Eight myths surrounding organic matter 

Organic matter is a crucial element for a well-functioning soil and therefore for good crop growth as well as a lively farm ditch. The crucial role of organic matter is not discussed in the entire field of work by agronomists, soil scientists, hydrologists and climatologists.

Organic matter plays a role in the availability of nutrients and water, the fixation of carbon in the soil, in the formation of structure and in the soil's resistance to diseases and pests. For some reason, however, the idea has taken hold among all kinds of researchers, advisers and farmers themselves that we have a problem with organic matter in the Netherlands.

We are allowed to supply less via animal manure and compost and we experience problems with soil structure and soil diseases. In practice, this even leads to bold statements such as 'soil fertility is declining', often referring with a reproachful look to intensive farming and the use of fertilizers and low-emission application techniques.

Even in the House of Representatives remarks sound like 'the current manure policy is impoverishing our agricultural land and must be stopped'. From this angle, a strong plea is then made for the supply of more organic matter. In order to increase the content in the soil. Because more is always better, right? No, that's not it.

More organic matter in the soil is not always better. And for most agricultural soils in our country it is not necessary at all to increase it. Below is a brief overview of the most well-known misconceptions about organic matter for the Dutch situation. 

Myth 1. More organic matter increases crop yield 
In 2017, a report was published that based on literature data indicated that a slight increase in organic matter can lead to significantly higher yields. It led to a lot of attention in the trade journals. For beets and potatoes (our most important crops), a yield increase of 10% could be possible. The fact that this conclusion was derived from foreign experiments on super-poor soils with very little organic matter - soils that do not occur in the Netherlands - was irrelevant for the moment. The tone was set.

However, a recent worldwide meta-analysis by Oldfield et al. (2019) shows that this effect is not visible in most long-term trials and only occurs if the amount of carbon in the soil is lower than 1,5 to 2%. Above 3 to 4% organic matter (OS), increasing the OS content therefore has no added value. In the Netherlands, 75% of all arable land has an OS content higher than 3,2% (NMI database).

For grasslands, the OS content is even substantially higher. Long-term trials with grass and maize in Gortel, where huge amounts of farmyard manure (up to 10 tons/ha) have been applied for more than 200 years (Prins & Brak, 1977), show that the effect of the (extra) organic matter is especially visible in a higher N-delivery. So it is the extra nitrogen that ensures a possible increase in crop yield and not so much the organic matter (Heijbeek, 2017).

An analysis of long-term trial data in Germany by Körschens et al. (2005) shows that an organic matter content of 1,5 to 2% is sufficient for an optimal crop yield. An annual application of 20 to 35 tons of slurry (or comparable manure) proved to be sufficient to maintain soil life. Similar results can be seen in the SANS project 1993-1995, where the N-supplying capacity of fields decreased drastically when the supply of animal manure was stopped (Ros, 2019). So the conclusion here is: it is not the amount of organic matter that is important for crop yield and soil quality, but the supply of nitrogen from the soil and freshly applied organic matter. 

Myth 2. More organic matter is good for the climate 
More organic matter in the soil also means less CO2 in the atmosphere. Both from the point of view of adaptation and mitigation there are therefore all kinds of wishes to increase the OS content of the soil. Compared to burning carbon, that's correct. In reality, a higher OS content in the soil also automatically means a higher CO2 production from the same soil.

Increasing the OS content in the soil therefore increasingly requires larger amounts of fresh organic matter and thus also increases the risks of nutrient leaching, nitrous oxide formation and the spread of contaminants and pathogens (TCB, 2016). From the perspective of carbon sequestration for climate, a strategy aimed at preserving already sequestered organically bound carbon seems more feasible.

A well-thought-out construction plan, with which 50 to 60% of the decomposition of development plants can be compensated, is more relevant than the use of all kinds of 'new' organic residual flows. Recent discussions in scientific publications show that the carbon sequestration potential is related to the amount of nitrogen in the soil (de Vries et al., 2019). The greatest effect of C storage in the soil can therefore be achieved in soils that are relatively poor in nitrogen and phosphate, a situation that is rare in the Netherlands. 

Myth 3. More organic matter holds a lot of water 
Organic matter is believed to have an impressive effect on the water storage of a plot. With an increase of 1% organic matter, a sandy soil can store up to 6,8 mm more water. For a clay soil this can amount to almost 10 mm more water. In practice, this means that watering can be postponed for a few weeks in the event of drought.

The underlying test data between available moisture and the OS content are almost always collected at different locations, making it impossible to establish that the organic matter content is the cause of the changes in available moisture (Groenendijk et al., 2017). This is a classic example where the concept of correlation is confused with causation.

However, a recent meta-analysis by Minasny & Mcbratney (2018) based on more than 50.000 soil analyzes shows that the amount of plant-available water increases by a maximum of 0,6 mm (per 10 cm) with an increase of 1% organic matter. And this effect is especially noticeable on sandy soils with an OS content of less than 2%. The maximum amount of additional water storage increases by a maximum of 4 mm. In practice, this effect often turns out to be smaller (Groenendijk et al., 2017). The added value of organic matter for water availability and water storage on Dutch agricultural plots is therefore minimal. 

Myth 4. More organic matter makes soils less prone to drifting 
The light soils in the Netherlands are generally sensitive to spraying when their surface is uncovered and the weather conditions are favourable. This mainly occurs in the Veenkoloniën and the bulb region. The supply of fresh organic material helps to reduce the risk of spraying.

This observation is correct, although this is not primarily related to the level of OS content. The rare experimental data available on this subject (Spek, 1950) show that the OS content has a very small effect. And especially if less than 2,5% organic matter is present. Again, most parcels within the Netherlands do not meet this criterion. 

Myth 5. More organic matter greatly increases infiltration 
The effect of a higher OS content is that due to a more intensive soil life the infiltration rate increases, so that heavy rain outside can be better buffered in the soil, the risk of runoff is reduced and rooting is stimulated. The underlying assumption here is that the amount of fresh organic matter limits the activity of the soil life.

Given the current fertilization practice in the Netherlands, I dare to doubt this assumption. Test data from the literature also show that this effect is especially visible on sandy plots with less than 2% organic matter. And most plots in the Netherlands do not yet meet this criterion. 

Myth 6. More manure or processed clippings are needed to compensate for decomposition 
The decomposition rate of organic matter in the soil varies between 1 to 5% per year and depends on the amount and quality of the organic matter, the type of soil, the drainage condition of the plot and the pH. Intensive land use can lead to increased decomposition of organic matter, because too little is supplied, the supplied organic matter is easily degradable and frequent soil tillage stimulates the decomposition.

Despite this, the organic matter content remains stable in almost all Dutch agricultural plots (Reijneveld, 2009; Tol-Leenders, 2019). Not because so much manure is supplied now (because the fertilizer doses have only decreased since the 80s), but because good soil management and a well-thought-out crop plan are used by farmers to achieve good soil quality.

The key to maintaining the OS content therefore does not primarily lie in the supply of compost, bokashi, roadside clippings or manure, but in soil management and crop rotation. Of course, the quality of the fertilizer supplied does play a role: the supply of stable organic matter is much greater via compost than via bokashi or fresh organic material. 

Myth 7. More organic matter retains nutrients in the soil
Compared with fertilizers, the frequent use of organic fertilizers increases the organic matter content and the N-mineralization and denitrification capacity of the soil. The quality of organic matter plays a major role in this. For example, micronutrients such as iron, zinc and copper can adsorb to the slightly negatively charged surface of organic matter.

As a result, they do not wash out and remain available for crop growth. While organic matter can buffer and complex all kinds of cations and metals, it is also an important source of nitrogen. In the peat meadow area, for example, the N supply from mineralization of organic matter can rise to 200 to 400 kg N per hectare (Ros & Van Eekeren, 2016). An increase in the OS content therefore also increases the risk of nitrate leaching.

This relationship is not very clear and is influenced by all kinds of factors. The interaction between the type of fertilizer, the level of N application, the N-supplying capacity of the soil, crop uptake during the season, soil type and weather ultimately determines how much nitrogen is lost through leaching of nitrate and through denitrification. The common thread is: the more organic matter, the higher the risk of additional N leaching (CDM, 2017). 

Myth 8. Increasing Organic Matter Is Simple 
It is not easy to increase the organic matter content in the soil. In the first place, there is already a large amount of organic matter in the soil and an increase of 1% requires an enormous amount of organic fertilizer (or compost or clippings) to be supplied. That is impossible under current law. Rightly so, because the organic products also produce enormous amounts of nitrogen and phosphate.

In general, the more stable the organic matter is in the supplied product, the more it contributes to the content in the soil. Compost and old farmyard manure contain more effective organic matter per ton than roadside clippings, slurry or fresh crop residues. Increasing the organic matter content (in small steps) is therefore better with compost and old farmyard manure than with crop residues. To increase the organic matter content by 1% you have to take into account a period of five to 10 years for most building plans, if not longer. The fastest way to build up a lot of organic matter is to grow grass. 

But still... 
And yet I think it's fantastic that so much attention has been paid to organic matter in recent years. Because we're looking at the bottom again. Because we are looking for solutions for sustainable soil management. Because the answer to many complex environmental challenges lies in the soil. 

Gerard Rose
Researcher / data scientist at Nutrient Management Institute 

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