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I've seen this repeatedly and the cause is usually the same. The gardener was in too big a hurry to do a first rate job of soil preparation. Some excellent gardeners say that they focus on growing soil not veggies.
This idea of "growing" soil is not really far-fetched. Soil is more than just its mineral component of sand, silt and clay. A good healthy garden soil is actually a complex community of interrelated organisms. This living part of the soil includes bacteria, fungi, nematodes, protozoa and tiny arthropods (insects and their relatives), as well as earthworms and the bigger arthropods that we commonly see.
These soil organisms are extremely important. They break down organic residue to release nutrients. Certain bacteria are able to take nitrogen from the air and make it available to your plants. Some fungi form associations with roots and use their extensive network to collect and deliver nutrients and water to the plant. Good fungi and bacteria compete with and suppress the ones that cause plant diseases.
These soil organisms also create good soil structure. Ideally, your soil's mineral particles are clumped together in tiny chunks called aggregates. These allow air, water and plant roots to move easily through the spaces between them. The thread-like fungi bind the soil into aggregates and bacteria waste products act like glue to clump the soil. Larger worms and arthropods help soil structure as they burrow and pass soil particles through their gut.
You can nurture this web of life in your soil by adding organic matter. Organic waste is the fuel that keeps this system going. Decomposer organisms will break it down and multiply and then serve as food for the predator organisms.
There are lots of ways to add organic matter. In a natural ecosystem the ground is usually covered with leaf litter. Gardeners can copy that and spread organic mulch around their plants. Worms and other critters will slowly attack it from below. Then you can till it in at the end of the growing season.
The best way to add organic matter is to mix compost into your soil. Compost is already partially decomposed and goes right to work feeding the soil organisms. Try to add about 2 inches of compost a year to keep your soil in peak form.
It's best to make your own compost on-site to recycle your garden wastes. If you haven't produced enough homemade compost, there are lots of commercially available composts that can be purchased by the bag or the truckload, such as composted steer or dairy manure, Cedar Grove compost, Zoodoo and composted municipal wastes. The latter, which are also called biosolids, may or may not be appropriate for edibles. Ask before you buy.
The third and last way gardeners add organic matter is by growing and tilling in green manures. These crops are grown specifically to add to the soil, although they have other benefits. Vetch, field peas, annual rye, fava beans, crimson clover are frequently planted in the fall and then turned under about now. In a couple of weeks they will have decomposed sufficiently to allow planting. (I'm often in a hurry to plant, so I pull the green manure plants out and add them to my compost pile. Then I mix finished compost with the soil and I'm ready to go. Some experts believe this works best and that we should call green manures "compost crops.")
Adding organic matter will cut down your need for fertilizer, but it doesn't completely replace it. Have a soil test run every few years to determine your soil's fertility needs. If you don't have time to have a test done, try 5 - 6 pounds of complete organic fertilizer per 100 square feet of bed. (That's about 1 cup per 10 square feet.)
Hopefully, you added lime last fall, if it was needed. Lime mixes lots better with a drier soil and takes months to sweeten the soil. Lime should be added in about the same amount as the fertilizer, but only every other year. Again a soil test will tell you exactly what is needed.
Hortsense: Managing plant problems with Integrated Pest Management
