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What about those new soil polymers?

What about those new soil polymers?

A revolutionary group of soil amendmentscalled super-absorbent polymers is making its appearance in the West. Like the one you see across the top of the page, these gel-like polymers can absorb hundreds of times their weight in water. In the soil, the gel supplies water and holds nutrients for plant roots.

Because the gel retains water that normallydrains from the soil, plants still have a source of moisture when the soil gets dry. This eliminates wide fluctuations in moisture levels between waterings, and helps plants survive temporary periods of drought.

Especially in sandy soils that retain littlemoisture, polymers can provide a valuable source of water to prevent drought stress.

As the pieces of gel absorb and releasewater, they expand and contract. With this action, they also can improve soil structure and increase air spaces in the soil. Since roots need oxygen for growth, bigger air spaces help their development, especially in fine-textured soils.

Along with the water, the gel absorbsnutrients from fertilizers and releases them to plants. Usually, water quickly moves such nutrients out of the plants' root zones; holding them is a particular benefit to plants in containers (see the photograph at top right of page 253).

Not all polymers are alike

There are three basic types of soil-treatingpolymers: starch, polyvinyl alcohol, and polyacrylamide. Only one type--a polyacrylamide called Broadleaf P4 (shown above)--is available in retail stores. Others are used commercially.

Depending on the type and manufacturer,these polymers all have slightly different characteristics. For instance, starch polymers absorb up to a thousand times their weight in water but last only several weeks in the soil. For the home gardener, the polyacrylamides are more useful since they last longer.

But even they don't all perform equally.Depending on the salt content in the water, Broadleaf P4 absorbs from 200 to 400 times its weight in water and lasts at least 5 to 10 years in the soil. But one of its most important characteristics is that it will release over 95 percent of the water to the plant. Other polyacrylamides only absorb 20 to 40 times their weight in water and release less than 60 percent.

Broadleaf P4 is nontoxic and is classifiedas nonhazardous by the Environmental Protection Agency. It eventually breaks down to carbon dioxide, small amounts of nitrogen, and water.

Which plants will they benefit?

Whether you're starting seeds, plantingvegetables in containers, or planting a lawn, polymer gel can benefit almost any kind of plant. The more sensitive the plant is to moisture stress and transplant shock, the more it can benefit.

Last summer and fall, we tested the polyacrylamideBroadleaf P4 on several different types of indoor and outdoor plants. Lettuce seedlings were grown in containers with and without this polymer mixed into the soil. As shown in the lower right photograph, the lettuce grown without out it wilted about seven days earlier than the lettuce with polyacrylamide.

We conducted a similar experiment witha house plant. Spathiphyllum plants were moved into larger containers with and without this polymer. After allowing a few weeks for the roots to migrate to the polymer, we stopped watering. Six days later, the one without it started to wilt. It took 15 days for the plant with the polymer to start wilting.

Maidenhair ferns are particularly sensitiveto moisture stress and low humidity. In our test, a fern with the polymer in the soil thrived, while over a third of the foliage on a plant without it shriveled.

They're easy to use

You can add the polymers to the soil dryor hydrated. They're easier to mix uniformly when hydrated, but it may not be practical if you're using large quantities such as under a new sod lawn.

When using polymers on established containerplants, you'll get the most uniform distribution if you move the plants into larger containers and backfill with a mixture of polymer and potting soil (as in the photograph at top right on the opposite page). If the plant has a large rootball, you should also poke some dry polymer into several holes.

Make sure your proportions of polymerand soil are accurate, especially when using the polymer dry. If you put too much into the soil and then wet it, the expanding gel can force the plant out of the ground or pot.

To wet Broadleaf P4, add 1 ounce ofcrystals to 4 quarts of water or one blister pack (4 grams) to 1 pint of water. Stir and then let sit about 20 minutes to allow crystals to absorb the water. When the gel has formed, mix at a ratio of 1 part gel to 6 parts soil.

To incorporate it into the soil dry, use 1ounce of polymer per 60 pounds of soil or 2 pounds of polymer per cubic yard of soil. To help distribute it more evenly, premix P4 with soil--1 ounce with 3 pounds of soil or 2 pounds with several cubic feet of soil--then mix it into the remaining soil. Drench the soil after planting, let stand about an hour, and drench again.

To apply it dry to existing containerplants, push holes into the soil with a pencil to about 2/3 the depth of the pot (this is less efficient than transplanting and mixing it into the backfill). For a 6-inch pot, make four holes and distribute 1/2 teaspoon among the holes (see photograph on page 252); for an 8-inch plant, make six holes and use 1 teaspoon; for a 10-inch pot, make eight holes and use 2 teaspoons. Cover with soil and drench.

Whichever way the polymers are added tothe soil, it takes at least two weeks for the roots to attach themselves to the gel. (Roots of annuals grow into it faster than roots of shrubs and trees.) Once they're attached, you don't need to water as often.

Watering polymer-treated soil is differentfrom watering regular soil. You can't judge water need by feel, since the soil can feel dry to the touch while the polymers still contain water.

If you previously watered a plant once aweek, it may now need it only once every two weeks. But, on the other hand, since you can't waterlog the soil as easily, you don't have to worry about overwatering. One of the main benefits of polymers is that if you go on vacation and skip an irrigation or the soil dries out more quickly than expected, the plant won't die.

Where to buy polymers

Broadleaf P4 is available at many nurseriesand supermarkets. A package of five blister packs that treats ten 6-inch pots costs about $4. A 5-ounce jar (enough for 300 pounds of soil) costs about $8. Larger quantities are also available. If you can't find it locally, write to Broadleaf Industries, 6150 Lusk Blvd., Suite B-103, San Diego 92121.

Photo: For plants in pots, punch holes in soil to 2/3its depth, pour measured amount of granules into holes, refill with soil, soak with water

Photo: To demonstrate how polymers work in soil, pour a packet of the sugar-like granules into water (left); 10 minutes later (right), they've expanded into clear, translucent 1/4- to 3/8-inch pieces of gel

Photo: Mixing with potting soil: put a pint of the gel pieces into a bowl with 6 pints of potting soil. Blend together well

Photo: Tomato plant roots at left had gel added to soil; same-age roots at right didn't. Gel held onto and supplied moisture to roots, greatly increasing growth. Look closely at roots at lower left to see how they grew into and through pieces of gel

Photo: Strong tops come with strong roots. Africanviolet at left has polymer gel in soil, otherwise-identical one at right doesn't

Photo: After eight days without water, lettuce growing in plain soil (right) wilted; lettuce growing in mixture of soil and polymer (left) lasted another week without water

Photo: Close-up of rootson a grass plant shows how they grow into and through squarish pieces of translucent polymer gel
COPYRIGHT 1987 Sunset Publishing Corp.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1987 Gale, Cengage Learning. All rights reserved.

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Date:Apr 1, 1987
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