L earning Objective 12

12. Explain soil structure, its source, function, and its role in plant growth.

I. The Function of Soil for Plants.

Soil (the term soil is different from "dirt" which refers to many substances, not necessarily soil in origin) supports plants, holds water and oxygen, and provides minerals necessary for growth.

II. SOIL COMPOSITION

Soil is made up of biotic and abiotic elements.

A. Biotic elements include bacteria, fungi, nematodes, insects, earthworms, and rodents, to name a few.

Also all dead remains of plant and animal life form a substance called HUMUS which increases the organic content of the soil. Humus is an imporant part of the recycling of chemicals needed for life as well as adding important beneficial properties to soil, such as water-and air-holding capacities.

B. Abiotic elements forms from the parent rock which underlies the soil. The parent rock weathers and breaks into small pieces. (See figure 12-A)

These elements provide structure of the soil as well as mineral, such as nitrogen, phosphorus and potassium.

A. Particles are classified by size such as: large (like sand), small (like clay) and many in-between sizes.

B. Particles of different sizes have different water-holding and air-holding capacities. (See figure 12-B)

 

1. For example, sand provides many air spaces but holds water poorly. What "problem" does the plant have in sandy soils and how is it manifested?

 

2. Clay, or small particles, on the other hand, hold water well,but provide few air spaces (which contain oxygen). What "problem" does the plant have in clay soils and how is it manifested?

 

C. The best soil combination is loam with particles of all sizes. What impact does loam have on plant growth and why?

D. Humus or any organic material both holds water, oxygen and slowly releases minerals into the soil. What are the benefits it provides the plant and how does it effect growth? How is humus supplied naturally?