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  MG Manual Reference
Ch. 2, pp. 9 - 12
[Soils: soils | properties | classes | caliche | depth | components | pH ]


1. Loam—When rubbed between the thumb and fingers, approximately equal influence of sand, silt, and clay is felt. Makes a weak ribbon (less than 2.5 cm long).
Textural Group
Textural Classes
Sandy Coarse Sand
Loamy Sand
Loamy Moderately Course Sandy Loam
Medium Loam
Silt Loam
Moderately Fine Clay Loam
Sandy Clay Loam
Silty Clay Loam
Clayey Fine Sandy Clay
Silty Clay
2. Sandy loam—Varies from very fine loam to very coarse. Feels quite sandy or gritty, but contains some silt and a small amount of clay. The amount of silt and clay is sufficient to hold the soil together when moist.Makes a weak ribbon (less than 2.5 cm long).
3. Silt loam—Silt is the dominant particle in silt loam, which feels quite smooth or floury when rubbed between the thumb and fingers. Makes a weak ribbon (less than 2.5 cm long).
4. Silty clay loam—Noticeable amounts of both silt and clay are present. Makes a medium ribbon (2.5 to 5 cm long).
5. Clay loam—Clay dominates a clay loam, which is smooth when dry and slick/sticky when wet. Silt and sand are usually present in noticeable amounts in this texture of soil, but are overshadowed by clay. Makes a medium ribbon (2.5 to 5 cm long).
In general, the finer the texture: the more difficult a soil is to work or till, the greater the water holding capacity, the slower water will enter and move through the soil profile, the more difficult plant root penetration, the more readily surface soil will crust, and the more nutrient rich the soil.
Regardless of textural class, all soils in Arizona contain sand, silt, and clay, although the amount of a particular particle class may be small.
Soil Classes
Other textural designations of surface soils are sands, loamy sands, sandy clay loams,silty clay, siltss, and clays. In each textural class there is a range in the amount of sand, silt, or clay that class may contain. These ranges can be expressed as a percentage for each soil texture. The percentages converge on the soil triangle to determine soil textural classes. The composition of each textural class does not allow for overlap from one class to another.
Single Grain
Single Grain
Usually individual sand grains not held together. Found in sandy or loamy textures.
Texture of soil influences many different characteristics. A brief comparison between sandy and clay soils will highlight these points. In general coarse-textured or sandy soils allow water to enter at a faster rate and to move more freely in the soil. In addition, the relatively low water-holding capacity and the large amount of air present in sandy soils allows them to warm up faster than fine-textured soils. However, sandy soils must also be irrigated more often than high clay soils. Sandy soils are also more easily tilled. They are well-suited for the production of vegetables, landscape plants and many types of fruit.

Cation ExchangeTop
Clay psrticles are composed of minerals that, in general, possess a negative electrical charge. In some mineral particles this charge is built in to the clay crystals, and is unaffected by changing soil conditions. This is called fixed charge. In other clay minerals, and in organic particles, the charge is altered by changes in soil acidity. This charge is called variable charge. Variable charge is greatest under alkaling conditions, and least under acidic conditions.

The negative charges of clay particles give them the ability to attract and hold positively-charged molecules ( cation). Nutrients such as calcium, magnesium, potassium, and many other soil elements are cations. The ability of the soil to hold on to cations is cation exchange capacity (CEC). Nutrients retained by CEC are prevented from leaching out of the rooting zone, yet are held lossely enough to be available to growing plants.

The soil properties most closely related to CEC are soil texture and organic matter content. Fine textured soils have more CEC than coarse-textured soils, and soils high in organic matter have more CEC than low organic matter soils. Also, non-acidic soils have more CEC than acidic soils.

Some soil particles have a small positive electrical charge, and can attract anions (negatively-charged molecules.) Anion exchange capacity is usually insignificant in Arizona soils.
Porous granuals held together by organic matter and some clay. Found in A horizons with some organic matter.
Most soil particles are grouped together to form structural pieces called peds or aggregates, although coarse soil particles may be unaggregazed and exist as single grains. In surface soil, the structure usually will be granular unless it is very sandy. The soil aggregates will be rounded and vary in size from that of a very small shot to that of a large pea. If organic matter content is low and the soil has been under continuous cultivation, the soil structure may be quite indistinct. If the soil is fine-textured with high organic content, it may have a blocky surface structure. Substance aggregate types are platy, blocky, prismatic, and massive.
Aggregates that have a thin vertical dimension with respect to lateral dimensions. Found in compacted layers and sometimes E horizons.
Air and water movement within the soil is closely related to its structure. Good structure allows rapid movement of air and water, while poor structure slows down this movement. Other things being equal, water can enter a surface soil that has granular structure more rapidly than one that has little structure. Soil compaction due to excessive vehicle or foot traffic or excessive tillage can greatly reduce soil aggregation and water infiltration. Since plant roots move through the same channels in the soil as air and water, good structure allows extensive root development whereas poor structure discourages it. Water, air, and plant roots move more freely through subsoils that have blocky structure than those with a flaky horizontal structure. Good structure of the surface soil is promoted by an adequate supply of organic matter. Soil structure can be protected, by working the soil only when moisture conditions are correct.
Roughly equidimensional pads usually higher in clay than other structural aggregates. Found in Bhorizons with clay.
Growing plants also change the soil structure as they send their roots into the soil for mechanical support and to gather water and nutrients. The roots of plants, as they grow, tend to enlarge the openings in the soil. When they die and decay, they leave channels for movement of air and water. In addition to the plants that we see, there are bacteria, fungi, nematodes, and other very small organisms growing in the soil which can be seen only with the aid of a microscope. Even these organisms enrich the soil as they die.
Structural aggregates that have a much grater vertical than lateral dimension. Found in some B horizons.
Drainage Top
Soil drainage is defined as the rate and extent of water movement in the soil, including movement across the surface as well as downward through the soil. Slope is a very important factor in soil drainage. Other factors include texture, structure, and physical condition of surface and subsoil layers. Soil drainage is indicated by soil color. Clear, bright colors indicate well-drained soils. Mixed, drab, and dominantly gray colors indicate poor drainage. Low-lying areas within the landscape receive run-off water. Frequently, the water from these areas must escape by lateral movement through the soil or by evaporation from the surface, as poor structure and other physical influences do not allow drainage through the soil. Continuous, cemented hardpan layers such as caliche also greatly reduce the internal drainage through a soil profile.
No definite structure or shape; usually hard. Found in C horizons or compact transported material.
Too much or too little water in the soil is equally undesirable. With too much water, most plant roots will suffocate due to a lack of oxygen. Where there is too little water, plants will wilt and eventually die. The most desirable soil moisture situation is one in which approximately one-half of the pore space of the soil is occupied by water.
To test soil drainage, dig a hole the size of a five-gallon bucket. Fill the hole with water. Let the soil absorb the water for an hour or two. Then fill it again with water. The hole should drain within 24 hours. If not, then plants should not be planted in that location. Perhaps caliche is a problem or some other physical barrier is present.

Compare How Much Water Different Soils Hold
Students will be able to:
  1. determine that soils higher in organic matter have higher water holding capacities,
  2. examine texture and structure influence water holding capacity.

You will need 2 cans of equal size (coffee cans will do); two 18-inch squares of cloth; some heavy string; a package or similar scale that weighs up to 64 ounces or 2,000 grams; and a container of water, such as a 2 or 3 gallon bucket or a 5-quart oilcan with the top cut out; old fashioned lamp chimneys or cylinders.

Put equal volumes of soil in the two cans. Take the soil for one from a field or garden that has been cultivated for several years and that shows lack of organic matter. This sample should be hard and cloddy. Get the other from a well-managed field where grasses and legumes have been grown, or from a good pasture or similar location. This sample should be crumbly and free from clods.

First allow the soils to dry.
Diag. 1

Diag. 2
Empty the two soil samples on the cloth squares, pull the corners together, and tie with a heavy string. Weigh each sample and record the weight.

Saturate each bag of soil by holding it in the water long enough to soak thoroughly. Remove the soil samples from the water and allow them to drain off the free water for a few minutes. Then weigh again and record the weights. Calculate the difference in weight.

Another way to measure the water-holding capacity of soils is to use two old-fashioned lamp chimneys or cylinders. Tie a cloth over the top, turn them upside down, and fill them about two-thirds full with the same two soils. Be sure the soils are both dry.
Place the chimneys in small-mouth fruit jars, as shown in the drawing. Pour a pint of water into each chimney. Then note how long it takes the water to begin to drip into the jars, how much water comes from each soil, and how long the water continues to drip.

Extension: add sawdust or other organic matter to soil and run the test again. Compare the results.

  • Which soil held more water?
  • Which soil took longer for the water to infiltrate?
  • Which soil would have more erosion?
  • Which soil would plants grow in best?
  • What did adding organic matter to the soil do?

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