From Marie Iannotti,
Your Guide to Gardening.
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First it should be pointed out that dirt is always called soil in gardening. Soil is arguably the most important component in a successful garden, so not calling it dirt is a show of respect. However, it is still dirt when it gets on your clothes.

What is Good Garden Soil?

Soil is generally evaluated on fertility and texture. Fertility is a combination of essential nutrients and a pH that makes these nutrients available to the plants. Texture refers to the size of the soil particles and their cohesiveness.

Nutrients
The three primary nutrients used by plants are nitrogen, phosphorus and potassium.

Nitrogen is largely responsible for healthy leaf and stem growth. In the soil, nitrogen is made available to plants by nitrogen fixing bacteria which convert nitrogen into nitrates, a form plants can use. Nitrogen does not remain in the soil for long. It gets used up by your plants and by decaying matter in the soil. It is also water soluble and can wash out of the soil rather quickly. Even so, an excess of nitrogen will cause a lot of foliage growth at the expense of flowers and fruit.

Phosphorus is very important for root growth. Flowering bulbs and root crops can always use some phosphorous. That’s why bone meal is often recommended for fall bulb planting. It also is crucial for producing flowers and you will sometimes see fertilizers with a high phosphoreus content advertised as flower boosters.

Potassium is needed for overall plant health. It keeps the plants growing and aids their immune systems. Like nitrogen, potassium is also water soluble and needs to be replenished from time to time.

Besides the three primary nutrients, there are several trace elements that are necessary for good plant health like: calcium, magnesium, zinc, molybdenum, etc.

pH
A lot is made of soil pH. In laymen’s terms, pH is a measure of the soil acidity or alkalinity. The scale goes from 1.0 to 14.0, with 7.0 being neutral. The lower the numbers go from 7.0, the more acidic the soil. The higher they go above 7.0, the more alkaline. The reason soil pH matters is that nutrients in the soil are only available to plants if the soil pH is within a certain range. Many plants like a pH in the low acid to neutral range (6.2 - 6.8), but that’s not true for all plants. Rhododendrons, heathers and blueberries favor very acid soils and lilacs and clematis will thrive in alkaline or even chalky soil. The only sure fire way to know where your soil’s pH falls is to have it tested. Keep in mind that it takes time to alter soil pH and your soil will tend to revert to its old pH over time, necessitating repeated treatment.

Texture
Soil texture is a little trickier to amend than soil fertility. Texture refers to the size of the soil particles. Sandy soils have very large particles. Water, air and plant roots can move freely in sandy soils, sometimes too much so. At the other end of the spectrum is clay. Clay particles are so small they pack together tightly and leave little room for water, air or roots. If you’ve ever tried to garden in baked clay you know it also leaves little room for a shovel blade.

An easy test for soil texture is to make a ball of damp garden soil. If it breaks apart when you tap it, it’s sandy. If you can press it between your thumb and finger and make a ribbon, it’s clay.

Most soils are somewhere in-between. What you are ideally going for is called a sandy loam. It should be light and allow for air and water movement, but have some tilth, a kind of fine bread crumb like texture, which usually occurs when there is plenty of organic matter in the soil.

Don’t try to change your soil texture by adding sand to clay or vice versa. That is a recipe for cement. Some amendment recommendations for clay do include a portion of very fine sand, but there are better ways to change your soil texture.

Organic Matter
Like soil pH, organic matter gets a great deal of press. Organic matter is dead plant or animal material. There is always some organic matter in your soil, but usually not enough for a plant’s needs. Decaying organic matter, or humus, will help give your soil tilth. It helps sandy soil by retaining water that would otherwise wash away and it corrects clay soil by making it looser, so that air, water and roots can penetrate. In all soils, it encourages beneficial microbial activity and it provides some nutritional benefits. Humus is natures way of feeding the circle of life.

How do you know if you have bad soil?