Nutrient uptake by plants

The nutrient uptake refers to the process of nutrient movement from an external environment into a plant. It is one of the fundamental demonstrations of plant’s life which involves especially a qualitative change where an abiotic material becomes a component of a cell capable of further assimilation processes, resulting in production of new mass. Plants receive mostly carbon and oxygen in a form of CO2 from air and partly hydrogen. These nutrients enter a plant in a molecular form. Thanks to leaves, stems, and eventually generative organs (klasů, lat, crops) most plants can also receive other nutrients, such as N, P, K, Ca, Mg, mikroelements, etc., mostly in a form of soluble salts of certain concentration. Such way of nutrition is marked as foliar (folic) nutrition. Besides, plants receive all nutrients and water by roots.

Nutrient uptake by roots

Nutrient uptake by roots is shared by all young root parts and especially by a zone of root hairs which up to hundreds of times increases the surface area of a root. The number of root hairs in 1 mm fluctuates according to humidity, soil areation and plant type. For instance, regarding alfalfa, maize and English ryegrass grown on a light soil 1 mm of the root length comprises the number of hairs of 105, 161, 88 and their total length reached 37, 146, 99 mm.

Root hairs have a limited lifetime (about 10-12 days). Gradual growth of new hairs enables new and new connection of a plant with the soil environment. It also results in a mutual interaction of the nereast soil environment by root excretions. The excretions consist of organic and inorganic substances. Significant amount of ions is also secreted together with organic substances into a root environment.

By its presence in the soil environment, the root excretions influence the solubility of some mineral substances (phosphorus), and with the content of sugars and amino acids support the development of soil microflora (mycorrhiza). There are continuous changes in the composition of the soil solution and the content of acceptable nutrients due to the processes listed above. Plants can take up nutrients through roots in a form of small molecules without an electric charge or in a form of positively or negatively charget ions of a range of elements. Moreover, some nutrients can be taken up in several different forms (e.g. nitrogen).

Foliar nutrition of plants

Leaves are organs specifically designed to exchange gases, nevertheless, they can also pose a place where a foliar nutrition of plants can take place. Foliar nutrition of plants is perceived as an uptake and utilization of mineral (as well as organic) nutrients applied in the aboveground plant part in a form of aqueous solutions. Literature ordinarily uses the term foliar nutrition due to the fact that most applied solutions stick to the leaves where there is also the biggest amount of nutrients taken up. It has been demonstrated that other aboveground plant parts including crops are capable of taking up nutrients from a solution. The stated kind of nutrition is necessary to be understood as a supplementary nutrition which enables operative correction of the nutritional status of plants not only according to visual symptoms, but especially based on the analysis of plant biomass.

The foliar nutrition cannot entirely replace the root one since the amout of taken up nutrients is low. It has been demonstrated that plants reliant only on this kind of nutrition lag in development and strongly inhibit the creation of generative organs (blooms, crops). The virtue of foliar nutrition is an elimination of interaction among ions which could significantly affect the acceptability of nutrients during their application in soil and, by that, also the efficiency of supplied nutrients. Nutrient application is also possible connect with (especially regarding nitrogen fertilizers) the treatment of crops by pesticides.

When entering the aboveground plant part, the main obstacle for nutrients is a cuticle, an acellular layer on their surface protecting a plant against water evaporation. Thanks to the moistening of the surface of leaves, which is strengthened by adding detergents (wetting agents) to the fertilizer solution, the cuticle spreads out and enables a contact of a solution with the cells of a leaf.

The uptake speed of single nutrients is considerably unlike. Cations penetrate membranes faster than anoints. Plant leaves can absorb all main nutrients and microelements with the spped listed in the following table.

Absorption speed of single nutrients by plant leaves (Hudská 1976)

Nutrients

Time at 50% absorption

Nitrogen in urea

1/2 to 2 hr.

magnesium

2-5 hr

potassium

10-24 hr.

calcium, manganese, zinc

1-2 days

phosphorus

5-10 days

iron, molybdenum

10-20 days

The uptake speed of ions in a plant affects the efficiency of foliar nutrituion. Received nutrients by a plant are destinquished by varying mobility. Regarding immobile nutrients it is therefore necessary to repeat spreys at the time when the plant needs it the most. A range of nutrients which are slowly received and relatively immobile in plants can be extremely efficient in a form of foliar nutrition and can be a precaution to ensure or eventually eliminate their deficiencies.

Absortion and relative mobility of foliarly applied nutrients (Wittwer a Bukovac 1989)

Adsorption order

Mobility order mobility

Fast:     N (urea), Rb, Na, K, Cl, Zn

Mobile:   N (urea), Rb, Na, K, P, Cl, S

Medium fast: Ca, S, Ba, P,Mn, Br

Partly mobile: Zn, Cu, Mn, Fe, Mo, Br

Slow:   Mg, Sr, Cu, Fe, Mo

Immobile:   Mg, Ca, Sr, Ba

The foliar nutrition can prevent soil nutrient overloading and lower the risk of environmental threats. During the foliar nutrition, nutrient efficiency can reach up to 85%, whereas application of fertilizers through soil only 30 - 60% of efficiency depending on nutrient type.

The efficiency of foliar nutrition is dependent on concentration and the dose of solution which cannot be too high to prevent leaves from burns. Regarding macrobiogenic elements, average 2% solutions are recommended, regarding microbiogenic elements an optimal concentration is from 0,1 to 0,5 %.

Factors of an external environment have significant importance: humidity, temperature, light. The higher a relative humidity is, the longer the solution remains on the surface of leaves and the amount of nutrients entering leaves increases. After water evaporation at higher temperature, the uptake of ions is limited and burns of leaves may occur.