A crumb cake is like a perfect soil, moist, but crumbly when you touched. Too wet and it is a mushy mess, too dry and it is hard as a rock.
Soil compaction is a serious issue for farmers who use machinery, but is real problem in the garden as well. Soil compaction usually occurs when soil is worked when it is too wet.
The structure of soil is composed of soil particles, water and air (or rather, gasses). When a soil is saturated it is holding as much water as it can. When a tractor tire or your foot compresses this soil, the soil particles press closer together and the pores holding the water are made smaller. This dense soil makes it harder for roots to grow and will impede growth. The technical term for this is "Bulk Density". Bulk Density is the amount of oven-dry soil in a standard volume. The higher the mass, the higher the density, and the less room for water, gasses and roots.
Soil pores are what allow water to infiltrate. The larger the pores, the more water will be allowed to enter the soil. These pores move water throughout the soil strata and make the water available to the plants you have growing. These same pores allow for the exchange of gasses in the root zone.
Here is how you avoid soil compaction:
1) Do not till or walk in your garden when it is wet.
Walking on wet soil is not only a good way to get your shoes caked with mud, but also it is the best way to compact your soil.
2) Add organic matter such as compost.
Compost has a low bulk density, there is a lot of pore space in its soil structure and therefore it can hold a lot of water. Amending heavy soils with compost is the best way to improve its density.
3) Mulch walkways
High traffic areas are the most susceptible to compaction. Mulching your garden with woodchips (not mulch) will greatly reduce the compaction happening below your feet. As you walk, the woodchips will absorb much of the pressure being exerted. Playgrounds put woodchips to absorb the pressure of a child falling onto the hard ground; mulched walkways work in the reverse of this.
4) Convert to using a tool like a broadfork to cultivate your soil rather than a rototiller
Rototillers make very nice seedbeds. They also chop up everything in their zone of impact. This includes worms and other life. Over time, a hardpan will occur below the maximum depth of the tiller. The tiller only reaches 8" maximum, right below that is soil that never gets tilled. After repeated tillage this soil becomes heavily compacted and forms a "plow pan." A broadfork is a wide tool with pointed tines that break up the soil rather than mixing it. This tool helps establish deep pores and improves the soil structure.
5) Drainage systems
In extreme cases of soil compaction, installing drainage systems may be the only option. Water infiltrates layer-by-layer in the soil spectrum. Which means it first fills the top layer until saturation then, starts to work down to the next lower layer, then the next lower layer. By trenching into the subsoil layer and adding a drain pipe, crushed stone or other drain, the water will move to that point after saturating the upper layer removing the long term puddles of a compacted soil.
And now you know!