Soil crusting is an insidious problem that is nearly universal. The only soils I've seen that seem really immune are basically sand. Below are my written and video takes on the subject with some possible innovative solutions using biochar and manure mats.
One of the first subjects I would try to explain to any beginning gardener is the evils of soil crusting. Ideal soil for growing most things has a structure or openness to it. There is space incorporated into the soil which allows the infusion of air. Since the pore spaces make the soil more friable, roots and organisms can make their way through it more easily. The structure of soil is created over time as creatures move through it, roots penetrate it and then eventually die, and worms wend their way about leaving their neatly formed earth filled droppings behind. When disturbed very much by digging and pulverizing (especially if too dry or too wet), and left exposed to the open environment without the covering of living or dead vegetation found in most natural environments, most soils form more or less of a crust. The crust is made of tightly packed small mineral particles which are no longer formed into the structures that make up good aerated soil. An extreme example would be to take some soil mix it in a blender with water, and then pour it out and form a sort of slurry that would dry to a packed smooth surface. That may be extreme, but many of our garden practices, some avoidable and some less so, can do nearly the same thing.
So, what’s the problem? it’s still soil right. Sort of, but is it functional soil? We have goals in gardening. We want plants to grow well for the amount of work we put in. Soil crusting can inhibit our gardening goals.
Slow entry but a ready exit. A soil crust resists the penetration of water. This effect will certainly vary with the soil, but it is more or less true across the board. Watering badly crusted soil by hand is frustrating. The water pools instead of sinking in and you just have to wait for it to soak down, add a little more and wait again, etc. In extreme cases the lower penetration leads to waste of water because it runs off instead of soaking in. The water that does soak in may not make it as deep as it could under better circumstances. If the water does not penetrate easily, then it does not penetrate evenly either and you may be getting less water down to the root zone, or none at all depending on how long you water for. Often only the top of the soil will be wetted, even with what seems like should be a reasonable amount of watering and water. Soil crusts increase the chances of ending up with a layer of wet soil on top and dry soil beneath it, which is a common beginner gardening mistake. Once that happens, it’s difficult to water enough to resaturate the bed without putting a sprinkler on if for a long period of time.
So, the water doesn’t penetrate, but to make it worse, it leaves the soil faster as well! Water travels easily through compacted soil. If the soil is broken up, the water can’t travel from particle to particle as easily and evaporation from the soil surface is minimized. The closely packed soil particles however are like a wick that speeds water to the surface by something like capillary action and back out into the atmosphere. Long standing farming wisdom says that if left uncovered, soil should be broken up to form a pulverized mulch to prevent this effect and keep the water in the soil. I have seen the practice of cultivation as a form of water conservation called into question before, but long tradition and my own observation seem to support the idea that compacted soil, and particularly surface crusting, speed the loss of water from the soil.
But wait, there’s more bad stuff! There is often more talk about getting air out of the soil when planting plants than getting air into the soil, but it should probably be the other way around. Plants may not appreciate large air pockets that their roots encounter, but very few plants are well adapted to survive or thrive in completely air free soil. In fact, ideal soil for most of the stuff we grow has a good bit of pore space and a ready exchange of air and gasses. Ever stepped on a garden bed? Your foot should sink a good divet into the bed as it crushes the soil structure and closes up the air spaces in the soil. Though I have never formally tested the proposition, it seems to me that plants grow much better if the soil surface is kept very open. Soil “breathes”, or at least it should, in order to keep the gajillions of living things in healthy soil thriving. Soil should be like a sponge containing a portion of air rather than like a uniform adobe brick. In fact if we are making adobe bricks or pottery, it is essential to thoroughly destroy any structure forming a homogenous mix where the clay particles are smeared over every grain of sand locking them together like glue. Digging and cultivation can ruin the structure of soil causing the air spaces to collapse. Generally the worst times to dig soil are when it is very wet, or very dry and powdery. One makes mud, the other makes powder that can turn into mud when the soil is eventually watered or rained on. Either one can lead to soil crusting. The spectrum in between when the soil is moist, but not too wet or dry is when you should do your digging and cultivating if possible.
Okay, so there it is. Soil crusting makes it harder to get water into the soil but the water evaporates more readily. Soil crusting also inhibits penetration of the air essential to keep the life of the soil buzzing along. So, what do we do about it? Here are the options.
Cover the soil
Any covering can help a little bit, no matter how small, like even a thin layer of coffee grounds. It could also be a heavy mulch, or a covering of live plants. Mulches and coverings basically prevent crusting by breaking the fall of water droplets so that they don’t hammer and pulverize the soil structure. They also slow the flow of water across the bed giving it time to sink in and encourage insect activity that can loosen the soil surface. If cultivation is not practiced, or at least minimized as much as possible, mulches will add organic matter to the top couple of inches of the soil where it does the most good at preventing soil crusting. I use coarsely sifted compost, but also coffee grounds, ashes, nut husks, pea pods, onion skins and various small stuff like that. Unless buying it in, it is difficult to find enough compost, so I don’t usually have as much as I’d like. I don’t like thick mulches of leaves and straw and such in my garden though because I just end up with too many bugs and rodents. Thick mulches work better for protecting the soil and preventing moisture loss though and they seem to work for some people.
Manure mats work extremely well to prevent soil crusting and conserve moisture. More on those soon, but my article from a few years back covers that subject pretty well.
Live plants can be used to cover the soil too and it really does help to have a full canopy of leaves to break the fall of the water. The BioIntensive method of gardening seems to rely primarily on plant cover to cover the soil and go out of their way to produce large healthy seedlings that will cover the bed quickly. They use close spacings for the same reason. I find it very hard to organize well enough to grow stuff just right to not have open soil for very long. Also, I don't like planting everything that close. Some stuff does better with open spacings and air circulation. It is good though when it all works out.
Breaking up the soil surface by cultivation has to be carried out regularly to keep the soil developing a thick crust. If best results are to be had, it is preferably done soon after every significant rain or watering, though it certainly doesn't have to be under many circumstances. Breaking the soil up into small chunks or even dust slows evaporation and allows water to penetrate during the next watering. Of course pulverizing the soil can make it crust easier the next time, so you have to keep it up. It sort of causes or exacerbates the problem it attempts to solve. A compromise I use a lot is to drag a hula hoe (aka strap hoe, or stirrup hoe) under the soil surface to crack it up but not pulverize it. That leaves any surface covering I do have somewhat intact. You can only do this maneuver so many times before it all gets mixed up though.
Increase organic matter
With high enough organic matter you soil becomes like potting soil and will either not crust up, or do so more slowly or less drastically. However, although any amount may help, we are talking about a crap ton of organic matter to actually prevent the problem. It’s a lot of work to gather, compost, sift and dig it all in and it rots away so you have to keep adding more. I’ve rarely seen this effect really work and it pretty much requires that you buy in, or somehow gather material to make, huge amounts of compost. Some raised bed gardeners will buy compost to fill the raised beds, but on any serious scale it's out of the question.
Improve soil structure by digging
Good luck. It doesn’t just happen in my experience. Digging and gardening have more often destroyed soil structure than built it for me. Minimal or no digging seems to let the soil slowly form it’s own structure down there, while digging very much can wreck it. There may be a sweet spot, between digging in organic matter and aerating the soil and over-digging and destroying the structure, but my experience is that it’s easier to mess it up than create it. If you cultivate regularly, then you are forming an artificial pore space, until it collapses and you have to cultivate again. Even if you manage to the lower soil structure, you still have the same crusting problems up top.
Improve soil structure by changing the make up of the soil
Adding sand or porous materials may help a heavy soil, but every light soil I’ve gardened in crusts as well, sometimes more quickly or worse than heavy soils. If you add too much sand, then you are basically gardening in sand that can’t crust up because it contains so few fine components, then you will have all manner of new problems like poor water and nutrient retention. I think using charcoal holds a lot of potential. It seems to make the clay less effective at sticking stuff together. A bed in which I’ve put 50% charcoal in the top layer seems to have little or no problem with crusting so far. 50% is a ton of charcoal to be sure. Since charcoal doesn't rot though, it is a one time application and may be feasible in some scenarios. Think of all the prunings and pallets and scrap wood that are ground up and composted every year, or simply burned to ash to get rid of them. If much of that was diverted into charcoal production, it would be a lot of stuff. Now imagine we had been doing that since the 1950s. Eventually if it becomes more available and cheaper, buying a dump truck load of charcoal may be a reasonable investment for a small permanent garden. Now people buy in many yards at a time of compost to build back yard beds but it just rots away steadily. The value of a very fertile, water efficient, low maintenance bed that always has loose soil is very high and might easily pay for itself over time with increased production and labor savings. There are other consideration like whether charcoal plays well with your soil or crops and what amount is beneficial or even detrimental, but it's worth experimenting with, especially for people with a few high intensity raised beds, which are often filled with compost anyway. For now, I'm just interested in whether it works and feasibility under various circumstances can be assessed later.
Soil crusting should get more play. I see it as a major gardening problem to solve, which either requires the use of energy and materials to constantly correct, or wastes water and results in suboptimal plant growth. In many cases, it will always be a problem and just has to be dealt with as it happens. A farm can't cover the entire soil surface with anything, or use enormous amounts of charcoal. Those things are just not practical on that scale. Gardeners have more options though and backyard gardeners with a couple of raised beds have even more. As always, context is king. Manure Mats (aka shit mulching) works very well, and charcoal may have some special properties useful for solving this problem, aside from the fact that it acts very similarly to coarse organic matter to create a sort of artificial pore space all the way to the soil surface if enough is used.
When you walk around, think of the ground as a sponge infused with air and exhaling gasses rather than as a line where the air ends and something solid begins. If the soil were viewed as an organism, a thick soil crust is like making it breathe through cotton batting or something like that. I don't assume that more air is better. I'm not really sure. Soils and plants are very adaptable and soils contain a lot of anaerobic organisms as well as aerobic ones. And granted, the exchange of air and gasses between the soil and atmosphere is minimal compared to the open air or the respiration of organisms such as ourselves. I think it is important though and farming wisdom and my personal experience seem to indicate that keeping the soil surface open and encouraging a soil structure with pore space are beneficial to soil health and crop growth.