MAPPING SOILS IN THE SEMI-ARID REGION OF TEXAS.

By M. W. Beck, U. S, Bureau of Soils.

Texas with an average yearly rainfall of more than 50 inches inthe eastern part and less than 10 in the extreme western sections hasa great variety of soils. On these, the climate has left its mark,and in many instances, the effect is plainly seen in the present soilcharacteristics. The humid region of the state extends westward toabout longitude 98°, where the average rainfall drops to approximately30 inches. The yearly average continues to decrease westward until atabout longitude 102° the rainfall averages around 20 inches. The zonebetween the 30 and 20 inch precipitation lines is the sub-humid beltwhile that west of the 20 inch line is semi-arid. Much of the semi-aridregion country is so dry as to constitute desert* The rainfall dropsto 10 inches or less west of the Pecos River. With changes in climaticconditions to the west there are also corresponding changes in the con-ditions of the soil, with series quite different fron those in thehumid sections*

The soils in this section are buff colored, and high in lime.They are also hard, when dry, immediately below a thin surface layer.AS you go east, the soils gradually become red* brown, or black. Thesoils of the semi-arid region are characterized by a surface mulch,or desert crust of from 1/16 to 1/8 inch in thickness. Immediatelybeneath this mulch is a crumbly granular layer of from 3 to 8 inchesin thickness and this is underlain by a hard subsoil, when dry.

Mapping soils in the sub-humid and semi-arid regions is carriedon in much the same general way as in the humid region. Here norecent base maps are available and it is necessary for the soil sur-veyor to construct his own map. In detailed soil survey work, two men^el along to best advantage using a Ford car; one man operating theplane table, while the other maps the soils. The plane table man alsoaids in examining the soils when complexities arise. After the publicroads are surveyed, and the soils mapped along them, the interveningareas are mapped, either by talking, or by driving across the treelesscountry, keeping located-by sighting ?«.nd measuring on some such objectas a windmill. Owing to the rather general uniformity of soils, andtheir occurrence in good sized areas in this region, it is frequentlypossible to cover large areas in a day; sometimes as much as 5 to 10square miles in some localities. This speed is due in part also tothe possibility of driving nearly anywhere at will. It is advisableto use the puncture proof type of automobile tire, as nearly all thevegetation, which is generally shrublike, is thorny. Such scatteringgrowth as the mesquite, chaparral, cacti, Spanish dagger, guajillo,allthorn, and many others present a constant menace to tires* Vegeta-tion in this dry country aids materially in mapping the soils sincemany of the types have a characteristic grass, weed, or shrub growth,

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and sometimes these are so distinct and constant that soil boundariescan be drawn from sane distance with certainty of being approximatelycorrect. As, for example, in Reeves County, Texas, the deep phase ofthe Reeves silty clay loam in the natural state, practically everywheresupports a growth of low greasewood, and green sage, Awhile another soil,the Verhalen clay, supports only Tobossa grass.

In the region west of the Pecos farming is not likely to succeedwithout irrigation. Some small areas of rich soils, such as those ofthe Toyah Valley, at the foot of the Davis mountains, are irrigated,and give excellent yields, Large areas of land topographically favor-able to irrigation contain so much water soluble salts, that even withirrigation, a very efficient system of drainage must be provided,before successful farming can be carried on, because of the probabilityof damage to crops by excessive accumulation of alkali upon the surface.This does not apply to soils having naturally good underdrainage, asthose in the Toyah Valley* In mapping soils here, it is, therefore,necessary, to keep a careful eye upon the subsoil porosity, as wellas the presence of hardpan and other subsoil conditions that may retardunderdrainage and cause "subbing", or allow a too rapid percolationand loss of irrigation water.

It is necessary here to make frequent determinations of the al-kali, particularly in the stream bottoms, where the land is largelyused or attempted to be used for crop production. These determina-tions are made by the Wheatstone Bridge in the usual way. The contentof alkali varies so much within narrow limits that it is generallybest not to undertake to map out the variations, but instead to showthe results of occasional determinations on the map, and to bring outin the soil survey report the general condition of each type with respectto alkali. Scoe types and phases run pretty consistently high or lowin soluble salts, and it is possible in many localities to give suffi-cient information relating to the alkali condition by report descrip-tions.

Among the richest soils of the Pecos Valley are the alluvial soils.Considerable areas of these under irrigation have given much troubleowing to alkali accumulations as a result of failure to provide ade-quate drainage outlets in the beginning. Reclamation of them has beencarried out near Barstow with success.

It is important in this region to watch the mineral composition ofthe soils, such as the presence of large amounts of gypsum. Some ofthe types hare, such as the Pecos and the Arno of the Pecos River bottom,are almost duplicates of other soils found further east in Texas in theirsuperficial appearance; but when examined closely it is seen that theycontain an enormous amount of small gypsum crystals, which make thesetypes markedly different chemically from those of the more humid regionsSoils are found which in color and texture resemble closely the Norfolksand of east Texas-, which is a highly quartzose sand, Submitted to the

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final test of chewing he material, it is discovered that the graincrushes to an inpalpable powder without disclosing any real grit, Suchsands are composed entirely of gypsum particles.

In the lower Rio Grande Valley a very unusual soil condition isencountered in the hillocks and ridges of clay occurring on the leasides of the Valle Saladas or low salt flats which are intermittentlycovered with water. These elevations frequently rise to 40 or 50 feetabove the prevailing flat plain; the sides facing the salt flats arecharacteristically steep but they fall away from the flats by longgentle slopes to merge with the surrounding flat country in such away that the boundaries are determinable only by soil differences.Through these clay hillocks thin seams of sandy material are frequentlypresent. The question is how did such material accumulate in thisregion of low marine development.

The process is as follows: when the salt flats are dry, the clayeymaterial assumes a loose, f lu f fy structure subject to easy transporta-tion by wind. Particles of this material are driven by the Texasmonsoons toward the north sides of the flats and up and over the ad-jacent country. In this way the clay dunes are built. The seams ofsand undoubtedly are due to deposition of sand driven by heavy windsfrom the more remote sandy lands near the beach. At first glance thedunes resemble water deposited material but it is to be observed thatthe sandy and clay layers do not lie in a horizontal position butslope downward from the higher part of the dune.

There are real dune sands in this western region, and very largeareas of thea as in the vicinity of Monohan, just east of the PecosValley. These are composed largely of fine quartz sand, and atfirst appearance resemble the loose Norfolk sands of east Texas, andother parts of the Coastal Plain, Wien one undertakes to bore intothe soil, however, it is found that the sand is not loose but isnoticeably hard. It would seem that such sandy land would be remark-ably deficient in moisture, and under natural conditions, it prob-ably is, but some of the sandy soils of the general region under cul-tivation conserve moisture very much better than the associated heavylands. In Andrews and Gaines Counties extensive areas of sandy landare used in the production of the sorghums, corn and cotton. This isa very dry locality, yet frequently good crops of corn are producedon these sandy soils, even though the region is generally decidedlytoo dry for success with this crop. These corn lards are not realdune sand but they are decidedly sandy soils, with a sandy clay sub-soil at depths of usually about 2 feet»

There is an interesting type of road building in these sandylands. Travel is begun through the thick shin-oak growth, and soonthe vegetation is destroyed, after this the wind picks up the loos-ened sand and blows it out of the original road until finally thehard sandy clay subsoil is reached and this gives an excellent roadbed*

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In the serai-arid region the soil surveyor finds problems that areentirely dissimilar to those of the humid region. His soils stay sodry for such long periods that he has at times to use crowbars, shovels,picks, and posthole diners to make a hole. Determination of texture isalso rather difficult, and due to high lime content, it is probablethat the clay, and silt grains art; cemented together, giving the soila sandy feel when in reality there is very little sand present. Ithas been found necessary to resort to an effective but unsanitary method,which may be shocking to the effete easterner. I refer to the readyapplication of saliva to the soil to moisten it. This is not a joke,but really a. valuable way to determine the texture in a accents time ina region where water is so scarce that a man can drive 40 miles andnever see a drop.

In this region, railroads are scarce, towns are very far apart,and the country is very thinly settled. The soil surveyor needs tocarry extra supplies of water, gasoline, oil, and whatever car partsare most liable to be needed lor repairs on the trail. On the otherhand, the climate is favorable for work many days of the year, thehealthfulness of the region is unsurpassed and the hospitality of therancher is as broad as his native plains.

The soils of the sub-humid region are characterized by a richdeposit, or beds of soft, white material, consisting mainly of calciumcarbonate, This lies at from 18 inches to 3 or 4 feet "below the sur-face, depending on the location and soil. Going westward, this mater-ial appears nearer the surface, and as the arid region is approached,the material lies at the surface in many places. Here also, itcarries other salts, such as calcium sulphate, sodium chloride, andsodium sulphate,

This material, on exposure to air becomes very hard and looks muchlike limestone. It is called caliche, and cones from the soil abovennd subsoil below.

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