Mobile vs Immobile Nutrients

Fig. 1   Older leaves on celery turning yellow while the growing points in the center remain green.

The last time I looked there were seventeen known essential elements for plants.  Each element performs a specific function.  When conditions are at optimum levels we see a healthy plant, but when one or more element is deficient we see a "needy" plant.  To determine which of these various elements is lacking, one has to begin by understanding where the plant stores all its limited reserves.  Some elements are like cash - they can be used anywhere, while some are like gift cards - which can only be spent in specific stores.  Plant nutrients are either plant-mobile or plant-immobile.   Understanding these two characteristics is important because it helps gardeners interpret deficiency symptoms more accurately.

What are plant-mobile nutrients?

Plant-mobile nutrients are those that are capable of being translocated within the plant.  When a plant is deficient of these elements, the nutrient that is already within the plant will be transported to where it is needed most - the young tissues.  Deficiency symptoms of plant-mobile elements are observed on the older leaves first.  One example of a plant-mobile nutrient is nitrogen.  If nitrogen is deficient in a plant, older leaves would turn yellowish first while the newer leaves remain relatively green (Fig. 1). The plant directs the nutrient where it is most needed to prolong the life of the stressed plant.

Examples of plant mobile nutrients are nitrogen (N), phosphorous (P), and potassium (K).  Manganese and sulfur are moderately mobile

What are plant-immobile nutrients?
By now, the answer to this question is obvious.  Plant-immobile nutrients  cannot be translocated from older tissue to a new one due to the nature of the elements and sometimes other conditions.  In other words they are stuck where ever they landed the first time.   They have reached their destination.  Deficiency symptoms for these elements are observed in the young plant parts.  Calcium is an example of plant-immobile elements.  It plays an important role in cell expansion.  When calcium is deficient, the young shoots and flower buds exhibit the devastating effects.  If the condition is not corrected the shoots and bud get aborted eventually.

Examples of plant-immobile nutrients are:  Iron (Fe), Calcium (Ca), Manganese (Mn), Zinc (Zn), Copper (Cu), and Boron (B). 

Plant nutrients are like humans.  Some are always moving to where the activity is going on while others just settle where they landed the first time until they go back to the ground. 

What are Fertilizers

Myth:  Fertilizers are plant food.

Truth: Plants produce their own food.

Plants are autothrops, which means they are self -feeding.   They produce their own food.  While humans and animals consume other living organisms to survive, plants photosynthesize.  Plants contain chlorophyll that allows them to capture solar energy needed to convert carbon dioxide and water into sugars, carbohydrates and proteins that are necessary food for all organisms.

If fertilizers are not plant food then what are they?

Fertilizers are materials that contain one or more of the essential elements used as soil amendment.   Fertilizers can be applied to the soil or by foliar application, i.e. spraying to the leaves.   The essential elements become integral components of the food manufactured through photosynthesis.  Without one or more of the essential elements, the plants will not be able to complete their growth cycle.  That is the reason that they are called "essential" elements. 

Plant nutrition - refers to the plant’s need for and use of the essential elements for growth and development.  The essential elements are also referred to as nutrient elements.

Availability of these elements in the soil depends on soil fertility, which is the inherent capacity of the soil to supply nutrients to plants in adequate amounts and suitable proportions.  The soil naturally contains all these elements but the condition of the soil determines whether such elements are in available or bound in form.  To give you very simple example, nitrogen can be present in the soil but when soil moisture is limiting, nitrogen cannot be absorbed by plants.  Then there is soil chemistry that determines how these elements are attached to each other. Soil acidity, better known as soil pH, plays an important role is the availability of a number of the micro nutrients in the soil.  When the availability of these elements is limited in the soil, fertilization becomes necessary.  Fertilization is the application of nutrient elements to the environment around a plant to meet its nutritional need.

Beneficial Elements

Beneficial Elements are elements that help optimize the growth and development of plants but they are not essential for growth.  When they are absent in the soil, plants can still live a normal life.  Here are some criteria that separate beneficial element from the essential ones:

1. It can compensate for the toxic effects of other elements.

2.  May replace mineral nutrient in some other less specific function such as the maintenance of osmotic pressure.

3.  May be essential to some but not to all plants.

Examples of beneficial elements are:

Silicon (Si)

Silicon increases the resistance of plants to pathogen and pests.  It also increases drought and heavy metal tolerance of plants.  Overall it improves the quality and yield of agricultural plants. 

Cobalt (Co)
Cobalt is essential for the growth of Rhizobium bacteria for N fixation and thus beneficial for the plant.  Nitrogen fixation is the process by which the atmospheric molecular nitrogen (N2) is reduced to form ammonia (NH3).  This process is carried out by nitrogen-fixing bacteria which are found in the roots of most leguminous plants.  Ammonia is the form of nitrogen that is used by plants and other living systems in the synthesis of organic compounds. 

Lithium (Li)
Affects transport of sugars from leaves to roots.  Production of food (carbohydrates and sugars) happens in the leaves during photosynthesis.  This food will be transported to the different parts of the plant such as the roots, fruits, new shoots, and stems.  Lithium enhances the transport of such food to the roots.  

For the backyard gardener, there is no need to worry about artificially applying them to the soil or any growing media that you might use.  Your plants will still survive.  In case you want to insist, then the best thing for you to do is apply compost around your plants. 

Beneficial elements are not essential for plant growth.  Lithium for example, is indirectly beneficial to the plant by its benefits to the bacteria rhizobium.  Plants don't need the bacteria for growth.  They need nitrogen.  

Essential Elements

There are seventeen elements known to be necessary for plants to complete their life cycle.  They are called essential elements.

What Makes An Element Essential?
Arnon and Stout (1939) proposed the following criteria:
1. A plant must be unable to complete its life cycle in the absence of the mineral element.
2. The function of the element must not be replaceable by another mineral element.
3. The element must be directly involved in plant metabolism.

Of the seventeen essential elements, hydrogen (H), oxygen (O), and carbon (C) come from the air and water and by nature, readily available.  Although nitrogen can also be found in the air, its gaseous form is not useable by plants.  Along with nitrogen, the rest of the elements are found in the soil.  Depending on the soil properties, however, some of these elements may be present but not in forms that are useable for plant use.   Still some soils may lack one or more of these elements. 

When one or more of the essential elements is deficient, plants cannot complete their life cycle.  Such deficiency will be expressed in abnormal behavior in plant growth.  For this reason, fertilizers are used to supply the elements that are deficient within the growing area. 

Macronutrients 
Macronutrients are essential elements that are required by the plants in huge quantities (parts/100 of dry plant matter).  Macro nutrients are not in any way more important than the rest of the 17 essential elements. 
1. Nitrogen (N)
2. Phosphorous (P)
3. Potassium (K)
4. Calcium (Ca)
5. Magnesium (Mg)
6. Sulfur (S)

Micronutrients
Micronutrients also known as trace elements are elements that are required by plants in small quantities (ppm of dry plant matter).  Micronutrients should not be mistaken as less important than their macro counterparts. 
1. Boron (B)
2. Chlorine (Cl)
3. Copper (Cu)
4. Iron (Fe)
5. Manganese (Mn)
6. Molybdenum (Mo)
7. Nickel (Ni)
8. Zinc (Zn)

Scientists continue to evaluate the essentiality of more elements that are not included in the list.  Nickel for example, recently won the status of an essential trace or micro element.