Plant nutrition and soils – Chapter 29
Mineral nutrition - an overview
 Plant require mineral nutrients
 Mineral nutrition has important consequences at agricultural, environmental
levels (e.g. crop and ecosystem productivity)
 Plants must acquire and transport nutrients from soil
Essential nutrients
 Essential nutrients are required to complete life cycle of plant
 Major Categories: mineral (inorganic) and non-mineral (organic, Carboncontaining) nutrients
 9 macronutrients are required in large amounts; 8 micronutrients in small
amounts
Investigating mineral nutrition
 Soil is a complex growth medium, so hydroponic growth with nutrient growth
solutions are used to investigate mineral roles
 Mineral deficiencies (diseases) can be readily observed, (e.g. nitrogen and
iron deficiency lead to chlorosis)
 Nutrient deficiency symptoms depend on function and mobility of essential
nutrient
Soil
 Weathering of rocks provides the inorganic nutrients used by plants
 Soils consist of layers called horizons
Soil properties and ion exchange
 Soil consists of particles and pore spaces (with air and water)
 In order of particle size (sand > silt > clay), and organic humus

The best soil is a mixture
Soil properties
 Minerals exist in soil solutions as charged ions
 Colloidal properties of soil particles (hydrated, negatively charged) provide
high cation binding capacity
 Cation exchange (abiotic and biological), H+ or K+ ions from plant roots
displace positively charged ions bound to soil particles
Ion exchange
 Exchangeability of ions varies according to binding affinity
 Al > H > Ca > Mg > K = NH4 > Na
 Negatively charged particles (NO 3-) are weakly bound and tend to leach from
the soil
Ion exchange
• Acid pHs (high H+ concentration) tend to displace positive ions from the soil,
liming, acid rain
 High pH (above 6.8) - can lead to Fe deficiency- precipitates from solution as
Fe oxides or hydroxides; Mn, Zn, Mg, Ca, PO4 become less available or
unavailable
 Low pH - Fe, Mn, Al become very soluble and toxic; Mo unavailable; Mg, K
may be in short supply in acidic sandy soils
Nitrogen cycle
 Nitrogen plays an important role in biological systems; availability often limits
productivity
 78% of atmosphere is gaseous N2
 the global nitrogen cycle - interconversion of nitrogen forms from atmosphere,
to soil, to biomass
Global nitrogen cycle
 Nitrogen fixation- reduction of N2 to ammonia by free living or symbiotic
prokaryotes
 Nitrate (NO3-) and ammonium (NH4+) converted to organic nitrogen forms
(amino acids) by plant
 Decaying biomass converted to ammonia by ammonifying bacteria-some
reenters atmosphere, some taken back up by plants
 Denitrifying bacteria denitrify nitrates to gaseous N2 which returns to
atmosphere
Nitrogen fixation
 Fixation of atmospheric N2 into usable forms
 Done by many free living bacteria (e.g., Azotobacter), and cyanobacteria (e.g.,
Anabaena)
Nitrogen fixation
 Many symbiotic Relationships exist:
 Cyanobacteria (blue green algae): Anabaena in Azolla (water fern)
 Actinomycetes (filamentous bacteria): Frankia in Alder trees
 Relationship is specific
Rhizobia
 Rhizobium is a nodulating bacterial associate of Legumes (beans, peas, locust
trees, alfalfa)
 Fix 25 – 60 kg N2 /hectare, vs. 5 kg/hectare for non-symbiotic fixation
 Complex exchange of signals establish and maintain symbiosis
Phosphorus cycle
 Earth’s crust is primary reservoir of phosphorus
 Amount of phosphorus required by plants is small compared to nitrogen