All vegetables need soil that is rich in vital nutrients. Different plants have slightly differing requirements but basically they all require a rich, well drained, organically enhanced soil. Some soils need a boost to yield a successful crop.
Start by testing your soil. One of the first things you should ascertain is the Soil pH - the soils acidity or alkalinity. When the soil pH is low, meaning it is too acidic, nutrients present in the soil become locked-up or unavailable. Regardless of what you amend into the soil - the plants will be unable to absorb it. Correcting the pH can have the same effect as adding fertilizer, it unlocks plant nutrients already present. A soil with a pH lower than 7.0 is an acidic soil, soil with a pH higher than 7.0 is alkaline. See - Tracking and Adjusting Soil pH
For most home gardeners, sending soil samples to be analyzed by a laboratory are at times costly and time consuming, and in reality not really necessary. Some local cooperative extension services will offer for a free test - and you might even get your results back before the season ends, but don't count on it.
Inexpensive soil test kits will give you a chemical break down of your soils composition some are better than others. There are also simple methods you can use to test your soil that you can do yourself. Soil Structure, Compaction, Soil Organisms, Water Infiltration, and root development can all be tested by the gardener without purchasing more equipment.
Organic matter is always the best amendment, in all scenarios. Adding organic matter to your soil improves structure in particular porosity. As the organic material decomposes it will release more nutrients for absorption by the plants as well as increase beneficial microbial activity. Organic matter can be aged manure or compost. Adding other organic material to the soil is advantageous as well, depending on what you are growing.
Wood Ash is a source of lime, potassium and many other trace elements that benefit plants. It is best scattered lightly, in moderation or blended into the compost. Wood ash produces salts and lye and if not used sparingly can damage or kill your plants
Eggshells provide large amounts of calcium, which many plants require in abundance. It also provides small amounts of nitrogen, which is beneficial to the growth and development of healthy plants.
Coffee grinds were once thought to benefit acid-loving plants, however, a study found that when mixed with yard waste or compost the ph was neutralized. So basically raw straight coffee grinds are not a great idea, the acidity varies, in some cases it is too high even for acid loving plants . When added to compost, it won't significantly increase the acidity, so a small amount well blended in will serve as additional organic material.
See Also - Test Garden Soil Yourself
If you have a clay soil or silt soil, adding compost high in fibrous material such as leaf mold or bark is best. Peat moss will help. So far as Peat Moss - although it breaks down in the soil relatively slowly, it has high water retention capacity which is great for sandy soil but not near as good for clay soils which drain slowly. Peat Moss can have negative effects in clay or silt soils if not used in moderation.Some people also add coarse sand, which I generally avoid. You need to add a tremendous amount to have any real effect.
If you have sandy soil, aged manure, peat moss, humus and even sawdust will help add structure to the soil. But also add that compost - don't ever forget the compost.
Other Soil Amendments include some items which are also used for mulching such as tree bark and wood chips. Leaf Mold, which is decomposed leaves adds both nutrients and structure to soil.
Plants primary nutrients are nitrogen (N), phosphorus (P), and potassium (K). These are available at varying levels in all fertilizers. The NPK number as it is called, on the label of any fertilizer bag will tell what percentages of each primary ingredient it contains.
Nitrogen (N) promotes foliage and stem growth. Fertilizers high in nitrogen should be used for leafy vegetables such as lettuce, cabbage, greens, broccoli and some herbs. fish emulsion or blood meal will also help boost your soils nitrogen.
Phosphorus (P) promotes root development and boosts the plants early growth. It is needed by flowering plants to produce blossoms and seeds and to develop fruit. Fruiting crops such as tomatoes, eggplants, peppers and cucumbers benefit from it. Bone meal and rock phosphate are common sources. Guano and crushed coral are other items amended into the soil to increase the phosphorous level.
Potassium (K) promotes root health and enhances disease resistance. It also improves the flavor of vegetables such as radishes, beets, carrots, onions, garlic and many root crops. Wood ash in moderation will increase potassium levels as will gypsum or kelp.
See - Understanding Fertilizer Labels
Organic fertilizers are always best, it is next to impossible to apply excess organic materials. Excess synthetic fertilizers will do more harm than good. They damage roots and reduce the availability of other nutrients.