Factors Affecting Nutrient Availability in the Soil – SS2 Agricultural Science Lesson Note (First Term)

Nutrient availability is how easily plants can take up the nutrients in the soil for growth and development. A nutrient may be present in the soil but still be unavailable to the plant.

Behavioural objectives

1. State the factors that affect nutrient availability in the soil.
2. Explain how soil pH, organic matter, soil texture and soil moisture affect nutrient availability.
3. Describe how fertiliser application and the concentration of other nutrients affect nutrient availability.
4. List the ways in which soil nutrients can be lost.

What factors affect nutrient availability in the soil?

The main factors are soil pH, organic matter, soil texture, moisture and temperature, and the concentration of other nutrients.

• Soil pH: the acidity or alkalinity of the soil.
• Organic matter and micro-organisms: the store of nutrients and the living things that release them.
• Soil texture: the mix of sand, silt and clay.
• Soil moisture and temperature: water and warmth for nutrient movement and microbial activity.
• Concentration of other nutrients: the balance between nutrients, including those added as fertiliser.

How does soil pH affect nutrient availability?

Soil pH is a measure of how acidic or alkaline the soil is, and it controls which nutrients plants can take up.

• pH scale: a pH below 7 is acidic, 7 is neutral and above 7 is alkaline.
• Best range: most crops take up nutrients best in slightly acidic to neutral soil.
• Solubility: different nutrients dissolve best at different pH levels, and plants take up only dissolved nutrients.
• Other effects: pH also affects chemical reactions, the form of nutrients and the activity of micro-organisms.

• Correction: farmers add lime to raise the pH of acidic soil.

How do organic matter and micro-organisms affect nutrient availability?

Organic matter stores nutrients, and micro-organisms release them in forms plants can use.

• Reservoir of nutrients: organic matter stores nutrients and releases them slowly as it decomposes.
• Soil structure: it improves crumb structure and the water-holding capacity of the soil.
• Micro-organisms: bacteria and fungi break down dead material and keep nutrients cycling.
• Result: soils rich in organic matter generally have greater nutrient availability than soils poor in it.
• Example: topsoil under a forest or a compost-treated farm is darker and more fertile than bare, eroded soil.

How does soil texture affect nutrient availability?

Soil texture is the relative proportions of sand, silt and clay particles in the soil.

• Effects: it controls water retention, drainage and the ability of the soil to hold nutrients.

How do soil moisture and temperature affect nutrient availability?

Soil moisture and temperature decide how well nutrients dissolve, move to the roots and are released by micro-organisms.

• Adequate moisture: water is needed to dissolve nutrients and move them to the roots.
• Excess moisture: waterlogging cuts off oxygen, which harms the roots and reduces nutrient uptake.
• Too little moisture: nutrients stay undissolved and plants cannot absorb them.
• Temperature: it controls the rate of microbial activity, which changes nutrients into forms plants can take up.

How does the concentration of other nutrients affect nutrient availability?

Nutrients affect one another, so too much of one can reduce the uptake of another.

• Nutrient imbalance: an oversupply of one nutrient crowds out others at the root surface.
• Example: too much potassium can reduce the uptake of magnesium and calcium.
• Excess fertiliser: too much fertiliser causes imbalance, leaching and runoff, so less nutrient is available to the crop.

Good practices that improve availability

• Soil testing: shows which nutrients are low and what the pH is.
• Balanced fertiliser use: supplies nutrients in the amounts the crop needs.
• Organic manure: adds nutrients and organic matter.
• Crop rotation: stops one nutrient from being drained year after year.

What other factors reduce nutrient availability?

Leaching, crop removal, oxidation and reduction, burning and erosion all remove nutrients or make them unavailable.

What is leaching?

• Leaching: the washing of soluble nutrients down below the root zone by water.
• Where it is worst: sandy soils and areas of heavy rainfall.

What is crop removal?

• Crop removal: nutrients are carried away when crops are harvested.
• Example: continuous cropping of maize without manure or fertiliser lowers soil nutrients.

What are oxidation and reduction of organic materials?

• Oxidation: organic matter breaks down quickly in warm, well-aerated soil, and part of its nutrients is lost.
• Reduction: in waterlogged soil, some nutrients change into forms plants cannot use or are lost as gas.

How does burning reduce nutrient availability?

• Burning: bush burning destroys organic matter and kills useful micro-organisms.
• Loss: some nutrients, such as nitrogen, escape into the air.
• Exposure: bare, burnt soil is open to erosion.

How does erosion reduce nutrient availability?

• Erosion: wind or running water carries away fertile topsoil together with its nutrients.
• Control: cover crops, contour ridging and mulching reduce erosion.

In what ways can soil nutrients be lost?

Soil nutrients are lost through crop removal, erosion, leaching, burning, and the breakdown of organic matter, and some factors make them unavailable.

1. Crop removal: harvested crops take nutrients out of the soil.
2. Erosion: topsoil is washed or blown away.
3. Leaching: soluble nutrients move below the reach of roots.
4. Burning: nutrients such as nitrogen escape into the air.
5. Soil acidity: it makes some nutrients unavailable and encourages the loss of others, such as calcium and magnesium.
6. Excess of other nutrients: an oversupply of one nutrient reduces the uptake of another.
7. Oxidation and reduction of organic materials: nutrients are lost as organic matter breaks down.

Evaluation

1. List five factors that affect nutrient availability in the soil.
2. Name two nutrients whose availability is limited in acidic soils.
3. Why do sandy soils have lower nutrient availability than soils with high organic matter?
4. State four ways in which soil nutrients can be lost.

Assignment

Explain how soil pH and soil texture each affect the availability of nutrients to plants.