Understanding Soil Chemistry and Secondary Nutrient Management

The Basics of Soil Chemistry

Soil chemistry plays a crucial role in determining the health and productivity of crops. By understanding the interactions between soil particles and nutrients, farmers can optimize their fertilization practices to improve crop yield and quality.

Soil chemistry revolves around the interactions between soil particles and various ions. These interactions are primarily governed by electrostatics, where positive and negative charges attract each other, and like charges repel. This fundamental principle helps explain two key processes in soil chemistry: precipitation and adsorption/desorption.

Precipitation

Precipitation occurs when cations (positively charged ions) and anions (negatively charged ions) in the soil solution come together to form a salt, which then precipitates out of the solution. This process is crucial for the formation of minerals in the soil. For example, when cations like calcium and magnesium combine with anions like carbonate and sulfate, they form precipitates such as calcium carbonate and magnesium sulfate.

Adsorption and Desorption

Soil particles, particularly clay and organic matter, have charged surfaces that attract and hold onto cations and anions. For example, negatively charged clay particles can adsorb cations like potassium, calcium, and magnesium. This process is essential for maintaining nutrient availability in the soil. Adsorption refers to the attachment of ions to the surface of soil particles, while desorption is the release of these ions back into the soil solution.

Soil Texture and Composition

Soil texture, determined by the proportions of sand, silt, and clay, significantly affects its chemical properties. Clay, organic matter, iron and aluminum oxide and carbonates are the most important components of soil which impact its fertility.

Clay Particles

Clays have a high surface area and mostly a strong negative charge, making them effective at holding onto cations. Different types of clays, such as kaolinite and montmorillonite, vary in their charge intensity and nutrient-holding capacity. Clays are composed of tiny mineral particles that have a significant impact on soil chemistry. Kaolinite, for example, has a relatively weak negative charge, while montmorillonite and illite have much stronger negative charges. This difference in charge affects the soil’s ability to retain and exchange nutrients.

Organic Matter

Another crucial component of soil, organic matter, consists of large molecules with negatively charged sites, such as carboxylic and phenolic groups. These negatively charged sites play a vital role in nutrient retention and soil fertility. Organic matter also improves soil structure and water-holding capacity. The humic fraction of organic matter contains large, hydrophobic molecules, while the fulvic fraction consists of smaller molecules that can be either hydrophobic or hydrophilic.

Iron and Aluminum Oxides and Hydroxides

These oxides can be either crystalline or amorphous. Amorphous oxides are considered young oxides that are in the process of maturing into more crystalline forms. They typically carry a positive charge and therefore can fix anions like phosphate and sulfate, which can affect nutrient availability in the soil.

Carbonates

Magnesium and calcium carbonates are prevalent in soils with high pH, they regulate pH, and release calcium and magnesium cations and can precipitate anions.

The nature of these soil components, the amount of each of them, the way they interact, it all defines the chemical properties of soils and how they interact with different ions in soil solution. To choose the right source of nutrients for a specific crop, it is necessary to understand this chemistry.

Soil pH and Reactivity

Soil pH is a critical parameter that influences soil chemistry and nutrient availability. It measures the acidity or alkalinity of the soil solution, with a pH above 7 being alkaline and below 7 being acidic. Soil pH affects the charge and reactivity of soil particles, which in turn impacts nutrient availability.

Variable Charge of Soil Components

Minerals, enzymes, and other reactive particles in the soil have a specific pH point at which their charge changes from negative to positive, known as the point of zero charge (PZC). For example, natural oxides typically have a PZC around 8.5-9, making them positively charged in most agricultural soils. For clays and organic matter, the PZC is much lower, around 2-3, meaning they are usually negatively charged in typical agricultural pH ranges and therefore adsorb cations.

This negative charge is not the same all the time, its intensity is very much influenced by pH. The lower the pH, the more protons there are in the solution and these protons neutralize part of that negative charge and the surface have less capacity to attract cations. When the pH is higher, there is more hydroxide and the negative charge is stronger. These surfaces have a higher capacity to attract cations.

Impact on Nutrient Availability

Soil pH influences the solubility and availability of nutrients. In acidic soils, phosphate anions can precipitate with iron and aluminum, while in alkaline soils, they can precipitate with calcium and magnesium. Adjusting soil pH can alter the anion and cation exchange capacity of the soils, i.e. the availability of nutrients.

Managing Soil pH in Different Regions

Soil pH diversity means that we cannot use the same types of fertilizers in all situations. Managing soil pH is essential for optimizing nutrient availability and improving crop yields. Different regions have varying soil types and pH levels, requiring tailored fertilization practices.

Alkaline Soils

In alkaline soils, characterized by high pH and active calcium carbonate, it is essential to use acidifying fertilizers like ammonium-based products to lower the pH and improve nutrient availability. These soils often have high nitrification rates and phosphorus fixation by calcium, requiring specific management practices. The presence of active calcium carbonate increases the activity of calcium ions, leading to higher nitrogen volatilization and phosphorus precipitation.

Acidic Soils

In acidic soils, which often have high aluminum and iron ion activity, it is crucial to supply sufficient calcium and magnesium to avoid nutrient deficiencies and reduce soil acidity. These soils may also require the use of non-acidifying fertilizers like nitrates that can slightly increase the pH, or also urea that have a neutral effect on pH. Acidic soils often have slower nitrification rates and strong phosphorus fixation by adsorption to oxides, making it important to manage nutrient applications carefully.

Importance of Secondary Elements

The importance of NPK for plant nutrition is well known but when focusing on secondary elements like calcium, magnesium, and sulfur we see, for example, that concentration of calcium that plants require is higher than phosphorus. Also, magnesium and sulfur are often required in same amounts as phosphorus. So calling them “secondary” is a bit of a misnomer.

Sulfur

Sulfur is only absorbed by roots as a sulfate anion. It is important for amino acid and protein synthesis. It is also crucial for the synthesis of vitamins, hormones, and defense compounds in plants. Sulfur is relatively immobile, so deficiencies typically appear in new growth and by an untrained eye can be mistaken for nitrogen deficiency. Until recently it was not a nutrient which was talked about as there were enough supplies in soil due to air pollution. However, this has dramatically changed and sulfur fertilization is nowadays a must.

Calcium

Calcium is absorbed as a cation. It is essential for cell wall structure, nutrient absorption, and root development. Deficiency symptoms include leaf tip burn, and blossom end rot in fruits like tomatoes and peppers. Calcium is immobile in plants, meaning it does not move from older to newer tissues, so deficiencies often appear in new leaves. Calcium also plays a crucial role in maintaining the structural integrity of cell membranes and facilitating cell division and elongation.

Magnesium

Magnesium is a key component of chlorophyll and an enzyme activator. Deficiency symptoms include yellowing of older leaves (interveinal chlorosis) and reduced photosynthesis. Magnesium is mobile in plants, allowing it to move to areas of new growth when needed. It is also involved in protein synthesis and the activation of various enzymes that are essential for plant metabolism.

Polysulphate: a unique source of sulfur, calcium, magnesium and potassium

Polysulphate is a natural mineral fertilizer that provides sulfur, potassium, magnesium, and calcium. It is mined exclusively by ICL from 1200m depth below the North Sea off the English coast. Its prolonged-release properties help in sustained nutrient availability and reduced leaching, making it an excellent choice for improving crop performance over time.

Prolonged-release Properties

Polysulphate dissolves gradually in the soil, providing a steady supply of nutrients over an extended period. This prolonged-release characteristic helps reduce nutrient losses due to leaching, especially in regions with high rainfall or irrigation. The gradual dissolution of Polysulphate ensures that nutrients are available to plants throughout their growth cycle.

Nutrient Composition

Polysulphate contains multiple essential nutrients, making it a versatile fertilizer for various crops. Its balanced nutrient profile supports overall plant health and improves the efficiency of other fertilizers used in combination. Polysulphate is not a mixture of different sulfates but a single mineral that naturally contains potassium, magnesium, and calcium sulfates in its crystalline structure.

Practical Applications

Polysulphate can be used in a variety of agricultural settings as a base fertilizer applied to the soil. As a natural mineral fertilizer, Polysulphate is suitable and certified also for use in organic farming systems. ICL has performed many field trials with Polysulphate on various crops. The results are available here.

Conclusion

Understanding soil chemistry and nutrient management is essential for optimizing crop yields and ensuring sustainable agricultural practices. By managing soil pH and addressing deficiencies in secondary elements, farmers can improve soil health and enhance crop performance. Innovative solutions like Polysulphate offer a balanced and efficient approach to fertilization, providing nutrients in a prolonged-release form that reduces leaching and supports long-term soil fertility.

Watch the whole webinar by Cristian Terrones our Agronomy & Product Specialist (the webinar in Spanish with English subtitles)