Soil is not just a passive environment where plants grow, but a dynamic, living system that plays a critical role in agricultural success. While it is often perceived as a vast reservoir of water, nutrients, microbes, and organic matter, assuming that plants will simply draw from these resources as needed, this viewpoint oversimplifies the true complexity of soil-plant interaction. For farmers seeking to optimise crop yields and soil efficiency, it is vital to grasp the fundamental processes that govern the relationship between soil properties, root systems, and nutrient availability. By deepening our understanding of these factors, we can unlock the full potential of our land, improve plant health, and maximise the return on agricultural investments.

The Importance of Soil Chemistry in Plant Growth

At the core of soil and plant interaction lies soil chemistry. The ability of soil to supply essential nutrients, facilitate water movement, and support microbial life directly influences plant growth and productivity. Understanding the chemical properties of soil, such as pH, cation exchange capacity (CEC), and organic matter content, allows farmers to make informed decisions regarding fertiliser application and soil management.

Soil pH, for instance, significantly impacts nutrient availability. In highly acidic or alkaline soils, certain nutrients become less available to plants, resulting in poor growth. By adjusting soil pH through the application of lime or sulphur, farmers can ensure that nutrients are in their most bioavailable form, enabling crops to take up nutrients more efficiently. Cation exchange capacity is another key factor, as it reflects the soil’s ability to hold and exchange nutrients. Soils with high CEC have a greater capacity to retain essential cations such as potassium, magnesium, and calcium, reducing nutrient leaching and improving plant uptake.

Soil Structure and Its Impact on Root Development

Soil structure—the arrangement of soil particles into aggregates—also plays a crucial role in plant growth. Well-structured soils have good aeration, water-holding capacity, and root penetration, allowing roots to explore a larger volume of soil and access more nutrients. Compacted soils, on the other hand, restrict root growth, reduce water infiltration, and limit oxygen availability, all of which negatively affect plant health.

Farmers can improve soil structure by incorporating organic matter, such as compost or cover crops, into their fields. Organic matter acts as a binding agent, promoting the formation of stable soil aggregates. Additionally, no-till or reduced-till farming practices can help maintain soil structure by minimising soil disturbance and promoting the activity of soil organisms like earthworms, which play a key role in creating channels for water and air movement.

Nutrient Uptake Mechanisms: How Plants Absorb Essential Elements

To understand how plants interact with soil, it is essential to explore the mechanisms by which they absorb nutrients. Nutrient uptake is not a passive process; rather, it involves a combination of root growth, soil properties, and chemical interactions. There are three primary mechanisms through which plants acquire nutrients: interception, mass flow, and diffusion.

1. Interception: Direct Root Contact with Nutrients

Interception occurs when plant roots physically encounter nutrients as they grow through the soil. Although this mechanism only accounts for a small percentage of nutrient uptake—approximately 1% for maize—it plays a role in absorbing nutrients such as phosphorus, potassium, and zinc. The efficiency of interception depends on root density, soil structure, and the availability of nutrients in the soil. In well-structured soils with an abundance of organic matter, root growth is more extensive, increasing the chances of direct contact with essential nutrients.

2. Mass Flow: Nutrients Carried by Water

Mass flow is the movement of nutrients dissolved in soil water towards the roots as plants take up water. This mechanism is particularly important for the uptake of nutrients that are mobile in the soil, such as nitrogen (in the form of nitrate), calcium, and sulphur. In environments with adequate rainfall or irrigation, mass flow is a dominant mechanism for nutrient uptake. However, in dry conditions, the effectiveness of mass flow decreases, making it important for farmers to monitor soil moisture levels and adjust irrigation practices accordingly.

3. Diffusion: Movement of Nutrients from High to Low Concentration

Diffusion is the movement of nutrients from areas of higher concentration to areas of lower concentration, driven by a concentration gradient. This mechanism is particularly important for nutrients like phosphorus, potassium, and zinc, which are less mobile in the soil. These nutrients are often bound to soil particles and need to diffuse through the soil solution to reach plant roots. Farmers can improve diffusion by placing fertilisers close to the root zone, ensuring that nutrients are within reach of the growing roots.

The Role of the Rhizosphere in Nutrient Availability

The rhizosphere—the narrow region of soil surrounding plant roots—is a hotspot of biological activity and plays a critical role in nutrient availability. Root exudates, which include sugars, amino acids, and organic acids, are released into the rhizosphere and influence nutrient availability in several ways.

Root Exudates and Microbial Interactions

Root exudates feed soil microbes, which, in turn, play a vital role in nutrient cycling. For example, bacteria in the rhizosphere can convert organic forms of nitrogen into plant-available forms like ammonium and nitrate. Similarly, mycorrhizal fungi form symbiotic relationships with plant roots, extending their hyphae into the soil to access nutrients like phosphorus that would otherwise be out of reach.

Chelation and Nutrient Solubility

Certain root exudates, particularly organic acids, can chelate (bind) metal ions like zinc, iron, manganese, and copper, making them more soluble and available for plant uptake. This process is especially important in soils with high pH, where these micronutrients may be present but not in a form that plants can easily absorb. Chelation improves the bioavailability of these essential nutrients, promoting healthier plant growth and development.

pH Modification in the Rhizosphere

Plants can actively modify the pH of their rhizosphere by releasing hydrogen ions (H+) or bicarbonates (HCO3-), depending on the nutrient needs and soil conditions. For example, in response to low phosphorus availability, plant roots may release organic acids that lower the rhizosphere pH, increasing the solubility of phosphorus and making it easier for plants to absorb. By understanding how plants interact with their rhizosphere, farmers can tailor their fertilisation practices to ensure that nutrients are available in the right form and at the right time.

Precision Fertilisation: Maximising Efficiency and Minimising Waste

Precision fertilisation is a key component of modern farming practices that aims to apply the right amount of nutrients at the right time and place. By understanding the mechanisms of nutrient uptake and the role of the rhizosphere, farmers can make informed decisions about fertiliser application, ensuring that nutrients are used efficiently and sustainably.

For example, nutrients that rely on diffusion, such as phosphorus and potassium, are most effective when placed near the root zone at planting. In contrast, nutrients that are mobile in the soil, like nitrogen and sulphur, can be broadcast or applied through fertigation. Tailoring fertilisation strategies to the specific needs of the crop and soil conditions not only improves nutrient use efficiency but also reduces the risk of nutrient runoff and environmental pollution.

Conclusion: Enhancing Crop Performance Through Soil-Plant Interaction

Optimising soil and plant interaction is key to improving agricultural productivity and sustainability. By understanding the complex processes that govern nutrient uptake, farmers can implement more effective soil management and fertilisation practices. This knowledge allows for better utilisation of resources, reducing input costs while increasing crop yields. Kynoch Fertilizer’s range of enhanced-efficiency fertilisers, along with expert agronomic advice, can help farmers optimise their fertilisation programmes for long-term success.

For personalised advice on improving nutrient management and soil health, contact Kynoch Fertilizer’s experienced agriculturalists on 011 317 2000 or info@kynoch.co.za

Compiled by: Hentie Cilliers or Chris Schmidt from Kynoch Fertilizer

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References

• Barber, S.A. (1995). Soil Nutrient Bioavailability: A Mechanistic Approach. 2nd edition. John Wiley & Sons.
• Marschner, H. (1995). Mineral Nutrition of Higher Plants (2nd edition). Academic Press, San Diego.
• McKenzie, B.M., Mullins, C.E., Tisdall, J.M., Bengough, A.G. (2012). Root-soil friction: Quantification provides evidence for measurable benefits for manipulation of root-tip traits. Plant Cell Environ, 36, 1085-1092.
• Mengel, K. (1995). Roots, Growth and Nutrient Uptake. Department of Agronomy publication # AGRY-95-08 (Rev. May-95). Purdue University, USA. Link. Accessed on 27 August 2024.

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Like all living things, plants require a wide range of chemical nutrients to grow, thrive and reproduce. In the case of plants, these substances may be present in adequate quantities in the soil or might need to be added in the form of natural or synthetic fertilisers. To help fight pests and diseases, spraying crops with pesticides may also be necessary. However, plants are often exposed to other sources of stress, including extreme temperatures, high soil salinity levels and drought. To resist these so-called abiotic stresses, they rely on plant biostimulants.

These stress-resisting agents can be any material containing chemicals or microorganisms able to stimulate some natural process essential for plant growth and health. These abilities are independent of any nutrient properties they may also possess. As well as providing increased resistance to abiotic stress, these materials may enhance nutrient uptake or improve utilisation. They are typically required in relatively small quantities, contribute little or no direct nutrient value and consist of two main classes, microbial or non-microbial. Natural or synthetic organic compounds are the primary ingredients of non-microbial plant biostimulants.

The natural compounds are typically obtained from seaweed extracts, algae, animal protein hydrolysates, amino acids and chitin. The active compounds in these materials mimic the physiological actions of various plant hormones, including cytokinins, auxins and gibberellins. For example, cytokinins aid cell division, vascular development and nutrient mobilisation. Gibberellic acid stimulates germination, and auxins regulate the chemical processes involved in every stage of a plant’s growth. Other commonly used substances in the non-microbial category include humic and fulvic acids, ascorbic acid and other vitamins.

By contrast, microbial plant biostimulants consist of living organisms. These include bacteria, non-pathogenic fungi, protozoa and nematodes. The class also includes arbuscular mycorrhizal fungi, which can form a symbiotic association with a plant’s root system. In this case, the benefits to plants are derived from the products of the natural physiological processes that enable these various soil microorganisms to thrive. In practice, much of the evidence for the efficacy of these products is empirical, and the precise mechanisms involved remain uncertain. Nevertheless, the increased resistance to physical stresses and disease, vigorous growth and high yields experienced by farmers who choose to apply plant biostimulants to their crops speak for themselves.

Kynoch has conducted extensive research in this field. Based on our findings, we have developed two world-class products – KynoHumate – Black and KynoFulvate – Yellow. The former is a humic acid-based soil amendment that reduces leaching and increases nutrient availability and tolerance to salinity stress. The latter contains fulvic acid to promote germination, nutrient uptake, and improved growth. We invite you to download more information about these proven effective plant biostimulants.

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