MicronutrientsIron (Fe)Boron (B)Zinc (Zn)Copper (Cu)Manganese (Mn)Molybdenum (Mo)

Principal nutrientsNitrogen (N)Phosphorus (P)Potassium (K)

Secondary nutrientsCalcium (Ca)Magnesium (Mg)Sulfur (S)

Soil Chemistry and Plant Nutrients

Image: Jon Davis, author of this presentation (CC BY-SA 2.0)

Plants use soil nutrients to construct

• DNA and RNA: nitrogen, phosphorus)• Proteins: nitrogen (N), sulfur (S) • Many other metabolites (lots of elements) – i.e. chlorophyll, anti-herbivorous chemicals

Note: Plants use K (potassium) to regulate numerous metabolic processes and membrane permeability

Micronutrients in organic moleculesMagnesium in Chlorophyll

Chlorophyll is the green pigment, within chloroplasts, within cells, within a leaf

Image: http://www.flickr.com/photos/76944491@N00/2590848826/ by Melvin Pao. License: CC BY-NC-SA 2.0Image: https://commons.wikimedia.org/wiki/File:Chlorophyll_a.svg by David Richfield. License: Public Domain

Cations and Anions in Soil

• What are cations?– Positively charged ions (NH4

+, K+, Ca2+, Fe2+)

• What are anions?– Negatively charged ions (NO3

-, PO42-, SO4

2-)

Cation Exchance Capacity (CEC)

• Both organic and mineral components of soil are negatively charged

• Cations that are important for plants can bind to them.

• These bound cations are not leached from soil by water.

Cation Exchange Capacity

Image created by Jon Davis, the author of this presentation. License: (CC BY-SA-3.0)

Positively charged ions bind to soil

CEC depends on soil texture

Sands, light-colored: 3-5Sands, dark-colored: 10-20 Loams: 10-15Silt Loams: 15-25Clay and clay loams: 20-50Organic soil: 50-100

Units: milliequivalent of hydrogen per 100 g of dry soil(meq+/100 g)

Q: How can you maintain and improveCEC in sandy soils?

Maintain the organic layer Reduce tillage and erosion Use cover crops Maintain a diverse soil biota

Image: http://www.flickr.com/photos/soilscience/5105328004/ by Soil Science @ NC State. License: CC BY 2.0

Fuquay series soil profile from NC (sandy)

Reduced tillage or no-till

builds soil organic matter and increases cation exchange capacity.

Image: http://www.flickr.com/photos/nrcs_south_dakota/7489934494/ by USDA NRCS South Dakota License: CC BY-SA 2.0

Soil pH• What is soil pH?• A measure of H+ ions in soil water

H+

H+

H+

H+

H+ H+

H+

Image: https://commons.wikimedia.org/wiki/File:PH_Scale.svg by Edward Stevens (CC BY-3.0)

Soil pH around the world

Acidic Neutral Alkaline

Image: http://commons.wikimedia.org/wiki/File:World_Soil_pH.svg by Ninjatacoshell (CC BY-SA-3.0)

Why are acidic soils in wetter places?

• Rainwater is slightly acidic (pH 5.7)– Rain is also known as carbonic acid, due to a reaction with

CO2 in the atmosphere. This creates caverns around the world.

• Nitrification of ammonium in fertilizer releases H+

– Nitrification will be discussed later in this presentation.

• Plant roots emit H+ when taking up other cations– Cations are postively charged ions, like

• Weathering: Highly weathered soils contain lots of Al and Fe (as in tropical soils)

Problems of acidic soils and how to address them

• Aluminum toxicity• Deficiencies of calcium and magnesium

• Most common method of amelioration of acidity: adding lime

• Lime: calcium containing inorganic products (e.g. limestone, gypsum)

The nitrogen cycle

Image: http://commons.wikimedia.org/wiki/File:Nitrogen_Cycle.svgby Johann Dreo License: CC BY SA 3.0

Where does N from added fertilizers enter?

Image: http://commons.wikimedia.org/wiki/File:Nitrogen_Cycle.svgby Johann Dreo License: CC BY SA 3.0

And in what form do plants take it up?

Image: http://commons.wikimedia.org/wiki/File:Nitrogen_Cycle.svgby Johann Dreo License: CC BY SA 3.0

NPK

• The principal nutrients N: Nitrogen P: Phosphorus K: Potassium• Provided together in fertilizers• e.g., 15-15-15: the proportions of the element N and the

proportions of the oxides of P and Q

Image: http://www.flickr.com/photos/iita-media-library/8144240712/ by IITA image Library (CC BY-NC 2.0)

Laboratory activities

• For each soil sample, in each plot or treatment collected earlier, we will determine…– pH– CEC– N– P– K