However, to optimise the efficiency of soil chemistry and plant nutrition, it is imperative for farmers to understand the foundational dynamics between soil properties, crop roots, and their interactions.
Many farmers employ precision-farming techniques without fully comprehending these basics, yet this knowledge is crucial for maximising agricultural success. Therefore, we explore these concepts in more depth below.
In soil, nutrient dynamics and behaviour are governed by a complex interplay of factors. Nutrients interact with the soil environment in various ways, and they are absorbed by plant roots through distinct uptake mechanisms. But why is a thorough understanding of these mechanisms so crucial?
The reality is that farmers can extract greater value from the same inputs by strategically managing nutrients in accordance with their unique properties, ultimately leading to a higher return on investment. Research indicates that nutrients must be positioned at specific locations in relation to plant roots to enhance absorption (Barber, 1995).
There are three principal mechanisms through which plants absorb nutrients. Interception, the first mechanism, occurs when roots physically encounter nutrients during their growth.
Barber determined that approximately 1% of a maize plant’s nutrient uptake is facilitated by this method. Nutrients absorbed through interception include phosphorus, potassium, and zinc.
The second mechanism, mass flow, involves nutrients that are dissolved in the soil solution and not held by the soil’s exchange complex. These nutrients are carried through water to the roots. This mechanism plays a significant role in the uptake of nutrients such as nitrate, calcium, and sulphur.
The third and perhaps most critical mechanism is diffusion. Certain nutrients, like phosphorus, potassium, and zinc, are strongly bound to clay complexes and are present in relatively low concentrations within the soil solution. For these nutrients to reach the roots, they rely on a diffusion process.
As nutrients near the root are absorbed, a concentration gradient forms, causing nutrients to move from areas of higher concentration to areas of lower concentration, towards the root. This mechanism is responsible for the bulk of absorption for these essential nutrients (Barber, 1995; Mengel, 1995).
Plants also depend on rhizospheric processes to facilitate nutrient uptake. During photosynthesis, some products are exuded by roots through direct chemical reactions or via processes involving soil microbes.
Additionally, complex polysaccharides and glycoproteins resulting from root and microbial activities form a gel-like layer around the roots, which interacts with the soil on both chemical and physical levels.
This gelatinous layer enhances the movement of water between the soil and roots, thereby aiding water uptake in drier soils. The negatively charged groups on polysaccharides within this layer attract positively charged elements, such as calcium, assuming exchange complexes are present.
Moreover, this layer serves as a lubricant, allowing roots to penetrate the soil more efficiently (McKenzie et al, 2012).
Root exudates, including phenolic compounds (amino and organic acids), play a critical role in modifying the soil environment. They react with heavy metals, complexing them and reducing their mobility within the soil matrix. Through a process known as chelation, metal ions like zinc, iron, manganese, and copper become more bioavailable for plant uptake.
Notably, low iron levels can stimulate the release of root exudates (Marschner, 1995). However, sustaining this chelation process requires a substantial amount of by-products. In addition, enzymes released by roots facilitate the extraction of organic phosphates from various sources.
Roots can also alter the rhizospheric pH by releasing protons, bicarbonates, organic acids, and carbon dioxide, directly impacting the availability of cations on the soil’s exchange complex, including complexed phosphates.
It is crucial for farmers to understand these distinct nutrient uptake mechanisms. Nutrients absorbed primarily through diffusion are more effective when they are placed near the seed at planting, particularly in soils with low levels of phosphorus, zinc, and potassium.
Conversely, nutrients like nitrogen, calcium, and sulphur are more amenable to broadcasting. Nevertheless, the key principle that fertilisation must result in a higher nutrient concentration to be effective remains.
Employing more advanced fertilisation techniques and selecting appropriate fertiliser types can significantly improve nutrient efficacy.
Given that a significant portion of photosynthetic by-products are synthesised and released by plants, root development and volume become critical aspects of plant management that must be prioritised. The significance of roots and the various nutrient uptake mechanisms must always be considered during the planning stages of farming operations.
Compiled by Hentie Cilliers (082 315 5228; [email protected]) and Chris Schmidt (082 885 8134; [email protected]), Kynoch agriculturalists.
Sources:
- Barber SA. 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 BM, Mullins CE, Tisdall JM, Bengough AG. 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.
