SP FAQ - Slot micro-environment in dry soils
Yes, several. They ensure that the seed micro-environment can be controlled almost regardless of soil conditions, and they permit fertilizer to be placed in bands that are separated horizontally from the seed.
A cut-away of a typical Cross Slot slot in soil
In short, no! Seed-soil contact is often not enough under no-tillage and is why no-tillage often fails compared with tillage. On the other hand it is also why no-tillage, undertaken with Cross Slot openers can be more failsafe than tillage.
Soil that is not tilled has an atmosphere within its pore spaces of 100% relative humidity. Germination of seeds is greatly assisted by this humid soil atmosphere. In some cases the humidity alone is capable of germinating seeds without any soil-seed contact at all. But the relative humidity level needs to be at least 90% to do this.
Humidity entrapment in a Cross Slot slot
On the contrary! Seeds will take up water in both liquid and vapour (humidity) forms. Seed-soil contact is still important to maximize the liquid uptake, but the availability of vapour moisture in Cross Slot slots gives no-tilled soils an added resource to germinate seeds, which is why germination and emergence failures with Cross Slot openers are very rare events indeed.
Yes! It means exactly that. Provided there is a good mulch cover of surface residues it is not necessary to actually place the seed in damp soil. Close to it will be sufficient for the humidity to do the rest.
No! Relying on humidity alone (or even as the predominant mechanism) for germination will certainly delay germination a few days since uptake of vapour water by seeds is slower than uptake of liquid water. But it will occur nonetheless.
No! The tillage process aerates the soil so much that soil humidity escapes to the atmosphere and finds a new lower level that seldom approaches even 90% (let alone 100%) except when it rains. Seeds sown into tilled soils therefore mostly rely on absorbing liquid water from the soil and this is influenced greatly by soil-seed contact.
Further, tillage destroys the natural capillary channels in soil that facilitate upward movement of liquid water as the soil surface dries.
Because not all no-tillage openers (and the soil slots they create) are capable of harnessing the soil's humidity. Nor are many of them capable of even creating good soil-seed contact. Many no-tillage openers aim to disturb the soil in the slot zone on the assumption that disturbance is good. Although disturbance may assist seed-to-soil contact it breaks the capillary channels close to the seed and ensures that soil humidity escapes from the slot zone.
Although undisturbed soil, especially if it is covered with crop residue (which is the cornerstone of no-tillage) is always at 100% relative humidity, there is one disturbed zone in each otherwise undisturbed soil that is capable of loosing humidity. This is the slot zone created by the openers as they pass through the ground sowing seed and perhaps fertilizer. Therefore it is important that no-tillage openers create their slots in a manner that minimizes loss of humidity from the slot zone itself while at the same time ensuring the seed gets good contact with the soil.
The biggest issue is how the slot is closed. With horizontal slots (Cross Slot) two flaps of soil are raised by the wings of the opener as it travels along, creating two horizontal shelves, one on each side of a central disc. Seed and fertilizer are placed separately on each of these two shelves and the flaps of soil are then folded back over the seed and fertilizer. If the flaps of soil are also covered with crop residue this traps the humidity under the flaps.
With vertical slots there are no horizontal soil flaps to be folded back over the seed. The best that can be done is to try to squeeze the sidewalls back together so as to wedge the seed between these walls. But even then the best that will happen is that this will assist soil-seed contact and ensure water is taken up in its liquid form. Vapour water plays no part in germination in vertical no-tillage slots.
|V-shaped slots are often difficult to close, allowing water vapour to escape easily||It is easier to get some loose soil back into U-shaped slots but loose soil is a poor barrier against water vapour loss compared with surface residues. (see Q's 5 - 8 above).|
With slanted slots there may be a partial slanted soil flap created. If so this will trap some vapour water but it is not nearly as effective as horizontal slots in this respect.
Because it is has been the subject of numerous scientific studies that are reported in the international scientific literature that were subjected to peer-review by other international scientists over a period of 30 years. No-one has ever challenged these findings, which are also the subject of an international textbook on the interactions between soils, seeds and no-tillage openers. Indeed, other scientists have supported them.
Scientists tried this. Even although they found that the slot atmosphere did indeed remain at around 100% relative humidity, they also found fungal growth in the anaerobic atmosphere of the plastic-covered slots. From this they concluded that by ensuring soil gets covered with dead plant residues, nature has always provided a medium that retains moisture vapour but also lets the soil breath. Plastic cannot duplicate both functions. Cross Slot slots simply duplicate what nature has always done, so it is no surprise that seeds and plants respond favourably.
It is not an accident that most deciduous trees flower, set seed and then drop their seeds on the ground before they shed their leaves. The leaves then land on top of the seeds and create conditions similar to that created by Cross Slot no-tillage openers. All we are doing is duplicating nature except that we are choosing the seed type we want and putting it in rows.
With some angled disc openers dry soil can fall into the slot before the seed, which effectively embeds the seed in dry soil. This cannot happen with Cross Slot openers since the seed travels down the inside of the blades, which prevent other soil from falling into the slot until after the seed has been placed. Even if some dry soil did eventually fall into Cross Slot slots the high humidity still ensures that germination will take place.
With other openers the dry soil cushion prevents the seed from getting access to liquid-phase water.