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Excerpts from Jim Conrad's
Naturalist Newsletter

YUCATAN'S RED SOIL

from the January 29, 2012 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO
THE YUCATAN'S RED & BLACK SOILS

Just north of Pisté in a cornfield lying fallow for the dry season the soil is thin and red, with much limestone bedrock emerging from it, as shown above. Maybe a hundred feet away the soil is black, but anthills built of deeper-mined soil are red, as shown below:

YUCATAN'S RED & BLACK SOILS, RED ANTHILL IN BLACK SOIL

The Maya know that the black soil is richer and easier to cultivate. Black soil holds water longer and doesn't dry into hard, brittle clods as red soil does. Sometimes the Maya carry black soil to areas of red soil where they want to grow something special.

All this makes sense because black soil contains higher concentrations of organic matter -- decomposing remains of living organisms -- while red soil holds much less. And soil organic matter behaves like a sponge in soil, not only holding water there but also nutrients. Black soil dries into a crumbly state instead of bricklike like red soil. Water-saturated black soil also tends to be crumbly instead of soupy as with red soil.

The Yucatán's limestone-derived soil is red because the limestone from which it derives contains small amounts of iron. When iron is exposed to oxygen, iron oxide is formed, and iron oxide is reddish. In the mineral form iron oxide is known as the reddish hematite; on gate hinges it's known as rust.

Therefore, for as long as soil keeps developing from limestone there'll be red soil. However, black soil is fragile and easy to destroy. Expose it to air and heat and it converts to nutrients that leach away, or its molecules are oxidized and mineralized, losing their beneficial organic properties. In the Yucatán, red soil is replacing black soil.

A technical article in the journal Agronomy (vol. 22, #3, April 2002) reports on soil studies in the Yucatán where forests are cleared and burned, cornfields are planted, then, when the soil deteriorates and weeds and insects invade, the fields are abandoned and allowed to revert to forest. The whole cycle is referred to as slash-and-burn. Because of ever-increasing population pressure, the time allowed for fields to remain abandoned, or fallow, as vegetation re-enriches the soils, is ever briefer.

The study found that as the fallow time "is shortened from the traditional 25-30 years to 6-12 years, soil productivity declines... Black soils contained twice as much organic matter, more total P {phosphorus} and three times more available P than red soils. At the end of the cultivation cycle both soils had lost one quarter of their organic matter relative to the longest fallow."

Online you can search for this paper under the title "Soil fertility during shifting cultivation in the tropical Karst soils of Yucatan" at http://www.agronomy-journal.org/.

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