Anyone who has ever heard of organic matter will be impressed by this miracle cure, with so many benefits linked to one substance (see box). However, the question is how important organic matter really is, say Hein ten Berge and Wijnand Sukkel, researchers at Wageningen University in the Netherlands. They do not know whether it is the organic matter itself that is the source of the benefits, or whether other factors play a role in the reasoning for increasing the organic matter content.
Knowledge is important for choosing the right strategy. What should the organic matter content of the soil be, and how can this content be managed optimally in economic terms to get all the benefits? What exactly are the disadvantages when there is (too) little organic matter in the soil?
And then there is the question of whether the organic matter level itself matters, or whether it is more important to provide a regular supply of fresh organic matter, which makes the soil system work by supplying energy and nutrients.
Stable fractions of plant residues
Organic matter consists of plant residues that are transformed in the soil into a stable fraction: humus. Humus is important for soil quality, but a lot depends on the structure and age of the humus. Every year, organic matter is broken down into smaller and smaller particles, and when these particles are too small they lose their value and usefulness.
This means that the organic matter content of two different soil types with two different origins can be the same, but the effects will differ. Soils such as peat have a high organic matter content, but it is old and no longer plays an active role. Young polder soils, on the other hand, have very active organic matter, to which all the known benefits apply.
It is therefore important that there is enough young, active organic matter in the soil. But what is the right level of organic matter? A lot of active organic matter holds water well.
This is useful in a dry summer, but after a wet winter the soil dries more slowly and the farmer can only get onto the land later. In addition, the risk of nitrogen leaching increases when there is a lot of active organic matter in the soil. But a level that is too low is not good either: production seems to suffer, as seen on the Vredepeel and Valthermond experimental farms and in certain years.
Declining organic matter levels: a concern
Farmers are convinced of the usefulness of organic matter and fear a decline in organic matter levels when they have to comply with the new phosphate standards in 2015, which could reduce their income. The level of the standards is not yet known, but they will move towards balanced fertilisation: applying fertiliser at rates higher than the plants can absorb in phosphate will be prohibited, so that no phosphate can accumulate in the soil. This limits the amount of manure, and therefore the amount of organic matter, that farmers can spread.
Until now, according to data from the soil analysis laboratory Blgg AgroXpertus, no decline in organic matter levels has been found on average in the Netherlands. That does not mean that levels on individual farms are not declining. At the experimental and research farms in Vredepeel, Nagele and Westmaas, a decline in organic matter levels was recorded between 1996 and 2004.
Farmers on light soils in North Holland and bulb growers on sandy soils know that organic matter levels have fallen below the critical minimum. As a result, the land is harder to prepare and yields are falling. Farmers have therefore greatly increased their organic matter inputs and are already seeing positive effects.
According to Sukkel and Ten Berge, farmers will be able to apply enough organic matter, even under the future standards, to maintain acceptable levels. They will have to work more often with green manures and compost, and crop residues will have to be returned to the soil. Sukkel: "You increasingly see the trend of farmers trying to sell their crop residues.
As with beet for energy production. For the soil, this is a bad trend. They should simply leave the residues on the fields and work them into the soil."
These measures are expensive. And then the question immediately arises: what inputs are needed to maintain soil quality? Nobody can answer this question correctly.
And nobody knows what happens when the farmer applies too little organic matter, or whether production decreases or not. Ten Berge: "With the 'soilless cultivation' project, high technology achieves good production levels without any organic matter.
"But he immediately adds: "the soil is precisely a reservoir that acts as a buffer and replaces this high technology. So you can see that organic matter is necessary." Both Sukkel and Ten Berge are convinced that organic matter is important and that soil levels must not deteriorate.
And not only because of a possible drop in yield. Just as important is carbon storage, for climate reasons. Globally, there is about 3.5 times more carbon in the soil than in the atmosphere.
If the organic matter content of the soil decreases, the amount of greenhouse gases in the atmosphere increases. In addition, soil that is rich in organic matter can store more water. This is important given the current changes in rainfall due to climate change.
Sukkel therefore clearly warns farmers: "Make sure that organic matter levels in the soil stay good, otherwise you will have problems, perhaps not now, but in the future."
Source: Wijnand Sukkel and Hein ten Berge (Syscope / WUR), Wageningen University