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The soil must be crumbly

Not only bread needs a good crumb. Soil also needs to have the correct friability as this is crucial for optimal plant growth.

[Translate to English:] Jord med en god smuldreevne er afgørende for, om afgrøderne får en god start og giver et godt udbytte. Foto: Janne Hansen

In the spring when cereals and other crops show the first tentative signs of growth and the fields are gradually greening, you will be able to see patches that are either bare or have poor growth due to poor germination.

- When you look across the fields in Denmark, you can see that crops do not grow equally well everywhere. There is actually room for improvement in very many places, says senior scientist Lars J. Munkholm from Aarhus University. 

Just as with animals and people, early youth is very important for how you thrive the rest of your life. For crops, a good start is crucial for maintaining a high yield potential. 

- You can easily lose 25-50 per cent of the yield if the crops get a poor start. It is difficult to compensate for poor establishment later on, says Lars J. Munkholm. He studies the many facets of soil properties and can tell you that soil friability is very important for crop establishment and its success.  

- Soil with a good crumbliness is fundamental for crops getting off to a good start. It is fundamental for the establishment of crops and for achieving high yields and the full benefit of fertilisers and pesticides. It is one of the cornerstones of a good plant production, he says. 

The crumbliness – or friability – is such an important factor for the soil and crop establishment that Lars J. Munkholm has written a Doctor of Science thesis on the subject, which he defended on 17 May 2013 at AU Foulum. He described the concept of friability, how it is assessed and how soil properties and management factors affect it. 

- In short, the talk covered 14 years of research in half an hour, says Lars J Munkholm with a smile.  

Organic matter is important

Soil with an ideal friability will crumble of its own into suitable fragment sizes called aggregates. To maintain a good friability the soil has to contain sufficient organic matter, but on cultivated soils this is not a given thing. 

Particularly on arable farms where no animal manure or catch crops are used and where crops and straw are removed year after year, there is a large risk that the friability of the soil will become poor. 

- It can take a long time to restore a soil to good friability. This is why it is important to constantly maintain good soil conditions. It is a continuous process, explains Lars J. Munkholm and continues: 

- You cannot apply animal manure onto the soil in one year and expect that this will solve your problem. The soil needs something to work on all the time. The flow of organic material into the soil stimulates biological activity, which is important, he points out. Although it can take a long time to completely restore a good soil crumb, improvements to cultivation practice can lead to apparent changes after only a few years. So taking remedial steps is worthwhile, also in the shorter term. 

Poor soil friability leads to other problems

Poor soil friability will not only lead to a poorer substratum for crops. It will also create problems long before the seeds are put into the ground, since the soil becomes more difficult to work with and the farmer will have a shorter timeframe with optimal friability. The so-called water window becomes smaller if the soil has a low organic matter content and is very compacted. The water window is the period where soil water content is optimal for soil tillage. 

- If the soil is not very friable, it will quickly form a crust and be difficult to penetrate when it is dry, and will more readily become elastic – like modelling clay – when wet. The interval between “too wet” and “too dry” – the water window – becomes narrower, explains Lars J. Munkholm. 

The farmer may end up in a vicious circle where he drives on the soil when it is too wet because of the poor soil friability, which results in soil compaction. This will further narrow the window available for soil tillage and require a larger energy input for soil preparation when the soil dries. It may, for example require a pto-driven harrow for turning the soil instead of a tine harrow that just needs pulling. After soil loosening. soil with poor friability will normally have a poor structural stability. This means that it will easily collapse and is very sensitive to compaction. 

Quantification of friability

To assess soil friability you can kick at a sod of soil, crush a lump of soil in your hand, and you can feel what it is like to drive on the soil. These methods are successfully used by farmers, advisers and scientists, but the assessments are difficult to put a figure on – and numbers are needed if you are to provide clear answers to whether one cultivation practice is better than another. You also need to be able to quantify these aspects if the knowledge is to be used in computer-based decision-support systems.

You can measure on soil aggregates in the laboratory, but you cannot necessarily upscale these results to the field level where there are large variations and a number of other factors interact. Neither can you transfer the results to other field water contents.

One of the topics that Lars J. Munkholm has studied is the modelling of friability. He has taken measurements that link laboratory analyses and field measurements and has developed a model that incorporates both the variation in soil aggregate strength and soil water content.

With colleagues in Denmark and the United States he devised a formula that contains only four parameters. This is a big step towards a simple mathematical model that can be used to predict the range in soil water contents for optimal soil tillage and the energy required for the soil tillage.

- The model needs to be further tested on other soils before it can be implemented in IT-based decision-support systems. It is also relevant to find out whether soil water contents can be accurately enough predicted from weather forecasts, obviating the need to take actual soil measurements, says Lars J. Munkholm.

Another area that needs closer study is how much organic matter the soil needs to maintain good soil conditions. The optimal situation would be to be able to advise the farmer how much organic matter there should be in the soil and should be added within particular climatic zones. But this requires an extended research effort to be able to achieve this.

- In any case, the farmer should always ensure that the soil is regularly replenished with organic material, either in the form of animal manure, straw incorporation or the use of cover crops. He also has to avoid driving on the soil when it is too wet and use wide, low-pressure tyres to minimise soil compaction in the cultivation layer. This is simply of the utmost importance, says Lars J. Munkholm.

Further information:

Senior scientist Lars J. Munkholm, Department of Agroecology, e-mail: lars.munkholm@agrsci.dk, telephone: +45 8715 7727, mobile: +45 2515 2716