Gardeners like to compete with each other over who has the worst soil. You wouldn't think we'd be proud of this, but what can I say, we're a strange bunch. One will argue for his hard clay, baked in the sun, another for her sand, without a trace of organic matter. I've got my own candidate for the worst soil ever - the stuff in the beds around my house.
Oh, texturally, it is among the best I've got - sandy loam, warms up nicely, isn't too wet like much of the rest of my soil. It had some nice enough foundation plantings, and I mostly ignored it for the first few years I was here. But a couple of years ago, I decided that I wanted to make use of this growing space, and then I discovered that my soil, was, well...dead.
By dead, I mean there wasn't a living thing in it. Not a beetle or a spider, and especially not an earthworm. It was weird. I knew that some previous owners of our house were umm... shiny green lawn people, and I don't know if that has something to do with it, but this stuff was "Its dead, Jim" dead.
So we embarked on a campaign of soil improvement. Any kind of soil improvement has two parts. First, there's getting your soil up to speed. In some cases, this might not be much - maybe some ashes from your stove or a little lime to even out some acidity, or maybe a little rock powder for trace minerals, or a light dressing of the rabbit poop your rabbits make sure you get anyway. If your soil is basically in good shape - you've had a soil test and you know that it is high in organic matter and sufficient in macro and micronutrients.
But what if it isn't? What if you've got dead soil, like mine, or rock hard clay, or soil (also like mine) that has been leached of nutrients? Again, there are two projects here - the first is the short term building of soil so that you can get to gardening. The second is the long term maintenence of soil health, and the addition of more organic matter, so that eventually, your soil can hold enough organic matter to save the world - or at least sequester a bit of carbon. Plus, things will grow better. Win-win.
My favorite way to build soil on something that is completely unworkable is the lasagna method, which is pretty much sheet mulching (covering the soil with newspaper or cardboard, then laying on as much organic material as you've got, with some dirt or compost on it. This makes raised beds, which is good if what you have is either wet or rock hard, or if you are, say putting your dirt on gravel or something toxic. It might be tough in a dry, hot climate though - raised beds dry out and warm up in the spring earlier, and keeping them wet might be tough. In that case, you might consider digging into the ground, creating sunken beds with the same mixture. We use leaves, grass clippings and farm bedding as the main ingredients in our lasagna beds, with a layer of find compost on top if I'm starting seeds in it.
If you need to amend soil, you'll have the choice of synthetic or natural soil amendments. Generally speaking, you'll want the natural ones. I'm not a complete organic purist - I think there are times when artificial fertilizer use is justified. But there's a price to be paid for its use, and care is needed - otherwise you can end up contaminating your water supply, wasting your money and depleting your soil overall. I don't generally use synthetic fertilizers, and if I were to use them, I'd use them only on untilled soil with plenty of organic matter added, in small and precise quantities. Most of the value of artificial fertilizers is presently lost into waterways and various other sites of contamination.
You can buy an organic fertilizer mix, or you can make your own. I generally use a mix of alfalfa meal, rock phosphate, and wood ashes, along with greensand and kelp, as well as occasionally special additions to deal with soil types or plant special needs. But I don't know about you, but I can't mine rock phosphate from my property, nor do I produce enough alfalfa to fertilize my garden. So this is not a long-term sustainable project. I use these amendments sparingly, where they are needed to bring soils up to basic fertility.
Then, we try to keep the fertility there. That means cover cropping a portion of our garden every year, integrating dynamic accumulator plants into our plantings (these are plants that bring up nutrients from the subsoil), undercropping with nitrogen fixers (these plants fix nitrogen from the air), mulching (we try to grow as much of our own mulch as possible in place - another good use for undercropping - a nice planting of buckwheat under tomatoes, or white clover under garlic can provide a living mulch and then the next planting cycle's mulching materials), and the heavy application of organic material - that is, compost and composted animal manures.
Every time we take something off of the soil, we are removing nutrients from our soil, and depleting, to some degree, the organic material available to them. High levels of organic material are essential for soil life and health - so faced with dead soil, the first thing I did was put my turkey poults in a chicken tractor on top of the border for a few days. The easiest way to move the poop to the garden beds is sometimes to move the poop makers there . Now since this was raw manure, I made sure there was plenty of bedding, and I wasn't planting food plants there right away. Had I needed to use it immediately, I would have switched to already composted manure, and gotten out the wheelbarrow.
Next, I planted the foundation plantings to annual alfalfa, since it was already summer, and warm. Different cover crops are specific to different seasons - they are spring, summer or fall sown. You sow the fall crops to overwinter - to hold soil in place, and add organic material. Winter rye, hairy vetch, fava beans (in some climates) are all common winter sown cover crops. Spring sown crops are generally cut down in summer, and either stay in place all season (things like red clover), providing multiple doses of fertility and green material, or they are cut down (oats, say) to provide organic material for the fall garden. Summer crops (buckwheat, annual alfalfa) can go in after the peas or the early lettuce, and grow fast and fill the space until fall. For a site you don't plan to get to for a year or two, perennial crops can do a lot to regenerate soil.
Cover cropping is very place specific - the best crops are specific to your climate, seasons and locality, so talk to your cooperative extension. They are a powerful tool for building fertility, adding organic matter and improving soil, and one that is worth getting to know.
My goal in the long term is for these beds to provide a warm, dry, moderately fertile site for mediterranean herbs and a few flowering perennials. That is, I wasn't trying to produce fertility for growing heavy feeders, like greens or corn. So after the alfalfa, I added some greensand and kelp, a light layer of compost, and planted into the mulch I'd already established. In went lavender, oregano, several marjorams and thymes, a rosemary that probably didn't survive the winter this year, and some plants that like or tolerate similar conditions of slightly dry soil, lots of sun and only moderate fertility - catmint, echinops and malva. And they've thrived.
Many perennial plants make wonderful fertility enhancers to annual gardens - whether perennial nitrogen fixing shrubs, whose leaf litter and root nodes enhance the trees and perennial plantings around them, comfrey and stinging nettle which can be cut for mulch or compost, small trees integrated into garden sites to provide leaf mulch, or perennial living mulches. This is one of those things that has potentially enormous long term yields, and has really only begun to be explored in a deep way.
The best soils for sequestering organic matter will be those that are in perennial plantings, that have constant inputs of organic matter - these include forests that are enriched yearly by leaf drops, permanent pastures which are manured by grazing animals (It has been calculated that Joel Salatin's grazed pastures sequester as much carbon as a similarly sized forest after decades of grazing), and perennial gardens that are carefully managed to provide their own needs.
I maintain fertility in the perennial planting I established in these beds by the occasional dumping of animal bedding on the ground, permanent mulch, wood ashes from our stove, and a strewing of kelp. I've also grown an annual crop of chamomile, a good dynamic accumulator, and left everything but the flowerheads in place. I give the whole thing an occasional boost of nitrogen by dumping dilute urine over it - urine is safe unless you have tularemia (in which case you have worse problems than not being able to fertilize with your pee) and diluted 1-7 (1-10 if you don't drink enough), it provides a real boost to plants. To be paranoid stop doing so 10 days before harvest.
More demanding annual feeders get composted chicken or goat manure, plant compost, weed and manure teas. Other plants might also get living mulches, and I rotate plants as wisely and carefully as I can, following the heavy feeders with nitrogen fixers or light feeders undersown with nitrogen fixing cover crops. My whole garden gets rotating quantities of worm casting to supplement the soil and improve its texture.
Meanwhile, in maintaining, we try to put back what we take off. Crop residues are left in place, either chopped down and incorporated into the permanent mulch or they are burned in our woodstove (for heavy, dense stalks) and returned as ash. Some of the nitrogen is returned in the form of urine. We mulch as much as possible with our own mulches - grass clippings, leaves and plants grown for compost or as mulch plants. We try not to steal too much from any one other place - but we gratefuly take things people discard, like leaves from yards when we venture into suburbia, or horse manure from our horse-keeping neighbors.
Animal manures have a very powerful role in gardening - in a perfect world, we'd compost all human manures until they were thoroughly pathogen free, and restore the soil with what we take off. But whether this is safe is debatable, and anyone who shares food will not want to risk a lawsuit. So composted animals manures are a powerful tool for maintaining fertility - one of the reasons that polycultures of animals and plants are generally more effective than either alone. We use composted human manures only on decorative and tree plantings.
Two particular ways of maintaining fertility deserve mention here - fungal soil support, by mycorrhizae (tiny fungus that colonize the soil) and biochar. Mycorrhizae have a symbiotic relationship with the roots of many plants, and can enhance the ability to plants to uptake nutrients and deal with water stress among other things. Many soils are fungi deficient, and an application of mycorrhizae can improve your plants ability to absorb the nutrients in your soil.
Biochar/Terra Preta is a fascinating subject - and one still uncertain. Terra Preta involves adding plant based charcoal (ie, not the briquets at the grocery store) to your soil. What this does is still a matter of speculation - it isn't clear, for example, whether the charcoal itself or the organic processes it enables are actually what creates the rich soil involved. Nor is it clear that all soils respond equally well to terra preta inputs - for example a study found that boreal forest soils did not seem to respond to biochar applications. That said, however, there have been some fascinating results - biochar supplemented soils seem to stimulate nitrogen fixing in legumes, for example, and while charcoal supplemented soils enable plants to take up more minerals, the soils deplete more slowly. This is a project still in the exploration stages, but one that home gardeners and small farmers can contribute to with experimentation.
We're not a closed circle by any means - we still take advantage, as long as they are available and we can afford them, of valuable amendments. But the idea is to lose as little as possible, while getting the best possible balance between improved soil, the health of the world, and a system in which you need to bring in a little less from offsite each year. This, it seems, is an entirely achievable goal.