomes lighter. It ensures that plants can take up minerals and it contains microorganisms that compete in order to suppress soil diseases.
Many materials are suitable for composting. On a biological tomato nursery these, for example, could be old crop remains and waste from pruning, combined with clover and lucerne grown on their own land. Therefore the way in which a compost develops is very dependent on the raw materials, the carbon:nitrogen ratio and the moisture level. It’s logical that the soil life in the compost behaves in different ways.
Resistance through competition
It is already known that certain elements in a compost can make a plant more resilient. The soil is crawling with organisms that work together or compete with each other. It is a matter of building up general resistance through competition. Microorganisms fight against each other. In the case of Pythium this happens by competing for food. Other microorganisms are antagonistic and work more specifically. In the case of Verticillium it’s known that non-pathogenic types exist that protect the root against the disease-causing types, Verticillium dahlae. Also Trichoderma protects roots against invaders. Administering these fungi is often carried out during the propagation phase or during planting. A few compost companies add Trichoderma during the final phase of the composting process.
If you can add nutrient to compost or soil that stimulate the useful bacteria then you can further increase the resilience of the soil. In theory the root knot nematode is a nuisance you can control in such a way.
Earlier research by Van der Wurff showed that a group of bacteria played an important role in resistance to nematodes because they eat chitin. The cell wall of a nematode contains this substance. Therefore if you add this substance to the soil the population of bacteria will increase with the chance that the root know nematode will not be able to develop any further. If this process is to succeed then the acidity has to be right. Chitin is found in the remains of prawns. Also mushroom compost, a by-product of mushroom production, contains a lot of chitin. A potting trial yielded good results.
This year commercial trials are being carried out by BioVerbeek using various compost compositions but it is not yet possible to draw any clear conclusions.
Also for conventional growing
The project is being carried out in biological production but of course it also has an impact on conventional soil grown crops. Here it is still possible to use chemical soil disinfection or steaming. However, these sort of rigorous measures disturb the biological balance of the soil.
If it becomes possible to introduce certain antagonistics or microorganisms into the soil via composts the necessity to fumigate the soil will decrease. The goal is to eventually develop a robust and resilient cultivation system for all crops, biological or conventional.
All signs indicate that it is possible to use compost to increase resistance to plant diseases. The addition of compost is already an important step towards improving the structure of the soil. A collaboration between two research institutes and entrepreneurs is focused on stimulating native antagonistics that can be added to composts and tested in practise.
Text: Pieternel van Velden. Photos: Louis Bolk Institute