Chemical Properties of Soils

Hi :) In this post we'll be discussing the chemical properties of soils. I'll explain cation exchange capacity, why it is important and how it changes with soil type. We'll discuss the cause and effects of soil acidity as well as the causes of salinity. In addition, we'll take a look at the nitrogen and phosphorus cycles.

Cation Exchange Capacity

Cations are positively charged ions which are attached to the edge of clay particles or organic matter by electrostatic forces. Clay particles are flat crystals that are made up of many thin sheets which are held together by hydrogen or water. This creates a large surface area which allows more cations to attach to the clay particles which are negatively charged. The total capacity of a soil to hold exchangeable cations is known as the Cation Exchange Capacity (CEC).

CEC is important because it influences the ability of the soil to retain essential nutrients and acts as a buffer against acidification. In addition, most of the exchangeable cations are needed by plants and animals. This includes ions such as calcium, magnesium, sodium and potassium ions as well as hydrogen, aluminium, and manganese ions as the soil becomes more acidic. The level of these ions and the balance between them may lead to imbalances, deficiencies or toxicities.

Soils with high clay proportions tend to have a higher CEC while sandy soils rely on the high CEC of organic matter to retain nutrients in their top soil. CEC varies according to:

• The amount of clay present
• The type of clay: smectites are the best clay type because they have the highest CEC.
• Soil pH
• The amount of organic matter present.

 Soil Acidity

Soil acidification is a natural process which has been accelerated by agriculture. Acidification has two main causes:

1. Inefficient use of Nitrogen: Nitrogen in the form of ammonia (NH4) is readily converted to nitrate(NO3 -) and hydrogen (H+) in the soil. If nitrate is not used by plants it may leach away, this results in an accumulation of H+ which ultimately reduces the pH.
2. Removal of plant material: Most plant material is slightly alkaline and the removal of this material by grazing or harvest leaves excess hydrogen ions in the soil. Over time, as this process is repeated, it leads to a decrease in pH.

 Soil acidity has several effects on the soil:

• When soil pH drops, aluminium becomes more soluble. In its soluble form, aluminium retards root growth and restricts access to water and nutrients.  
• Low pH also leads to decreased availability of nitrogen, phosphorus, potassium, sulphur, calcium, manganese and molybdenum. 
• A decreased pH also lowers microbial activity, especially the nitrogen fixing rhizobia. 

Acidic soils can be improved by the addition of lime which increases the soil's pH. 

Salinity

Salts are carried inland from the ocean by wind and rainfall and they have been accumulating in clay sub-soils for long periods of time. Before European settlement in Australia, native vegetation used up most of the rainfall. This kept the water table low and the salt remained deep in the soil profile.

When the Europeans arrived, the widespread clearing of vegetation and the use of shallow-rooted annual plants, which use less water than native vegetation, resulted in rising water tables. Salt rises with the water tables and evaporation leaves the salt at the surface. Rising salinity reduces soil productivity and kills vegetation.
 
Phosphorus

Phosphorus is essential for plant and animal growth and is one of the most critical and limiting nutrients in agriculture. Unfortunately, it is almost universally deficient in Australian soils in their natural state and this results in stunted plant growth.

Phosphorus fertiliser is added to soils in a water soluble form. This then reacts in the soil to form insoluble and more stable compounds which are inaccessible to plants. There is competition between the soil and plants for access to phosphorus as only 5-30% of the phosphorus applied will actually be used by crops.

Nitrogen

Nitrogen is needed by plants in larger quantities than any other nutrient. Most nitrogen in soils is present in an organic form, that is it's associated with organic matter, plant residues, organisms, and animal waste. Organic nitrogen is mineralised to inorganic forms such as nitrate and ammonia by microbes and this occurs slowly over the growing season providing a steady supply of nitrogen to the plants. For this course we only need a general understanding of the nitrogen cycle. This website provides an excellent explanation of the topic.


That's all for this post, if you have any questions please feel free to ask :)