While the role of chemicals in the soil is not clearly understood, we do know that:
- Too much nitrogen yields heavy leaf growth and shade, while too little produces hydrogen sulfide.
- Too little potassium makes plants vulnerable to drought and disease and yields grapes that are low in sugar.
- pH is not critical, because grapes grow in a variety of soil pH levels.
- Too much organic material yields too much nitrogen and water, but too little will not support many soil organisms (earthworms).
Growing grapes, a process known as viticulture, involves a complex interaction (terroir) between the following factors:
Soil - Soil influences how much water and heat are available. Grapes need a steady, but not excessive, water supply.
- Color - Dark soils tend to be warmer than light soils because they are better at absorbing and holding heat.
- Geology - Rocky or stony soils allow water to drain better than clay soils; rocks also help to absorb heat in the soil.
- Chemicals - The role of chemicals in the soil is not clearly understood (see sidebar).
Topography - This influences the amount of sunlight available (temperature) and the drainage (water supply).
Climate/Microclimate - This influences temperature, sunlight and water (rainfall, fog, mist). Some grapes, such as Vitis Vinifera, tend to grow best in areas where the seasonal temperature varies by about 30 to 35 degrees Fahrenheit (17 to 19 degrees Celsius).
The types of grapes that are planted and grown in any given area depend upon the terroir. In the Northern Hemisphere, grapes begin to bud in late March or early April. The grapes grow, bloom and develop fruit throughout the summer. The grower's goal is to keep the leaf growth small, which allows more sun in and keeps the grape clusters small yet numerous. The growers must also watch for signs of drought, disease and pests. In late September or early October, the grapes are ready to harvest. The actual times vary with the climate, latitude and judgment of individual growers.