A Little Dirt on Soil

By Jeff Rugg

April 30, 2014 5 min read

Q: We retired and moved south. I used to have great black garden soil, but here the soil is brownish-red. Other people do seem to have healthy vegetable gardens in this soil. Should I add a bunch of organic matter to try to change the soil to black? I was told the red color comes from rusty iron. Why are soils different colors?

A: Healthy garden soil is made from air, water, minerals, organic matter and live organisms — in that order. Soil pores make up about 50 percent of the volume and half of the pores are filled with air and half with water. Of the 50 percent of soil that is solid, less than 5 percent is organic matter.

Soils are colored by the minerals and organic matter in them. Iron compounds cause many of the colors. The brown, red and yellow colors of soils are caused by iron-oxide minerals. A brown color is from maghemite, grayish green is from hydromagnetite, red is from hematite and the yellowish soil is colored by goethite. Black soil is the color of the carbon in the organic matter.

Soil scientists know that iron, oxygen and other chemicals bind together to form these colored minerals depending on the ratios of the minerals, temperature and water content of the parent material used to form the soil. Even though the iron-oxide minerals color the soil, they may not make up a large percentage of the soil. The iron content may not be large enough to cause any problems for plant growth. In fact, the iron content of a yellow or red soil may be below 1 percent.

If you want to add organic matter to the soil, you should do so. You may want to try a season of growing to see what the soil is like without the extra organic matter. Check with your local university cooperative extension office to find your local Master Gardeners. They will be able to answer questions on what grows well in your new soil type and where you can have your soil tested for organic matter and the percentage of iron.

Q: A friend told me that I applied my crabgrass preventer too early. He said the soil had not warmed up enough. I didn't think that the soil temperature was as important as the amount of spring sunlight. Is he right?

A: He probably is right. Many garden and weed seeds need sunlight to sprout, but the soil temperature is very important. We know that days get longer in the spring, but an early spring or a late spring is determined by the air and soil temperatures. Soils warm up slower than the air. Plants, microorganisms and insects react to the soil temperature by growing faster as it warms up. Chemical reactions also go faster in warmer soil.

Each landscape has microclimates that warm up and cool off differently. The grass in my parent's backyard is on a slope facing the sun. It needs to be mowed a week or two sooner than the front yard even though the watering and fertilizing are the same. Crabgrass preventers would need to be applied two weeks earlier in the backyard because the soil is warmer.

Crabgrass and other weeds that are annuals come up from seeds each spring. A pre-emergent weed killer like Preen can be applied to stop the growth of the seed. Some pre-emergents last six weeks and some last six months. If you apply one too soon in the spring, it may not be as effective in the summer when additional weed seeds are still sprouting. Some crabgrass seeds start sprouting when the soil reaches the upper 50s at a depth of 1 inch. Other crabgrass seeds in the same lawn start sprouting when the soil temperature reaches the mid-70s, so you need a long period of coverage from the pre-emergent. Apply the pre-emergent when you see Bradford pears in bloom.

The soil temperature is important for vegetable gardeners who want to plant seeds or plants. Seeds planted too early in cold soil can rot before sprouting. Plants planted too early in the spring may have roots that stop growing for a while, thus delaying growth longer than if they had been planted when the soil was warmer.

How fast the soil warms up depends on the sun angle and the soil's components. The wetter a soil is, the slower it will heat up because water needs to absorb lots of energy to increase its temperature. Porous, sandy soil can heat up quickly.

Many people try to rush spring. It is often better to sit back and relax. Warm weather will come.

Email questions to Jeff Rugg at [email protected] To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

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