Lesson 44: Soil and Plants
In this lesson, we will talk about soil and its properties, the five soil
horizons, and the classification of soil by soil order. We will also learn
about how plants and mycorrhizae help in the production and maintenance of soil.
Soil consists of solid material of geological and biological origin that is
changed by chemical, biological and physical processes to support plant life.
As former President Franklin D Roosevelt said, "The nation that destroys its
soil, destroys itself."
90% of the world's food supply comes from
land-based agricultural systems. There are
4 textures of soil: sand, silt, clay and loam. Loam contains a mixture of 40%
sand, 40% silt and 20% clay.
There are 5 properties of soil:
water infiltration, water-holding capacity, nutrient-holding capacity,
aeration, and workability, or the ease with which soil can be cultivated.
Depending on the type of soil texture, there are different degrees to which
water passes through, how much water and nutrients can be retained, how much
air passes through the soil, and what types of plants can grow in that soil.
Soil Profile is a vertical slice through different layers or horizons of soil.
Taking a soil profile reveals a lot about the soil composition and its formation.
There are 5 soil horizons:
- O horizon: humus containing surface litter and decomposing plant matter
- A horizon: topsoil containing mixed humus and leached mineral soil
- E horizon:
zone of leaching containing less humus and minerals that resist leaching
- B horizon: subsoil containing leached minerals like iron and aluminum oxides
- C horizon: weathered parent material containing broken-down minerals
One of the main methods of classification for soils is soil order. There are 12
major soil orders worldwide with 4 common soil orders that are important for
agriculture, animal husbandry and forestry. They are mollisols, oxisols,
alfisols and aridisols.
Mollisols are
fertile, dark soils in temperate grasslands. This offers some of the best
agricultural soil and is located in the midwestern US, Ukraine, Russia and
Mongolia.
Oxisols are soils in tropical and subtropical rainforests. Alfisols are soils
in moist, temperate forest biomes. Aridisols are soils in drylands and deserts.
Green plants protect soil from erosion and
reduce water vapor loss. Keeping top soil is important for the health of
plants. Soil deteriorates or is renewed through the loss or addition of organic
matter.
Mycorrhizae are soil fungi that
draw nourishment from the roots of plants. They also pull out nutrients from
detritus or decaying matter in the soil and then transfer them to the plant.