14.2 Applications and skills
14.2.6 Lab: Nutrient content of soils (HL)
Soils are dynamic systems which shape and are shaped by their environments. Soil structure and nutrient content have an impact on the type and distribution of plants growing on them and, in turn, the organisms affect the structure of soils through death and decomposition.
Soil structure is generally determined by particle size. Clay particles have the smallest diameter, followed by silt and sand. Most soil is composed of a combination of these three types of particle, as shown in Figure 14.2.6a.
Figure 14.2.6a – Soil classification system.
All soils are composed of a percentage of small (clay), medium (silt) and large (sand) particles. Different plants grow best in different types of soil depending on their oxygen and nutrient requirements.
Application: Effects of waterlogging on the nitrogen cycle
- Soils become waterlogged when water saturation prevents oxygen from reaching plant roots.
- Waterlogging can occur because of excessive irrigation or poor drainage, often in flat clay soils.
- Waterlogging leaves anaerobic conditions in the soil, so the plant roots are not able to respire. In addition, carbon dioxide accumulates in the soil. The soil condition is said to be ‘anoxic’.
- This affects the nitrogen cycle because anaerobic conditions favour denitrification over nitrification. More nitrogen is released into the atmosphere.
- Increased denitrification leaves less nitrogen available for plants.
- This also affects the nitrogen cycle because waterlogging can cause runoff. Nitrogen in runoff water can also lead to eutrophication in other ecosystems.
|Nitrification||NH3 → NO2- and NO3-||Aerobic||
|Denitrification||NO3- → N2||Anaerobic||Pseudomonas|
Lab activity: Determining soil texture by feelHere is a quick activity you can do to help you understand the different structures of soils and why certain soils are more prone to waterlogging than others. All you need to do is obtain samples of different types of soil, then follow the instructions given below. Download Figure 14.2.6c as PDF >
Skill: Assessing nutrient content of a soil sample
- Nutrient content of a soil sample can be assessed indirectly by examining the plants growing there. Some examples are given in the table below.
Plant appearance Nutrient deficiency Green glossy leaves None Yellow leaves, especially older leaves, new leaves lighter in colour Nitrogen Leaf tips appear burnt, older leaves may be purple Phosphorus Interveinal chlorosis from edges moving inwards Potassium New leaves are distorted or irregularly shaped Calcium
Figure 14.2.6d – Interveinal chlorosis: yellowing between the veins and the edges of the leaf
- Testing the pH of your soil sample using universal indicator is another indirect method of indicating which nutrients are present.
- Electrical conductivity can be measured to indicate the concentration of dissolved salts in the soil. In general, the higher the salinity, the more nutrients are dissolved in the soil
Figure 14.6.2f – Electrical conductivity
Nitrates and dissolved salts increase conductivity of a soil sample.
- The easiest way to test for nutrient content of soils is to use a commercial soil testing kit like the one shown in Figure 14.6.2g. The chemicals in each of the tests indicate levels of nitrogen, potassium and phosphate.
Figure 14.2.6g – Commercial soil test kit
Chemical indicators change colour when nutrients are present.
- Explain the impact of waterlogging on the nitrogen cycle.
- Assess the nutrient content of a soil sample.
All soils are made of three sizes of particles
From Figure 14.2.6a:
- Identify the type of soil that is composed of 40% silt, 30% sand and 30% clay.
- What is the minimum amount of clay necessary for a soil to be considered at least partially ‘loamy’?
The sicklepod, Cassia obtusifolia, grows where soils are well aerated and is used as an indicator plant for soil quality.
- Which soil conditions are best for plant growth?
- What soil texture is most appropriate for different crops?
- How does soil structure affect respiration rates?
Figure 14.6.2j – Raised beds
Gardeners use raised beds to prevent waterlogging
Figure 14.6.2.k – Fertiliser showing NPK ratings
Fertilisers show NPK ratings to indicate how much nitrogen, phosphorus and potassium they contain. Different crops grow best with different ratios.
- At which pH is the availability of calcium highest?
- At which pH range is the NPK rating highest?
- Suggest the characteristics of plants grown in soils at pH4.5.