Restoring soil from bone dry to perfectly moist, isn't difficult, but, depending on how far gone it is – it may take a bit of time to awaken all those processes. Clay particles need to swell, salts dissolve and water pathways, reopen.
The easiest way to re-wet dried out soil - in my experience - is just dumping it all into a bucket and pouring water and then mixing it up until the soil re-wets. If you pour too much water, just leave the bucket for a few days and the soil will dry out to a more normal moisture level.
If the soil is hard because its dry and compacted, I would moisten the soil. Water it, let the water soak in, then water again, and repeat until the soil is deeply moist but not so much that water pools on the surface. Let it sit for a day. Any soil when moist is workable.
Refresh Old Potting Mix with New Ingredients
You can add pre-soaked coir (coconut husk fibre which helps retain moisture). I soak a compressed block in HOT water to speed up hydration and add liquid seaweed and molasses. Seaweed adds trace elements and both these ingredients feed soil microbes.
Restoring good vegetation to degraded soil helps to maintain its structure, as the roots become strongly anchored, generating a cycle of organic matter that can help to restore fertility.
“Another option for gardeners who have larger containers is to remove only the top 6 inches or so of the old potting soil and replace it with new potting soil,” he said. “Since they aren't replacing all the soil, they're saving money while still refreshing the upper area, which will aid in root growth.
Some tangible benefits from improved soil health may take 5-10 years to become visible or measurable, but others may appear within a single year.
If the soil has dried out, it can be rehydrated by adding water. However, it's important to check for other signs that the soil might have gone bad, such as a sour smell or the presence of mold. Also, if the soil has been stored improperly or for a long period, pests might have infested it.
It may take an hour or more to thoroughly re-wet the soil. Be careful not to leave pots soaking in standing water continuously. Check them after an hour or two and remove them when they are hydrated.
Soil may also become hard due to lack of water, rocky composition, or excessive foot, equipment or vehicle traffic. Over-tilling, working soil when it is sodden, and mixing sand into clay soil in an effort to lighten it can all contribute to compaction, resulting in soil that is hard and difficult to work.
Compost is the best substance to mix in with hard soil.
Some other materials you can mix in include grass clippings, straw, manure, sphagnum moss, or leaf waste. If your soil has a lot of clay, mixing in some coarse sand can help improve the hard texture, too.
Organic amendments like compost, manure, and peat moss have high water-holding capacity, acting as sponges that absorb and slowly release moisture to plant roots. Inorganic additives such as vermiculite and perlite create reservoirs within the soil, absorbing and holding water for plant uptake.
Initially, just moisten the soil. Thereafter, water well once every week during the growing season then allow the plant to rest and breathe before watering again. If they're not too far gone, you may be able to rehydrate container plants. Plants stressed from drought should be fertilized carefully.
Simply add organic debris to the soil surface and let your “underground herd” break it down. As it does, it will improve your soil's texture and make nutrients available to your plants. So don't be shy about applying copious amounts of organic residue to your soil.
Add Nutrients and Beneficial Microbes to Old Soil
Add a handful of a slow release fertilizer like manure. Mix in one part compost for every three or four parts of the old potting mix.
Create a healthy soil habitat (pH, soil structure, organic matter) to promote beneficial soil microbes. Building organic matter, maintaining optimal pH, and managing deficiencies in critical nutrients such as nitrogen, phosphorous, and sulfur are important for both your plants and soil microbes.
For an added nutrient punch, add approximately 1 tablespoon of time-released fertilizer per gallon of potting soil mix. Viola, the once spent potting soil is now ready to once again super charge plants providing them the nutrients needed for optimal growth!
You can put a 2- to 3-inch thick layer directly on top of existing soil before planting and just let nature do the rest of the work, or you can till it in. On top of that, you can amend your topsoil further with a couple of inches of compost.
It's generally fine to reuse high-quality potting soil if whatever you were growing in it was healthy. But can you reuse potting soil that's been home to pests? If you noticed pests or diseases on your plants, it's best to sterilize the mix to avoid infecting next year's plants.