Why do plants root deeply? – A question of optimum seedbed preparation?

Average reading time: 4 minutes

While driving the cultivator my superior asked me: “Why is the seedbed actually called seedbed?” I thought for a moment, and I realised that you could illustrate it with your own bed. The fine bed sheet on top, then the soft mattress – some like it harder – and then the slatted frame. It also becomes apparent that only the optimum coordination of all three components makes a good bed. But how can this be transferred to farming? More on that in a moment.

But first of all, we want to answer this question:

”Why do plants, or rather the roots, grow downwards?”

And above all: Why do they grow up to 120 metres downwards (longest root ever measured of a fig tree in South Africa)? There are different reasons:

First of all, deep down in the soil it finds what sometimes is essential for an optimum plant growth: water, oxygen and nutrients. The fact that roots can actively direct their growth in this direction is used for example in underground fertilisation and is also shown in the example of the fig tree. This increases the chance of water reserves in deeper layers, especially on dry sites. This is why the plant takes root in these depths.
Temperature and light also influence the growth of the roots. Maize, for example, roots more deeply if temperatures are higher.
Evolutionary, the roots of most plants instinctively grow from a soft horizon to a harder one.
The so-called gravitropism influences the factors the plant needs for growth and thus has a direct influence on the depth development of the roots.
The thickness of the roots sometimes depends on the time of sowing: according to the motto “as early as possible, as late as necessary” you can make optimum use of the vegetation period when sowing early – also with regard to the development of the root system. Concerning the root formation, the seemingly massive, above-ground leaf mass often gives a false impression. Therefore, a look into the soil is crucial. (?)
The architecture, too, is influenced by fertilisation. This is shown by the study of the “IST Austria” in which the cell behaviour of Arabidopsis was analysed in relation to the available nitrogen. Basically, there are two types of nitrogen that can be taken up by plants: ammonium and nitrate. If we provide plants, e.g. Arabidopsis, with nitrogen in the form of ammonium, the roots starts to grow in length due to cell extension. The reason is that many plants have the urge to take up nitrate rather than ammonium. If we now move the same plant to a site that is rich in nitrate, it starts cell division in the roots so that they form branches – for it has achieved what it has been striving for. Thus, farmers can also control root growth by fertilisation.

But now back to the seedbed. Let’s start with the bed sheet.

Structure of the seedbed

Bed sheet

The bed sheet stands for the area of seed placement. The objective is to create optimum conditions to push germination. As the radicles are very thin and relatively sensitive, the seedbed should allow for a good rooting. This guarantees that water is absorbed as quickly as possible by the radicles. Plants can almost only take up anions and cations (usually fertiliser) in dissolved form. The faster the root can absorb water, the faster it takes up nutrients. Therefore, the seed-soil contact and, thus, the capillary connection to the water supply and the ensuring of the capillary transport play an essential role and also mark the transition to the second layer, the mattress.

Mattress

The mattress is the A horizon of the soil. In agriculturally used soils you often call it the cultivation horizon. In this area, the roots are less sensitive than at the beginning. They take up a large part of the nutrients and root quite intensively, thus providing the plants with a strong hold. This layer at a depth of approx. 5 to 30 cm may be more compact. Disturbing layers with too loose soil cause a delayed growth of the plant and have a negative effect on the development, as does damaging compaction. They cause growth delays as well as malformations of the root system and they can cause stagnant water and thus oxygen shortage. This is the reason why the objective should be a sound balance.

Slatted frame

Now let’s go one layer deeper into the B horizon, also called illuvial horizon. It forms the slatted frame that virtually is the basis for anything above it. This is where we will find nutrients washed in from the A horizon which replenish there and can be absorbed by the plants depending on the thickness of the root and also the water that can be found in deeper layers.

What does the optimum seedbed have to look like?

In practice, the seedbed is usually prepared in such a way that no wet clods are taken to the surface, that the crumbling structure is in line with the respective crop (a little bit finer for sugar beet than for maize and soya) and that water is easily accessible for the seed.

At this point, we could discuss endlessly how large the clods may be, how much fine earth is required and how deep you can, should or may go with different machines. But if we are honest, this decision mainly depends on the respective site and soil (as well as on other influences).

Summary with regard to the structure of a seedbed

Water should quickly be made accessible to and be taken up by the roots.
The density of the seedbed should increase constantly from top to bottom to achieve an optimum deep rooting.
However, damaging compaction as well as over-loose horizons should be avoided.

Speaking of soil compaction: in our blog series Soil conservation vs. heavy machines (https://www.horsch.com/en/detail/i-soil-conservation-vs-heavy-machines) you will get practical tips to minimise soil pressure and why it pays off to pay attention to it.