Planting by the Law

Understanding a key law of soil physics is critical to providing optimal irrigation for improved plant growth.
By Carl Whitcomb

Figure 1. Every container retains considerable water at the bottom of the growth medium. If the pore spaces are small, a great deal of water is retained. If the pore spaces are large, less water is retained. Water stops draining from a container when the weight of the water at the top of the column is no longer sufficient to overcome capillarity attraction between water and mix particles and attraction of water to water to force water out the drain holes.

Everywhere you turn, there seem to be more and more laws. But knowledge of some laws can be beneficial and even useful. When planting plants grown in containers in the landscape or field, Darcy's law of soil physics directly applies. Darcy's Law summarized: Water moves readily from a coarse-textured material to a finer texture. Water will not move from a fine texture to a coarse texture until saturation.

Because of the unique environment of a container, in order to have sufficient drainage and oxygen for root functions, a mix more coarse in texture than any field soils is required. In a container, the mix or growth medium holds considerable water including a zone in the bottom that is often at or near the saturation point (Figure 1). However, once removed from the container and installed in field soil, the growth medium abruptly goes from holding too much water to holding very little. The reason for this abrupt change is Darcy's Law.

Practical applications

Figure 2. This drainage system works well for planting in heavy clay soils. The root-ball should be entirely above the upper limit of the perched water table created by the gravel. The gravel layer should be no more than 4 to 6 inches deep. Water will enter the drain tile only when the soil just above is very wet, but as long as the root-ball is above the saturated zone this works well.

How long it takes for roots to grow out into soil depends somewhat on plant species but especially how they were grown. Plants with circling roots, poor nutrition and few white root tips at time of planting may require several weeks to grow a few roots an inch or more. By contrast, plants grown in RootMaker containers and with proper nutrition have grown roots as long as 11 inches in 12 days and in huge numbers. (Figure 4).

Figure 3. When the backfill around the plant is soil, water is drawn out of the container mix and into the loosened soil (left). However, as soon as roots grow out into the surrounding loosened soil, moisture is generally available. On the other hand, if the soils around a container-grown plant are amended, water is drawn out of the container mix by the amended soil (since it is finer in texture), and in turn water is drawn out of the amended soil by the unamended soil (right). This leaves the plant "high and dry" because moisture will not move from the moist unamended soil back into the amended soil. When backfill is amended, roots have to grow much farther to reach unamended soil which is a must for long term survival.
Figures courtesy of Dr. Carl E. Whitcomb

Figure 4.These catalpa seedlings were grown in 18 cell RootMaker® containers. They were transplanted and allowed to develop roots for 12, 8, 4 and 0 days. How the plant was grown, energy in root tissues and extent of root branching play a huge role in how fast roots grow out and establish the plant.

There is an exception:

Figure 5. Water will percolate into a coarse-textured material more quickly than a fine-textured material. When a container grown plant is placed in a heavy soil, water will percolate into the planting hole through the container mix faster than the loosened heavy soil (left). However if soil amendments have been added to the heavy soil, water percolates even faster into the planting hole from the more coarse-textured amended soil (right). This can cause a sufficient "bath tub" effect to restrict or suffocate plant roots.

Water movement in soils is controlled by Darcy's law of soil physics. Keep this law in mind as it can be helpful in many ways. (For a detailed presentation of this topic including more drawings, photos and solutions to unique planting situations, see Chapters 7 and 9 in Establishment and Maintenance of Landscape Plants II. See www.lacebarkinc.com for ordering.)

Dr. Carl Whitcomb is president of Lacebark, Inc., a private research and plant breeding facility near Stillwater, Okla. Lacebark, Inc. celebrated 25 years in 2010. Prior to creating Lacebark Inc. he was a professor at the University of Florida and Oklahoma State University for 19 years. An inventor and entrepreneur, Dr. Whitcomb holds 30 USA patents and over 30 registered trademarks. He has written five books; four in the field of horticulture and plant science and most recently his autobiography.