2. Plant roots and soil relations.
Plants utilize the plant growth factors in the soil by way of the roots. The density and distribution of roots affect the amount of nutrients and water that roots extract from soil. Root growth is influenced by soil environment. By distribution and density it is possible to evaluate soil conditions. The roots at different location in the soil (topsoil versus subsoil) perform different functions or the same functions to varying degrees.
For example, most of the nitrogen will probably be absorbed by roots from the topsoil because most of the organic matter is concentrated there. In soils with acid topsoil and alkaline subsoil, deeply penetrating roots encounter a great abundance of calcium in the subsoil. Roots in alkaline subsoil may absorb more Ca than roots in an acid topsoil. Topsoil frequently becomes depleted of water in dry periods, whereas an abundance of water still exists in the subsoil. So the dependence on the subsoil for water and nutrients becomes greater. Subsequent rains may rewet the topsoil. This causes a shift to greater dependence on topsoil for water and nutrients. Most plant roots do not invade soil that is dry, nutrient deficient, extremely acid or water saturated and lacking oxygen. The available nutrient and available water are these that the roots can absorb. This absorption is dependent on the specific surface area of roots. Mathematically: uptake = availability times specific surface area.
3. Soil fertility and soil productivity.
Soil fertility is defined as the ability of a soil to supply essential elements for plant growth without a toxic concentration of any elements. Soil fertility refers to only one aspect of plant growth-the adequacy, toxicity and balance of plant nutrients. An assessment of soil fertility can be made with a series of chemical tests. Soil productivity is the soil’s capacity to produce a certain yield of crops or other plants with optimum management. The optimum management system specifies such factors as planting date, fertilization, irrigation schedule, tillage, cropping sequence and pest control. Soil scientists determine crop productivity of soils for various crops by measuring yields over a period of time. S. P. is an expression of all the factors, soil and nonsoil, that influence crop yields. For a soil to produce high yields, it must be fertile for the crops grown. But! It doesn’t follow that a fertile soil will produce high yields. High yields or high soil productivity depends on optimum management system. Many fertile soils exist in the semiarid part of Ukraine but without irrigation these soils are unproductive for most field crops.
What is soil? According to B.G.Rosanov, “soil is a complex polyfunctional open four-phased structured system located in the surface layer of the cortex of weathering which is a function of the factors of soil formation and endowed with fertility (productivity).
4. Soil texture.
Texture is the relative proportion of sand, silt and clay in a soil (Foth). The weathering and soil formation give a wide range in site of particles: stones, gravel, sand, silt and clay (including colloids). The particle – size distribution is important. It determines the soil’s coarseness or fineness, or the soil texture.
The size groups of particles are called soil separates (or fractions in Ukraine).
SOIL SEPARATES (Kachinsky’s classification)
Diameter, mm.
S
tones
>3
soil skeleton
G
ravel
3-1
C
oarse
sand physical sand 1-0.5
Medium sand 0.5-0.25
Fine sand 0.25-0.05
Coarse silt 0.05-0.01 fine earth
M
edium
silt 0.01-0.005
Fine silt physical clay 0.005-0.001
Clay <0.001
The clay fraction includes coarse clay 0.001-0.0005mm in diameter, fine clay: 0.0005-0.0001mm in diameter and colloids: less than 0.0001mm in diameter.
Surface area of the separates rapidly increases with decreasing particle size. The particles less than 1mm in diameter are called “fine earth” in Ukraine. The particles of stones and gravel form “soil skeleton”. According to the USDA classification system, sand is the 2.0 to 0.05mm fraction. Diameter of clay particles is below 0.002mm. The sand and silt separates of many soils are dominated by quartz. There is usually a significant amount of weatherable minerals, such as feldspar and mica, that weather slowly and release ions that supply plant needs and recombine to form secondary minerals such as clay. Clay particles tend to be plate –shaped, rather than spherical and very small in size with large surface area per gram. The clay fraction usually has a net negative charge. The negative charge absorbs nutrient cations, including Ca2+, Mg2+ and K+ and retains them in available form for use by roots and microbes.
Soil textural groups and subgroups (Kachinsky’s classification): “fine earth fraction” <1mm in size is divided into two subfractions: (1) physical sand and (2) physical clay (<0.01mm in diameter). To determine soil textural group by Kachinsky’s classification still functioning in Ukraine it is necessary to know the percent of either physical clay or physical sand and the prevailing type of pedogenesis (soil formation) for a given soil sample. The following nine soil groups were differentiated in order of increase of the percent of physical clay: (1) loose sand, (2) bound sand, (3) loamy sand, (4) light loam, (5) medium loam,
(6) heavy loam, (7) light clay, (8) medium clay, (9) heavy clay. To determine soil textural subgroup within a given group it is necessary to know the percent of the following fractions: (1) gravel (3-1mm), (2) sand (1-0.05mm), (3) coarse silt (0.05-0.01mm), (4) silt (0.01-0.001), and (5) clay (<0.001mm). Then it is necessary to determine the dominant fraction and the fraction immediately following it. This will allow to “develop” a subgroup name. For example, if the fraction of coarse silt is dominating and the fraction of sand immediately follows it, the subgroup name will be “sandy coarse silt”. If the group name for a given soil sample is “light loam”, then the full name of a textural class will be “sandy-coarse-silt light loam”. Such a name sounds a bit unusual for English-speaking public, but the tradition is still preserved in Ukraine.
USDA soil textural classes. They are determined on the textural triangle, which shows the limits of sand, silt and clay content of the various texture classes. The sum of the percentages of sand, silt and clay at any point in the triangle is 100. The soil can be placed in one of 12 major textural classes by which the area of a triangle is divided. Sieves can be used to separate and determine the content of the relatively large particles of the sand and silt separates. Sieves, however, are unsatisfactory for the separation of the clay particles from the silt and sand. In the USA the hydrometer method was devised for rapidly determining the content of sand, silt and clay in soil. In the hydrometer method a sample (usually 50 grams) of air-dry soil is mixed with a sodium pyrophosphate solution for about 12 hours to promote dispersion. Na4P2O7 is a good dispersing agent used in Ukraine too. The soil-water suspension is placed in metal cup with baffles on the inside and stirred on a mixer for several minutes to bring about separation of the particles. The suspension is poured into a specially cylinder and distilled water is added to bring the contents up to volume. The soil particles settle in the water at a speed directly related to the square of their diameter and inversely related to the viscosity of the water. A hand stirrer is used to suspend the soil particles thoroughly and the time is immediately noted. A specially designed hydrometer is carefully inserted into the suspension and two hydrometer readings are made. The sand settles in about 40 seconds and a hydrometer reading taken at 40 seconds determines the grams of silt and clay remaining in suspension. Subtraction of the 40-second reading from the sample weight gives the grams of sand. After about 8 hours most of the silt has settled and a hydrometer reading taken at 8 hours determines the grams of clay in the sample. The silt is calculated by difference.
Texture and the use of soils. Plasticity, water permeability, ease of tillage, fertility and productivity are all closely related to soil texture. Many clay soils expand and shrink with wetting and drying, causing cracks in walls and foundation of buildings. Maximum productivity for red pine (“Pinus resinosa”) occurs in coarser-textured soil than for corn. Loam soils have the greatest productivity for corn. But when soils are irrigated and highly fertilized, the highest yields of corn occur on the sand soils. A claypan is a dense compact layer in the subsoil. Claypan soils are well suited for flooded rice production. Tobacco likes sandy soils. Vines like skeletal loams with pebbles and gravel. Potatoes like sands and loamy sands and so do the strawberries. Ukrainian soils formed mostly in loess which is loam, silt loam or clay loam are ideal for wheat and other small grains. Sugar beet also shows good performance on such soils. Field method of soil texture determination will be considered at the next lecture.
LECTURE THREE.
Soil organic matter.
Study outline:
1. Sources and composition of SOM.
2. Residue decomposition and humus formation.
3. Agronomical and ecological roles of SOM.
4. Maintenance and balance of SOM.
Almost all life in the soil is depended on organic matter (OM)for nutrients and energy. People have long recognized the importance of OM for plant growth. Although OM in soils is very beneficial, Justus von Liebig pointed out about 170 years ago that soils composed entirely of OM are naturally infertile. SOM may be very much responsible for the formation of soil productivity and play a significant role in ecosystems. And the productivity of minerals, not organic soils is very much increased by colloidal OM (humus) in them.