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Plant/Animal Production
The physical and chemical characteristics of soil
Healthy soil is the basis of a well managed production system
This material addresses aspects of the following syllabus outcomes:
H1.1 explains the influence of the physical, biological, social, historical and economic factors on sustainable agricultural production.
H2.1 describes the inputs, processes and interactions of plant production systems.
The work presented in the following section contributes towards understanding the following syllabus content areas:
- the chemical and physical characteristics of soil
- techniques to maintain and/or improve soil fertility, including alternative strategies to the application of inorganic fertilisers
- the importance of agricultural chemical labels as they relate to safe practice and correct usage
Source: NSW Board of Studies Stage 6 Agriculture Syllabus
Soils have particular characteristics that can be categorised into two groups:
| Physical characteristics | Chemical characteristics |
|---|---|
| *structure | *acidity |
| *texture | *nutrient content |
| *bulk density | *cation exchange capacity |
| *porosity | |
| *permeability | |
| *shear strength | |
| *soil profile | |
| *colour |
It is important that a farmer who works with the soil is aware of these characteristics. These characteristics will determine the success or failure of any crop grown in the soil.
Chemical characteristics of soil
The pH indicates the acidity or alkalinity (basic) of the soil. Different plants have differing optimum soil pH requirements. The majority of plants prefer a pH of around 6 to 7, which is very slightly acid.
The soil pH is important in determining the availability of soil minerals. At pH extremes some minerals are "locked up" and cannot be used by the plants, whilst other minerals may become toxic. e.g. aluminium, iron, zinc, copper, manganese and boron become more soluble under acidic conditions and can reach toxic levels.
The application of different fertilisers can affect the pH of the soil.
If a soil is too acid then it can be modified by the addition of lime. If it is too alkaline then elemental sulfur may be added which is oxidised by specific soil bacteria to form sulfuric acid. Ploughing in green manure crops and animal manures also lowers soil pH.
Soil pH can have an effect on microbial activity in the soil.
Plant nutrients
Plants require a number of essential nutrients for growth and development. Both the soil and the atmosphere can provide these nutrients. Some of these minerals are needed in large amounts (major elements or macronutrients) and others are needed in smaller amounts (trace elements or micronutrients).
| Macronutrients | Micronutrients |
|---|---|
| phosphorus | copper |
| nitrogen | molybdenum |
| potassium | chlorine |
| sulfur | zinc |
| calcium | boron |
| magnesium | iron |
| hydrogen* | cobalt |
| carbon* | manganese |
| oxygen* |
Alternatively the elements are classified into 3 groups.
- Major Elements: N, P, K
- Secondary Elements: Ca, Mg, S
- Trace or Minor Elements: Fe, Mn, Cu, Zn, B, Mo
What do you think is different about the elements marked with an asterisk in the table?
Make sure you know the symbols for each of the elements.
Fertilisers are used to improve the production of plants and the animals that are dependent on the plants. If a crop is harvested, large amounts of elements are removed from the farm which must be returned to the soil for subsequent crops. Elements removed by animals grazing pasture plants are not returned to the soil if the animal products, such as wool and milk leave the farm.
Different plants require different amounts of different nutrients. Many vegetable crops, such as broccoli and lettuce, are very heavy feeders and when grown on the same land, continuously deplete the soil of valuable nutrients. Rotating the crops grown on a particular area and growing green manure crops helps in balancing the nutrients removed from the soil.
Fertilisers
Fertilisers may be classified into one of two groups:
- Organic fertilisers
These fertilisers include animal manures and animal or plant by-products. Animal manures contain small quantities of nitrogen, phosphorus and potassium, which vary with the kind of animal and its diet.
Organic by-products include blood and bone, bone dust, cotton seed meal etc. Organic by-products generally have a higher nitrogen content than animal manures.
Green manuring is also an organic method of improving soil fertility and the levels of organic matter.
- Inorganic fertilisers
These fertilisers are derived from inorganic sources and require more processing in their manufacture. They can be manufactured with varying mineral contents to suit particular situations. Some may provide mainly one element, these are called straights or they may contain a range of elements, called compounds or mixtures. They may come in various forms such as a powder, granule, liquid or gas. Some of the most widely used artificial or inorganic fertilisers include:- Superphosphate. This is the most common phosphorus fertiliser. It also contains calcium and sulphur.
- Sulphate of ammonia, ammonium nitrate (nitram) and urea. These provide nitrogen.
- Potassium chloride (Muriate of Potash) and Potassium sulfate. These provide potassium.
These three fertilisers are known as straights. - Starter, Grower and Banana Special are examples of artificial or inorganic fertilisers that are known as compounds and mixtures.
For a plant production system that you are familar with, write down the names of the fertilisers used and the elements contained in them.
Elemental analysis
Most fertilisers indicate on the packaging the percentage of the three major nutrients they contain i.e. nitrogen, phosphorus and potassium.
For example a fertiliser labelled 10:10:6 contains 10% nitrogen, 10% phosphorus and 6% potassium.
A bag of Starter 12 fertiliser is labelled with 12.4 : 22.3 : 3.0. What is the percentage of the major nutrients provided by this fertiliser?
Urea has an elemental analysis of 46 : 0 : 0. Suggest what effect the application of urea would have on a pasture.
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