Soil Chemistry

As plant material dies and decays it adds organic matter in the form of humus to the soil. Humus improves soil moisture retention while affecting soil chemistry. Cations such as calcium, magnesium, sodium, and potassium are attracted and held to humus. These cations are rather weakly held to the humus and can be replaced by metallic ions like iron and aluminum, releasing them into the soil for plants to use. Soils with the ability to absorb and retain exchangeable cations have a high cation-exchange capacity. Soils with a high cation-exchange capacity are more fertile than those with a low exchange capacity.

Hydrogen ion concentration in the soil is measured in terms of the pH scale.  Soil pH ranges from 3 to 10. Pure water has a pH of 7 which is considered neutral, pH values greater than seven are considered basic or alkaline, below seven acidic. Most good agricultural soils have a pH between 5 and 7. Though acidic soils pose a problem for agriculture due to their lack of nutrients, alkaline soils can pose a problem as well. Alkaline soils may contain appreciable amounts of sodium that exceed the tolerances of plants, contribute to high bulk density and poor soil structure. Alkaline soils are common in semiarid regions.

Soil is an essential natural resource, formed spontaneously from rock over an extensive period of time, and made up of soil solids (mineral particles and organic matter) and pore spaces filled with water and air.

Soil profile

Soil is made up of several distinct layers or horizons. These layers form what is known as the soil profile.

The top layer of soil or topsoil is the richest, having the most amount of humus (partially decomposed organic soil material). This phenomenon is largely due to the presence of decomposers (predominantly bacteria, fungi, and earthworms) that recycle dead organic matter (plants and animals) into humus. The subsoil is below the topsoil and is low in humus. However, this is where most soil nutrients are found. Below the subsoil is weathered parent material, which is full of rock particles and minerals with no humus. Parent material is the initial state of soil and can be bedrock, organic material or deposits from water, glaciers, volcanoes, or wind. Physical weathering over millions of years has broken down these materials into fine particles, and soil inherits physical and chemical properties from this parent material. Bedrock sits underneath the weathered parent material and is made up of solid rock. This solid rock will stay hidden and undisturbed until an earthquake or erosion expose it to the surface where some of it will be weathered to make way for the next batch of parent material, which starts the soil forming process all over again. However, soil is not the end product of weathering rock, it is simply a stage in the mineral cycle, and the process by which nutrients such as carbon, nitrogen, and calcium cycle between living things, and the atmosphere and soils.

What is soil structure?

The form that the soil takes based on its physical and chemical properties is termed soil structure. Mineral particles that make up soil ranges in size from fine to coarse and are categorised accordingly as clay, silt, and sand. The amount of clay and organic matter in a soil plays an important role in determining soil structure. Clays carry a negative electric charge and can attract positively charged cations and water molecules, forming small aggregates. Sand and silt do not have any charge, but are combined into these aggregates when their surfaces become coated with clay or organic matter. These small aggregates can then form larger aggregates with the help of fungal hyphae. The structure of the soil depends on the size, shape and arrangement of these aggregates, and on the pores between these aggregates.

Why is structure important in your garden?

A ‘well structured’ soil will hold large amounts of water and dissolved nutrients. The aggregates will withstand cultivation and will not ‘puddle’ when wet or become dusty or set hard when dry. The network of pores will ensure adequate drainage and aeration, which are essential for the health of plant roots. Additionally, good structure will provide an excellent medium from which seedlings can emerge and through which roots can explore for moisture and nutrients.

Grading soil structure in your garden

How to improve soil structure?

The most preferred soil structure for growing crops contains a ratio of 2:2:1 of sand, silt and clay, and is referred to as loam or balanced soil. However, a good soil structure can be obtained by adding organic matter to the soil.

Clay and hardpan type soils can be improved by adding gypsum to the soil. Gypsum tends to improve soil structure by loosening the soil, improving drainage and aeration and reducing crusting. However, you still must add organic material to the soil to add nutrients and assist good soil structure.

Note: Gypsum use on sandy soils can aggressively leach out nutrients.

Functions of organic matter

• Serves as a source of plant nutrients (especially nitrogen and phosphorus)

• Helps the formation of soil aggregates with the help of microorganisms (especially fungi), improving soil structure, aeration and water infiltration and resisting erosion
• Increases buffering of soils
• Provides sources of energy that affect the activities of both macro and microfaunal organisms
• Improves nutrient holding capacity (cation exchange capacity)
• Improves soil colour

From: www.uvm.edu