Soils Agronomy Principles and Practice

This unit of study deals with a very important aspect of agronomy soils Soil is one of the most basic and life essential resources of the world It has a wide range of influence on human society affecting the quality of our food our water And our lifestyles you Should now stop the VCR and study the section entitled soil forming factors in your text Then return to the VCR for further tutoring As You have just studied there are five soil forming factors climate vegetation Topography parent material and time Let's analyze these further These factors are not independent rather they interact with each other to influence soil formation For instance climate the amount of moisture Frost heat wind and sunlight all play a primary role in influencing the rate of weathering of basic minerals Climate can influence the kinds and amounts of plants or vegetation growing in an area thus exerting a secondary influence on soils The type of vegetation in an area can affect soil formation and soil characteristics for example short grasses and bunch grasses are found in the west central region of the US which receives a relatively small amount of precipitation therefore lime or calcium carbonate accumulates in the upper portion of these soils known as a ridah Sol's The shorter grasses in this region produce less organic matter which is limited to the top few inches of soil The climate in this area will support tall grasses thus distributing organic matter to greater depths in the soil in The eastern central region of the u.s.. There is more rainfall The lime is leached to greater depths in the soil and the soils are known as alpha Sol's The climate in this area will support tall grasses with deeper root systems thus distributing organic matter to greater depths in the soil Soils under forest vegetation are different from soils under Prairie vegetation Natural forest soils have a dense layer of organic matter on the soil surface In contrast to Prairie soils that have a greater distribution of organic matter in the soil The topography or slope of the land is the third factor which affects soil formation Slope influences the rate of soil erosion which in turn affects the depth of soil Slope can also influence soil drainage and indirectly the weathering or aging process a Profile or vertical cross section of soil may have different layers or horizons known as a B and C The a horizon is located in the upper layer of soil The B horizon refers to the subsoil or zone of clay accumulation and The C horizon refers to the parent material The characteristics of these horizons in a profile helped distinguish different soil types Parent material is the fourth factor in determining the soil type? Remember that our soils throughout the United States and the world were formed from the various types of parent materials ranging from sedentary to transported parent materials soils derived from different parent material have different characteristics which can influence crop management practices Time is the fifth soil determining factor The length of time helps determine the overall impact of the first four factors on soil formation Note the well defined a B and C horizons in the mature soil as compared to the young soil The mature soil has a definite profile development and the a B and C Horizons are not as well defined in the young soil The next section for study is the section on properties of soil you Should now stop the VCR and study the section on physical properties of soil and return to the VCR for further tutoring Now that you have studied the physical properties of soil let's review some major points First of all soils are composed of minerals organic matter air and water Soil tilth refers to the condition of the soil for crop growth The tilt of the soil is influenced by physical chemical and biological properties of that particular soil Texture is the proportion of different sizes of individual soil particles generally soils are said to be heavy if they contain a large portion of small particles such as clay and tend to be sticky when wet and cloudy when dry Soils are said to be light if they contain a large portion of large particles Such as sand And they generally are less sticky when wet and they are usually less cloudy than clay soils The soil textural triangle is used to help classify or describe soil texture for example suppose a soil has 30% sand and 60% silt the Intersection of these two lines falls in the textural class of silt loam and the soil would therefore be classified as a Silt loam soil From the soil textural triangle You can see that loam and silt loam soils have significant percentages of sand silt and clay This provides good soil drainage aeration and nutrient holding capacity an Ideal loam soil for optimum crop growth generally is composed of about 45% mineral solids 5% organic matter with the remaining 50% being pore space that could be filled with air or water preferably 25% air and 25% water for the best crop growth Soil structure is the way in which the individual mineral particles are arranged into aggregates or peds? Structure affects the water holding capacity water infiltration rate and soil permeability of water and air The root environment for optimum growth is greatly enhanced by good soil structure Which in turn can improve crop yields? Crop management practices and cropping systems can affect aggregate size and therefore affect soil structure and tilt Usually cropping systems that include a perennial grass, or leg you meadow are tilled less and have higher organic matter conditions that improve soil aggregate structure Soil structure can be maintained improved or destroyed Management decisions such as fertility level crop selection and rotation the timing selection and operation of tillage implements all affect the soil structure The next section for study is chemical properties of soil you should now stop the VCR and study soil fertility and mineral nutrition and return to the VCR for further tutoring Now that you have studied soil fertility and mineral nutrition let's review some important points Organic matter in addition to improving soil structure, there's also an important source of nutrients for crop and soil microorganism growth Soil microorganisms help decompose organic matter in the soil releasing mineral nutrients soil fertility and soil organic matter benefit each other good soil fertility practices stimulate higher soil organic matter and higher soil organic matter stimulates higher soil fertility an Example of the nutrient requirements for producing high crop yields is shown in your text the nutrients and substances supplied by air our water oxygen and carbon These are required in much greater amounts than the soil supplied nutrients The soil supplied nutrients can be broken into primary secondary and micro nutrients depending on the amount required by crops The primary and most heavily required nutrients are nitrogen phosphorus and potassium It can be very expensive in terms of money and energy to meet the high nutrient demand of crop plants There are many elements that our soil supplied and are essential or necessary for plants to grow and complete their life cycle These elements play many important roles ranging from components of proteins chlorophyll and cell walls to processes such as photosynthesis respiration and cell division When essential nutrients become limiting plants usually show outward or visible signs of nutrient deficiency in the case of nitrogen deficiency symptoms yellowing of leaves occurs first in older leaves and then if the deficiency continues in younger leaves This is because nitrogen is translocated from the older leaves To other younger more rapidly growing parts of the plant Legume plants with a phosphorus deficiency may have a darker green or blueish green cast on the leaves Grass leaves with the phosphorus deficiency have symptoms of turning reddish purple phosphorus deficiency often appears first in younger leaves of crop plants With potassium a deficiency symptoms usually occurs on older leaves In grasses the leaves of deficient plants may yellow at the leaf tip and around the margins Black spots on leaves may occur on some dicotyledons plants Leaves of legumes that are deficient in potassium are often speckled the degree of availability of soil nutrients for plant growth Is influenced and conditioned by many different factors an? Efficient fertility program is one that optimizes yields and maintains good soil tilth without excessive fertilizer use Soil fertility testing is one of the first essential steps in an efficient fertility program Fertility needs can vary according to soil type within a field from season to season from crop species to crop species and at different growth stages Knowing the fertility needs of your soils and crops Can result in optimum yields while keeping the cost to you and the environment at a minimum The next item for discussion is the section on soil acidity and alkalinity? You should stop the VCR study this section in your text and then come back to the VCR for further tutoring Now that you have studied the section on soil acidity and alkalinity Let's review some important points the pH of a soil expresses the degree of acidity or alkalinity of a soil and plays an important role in regulating nutrient availability in the soil when the soil is at a pH between 6 & 7 Most of the minerals needed by the crop are more available than at most other levels of pH If the pH is strongly acidic or strongly alkaline? several of the mineral nutrients become less available Many crops require a pH between 5.5. And 7 for high production however, there are some exceptions to this rule as indicated in your text and Sometimes this is related to disease organisms and other environmental factors of the crop One should also realize that soil pH affects soil organisms and that an optimum pH range for many crops is Also an optimum range for the desirable soil organisms Factors affecting soil pH can vary some factors can lower soil PH such as high rainfall or adding nitrogen fertilizer Others such as poor soil drainage or adding lime can raise soil pH Some factors can be adjusted by the farmer and others are inherently tied to the climate or parent material of the soil in General it is easier and less costly to raise soil pH by adding lime than it is to lower pH by adding acid Forming materials such as sulfur Iron is an example of an essential element whose availability is determined by soil pH Iron becomes less available in soils as soil pH increases To a point where iron chlorosis appears Iron chlorosis is a yellowing of the leaves due to iron deficiency Because it is difficult to lower the pH of the soil other remedies are used to lessen the damage One of the remedies is to breed varieties that are more resistant to the iron chlorosis found under high, ph soils Another possible solution is to supply iron as a foley. N't spray or one that is sprayed on the leaves of the affected plants The next item for discussion is crop rotations and soil fertility you should now stop the VCR and study this section in your text and return to the VCR for further tutoring Now that you have studied the section on crop rotations and soil fertility Let's review some important points in Chapter 2 you learned various types of cropping systems such as continuous cropping or multiple cropping systems Cropping systems can influence soil fertility in many ways as noted in your text For example nitrogen fertilizer for grass crops like corn can be reduced when grown after legume crops This is due to the nitrogen remaining in the soil from the nitrogen fixation activity of legume roots For instance that is estimated that soybeans supply one pound of nitrogen to next year's crop for every bushel produced The last section for study is on biological properties of soils you should now stop the VCR and study the section on biological properties of soils and return to the VCR for further tutoring Biological properties of soils Most people would think of soil as an inert body But actually it is a dynamic environment that is home to many different soil organisms It is estimated that one gram of healthy soil contains one to four billion bacteria 8,000 to 1 million fungi and up to 100,000 soil algae Earthworms in one acre of soil weigh between 90 and 455 kilograms or between 200 and 1,000 pounds Soil and crop management practices can influence the soil environment for macro and micro organisms which in turn can influence crop growth Some microorganisms such as bacteria usually prefer natural or slightly alkaline soils Rhizobium is a nitrogen fixing bacteria Thus legume nodulation and nitrogen fixation would be better under near neutral soil conditions than acidic conditions In addition crop diseases caused by fungi are usually more serious under acidic than alkaline soil Soil organisms decompose the plant and animal residues added to the soil Decomposing microorganisms such as some bacteria and fungi are important in breaking down crop residues Mycorrhizae are a special type of fungi that colonnades on root surfaces the fungal hyphae Extend into the soil helping to absorb more water in nutrients Creating a symbiotic relationship with the plant host Macro organisms such as earthworms are also important in organic matter decomposition as well as improving aeration drainage soil structure and soil tilth Availability of nutrients in the soil especially nitrogen can influence the population and growth rate of soil organisms and consequently the rate of residue decomposition and release of nutrients from organic matter In fields with a lot of crop residue mixed in the soil crop plants can show temporary nitrogen deficiency This is observed particularly if the crop residue is low in nitrogen and the planted crop is a non leguminous species The soil organisms are using up available soil nitrogen due to their rapid growth and the residue rich conditions after a period of time the soil Organisms will begin to die and the nitrogen in their bodies will be released into the soil to be absorbed by plant roots Hopefully from this chapter you have learned that soil is a dynamic world of physical chemical and biological processes It truly is one of our most basic and essential resources This is the end of the tape for the chapter entitled soils you should now be ready to try the self-evaluation Test at the end of the chapter in your text