Rich, ready to produce garden soil must be constantly replenished with nutrient rich additives to keep it fertile. Without replenishing nutrients the the soil becomes nutrient poor, no longer attractive to worms for irrigation or a source of food for your garden plants. Plants become weakened and diseased with limited or stunted growth. Applying fertilizer at this point is too little too late because they cannot be absorbed by the already depleted soil condition.
Most garden soils, without regenerative organic additives will be depleted within just 1 or 2 growing seasons.
To restore or improve your garden soil to a rich healthy plant supporting environment requires a simple plan of adding manure and/or compost. Whether you make your own compost or buy it, this along with manure, are the best organics and cheapest additives to beef up your soil.
The best time to bring in manure is in late fall or early spring, before or after the seasons growth is completed. If you live in an area near a farm or type of cannery, compost is usually there for the taking for free or at minimal "you haul" costs. Rural county offices often have a "hot line" to call to find a source for manure near your residence.
If you are preparing a brand new bed for the spring, till the manure in as deeply as the machine allows. 10" is a minimal starting depth. For mature beds, do not mix the manure into the soil, rather lay it over the surface of the soil approximately 2 to 4 inches deep. The worms and winter weather will get those nutrients to the roots naturally.
Be sure to wear a mask and gloves while spreading the manure to minimize the risk of any possible infection.
Home made compost is basically a year long, built up pile of grass clippings, shredded leaves, food scraps and any other organic waste that an average home generates. The simple way to check the quality of your compost pile is to check for earth worms. If a few digs into the pile reveals healthy, fat worms your compost is top notch and will be a powerful addition to your garden.
The best time to add compost is in the fall. Rather than adding an additional layer of manure after you till in the compost, add the manure into the compost earlier in the year. By the time it's needed it will have melded into the overall organic compost boost.
Before tilling, check the soil to ensure it is not too wet to till. To do this, gather some soil into your hands and try to make a ball with it. If a ball can actually be made, the soil is too wet. If it crumbles, it is ready for tilling. Set the roto-tiller to it's deepest dig level. Mix the compost in as slow and as deep as the machine allows. Rough turned garden soil is more receptive to absorbing moisture and organic additives.