I read that tilling wet soil can destroy soil structure and make it more difficult for seeds to grow roots through the soil.
How exactly do roots find their way through the soil? Can artificial soil structure be created for roots to grow more easily?
I think you're conflating two things - tilling wet soil and jumping straight to 'artificial' soil to create particular conditions for seeds to grow in,but the one doesn't necessarily lead to the other. The statement that tilling wet soil destroys soil structure is accurate, but tilling destroys soil structure whenever it is done, its just worse for seeds and plants if you do it when its very wet.
First, why you shouldn't till wet soil; the reason you don't is because, if the soil is very wet, when you cultivate it, you knock out the air spaces between the soil particles, and this can cause solid mud, particularly if your soil is clay based or heavy. Using hand tools such as spades or forks is slightly better than machine tilling in such conditions, but even hand cultivation is best done when the soil is not very wet. One of the purposes of tilling/digging is to increase aeration and reduce compaction,but tilling wet soil has the opposite effect,so its best to wait until the soil is merely damp, then till. Tilling very dry soil isn't great either, it tends to increase the amount of dust, but your question is about wet soil.
In terms of sowing seeds, they grow best if the soil is friable, which means reasonably loose with a relatively fine, crumb like structure - these are ideal conditions for seeds to germinate and put down roots easily. If the soil conditions aren't great because of excessive moisture, its possible to sow the seeds into seed and cutting potting compost bought from a supplier (I'm assuming this is what you mean by 'artificial' soil), prick them out into pots when they have two sets of true leaves, then plant out into the ground at a later date, when the soil is in a more friable condition.
Further reading about tilling soil generally here http://homeguides.sfgate.com/rototilling-wet-soil-95825.html
In regard to roots finding their way through soil, gravity (geotropism, also known as gravitropism, mentioned by someone else) does play a large part in their going downwards; roots push their way through soil primarily for reasons of stability, and to seek out moisture and nutrients. Obviously, if the soil is solid, compacted and just muddy, root penetration and spread will be compromised or even impossible. A lighter, damp and friable soil with many small air spaces makes it easier for roots to grow and thrive, and allows more bio diversity within the soil, making it a healthier place for plants to be.
Roots show positive gravitropism meaning that they grow towards the direction of gravitational pull. Vigorous plants like trees need to anchor themselves in compact soil, while small and delicate annual flowers prefer lighter soil.
When tilling, the capillarity of the soil is interrupted and this means that water will evaporate more slowly. If the soil is wet, it will stick to the metal parts of the equipment, if the soil is too dry it will be difficult to break it. That being said, every soil texture has a moment for optimal tillage.
Seeds of most food crops (beans, peppers, wheat etc.) need a proper germinative bed after tillage. This means that the upper layer of the soil should look like small fine bits to make sure the seeds get their water and that they can sprout easily. Of course, your seeds might be an exception, bu you'll have to tell us what seeds do you have to assess that.
As addition to Alina answer:
Wet soil it is easier to compact, and it carries less weight (think about tilling machines). So tilling will move up deeper soil (less rich), and it will compact the soil, which it is very bad.
Freezing or rotary tilling machines could help to have finer grains of soil, as needed to seed. But this problem is not specific of wet tilling, but in general to tilling.
There are three types of soil; clay, sand and silt. All soils are composed of these sized ROCKS and organic matter; Clay is FLAT, TINY TINY Tiniest. Sand is the largest and silt is somewhere in between, sometimes called loam. All soils are JUST FINE if one knows how to manage their soil.
Clay has a few attributes to understand. The flat surfaces are highly electromagnetic. Especially with H20 involved. The water is STUCK to the particles of clay to the extent plant roots have to work to use to get it to cross the epidermis of the root's outer surface cells. The particles are GLUED to each other by electromagnetism. Where do you think we get porcelain clay for ceramics...concrete!
Think about how concrete is made. The ingredients are; clay, gravel, water, gypsum and lime...sometimes differing amounts but these ingredients are put in a tumbler and KEPT tumbling (akin to tilling) to enhance the properties of the clay, the electromagnetism of the flat particles, the rest helps with air, consistency of the concrete. Makes concrete. Tilling wet soil WITH clay involved will make concrete.
All soils, like I said are GOOD SOILS. Management or improvement of any soil is the addition of only ONLY one thing...DECOMPOSED ORGANIC MATTER. Once the beds have been double dug using a spade and a few calories (during which I am throwing willy nilly this decomposed organic matter, or compost or mulch into the soil as I dig). At no time do I worry about evenly mixing. At no time do I worry about subsoils or the lack of organics within the topsoils. I will also add fertilizer depending on my soil tests. If I have to use commercial bagged compost I will increase the nitrogen as that stuff has lots of non decomposed chunks that need to be decomposed and the decomposers need nitrogen to do their work.
After creating my RAISED beds (no sides, just compacted down with plywood and jumping up and down on the soil...sometimes after double digging these beds are up to 3' in height, then I use a large heavy piece of plywood to get rid of large air pockets reducing the pile to a foot in height) and I dig trenches along the bottoms of my beds for the water to flow where I want it. Then I add 2 inches of compost for each bed (3' wide minimum) for plants already started. I hold back when seeding (broadcast form not single rows) until the plants are up a good foot. I don't use compost for fertilizer. That compost will be eaten by the micro and macro organisms in the soil. They go back down in the soil, poop all that great organic matter mixing it into the soil for you AND because there is DECOMPOSED organic matter available for their energy needs, these organisms start multiplying to make even better soil. The more life a soil has THE BETTER THAT SOIL IS.
Critical when making new beds for plants is a SOIL TEST. To not get a soil test is just lazy. This initial bed making is the only time soils need any manipulation. If one has sand or sandy silt loam or lower clay content and the soil is dry, a rototiller is fantastic. ONE TIME. Make those beds and have walkways for humans so they stay off of the beds with their weight, this is a one time workout. Continue to put DECOMPOSED organic matter on top of the soil and/or use a green cover crop for the winter...that will be all that is necessary to keep the tilth and correct amount of decomposed organic matter and the micro and macro organisms replenished for other crops.
Plants make their own food and/or produce flowers/fruit. They do not NEED FOOD. I hate these advertisements on fertilizers and soil that say they have food for the plants. This is why I won't use the word NUTRIENT as humans assume NUTRIENTS are FOOD. Fertilizer is NOT FOOD. We humans have to provide the chemicals necessary for the plants to provide their OWN FOOD. All gardens and landscapes by man are ARTIFICIAL. This takes a bit more knowledge to be successful than just plunking a plant or seed into the ground hoping for success.
Root apical cells go towards WATER. Gravity is NOT PART of the root's growth. Gravity does work on the water not to mention capillary action. If one wants their plants to be drought tolerant, they water deeply and allow that soil to dry out down 2" BEFORE watering again. Shallow rooted plants need frequent shallow watering, such as succulents and cactus.
Unless you have water plants that normally grow well in mud, marsh, stagnant water, soil that holds water like clay should definitely be raised to enhance drainage. Roots we need to worry about are within the top 4 to 6 inches of soil. Below that the air is greatly diminished and plants need air. Most (something like 95% of all) plant roots are found within this narrow zone.
Artificial soil is CRITICAL to use in artificial conditions such as pots. Freeze and frost have NOTHING TO DO WITH SOIL creation...well nothing at our human scale. What soil you have in the garden CAN NOT BE CHANGED SHOULD BE CHANGED. The ONLY way to improve ANY garden soil is by the addition of DECOMPOSED ORGANIC MATTER. You most certainly can use NON DECOMPOSED ORGANIC MATTER but like anything that was once alive and now dead needs to be dealt with by decomposing micro organisms (which btw don't operate in freezing conditions very well) and these organisms need nitrogen like crazy. Decomposed organic material is the ONLY THING that micro (bacteria, nematodes...) and macro (centipedes, earthworms) soil organisms are able to use for energy and reproduction. When large amounts of non decomposed organic matter are in the mix, large amounts of nitrogen are necessary. The decomposers will use the nitrogen up before any plant can use it.
That is my big beef with bark chips or non decomposed organic matter normally meant only for human aesthetics. That stuff does not FEED the soil! The soil includes everything to include the organisms. The finer the bark the quicker it can be decomposed. The sooner the soil organisms are able to use it, the more the population of soil organisms increases, the better the uptake for the plants as soil organisms work symbiotically with the plants.
For those of you who believe we are in global warming because of CO2 (sigh) you should know that one of the most HUMONGOUS of CO2 releases is from TILLING THE SOIL. Seriously.
If you have raised beds, trenches for excess water, plenty of decomposed organic matter being added to your beds, the proper amounts of fertilizer WHEN necessary, re-digging your trenches throwing the excess dirt on top of your mulch and beds, know at least one real soil test FOLLOWED by at least one more soil test a year later, have promoted certain beds for an acidic pH the others for neutral, rotate every single year (exception would be perennials like asparagus and strawberries)...purchase NON GMO certified seed and starts, your garden should be just fine.
Please, please please ask questions or hold me accountable. Soil is the basis for all plants. Hydroponics should only be attempted by those who can prove proficiency the NORMAL WAY in soil, first.
