Soil Conductivity in Hawaii

"Define "BAD"; as a certain president might say...

There is much folklore and confusion about Hawaii soil conductivity, so let's get some expert help in clearing this up. I talked with soil scientists at UH Manoa until things began to de-fog.

The basic unit of soil conductivity is decisiemen per meter. This is used because they used to use millimhos per centimeter. [.1/1 = .001/.01] So they are the same units by a different name. 1 still equals 1.

[FYI a siemen is a mho. It is called siemen to make it harder to remember than mho. Mho is Ohm spelled backwards, a clear clue to its meaning which is 1/ohms or reciprocal ohms. Since ohms is resistance, mhos is conduction. Mhos had too many people understanding what was going on so it was renamed siemen to make it sound really dirty and also keep ordinary people from understanding it easily. A deci-siemen is one tenth as dirty or 0.1 mho.]

Maile

Soils in Hawaii tend to be highly variable in conductivity They are often based on oxides, or are weathered igneous, volcanic rock. These are very inert chemical compounds. Unlike Kansas or other typical mainland sites, Hawaii soils are not usually full of reactive minerals like calcium carbonates from sedimentary rocks such as limestones. Our water is soft, not hard and mineral laden like in Kansas. So you would expect our glassy soils to be lousy conductivity vs. nice wheat fields in Oklahoma.

But wait, ocean wave action is constantly generating salt spray that evaporates and makes tiny specs of salt in our trade winds. Salt is a major problem with Hawaii soils. In fact, in regions that get trade winds bringing in ocean salt and because of local geography are in a rain shadow and dry, salt can build up to near lethal levels.

The Romans used to sow fields with salt to destroy the productivity of farm land and make conquered peoples dependent on them for food until rain could leach out the salt and restore the land's usefulness.

Such saline soil areas can accumulate conductivities over 10 decisiemen per meter. At that sea salt level not much can grow. Many plants can be done in by 3 decisiemen per meter if all that conductivity is due to sodium chloride from sea salt.

But not finished yet. Soils in Hawaii have terrible retention of nutrients in many cases. Stuff leaches right out of them with rain. So it is a balance of the salt arriving on the wind and the rain washing it out, a constant fight up and down, plus other more reasonable ions leaching from soil constituents or even fertilizer, that determines conductivity. Soil moisture and so called PH buffering of the soil contribute to the picture as well. Along with water retention.

You can also expect SWR changes on many antennas when it rains and soil moisture levels temporarily rise significantly. In fact, a very noticeable change in reception as well as loading characteristics can accompany a serious rain shower. The effective ground plane under the antenna changes both in conductivity and relative spacing. This can be quite obvious on difficult installations at low frequencies like 160 and 80 meters. It may not be too crazy to set up a sprinkler under your low band vertical system before that 160 meter contest starts!

So What Figures to Expect

Typical Hawaii soils should measure about 1 decisiemen per meter in undisturbed rain forests, about 2 decisiemen per meter in agricultural areas, and 3 decisiemen in areas like residential lawns or even higher, based on the increasing augmentation by extra organic material or even chemical fertilizers vs the undisturbed rain forest floor. Now keep in mind these are upland readings where less conductivity is due to sodium chloride from ocean water and thus such high conductivities do not mean toxic saline soil necessarily.

Look at the EZnec antenna modeling program that features "real" grounds. It assumes 5 millisiemens per meter as "ordinary" ground. Check this carefully. This is .05 decisiemens per meter. Very very low conductivity vs. typical Hawaii soil conductivity levels we are told to expect, and it is assumed to be good ground by the program.

After triple checking the number, it is clear. Soil scientists who teach and know this stuff, have long familiarity with the subject in Hawaii's special case, have seen tens of thousands of soil measurements, say we are more likely to be in the 1-4 decisiemen per meter range. EZnec thinks that .05 decisiemen per meter is pretty darn good. The 1 decisiemen per meter is equal to 10 ohms per meter resistance. The .05 decisiemen per meter is equal to 200 ohms per meter resistance. So where is the bad soil conductivity?

Ah! here it is. The folk lore that we have bad soil conductivity in Hawaii is TRUE!! What they (Agronomy types) mean is that it is too HIGH!, a danger sign of too much sodium chloride, poison for crops and BAD for agriculture. Now for antenna grounds, a high soil conductivity is not BAD, we consider that GOOD. This appears to be the basis of the confusion. When an soil scientist measures conductivity and starts to see ranges up around 4 decisiemen per meter, he starts to cringe and worry about crop failure due to too much sea salt in the soil. BAD.

You are much more likely to find high soil conductivity in Hawaii, 20 to 50 times the 5 millisiemen per meter that EZnec considers good, than poor soil conductivity. Even the base upland reading of 1 decisiemen per meter is 100 millisiemen per meter. And that is for well rained on, upland tropical rain forest soils.

But, also note that soil conductivity is highly variable. One reliable actual measurement was made in grid BL01, on Oahu near Waimea Bay. The location was fairly high up on the side of the hills above and miles back from the beach. It is in an area with lots of rain and trade winds that usually blow the salt from the nearby beach away. This soil conductivity measured about 3.6 millisiemen [.036 decisiemen] per meter. It meets the low conductivity profile described above, little windborne transport of salt into the area and frequent rains that keep the soil leached out.

In Fall of 1999 I was able to operate in Kalaupapa. It was a classic case. First I thought the horrible soil condition was due to drought. True the tip of the peninsula gets reduced rainfall close to the ocean. But the major problem is salt poisoning. In spite of lots of organic material in the form of horse and other droppings almost blanketing the ground, little would grow even with watering.

I erected a portable vertical antenna, a version of my Stake Stick, and got excellent results, especially after I watered the soil under the antenna's groundplane. The soil was salt poisoned all the way to the ocean about .3 miles away and the 1/4 wave vertical worked extremely well.

Can You Help?

A viable approach to ground rods in horrible soil conductivity areas has been to use chemicals in conjunction with the driven or buried ground rods. Books have describe putting an eight-foot ground rod in the bottom of a pit and then filling the pit with sand and chemicals like copper sulphate. Of course you will kill everything near the ground rod and poison the soil, but it works as long as the chemical lasts. A commercial version using a perforated copper pipe and more environment friendly chemicals is available called Valrods.

Do not over look the simple approach to soil conductivity found at your local lawn and garden center. Fertilizer will help the yard near your vertical or ground system and significantly help lousy conduction like 0.2 siemen/meter. You can get two kinds. Pellets are broadcast [as in cast over a broad area, people have been doing it with seeds for millennia]. Solubles can be sprayed with water, often using special inexpensive dispensers that attach to garden hoses. Score points with the XYL by doing "lawn work" just before the next 160 contest.

One recommended chemical is gypsum, Calcium Sulphate. It is relatively save to use on your lawn, dissolves slowly, lasts a long time, does not easily burn vegetation and if watered can jack soil conductivity up to 3-4 decisiemen per meter.

But the truth seems to be that in Hawaii, as one might expect from an island in the Pacific ocean swept by trade winds, salt in the soil is the real problem, not poor conductivity. If you are really worried about soil conductivity, water the lawn during the next big contest.


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08/99