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New
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Introduction The importance of the soil cannot be overemphasized when maintaining healthy plants. Taking care of the environment in which the plant root system grows is essential to producing productive plants and avoiding plant problems. Soil is the primary source of water and nutrients for the plant, and provide the physical anchor for the plant to stand upright. Managing soil and water properly and applying appropriate fertilizers when needed are basic. There are many different kinds of soils in New Mexico, from sugar sands to heavy clay to less prevalent loams. A thorough knowledge of soil characteristics is necessary to make proper decisions with regard to vegetable gardening, fruit production and/or landscaping. If the soil is to be modified, this knowledge is fundamental to soil improvement, planting, watering and fertilizing. What is Soil? Soil is produced when a parent material (rock) is acted upon by climate and vegetation over a period of time. It consists of weathered rock fragments (minerals) and decaying remains of plants and animals (organic matter). Soil also contains varying proportions of air, water and microorganisms. A soil in good condition for plant growth contains approximately 50 percent solid material and 50 percent open or pore space. The mineral component of the solid material is usually made up of many different kinds and sizes of particles, ranging from those visible to the unaided eye to particles so small that they can only be seen with the aid of a powerful electron microscope (Figure 3.1). This mineral material makes up about 45 to 48 percent of the total volume. The remaining solid material consists of organic matter, which in New Mexico makes up about 0.5 to 3 percent of the soil volume and may contain both plant and animal remains in varying stages or degrees of decomposition. Under ideal moisture conditions for growing plants, soil openings or pore spaces contain 25 percent air and 25 percent water based on total soil volume. The percentage of mineral matter and organic matter in a cubic foot of soil varies from one soil type to another and even within the same soil, depending on the kinds of plants grown, frequency of cultivation and moisture or drainage of the soil. Organic matter will usually be highest in soils that have not been cultivated over long periods of time. Soils that are frequently tilled and have relatively small amounts of plant residues left that are worked into the soil are usually lowest in organic matter. Tilling and plowing increases the amount of air in the soil, which also increases the rate of organic matter decomposition. Soils with poor drainage or high water tables usually have a higher organic matter content than those that are well-drained because the water excludes air from the soil mass and prevents oxidation of organic matter. Generally, the hotter and drier the climate, the lower the organic matter content will be in the soil. Since either air or water fills pore spaces, the amount of air in soil at a particular time depends on the amount of water present in the pore spaces. Immediately after a rain, more water and less air is present in the pore spaces. Conversely in dry periods, a soil contains more air and less water. Increasing organic matter content usually has the effect of increasing water holding capacity, but the addition of large amounts of undecomposed organic material can reduce water capacity until the material has partially decomposed. Physical Properties of Soil The physical properties of a soil are those characteristics that can be seen with the eye or felt between the thumb and fingers. They are the result of soil parent materials being acted upon by climatic factors (such as rainfall and temperature) and affected by relief (slope and direction or aspect) and vegetation (kind and amount, such as forest or grass) over a period of time. A change in any one of these influences usually results in a difference in soil formation or development. The important physical properties of a soil are color, texture, structure, drainage and depth. Soil physical properties (texture, structure, drainage and depth), chemical composition (fertility) and surface features (stoniness, slope and erosion) largely determine the suitability of a soil. Soil fertility, to a limited extent, governs what it may be used for, and to a larger extent it determines expected yields. However, soil fertility alone is not indicative of a soil’s productive capacity. The physical properties of soil usually control its suitability as a growth medium. It is easier to adjust soil fertility than it is to change the physical properties of soil. Color When soil is examined, color is one of the first things noticed. In itself, color is of minor importance, but it indicates or reflects other soil conditions that are extremely important. In general, color is determined by the following: (1) organic matter content, (2) drainage conditions and (3) degree of oxidation or extent of weathering. Soil colors vary from almost white to shades of brown, gray and black. Light colors indicate low organic matter content, while dark colors can indicate higher organic content. Light or pale colors in the surface soil are frequently associated with relatively coarse texture and highly leached conditions. These colors occur in areas that have high annual temperatures. Dark colors may result from high water table conditions (poor drainage), low annual temperatures or other influences that induce high organic matter content while at the same time, slow the oxidation (burning) of organic materials. Dark colors may also result from colors imparted by the parent material. Shades of red or yellow, particularly when associated with relatively fine textures, usually indicate that subsoil material has been incorporated in the surface. In general, subsoil colors indicate air, water and soil relationships and the degree of oxidation of certain minerals in the soil. Red and brown subsoil colors indicate that the soil allows relatively free movement of air and water. If these or other bright colors persist throughout the subsoil, it indicates favorable aeration. Some soils with mottled subsoils, especially when the colors are shades of red and brown, are also well-aerated. Soils with yellow subsoils usually have some drainage impediment. Most soils that have mottling in the subsoil, especially where gray predominates, have too much water and too little air. The red-to-brown color of subsoils comes from iron coatings under well-aerated conditions. In wet soils with low oxygen levels, the iron coatings are chemically and biologically removed, leaving the gray color of background soil minerals. Texture The relative amounts of differently-sized soil particles, or the fineness or coarseness of the mineral particles in the soil, is referred to as texture. Soil texture depends on the relative amounts of sand, silt and clay. The proportions of sand, silt and clay vary in each texture class.
Although there are about 20 kinds or classes of soil texture, most soils fall into six general textural classes. Each class name indicates the size of the mineral particles that are dominant in the soil. Texture is estimated in the field by rubbing or feeling moist to wet soil between the thumb and fingers. Accurate results can be provided by laboratory or mechanical analysis or by separation into clay, silt and various size sand groups (called separates). Regardless of textural class, all soils contain sand, silt and clay although the amount of a particular particle size may be small. Determining the amounts of sand, silt and clay can easily be determined by thoroughly mixing a measure of the soil with water and allowing the particles to settle out. The largest particles will settle out first and the last will be the clay which may take some time to settle. By comparing the relative amounts of the three main particles one can determine what the soil is. Principle Soil Classes
Soil texture influences many different characteristics. Coarse-textured or sandy soils allow water to enter at a faster rate than fine-textured or clay-dominant soils. 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. Sandy soils are also more easily tilled. They are best suited for the production of special crops such as vegetables, peanuts and certain fruits. Structure Soil particles are grouped together in the formation processes to form structural pieces called peds or aggregates. Soil structure will usually be granular unless it is disrupted. The soil aggregates will be rounded and will vary in size from very small shot to a large pea. If organic matter content is low and if the soil has been under continuous cultivation, the soil structure may be quite indistinct or massive. If the soil is fine-textured with high organic content, it may have a blocky structure. Soil structure is closely related to air and water movement within it. Good structure allows favorable movement of air and water, while poor structure slows down this movement. Water can enter a soil with granular structure more rapidly than it can enter one with little structure. Since plant roots move through the same channels in the soil as air and water, good structure allows extensive root development while poor structure discourages it. Water, air and plant roots move more freely through soils that have blocky structure than those with platy or flaky horizontal structure. Good soil structure is almost always promoted by additions of organic matter and by working the soil when it is not so wet that large clods form. Growing plants also change the soil structure as they send their roots into the soil for mechanical support and to gather water and food. As plant roots grow, they 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 visible plants, there are bacteria, molds, yeasts, and fungi and other very small organisms growing in the soil that may only be seen with the aid of a microscope. They also enrich the soil as they die. Drainage Soil drainage is defined as the rate and extent of water movement in the soil. This includes movement across the surface as well as downward through the soil. Slope or lack of slope is a very important factor in soil drainage. Other factors include texture, structure and physical condition of soil and subsoil layers. Soil drainage is often indicated by soil color. Clear, bright colors indicate well-drained conditions. Mixed, drab and dominantly gray colors indicate imperfection in drainage. Low lying areas within a field or landscape receive runoff water as well as precipitation. 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. Too much or too little water in the soil is equally undesirable. With too much water, most plants will suffocate. However, when there is too little water, plants will wilt and eventually die. For most plants, the most desirable soil moisture situation is one in which approximately half of the soil pore space is occupied by water. Soil Depth The effective depth of a soil for plant growth is the vertical distance into the soil from the surface to a layer that essentially stops the downward growth of plant roots. The barrier layer may be rock, sand, gravel, heavy clay or a partially cemented layer. Terms that are used to express effective soil depth include the following:
Soils that are deep, well-drained and have desirable texture and structure
are suitable for the production of most crops, especially fruit and nut trees.
Deep soils can hold much more plant nutrients and water than shallow soils with
similar textures. Soil depth and its capacity for nutrients and water frequently
determine the yield from a crop, particularly annual crops grown through summer. Soil Organic Matter Organic matter in soil consists of the remains of plants and animals. When temperature and moisture conditions are favorable in the soil, earthworms, insects, bacteria, fungi and other types of plants and animals use the organic matter as food. They break it down into humus (the portion of organic matter that remains after most decomposition has taken place) and soil nutrients. Through this process, nutrients are made available for use as food by growing plants. The digested and decomposing organic material also helps develop good air-water relationships. In sandy soil, organic material occupies some of the space between the sand grains, binding them together and increasing water-holding capacity. In a fine-textured or clay soil, organic material creates aggregates of the fine soil particles, allowing excess water to drain more rapidly and oxygen to move into the soil more easily. This grouping of the soil particles into small pieces (aggregates) causes the soil to be easier to work. Organic matter content primarily depends on the kinds of plants that have been growing on a soil, long term management practices, temperature and drainage. Soils that had native grass cover for long periods usually have higher organic matter content. Likewise, those soils that have native forest cover usually have lower organic matter content. In either case, if the plants have grown on soil that is poorly drained, the organic matter content is usually higher than if the same plant is grown on well-drained soil. This is because of differences in available oxygen and other substances in well-drained soil that are needed by the organisms that attack and decompose the organic material. Soils in cold climates usually have more organic matter than those in warm climates. Most New Mexico soils have less than one percent organic matter. Thus, additions of organic matter (i.e., composted manures and wood products) to the soil is a primary key to successful gardening in New Mexico. Improving Soil Structure Additions of Organic Matter Growing Cover Crops Soil Microorganisms and Earthworms Soil microorganisms are microscopic plants and animals living in the soil. They include bacteria, fungi, actinomycetes, algae, protozoa, yeast, virus and nematodes. There are about 50 billion microbes in 1 tablespoon of soil and 900,000,000,000 (nine hundred billion) microorganisms per pound of healthy soil. A typical soil may contain these estimated numbers of organisms found in each gram:
NOTE: 1 gram is the approximate weight of a standard paper clip. The microorganisms’ primary job is to break down the organic matter; first
into humus, then humic acid and ultimately into basic elements. Microbes must
have a constant supply of organic matter or their numbers will be reduced. In
addition, certain microorganisms also have the ability to fix nitrogen from the
air. Nutrients from the soil’s organic matter is made available for plant food
through microbial feeding, as microbes are constantly being born and dying.
Their dead bodies are actually an important source of not only nutrients but
also of organic matter. Earthworms. Mulching The regular addition of manures, compost, cover crops, mulches and other organic materials can raise the soil nutrient level and enhance the physical structure to such a point that the need for synthetic fertilizers is greatly reduced. These highly desirable soil qualities do not come with a single or even several additions of organic material, but rather these qualities require a serious soil-building program. A major step toward such a program is building a home compost pile. Soil Water Soil Water Terms Available Soil Water Soil Moisture Storage The size and total volume of pore space are a function of both the soil’s texture and structure. Clay soils can hold a significant amount of water because of the relatively large surface areas of individual clay particles and the large number of very small pores. Sand particles, on the other hand, have relatively small surface areas, and sandy soils contain a smaller number of pores that are larger in size. Water drains more easily from these larger pores because of gravity forces. Figure 3.5 illustrates the relationship between soil texture and the amount of water held in the soil. Both the amounts of available and unavailable water increase as the clay content of the soil increases. Thus, sands have a much lower water holding capacity than clay soils. Knowing the water holding capacity of soils is important in determining both the amount and frequency of irrigation. Soils with low water holding capacity must be irrigated more frequently with smaller amounts of water than soils with higher water holding capacity. In Table 3.2, numerical values of approximate water storage capacities are listed for soils. These values can be used as a general guide in absence of specific field data. A good source of specific soils information is contained in the county soil survey reports published by the Soil Conservation Service. It must also be noted that many garden soils have been modified significantly. Additions of organic matter, such as compost, shredded pine bark and other soil amendments can dramatically increase the water-holding capacity of a soil. Although it is generally recommended to amend garden soils, be aware that the values in Table 3.2 may vary significantly from improved garden soils. Properly improved garden soils will generally hold more available moisture, thus requiring less frequent irrigations of increased amounts. Philosophy of Irrigation In New Mexico, irrigation is considered a supplement to natural rainfall. In the absence of rainfall, landscapes, vegetable gardens and fruit orchards are irrigated. However, in arid areas of the state, rainfall may seem to only be a supplement to irrigation. Nevertheless, effective and efficient use of water resources should remain a primary concern for home gardeners. Specific recommendations for scheduling irrigation and soil improvements will be discussed in chapters on vegetable gardening, home fruit production, lawn care and landscape horticulture. Plant Nutrients and Soil pH The other 12 essential elements are: iron, calcium, phosphorus, potassium, copper, sulphur, magnesium, manganese, zinc, boron, chlorine and molybdenum. These elements come from the soil. With the exception of nitrogen, calcium, magnesium, phosphorus and potassium, there is usually enough of these elements in the soil for most plants. Other elements also associated with plant growth are cobalt, silicon and selenium. To thoroughly comprehend plant nutrients and fertilizing, an understanding of soil pH is crucial. Soil pH The measurement scale is not a linear scale but a logarithmic scale. That is, a soil with a pH of 9.5 is ten times more alkaline than a soil with a pH of 8.5, and 100 times more alkaline than a soil with a pH of 7.5. The pH condition of soil is a major soil characteristic that affects the quality of plant growth. A near-neutral or slightly acid soil is generally considered ideal for most plants. Some type of plant growth can occur anywhere in a 3.5 to 10.0 range. With some notable exceptions, a soil pH of 6.0 to 7.0 requires no special cultural practices to improve plant growth. The major impact that extremes in pH have on plant growth is related to the availability of plant nutrients and the soil concentration of the plant-toxic minerals (Figure 3.7). In highly acidic soils, manganese and aluminum can concentrate at toxic levels. At low pH values, calcium, phosphorus and magnesium are less available. At pH 7 and above, phosphorus, iron, copper, zinc, boron and manganese become less available. By applying certain materials to the soil, adjustments can be made in pH values. Soils can be made less acidic by applying a form of lime; ground agricultural limestone is the most frequently used. The finer the grind, the more rapidly it becomes effective. Different soils will require a different amount of lime to adjust the reaction to the proper range. The texture of the soil, organic matter content, the crop to be grown and soil type are all factors to consider in adjusting pH. For example, soils low in organic matter require much less lime than soils high in organic matter to make the same pH change. *(In New Mexico most soils are very alkaline and lime is rarely, if ever, required.) If pH is too high, elemental sulfur, concentrated sulfuric acid or aluminum sulfate can be added to the soil to reduce alkalinity. Many ornamental plants require slightly to strongly acidic soil. These species develop iron chlorosis when grown in soils in the alkaline range. Iron chlorosis is often confused with nitrogen deficiency since the symptoms (a definite yellowing of the leaves) are similar. This problem can be corrected by applying chelated iron sulfate to the soil to reduce the alkalinity and add iron. *(Note. Dr. Flynn of NMSU has recently added that of three possible chelating agents only the one employed by Millers contains an agent that is not inactivated in soil with a pH of 7.5 or higher. All three chelating agents are available from the Albuquerque Garden Center shop.) * This information added by Editor, RB)
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