Experimenting with Biological Tillage for Septic Drainfield

August 1, 2014


A client’s property located in Sherwood, Oregon, has NO suitable soil for an approvable septic drain-field. With overland flow onto the site, horse & cattle traffic, a slope of 1 to 2 percent on top of about 8 inches of manure & Silty Clay over a dense layer of Clay, it couldn’t be much worse for gaining approval. None of the traditional methods of de-watering the site are likely to make a difference.



Over 36 years in this field tells me – “YES”.  But getting a permit could be another matter.  We don’t like to give up on our client’s property & this approach could create conditions that lead to an approval.  Where the client is informed & willing, EMS is eager to attempt to create a new reality.  Question:  Can we make an “impossible” site work?  We are working with a client to do just that, using BIOLOGICAL TILLAGE. Will deep rooting plants successfully open up clay / hardpan soils and increase hydraulic conductivity?  We shall see.

There are three goals to be accomplished: 1) improve the soil’s capacity to infiltrate water by increasing the surface area along the clay boundary, 2) establish an extensive root structure for improved wastewater contact with the soil & effluent treatment and 3) keep the hydrology moving through the site.



The procedure will consist of the following phases:

1. Drainage Enhancement. Divert the surface & shallow soil water running into the site via a French Drain / Ground-Water Interceptor.

2. Plowing in Compost. During dry summer conditions, the soil can be altered without causing it to turn into soup, only to dry into a big brick. Compost, Not Sand can increase water movement in soil. Regardless of treatment, the water will need to go somewhere, so ensuring a consistent slope in the surface & especially in subsurface interface with the clay layer will be critical.

3. Fall & Winter (Wet Season Evaluation) Phase. Tillage Radishes, w/ deep tap roots will be planted to create large holes into the clay layer. In the fall, a cover crop of common vetch and rye grass with fibrous roots will be used to create an abundance of smaller pores in the soil matrix.


Client Implementation.

Before sowing the cover crop seed, the client will prepare the drainfield area with shallow tilling to prevent channeling of flow caused by low and high levels of clay. Be careful not to change the existing gradient. Use stakes and string to ensure depth of tillage (depth to undisturbed clay) is consistent throughout the drainfield. Create a tapered effect on the downhill edge of the drainfield to avoid pooling conditions (Figure 1).

tapered plow Figure 1: Tapered plow along lower edge of drainfield area.

Critical Factor: Make Sure that the Subsoil Does Not Pond.

(NOTE: actual slope on this site = 1 to 2 %)


The recommended cover crop for the spring is Tillage Radish, (Raphaus sativus var. niger). This radish is known for producing tap roots 4” in diameter that go as deep as 24”. Irrigation will be required until the radish is established (when the tap root reaches the water table). Let it grow!


Farmer holding a 21″ GroundHog Radish. This Field was seeded after a field bean crop. Radishes were sown at 5#/acre along with Hairy Vetch. Source: Covercropsusa.com

After adding the soil blend, sow a cover crop of Cereal Rye Grass (Lolium multiflorum spp.) and Common Vetch (Vicia sativa L.). These two plants are known to perform well in hydric soils and will not require irrigation. As the radishes decompose the fibrous grass roots will enter the large holes and penetrate the clay boundary. The vetch will provide the rye grass with the requisite nitrogen to promote an extensive root structure.

measuring roots


Cereal Rye Grass roots can grow up to 4 feet deep.  Source: Farmprogress.com

common vetch

Common Vetch (Vicia sativa L.)

Source: www.uwyo.edu/plantsciences/uwplant/forages/legume/common-vetch.html


After we install the drainage system, amend the soil with compost & plant the tillage crops – we monitor the hydrology.

Monitoring wells with liquid level sensors will record readings 4 times / day.

In late spring or early summer of 2015, test pits will be dug to observe the soil matrix & evidence of water table below the drainfield area. At this time the soils should exhibit a healthy biological environment conducive to wastewater effluent treatment.

This is when the County Sanitarian (Environmental Health Specialist) will review & determine whether the site can be approved for an onsite wastewater treatment system.

We hope for an early success, but realize that nature sometimes operates on its own time-line.  This could take more than one season.  Expect more observations as this project evolves.


Comments & Advice are Always Welcome!