All living organisms require nutrients for adequate growth, development and functionality in order to survive. Plants have the capacity to produce their own food through the conversion of light energy into a sugar (glucose), in a process called photosynthesis, which is the base of biomass formation. For this, plants require sufficient light, suitable temperature, substances such as CO2, oxygen and a number of nutrients (FAO, 2006). As plant constituents, nutrients play a vital role in biochemical reactions, and the production of organic material. To obtain high agricultural yields an optimal nutrient program is required, whereby plants absorb nutrients from soil reserves or external sources, where water is the main carrier. Nutrients can be added either to the soil or leaves (foliar application) by using granular or liquid fertilizers, organic manures, soil amendments, among other sources.
Plants absorb most of their nutrients through their root system; hence, it is important to develop healthy and strong roots. Nutrients are absorbed and transported to aerial parts of the plant, and stored or used for metabolic processes. Despite the importance of applying external sources of nutrients to ensure a good nutrition program, there is a chance of over applying and reaching toxicity levels. Figure 1 shows a relationship between the concentration of nutrient in tissue and the growth of plants. A critical concentration exists, which when below, signals deficiency levels and visual symptoms are observed. There is also a critical nutrient range that is the optimum level of nutrients required for plants to grow adequately. When the level surpasses this range, there is a phase of luxury consumption, followed by toxicity levels, which show visual symptoms. Therefore, a good crop nutrition program must always aim for the critical nutrient range for optimum growth.
It is known that plants require at least 16 elements for growth and full development (Arnon and Stout, 1939). Plants utilize three basic nutrients, carbon (C), hydrogen (H) and oxygen (O), which are considered non-mineral nutrients because they are derived from the air and water. Other important nutrients are classified as macro and micro nutrients, as shown in figure 2.
It is important to understand the role of each element or nutrient in the plant metabolic functions. Below is a very brief description of the main functions of them in a plant (Salisbury and Ross, 1991).
Carbon (C) – Constituent of carbohydrates; necessary for photosynthesis.
Hydrogen (H) – Maintains osmotic balance; important in numerous biochemical reactions; constitutes carbohydrates.
Oxygen (O) – Constituent of carbohydrates; necessary for respiration.
Nitrogen (N) – Necessary for formation of amino acids and proteins; vital for plant growth; directly involved in photosynthesis.
Phosphorus (P) – Involved in photosynthesis, respiration, energy storage and transfer; promotes early root formation and growth; increases water-use efficiency.
Potassium (K) – Increases photosynthesis, water-use efficiency; important in fruit formation, quality of seeds and fruit; increases disease resistance.
Calcium (Ca) – Important in cell division and formation; involved in nitrogen metabolism; reduces plant respiration; increases fruit set.
Magnesium (Mg) – Important in chlorophyll formaton; improves utilization and mobility of phosphorus; influences earliness and uniformity of maturity.
Sulfur (S) – Necessary in chlorophyll formation; helps develop enzymes and vitamins; promotes nodule formation on legumes.
Boron (B) – Essential for seed and cell wall formation; promotes maturity; necessary for sugar translocation.
Copper (Cu) – Performs a major function in photosynthesis and reproductive stages; increases sugar content; plays an indirect role in chlorophyll formation.
Iron (Fe) – Promotes formation of chlorophyll; acts as an oxygen carrier.
Manganese (Mn) – Increases the availability of phosphorus and calcium; aids in chlorophyll synthesis.
Molybdenum (Mo) – Aids in the formation of legume nodules; plays a role in reducing nitrates to ammonium in plants.
Zinc (Zn) – Necessary for chlorophyll production, carbohydrate and starch formation; aids plant growth hormones and enzyme system; aids in seed formation.
Silicon (Si) – Increases resistance to fungal diseases.
Cobalt (Co) – Component of enzymes and increases drought resistance of seeds; important for nitrogen fixation by the bacteria that associate with legumes.
Nutrients within a plant may be classified by their mobility. There are some that are mobile, very mobile or immobile. Their mobility can assist in determing where and which nutrient may be deficient. Figure 3 shows where in a plant and which nutrient could be diagnosed. Differentiating where the symptom appears is important; for example, yellowing of lower leaves is typical of nitrogen deficiency, and yellowing of upper leaves is sulfur. Some nutrients may present similar symptoms which require further analysis and expertise.
In a way, plants speak to us visually; understanding their environment and requirements is key in formulating a crop nutrition program. Regular soil and leaf analysis is recommended to maximize crop yields.