Soil Ecology and Restoration Group


Addressing soil strength problems on restoration projects

David Bainbridge, Janaira Sawyer, Michael Gates, and Ian Delgado

One of the common adverse effects of disturbance is an increase in soil strength. Soils are often compacted or made more dense by the weight of vehicles, tire or track slip and even the footprints of animals and people. Disturbance may also affect many other soil characteristics that can increase soil strength. Tillage, vegetation removal and grazing reduce litter input into the soil and adversely affect soil organisms, further increasing soil density and soil strength.

These increases in soil strength can make it very difficult for the roots of seedlings to penetrate soil. The effects of even limited increases in soil strength can be dramatic even in favorable environments. In one study, equipment operation increased soil bulk density only 12% on logging decks and skid roads, but reduced seedling survival 90% (Lockaby and Vidrine,1988). In the desert or in dry lands, increases in soil strength are even more critical as they can prevent roots from keeping up with the drying front as the moist soil dries out after a rain event.

Measuring soil strength precisely is difficult because it involves many factors. Fortunately, the interaction of these many factors can be compared easily using soil penetrometer. A soil penetrometer is simply a pin, cone or bar that is pushed into the soil while the force needed to drive it into the soil is measured These instruments range in cost from 20-30 thousand dollars or more for a truck mounted hydraulic/electric system to only 20-30 dollars for a simple spring-scale mounted pin. We developed an impact penetrometer that could be built for less than $100, but provides accurate soils strength data up to 20 inches. These soil tests are very useful for comparison between sites, and because soil strength is often not as variable as soil chemistry, figure 5.1, these tests can be helpful without extensive replication.

Figure 5.1. Soil strength trace, USIU Selaginella area.

More impacts per unit depth indicate increasing soil strength.

In 1998-99 as part of our survey of disturbance effects on soils, we conducted many tests of soil strength. A representative graphs are shown in figure 5.2. These tests revealed the adverse changes that occurs as a result of human disturbance. Increases in soil strength further compound the problems cause by decreases or unbalanced fertility and reduced infiltration. Adverse changes in soil properties add to more difficult above ground conditions,such as greater temperature extremes, increased herbivory, wind damage, and sandblast.

Figure 5-2. Soil strength comparison, Elliot Reserve and USIU

Fortunately these problems can be remediated to some extent by deep ripping, spading or other methods of decompacting soil,figure 5.3.

Figure 5.3. Soil strength comparisons native, disturbed, and ripped

Treating soils with abnormally high soil strength characteristics requires addressing the problems that contributed to problem. These include compaction, reduced infiltration,reduced organic matter, and changes in soil surface configuration and fertility. The common use of deliberate high levels compaction to reduce erosion risk can prevent or stunt plant growth.

Treatment option including scarifying, deep ripping, spading,rototilling and surface shaping (pitting, imprinting, swaling) to increase water retention on site. Scarifying with shallow teethor deep ripping with a D-4 cat and 24+" rippers addresses the soil strength problem in the short term, but soil can reconsolidate if organic matter is not added. Rototilling provide an appealing looking fluffy soil, but mixes subsoil sand surface soils and breaks soil into small particles that reconsolidate and erode easily. Spading provides the best treatment for high soil strength soils yet remains little known outside of Europe.

There are two types of spaders: rotary and reciprocating (Coleman, 1995). Rotary spaders have spade shaped blades on arm that are much longer than a rototiller. These move more slowly through the soil and are less harmful than the high speed tines of a conventional tiller.

Reciprocating soil spaders use a mechanical drive to simulate the action of spading with a shovel. The reciprocating soil spaders are more gentle to the soils and should be considered the tillage tool of choice on most projects. They are made by several European manufacturers (Celli, FALC, Imants) in a range of sizes from 39-122 inches wide. The larger machines require a 100-240 horse power tractor but can spade up to 20" deep. Reciprocating spaders work at a relatively slow ground speed, but a large machine can still treat 2 acres per hour. High speed rotary spaders like the Imants models with fewer shovels and a larger tunnel diameter can operate at higher ground speeds, but speed is rarely critical for restoration work.

Spaders produce results much like hand double digging, a tedious but effective method of rehabilitating degraded lands widely used in organic gardening, permaculture and French Intensive gardening. Anyone who has used this method can appreciate the tremendous benefit it provides for the soil and the striking difference it makes in plant growth. The mechanical spader does days of hard work in a matter of minutes. Like double digging, the reciprocating spader doesn't invert or mix the full soils profile like a rototiller and this can be very desirable in arid lands, where nutrients and organic matter are concentrated near the surface.

Spaders produce a loose broken topsoil with well mixed crop residues and soil amendments. In trials comparing 3 rototillers and a spader the spader provided the best distribution of material through the soil profile. Deep digging with a larger spader can eliminate old plow or rototiller pans and will open up the subsoil, admitting air and moisture which help increase water holding capacity, improve fertility, and enable extensive root systems to develop. Spaders can work in soils with loose rocks where a tiller would jam or bounce. The spades help push the tractor forward and this reduces the energy required to work afield and minimize the chance of getting stuck even in marginal conditions. This reduces soil smearing and snealing, rutting, and compaction from wheel or track dynamics. The spaders ability to penetrate hard surfaces allows it to work dry compacted soils where rototillers, plows, and discs are not effective (see following paper).

The uniform incorporation of large amounts of compost, green manure, weeds or amendments is important on degraded sites. The percent of organic matter in Southern California oak woodlands may reach 20%, in healthy bunch of grass and shrub lands it maybe 4-6%; but in degraded areas it is usually below 2% and may drop below 1% (SERG data). Over an acre this means that 50-300+cubic yards of organic matter should be added to the top foot of soil, and preferably not 100-600+ cubic yards to the top two feet. Adding 50 cubic yards is not difficult, this is only a 3/8 inch thick layer, but adding 300 cubic yards, would require 2.5 inches or mulch. Adding 600 cubic yards, 5 inches, would be very difficult with conventional tools. This is much easier with a spader. Spaders can incorporate large dense masses of vegetable matter in a single pass, making them ideal for rapid reclamation of severely degraded soils.

Adding composted organic matter or green manure crops(nitrogen fixing plant like beans or clover) can restore soils conditions favorable for fast root growth of native plants. The rapid deep rooting increases plant vigor and reduces droughtrisk. These benefits can be achieved in a single pass with a spader. Shallower digging spaders work better if cover crops are mowed first, while deeper digging models can often take down even very tall standing material.

If a finer seedbed is desired in the same pass a secondary tilling attachment can be mounted on the rear of spader. This smaller, straight-tined tiller breaks up surface clods into smaller clods by striking them with blunt tines. The secondary tiller is adjustable so that only the top inch is treated and very small shallow seeded plants can be sown.

Spaders cost more than a comparably size rototiller, but cost less to operate and provide much improved soil. The spades eventually wear and need to be replaced, but with good maintenance the machine should have a long service life. The increased purchase cost is offset by the saving in operation,maintenance and improved plant survival and growth.


Canales, E. nd. Spader tech. Internet Site adapted from an article in Biodynamic Farming. (revised version follows this paper)

Coleman, E. 1995. The New Organic Grower. Chelsea Green, White River Jct., VT.

Lockaby, B.G. and C.G. Vidrine. 1984. Effect of logging equipment traffic on soil density and growth and survival of young loblolly pine. Southern Journal of Applied Forestry 8(2):109-112.