Drainage & Irrigation

Tilth, or the physical condition of the soil, is the result of the arrangement of a soil’s texture and aggregated structures as well as plant material and other organic matter. This arrangement creates a mix of large and small pores that air and water move through. The size and connectivity of the pores control water and air infiltration into the soil, storage of water within the soil, and ultimately drainage from the soil. 

Approximately 50% of a healthy soil’s volume is open pores where water can flow. The smaller the pores, the easier it will be for the soil to store water against gravity but the slower the water will move. The larger the pores, the faster the water will move but the less will be stored. We can generalize water behavior in a soil based on its texture alone. In the graphic below, the curve labeled “field capacity” is the maximum amount of water a given soil texture can hold against gravity. The percent of water above this line will drain out of the soil due to gravity and is called gravitational water. All of the water below the curve labeled “wilting point” is unavailable for plants as it is tightly held by the small pores. The volume of water between field capacity and the wilting point is available for plant uptake and other soil organisms.

By looking at the graphic we can see that loamy soil textures provide the most water storage for our plants. But texture alone doesn’t tell us everything — organic matter leading to healthy soil structure can dramatically increase the water storage of sandy soil while improving the drainage of clayey soils.

Plants need both water and air to survive, so avoiding flooding as well as overly dry conditions is essential for the health of most plants. Understanding how water moves on your site can help. Consider whether rain barrels or other rainwater catchment strategies might be useful to store water for the times when your soil and plants need it.

Once drained, the pores that held gravitational water will allow air into the soil, but texture matters – drainage can be particularly slow with fine textured soil and extremely fast with course textured soils. While medium textured loams offer good drainage and a large volume of available water, all soils will encounter dry conditions and may require irrigation.

Consider your soil’s tilth when choosing the best timing for irrigation and the amount of water needed:

• For courser soils, frequent watering but in limited amounts is often a successful approach.
• For fine textured soils, slowly adding larger volumes of water with less frequent applications will be more successful.  
• If it’s an option for your site, drip irrigation can reduce water use and get the water directly where it’s needed.
• While it may take time, reducing disturbance and adding compost or mulch will also encourage plant and soil organisms to build healthy and stable soil structures making water management easier and more reliable for years to come.