What Does Light, Medium and Heavy Mean?

Q. I am confused by the terms, “light, medium, and heavy”?

Desert soils can be confusing. Even though this soil may be difficult to dig, it is a sandy loam soil; a mixture of sand silt and clay but the clay is not present in high enough amounts for it to be considered "a clay soil".

A. These terms usually describe soils, but could also be used to describe watering, such as a “light watering”. A light soil is a sandy soil. A medium soil is a soil has a mixture of sand, silt and clay but still drains water in a few hours to overnight. A heavy soil is a soil that is dominated by clay. It drains very poorly. These “heavy” clay or sticky soils hold water and don’t drain well. Oftentimes planting in these soils is a disaster for xeric plants because of the amount of clay and they hold water for a long time. 

This soil has a fair amount of clay in it. The type of clay makes it "expansive" or not. Expansive soils like this one are considered to have a higher than normal "shrink/swell potential" and cause problems when building or construction. Montmorillonite clay is an example of an expansive clay. Some clays are not as expansive as others.

To make a heavy soil “lighter”, about 90% more sand (v/v) is needed. The other option is to grow plants higher than the surrounding soil so their roots can “breathe”. Small plants need about a foot of soil higher (about 12 inches in diameter) than the surrounding soil. Large trees may need two or three feet tall mounds that are 6 to 10 feet in diameter.

What is a light watering?

            A “light” watering is an irrigation that wets the soil only 3 or 4 inches deep. It might be good for very small plants. This type of watering encourages lots of shallow roots to flourish and makes this plant less tolerant of hot summers. Shallow rooting oftentimes happens with “hose” watering; watering with a hose or a “breaker” at the end of a hose. 

Hose irrigation of corn in raised bed. Water not penetrating deep enough.

Hose watering raised beds and apparent adequate water until examined closely. Water was applied so quickly that it was only getting three or four inches deep but appearing wet, enough for shallow rooted crops but not deep enough for deeper rooted crops like corn. Drip irrigation is a better choice.

To water deeper (a.k.a. medium or moderate) requires watering the same soil about 10 to 20 minutes later but while it’s still wet. This drives the irrigation deeper in the soil and can wet the soil to a depth of 6 to 8 inches. A “deep” watering usually requires some sort of “moat” or “donut” around the plant to capture the water and cause it to go deeper. A third watering like this frequently doesn’t work but requires a “moat” or “donut” instead. Drip irrigation can release the water slowly, so it travels deeper. That’s one of the reasons why single emitter, nonadjustable drip irrigation is preferred in home landscapes and raised vegetable beds.