Plant Nutrition

Pla nt Nu tr iti on

 Plants require certain chemical elements to complete their life cycle  Plants derive most of their organic mass from the CO2 of air  But they also depend on soil nutrients such as water and minerals CO2, the source of carbon for Photosynthesis, diffuses into leaves from the air through stomata.

H2O

CO2

O2

Through stomata, leaves expel H2O and O2.

O2

Minerals Roots absorb H2O and minerals from the soil.

CO2 H2O

Roots take in O2 and expel CO2. The plant uses O2 for cellular respiration but is a net O2 producer.

Ma cro nutrients a nd Mic ronutr ie nts

 More than 50 chemical elements

 Have been identified among the inorganic substances in plants, but not all of these are essential

 A chemical element is considered essential  If it is required for a plant to complete a life cycle

 Nine of the essential elements are called macronutrients  Because plants require them in relatively large amounts

 The remaining eight essential elements are known as micronutrients  Because plants need them in very small amounts

Co mmon Deficie ncies  nitrogen, potassium, and phosphorus Healthy

Phosphate-deficient

Potassium-deficient

Nitrogen-deficient

Nit rogen F ix ation  Nitrogen is often the mineral that has the greatest effect on plant growth  Plants require nitrogen as a component of  Proteins, nucleic acids, chlorophyll, and other important organic molecules

Soi l B acteri a and Nitrogen Avai labil ity  Nitrogen-fixing bacteria convert atmospheric N2 to nitrogenous minerals that plants can absorb as a nitrogen source for organic synthesis Atmosphere N2

N2

Atmosphere Soil

N2

Nitrogen-fixing bacteria

Denitrifying bacteria

H (From soil) +

Soil

NH4+

NH3 (ammonia) NH4+ (ammonium)

Nitrate and nitrogenous organic compounds exported in xylem to shoot system

Nitrifying bacteria

NO3– (nitrate)

Ammonifying bacteria

Organic material (humus)

Root

So il  Soil quality is a major determinant of plant distribution and growth  Along with climate  The major factors determining whether particular plants can grow well in a certain location are the texture and composition of the soil

 Texture  Is the soil’s general structure

 Composition  Refers to the soil’s organic and inorganic chemical components

Te xtu re a nd Co mposit ion of So il s  Various sizes of particles derived from the breakdown of rock are found in soil  Along with organic material (humus) in various stages of decomposition

 The eventual result of this activity is topsoil  A mixture of particles of rock and organic material

 The topsoil and other distinct soil layers, or horizons  Are often visible in vertical profile where there is a road cut or deep hole The A horizon is the topsoil, a mixture of broken-down rock of various textures, living organisms, and decaying organic matter. A

Figure 37.5

B

The B horizon contains much less organic matter than the A horizon and is less weathered.

C

The C horizon, composed mainly of partially broken-down rock, serves as the “parent” material for the upper layers of soil.

 After a heavy rainfall, water drains away from the larger spaces of soil  But smaller spaces retain water because of its attraction to surfaces of clay and other particles

 The film of loosely bound water Soil particle surrounded by

 Is usually available to plants film of water

Root hair Water available to plant

Air space (a) Soil water. A plant cannot extract all the water in the soil because some of it is tightly held by hydrophilic soil particles. Water bound less tightly to soil particles can be absorbed by the root.

 Acids derived from roots contribute to a plant’s uptake of minerals  When H+ displaces mineral cations from clay particles Soil particle K+

– –

Cu2+

–

– K+

–

–

–

– + K – Ca2+

Mg2+

H+ H2O + CO2

H2CO3

Root hair

HCO3 + H+ –

Cation exchange in soil. Hydrogen ions (H+) help make nutrients available by displacing positively charged minerals (cations such as Ca2+) that were bound tightly to the surface of negatively charged soil particles. Plants contribute H+ by secreting it from root hairs and also by cellular respiration, which releases CO2 into the soil solution, where it reacts with H2O to form carbonic acid (H2CO3). Dissociation of this acid adds H+ to the soil solution.