SCSB# 395
MLRA 86: Texas Blackland Prairie
C.L. Munster and R.L. Rhykerd
Texas A&M University

Chapter Contents

The Texas Blackland Prairie (MLRA 86) is located in east central and south central Texas and consists of about 49,700 km2 or 12.3 million acres.

Annual precipitation in major land resource area (MLRA) 86 averages approximately 1000 mm with over 50% falling between April and September, which includes the growing season for most crops. Snowfall ranges from 76.2 mm per year in the northern counties to very rarely occurring in the southeast central counties where there is no measurable snowfall in 80% of the winters. The average relative humidity is about 60% during midday and 85% at dawn. During the summer months, the sun shines 75% of the time possible and 50% of the time during the winter months. Winds prevail from the south-southeast. Highest average wind speeds occur during April and range from 20 to 25 km/h. In the winter months the average daily high temperatures range from 7°C in the northern counties to 11°C in the southern counties, and daily low temperatures range from 0°C in the northern counties to 5°C in the southern. In the summer, the average daily temperature is about 28°C. Average annual temperature is approximately 18°C. Frost-free days range from 275 in the southern part of this region to 232 in the north. The average number of growing degree-days ranges from 5751 in the northern to 6961 in the southern part of this region.

Geology and Topography
MLRA 86 is mostly gently sloping with some parts nearly level and some moderately steep to steep. Elevation ranges from 60 to 250 m.

The soils formed under timbered savannah, post oak, and grass. The soils formed under timber are light colored fine sandy loams and loamy fine sands, while those formed under grass are dark fine sandy loams, clay loams, and clays. If unprotected, these soils are subject to erosion.

Nearly all of MLRA 86 is in farms or urban usage. Of the agricultural area, 10.8% is cropland and 63.4% is classified as mixed in various combinations of cropland, improved pasture, range, and woodlands. Farming and livestock operations provide the livelihood for much of this area. The main agricultural enterprises are beef and dairy cattle and row crops, which includes corn, cotton, grain sorghum, wheat and peanuts. During early settlement much of the land was cultivated. How-ever, soil erosion from many of the rolling and slopped soils has reduced yields of row crops and some of this land has been removed from use for row crops.

Natural Resources
Soil is a valuable resource in this MLRA. In addition, petroleum and natural gas are major resources. Their production provides income for many landowners and is very important to the overall economy of this region. Sand and gravel pits provide further useful resources.

Water Resources
Water is relatively abundant throughout the region. Precipitation is adequate for crops in most years. In 2 out of 10 years, rainfall from April to September is less than 400 mm, which leads to reduced yields and in other years excess rainfall can delay planting or interfere with harvesting. About 30 thunderstorms occur per year, mostly during the summer. Additional water is available from underground sources and from lakes and reservoirs such as Lake Summerville and Lake Texoma.

Most of the soils in this region are well drained and moderately well drained, clayey and loamy soils on uplands. The major soils are of the Houston Black, Crockett, Heiden, Wison, and Austin series, which represent approximately 42% of the soils in this resource area (Table 1). STATSGO soils are presented in Fig. 1.

Fig. 1. STATSGO soils of MLRA 86 Texas Blackland Prairie.

The Houston Black series consists of deep, clayey soils. These soils formed in calcareous clays and marls. The Crockett series consists of deep, loamy soils on uplands. These soils formed in alkaline marine clay. The Heiden series consists of deep, clayey soils on uplands. These soils formed in clayey marine sediment and range. The Wilson series consists of deep, loamy soils on nearly level and gently sloping terraces. These soils formed in ancient alluvium. The Austin series consists of moderately deep, clayey soils on uplands. These soils formed in chalk. Various physical properties of these soils are presented in Table 2. Many of these clay soils show a significant temporal change in hydraulic conductivity depending on the water content of the soil and may have a very high initial infiltration rate depending the moisture status of the soil. Infiltration rates after prolonged dry spells may be 100 to 200 micrometers per second (Lin et al., 1998; Lin et al 1997).

Literature Cited
Lin, H.S., K.J. McInnes, L.P. Wilding, and C.T. Hallmark. 1998. Macroporosity/moisture effect on infiltration rates in vertisols and vertic intergrades. Soil Sci. 163:2-8.

Lin, H.S., K.J. McInnes, L.P. Wilding, and C.T. Hallmark. 1997. Low tension water flow in structured soils. Can. J. Soil Sci. 47:649-654.

USDA, 1998. NRCS. National Cartogrophy & Geospatial Center, Fort Worth, Texas.

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