Soils for plant growth Revision Notes

Plants require specific mineral ions absorbed from the soil to carry out vital functions such as growth, energy transfer, and cell division. The three primary mineral ions are nitrate ions (NO3-), phosphate ions (PO43-), and potassium ions (K+).

Nitrate ions supply nitrogen, a key component of amino acids and proteins. Without sufficient nitrate, plants show stunted growth and yellowing leaves due to reduced chlorophyll production.

Phosphate ions provide phosphorus, essential for energy transfer molecules like ATP and for root development. Phosphorus deficiency often results in poor root growth and delayed maturity in plants, visible as dark green or purplish leaves in some species, such as maize or beans. For example, a farmer noticing weak root systems in crops might suspect phosphate deficiency and test soil accordingly to apply phosphate fertilisers if needed.

• Nitrate ions (NO3-) supply nitrogen for proteins and chlorophyll.
• Phosphate ions (PO43-) support energy transfer and root growth.
• Potassium ions (K+) regulate water balance and enzyme activation in plants.

Organic matter consists of decomposed plant and animal residues, microorganisms, and humus. It improves soil fertility by releasing nutrients slowly as it decomposes, providing a steady supply of mineral ions.

Additionally, organic matter enhances soil structure by binding soil particles into aggregates, improving aeration and water retention. This structure facilitates root penetration and microbial activity, which further aids nutrient cycling.

For example, adding compost to garden soil increases organic matter content, making the soil crumbly and easier to work with, while also boosting nutrient availability for plants.

• Organic matter releases nutrients gradually through decomposition.
• Improves soil texture and aggregation, aiding root growth and water retention.
• Supports beneficial microbial populations that assist nutrient cycling.

Soil pH measures acidity or alkalinity, influencing chemical forms of nutrients and microbial activity. Most plants prefer a pH between 6 and 7.5, where nutrient availability is optimal.

If soil is too acidic (low pH), essential nutrients like phosphorus become less available, and toxic metals may become soluble, harming roots. Conversely, alkaline soils (high pH) can cause deficiencies in iron, manganese, and phosphorus.

Farmers often test soil pH and apply lime to acidic soils to raise pH or sulphur to acidic soils to lower pH, aiming to optimise nutrient uptake. For example, blueberry plants thrive in acidic soils (pH 4.5-5.5), illustrating how different plants have specific pH preferences.

• Optimal pH range is generally 6–7.5 for nutrient availability.
• Acidic soils reduce availability of phosphorus and increase toxic metals.
• Alkaline soils limit availability of iron, manganese, and phosphorus.
• Soil pH can be adjusted with lime or sulphur to improve plant growth.

Roots require oxygen for respiration, which is supplied by air spaces in the soil. Well-aerated soils allow roots to respire efficiently, supporting healthy growth.

Water content must be sufficient to supply hydration but not excessive. Water dissolves mineral ions, making them available for root absorption. However, too much water fills air spaces, reducing oxygen availability and causing root suffocation.

Good drainage prevents waterlogging, which leads to anaerobic conditions harmful to roots and beneficial microbes. For example, heavy clay soils often drain poorly, causing waterlogging after rain, while sandy soils drain quickly, reducing waterlogging risk but sometimes causing drought stress due to rapid water loss.

• Air spaces in soil provide oxygen for root respiration.
• Water dissolves nutrients and hydrates plants but excess causes waterlogging.
• Good drainage prevents root suffocation and supports microbial life.
• Soil texture influences drainage and water retention properties.

Ease of cultivation refers to how readily soil can be tilled or worked for planting. Soil texture, determined by particle size, strongly affects this property.

Sandy soils have large particles and feel gritty. They drain quickly, reducing waterlogging risk, and are easy to cultivate because they do not compact easily. However, they hold fewer nutrients and water, requiring more frequent irrigation and fertilisation.

Clay soils have very small particles and feel sticky when wet. They retain water and nutrients well but drain poorly, increasing waterlogging risk. Clay soils are harder to cultivate because they compact and become heavy, making ploughing difficult, especially when wet. Farmers often add organic matter to clay soils to improve structure and ease cultivation.

• Sandy soils: large particles, quick drainage, easy to cultivate, low nutrient and water retention.
• Clay soils: small particles, slow drainage, hard to cultivate, high nutrient and water retention.
• Organic matter improves structure and cultivation ease in clay soils.
• Choosing soil type affects crop choice and management practices.