It is widely accepted that plenty of organic matter in the soil is the best guarantee for good quality soil. But is it true, and how much is plenty? We take a look at the benefits of organic matter, but also at what isn’t yet known.
Anyone who hasn’t heard of organic matter before is bound to be impressed by this miracle cure. So many benefits in one substance (see box)! But the question is how important organic matter actually is, say Hein ten Berge and Wijnand Sukkel, researchers at Wageningen University & Research. They don’t know whether it is the organic matter itself that delivers the benefits or whether other factors also play a role in the benefits to be gained by increasing organic matter content in the soil. This knowledge is important in choosing the right strategy. For example, how much organic matter do you need in the soil to be able to exploit all its benefits, and do the benefits outweigh the investment needed? What exactly goes wrong if there is too little organic matter in the soil? And then there is the issue of whether it is the content itself that is important or whether it is rather a regular supply of fresh material that provides the soil system with nutrition and energy.

>> Stable fractions of plant residues
Organic matter consists of plant residues that have been transformed in the soil into more stable fractions, known as humus. Humus is important for soil quality, but a lot depends on its age structure: organic matter is broken down into smaller and smaller pieces every year, but when the particles become very small they lose their utility value. This means that the organic content in two different soils can be the same but the effects can differ. For example, old peat soils in Dutch peat beds have a high organic content but this matter is old and is no longer active. Young polder soils, on the other hand, have very active organic matter which delivers all the well-known benefits. So it is important to ensure that there is enough young, active organic matter in the soil. But how much do you need? High levels of active organic matter boost water retention in the soil. This is useful in a dry summer, but after a wet winter the soil dries out more slowly, which means the farmer can’t get out onto his land until later in the season. What’s more, high levels of active organic matter in the soil increase the risk of nitrogen leaching. But too little is also not a good thing: this seems to cause production to suffer, as can be observed on the Vredepeel and Valthermond experimental farms in some years.

 >> Concern about falling levels
Farmers are convinced of the usefulness of organic matter and are worried that complying with the new phosphate standards from 2015 onwards will cause levels to drop and will impact on their yields. The level of the standards is not yet known but they are designed to achieve balanced fertilisation: in other words, not applying more phosphate in fertiliser than the plants can absorb so as to avoid an accumulation of phosphate. This limits the amount of animal manure and therefore the amount of organic matter a farmer can apply. So far, according to data from the soil analysis lab BLGG AgroXpertus, this has not yet resulted in a reduction in organic matter in Dutch soil on average. But this doesn’t mean that the content on individual farms is not declining. According to the farm system survey conducted at Vredepeel, Nagele and Westmaas, a decline in organic matter content was observed between 1996 and 2004. Farmers on the light soils in Noordoostpolder and bulb growers on sandy soils are finding that organic matter levels have dropped below the critical minimum, making the soil more difficult to work and apparently reducing yields. Farmers have therefore started to apply much more organic matter and are seeing positive effects. Sukkel and Ten Berge expect that farmers will still apply enough organic matter to maintain levels in the soil even with future standards. However, they will have to start using more green manures and compost and working their crop residues into the soil as much as possible instead of selling them. Sukkel: “We are seeing a growing trend whereby farmers try to convert their crop residues into cash, such as beet leaves for energy production. That’s a bad deal for the soil. What they should be doing is leaving high crop stubble and working their crop residues into the soil.”

>> How much do you need?
Measures such as these are costly. And then the question arises: how much do you need to apply to maintain good soil quality? Nobody knows exactly the right answer to that question. And nobody knows what happens if the farmer applies too little: in other words, whether production drops or not. Ten Berge: “In the ‘Soilless Cultivation of Outdoor Crops’ project, they have finally managed to achieve high yields with a lot of technology without using any organic matter at all.” He immediately adds: “But the soil is a buffer that can absorb a lot itself without all this control technology. You do need organic matter for that.” Both Sukkel and Ten Berge are convinced that organic matter is important and that levels in the soil should certainly not fall any further. And not only to prevent a potential drop in yields: at least as important is carbon storage in the soil as a climate measure. Globally there is about 3.5 times as much carbon in the soil as there is in the atmosphere. A reduction in the organic matter content of the soil will cause an increase in the amount of greenhouse gases in the atmosphere. What’s more, soil containing organic matter can store more water. This is important in terms of the changing pattern of rainfall that is predicted as a result of climate change. Sukkel therefore emphatically warns farmers: “Make sure you maintain the organic matter content of your soil at a good level, otherwise you are bound to have problems – maybe not now, but certainly in the future.

What organic matter in the soil delivers:
• Resistance to pests and diseases;
• Good soil structure, which in turn has a positive effect on aspects such as plant rooting, workability and bearing strength of the soil and creating a good seed bed;
• Less leaching of pesticides and nutrients;
• Supply and buffering of nutrients;
• Good infiltration and storage of water;
• Carbon storage, which helps mitigate climate change;
• Plentiful and varied soil life.

 Source: Wijand Sukkel and Hein ten Berge (Syscope / WUR)