Potassium

Chapter 5 in Lecture Manual

K fixation

the process of reducing plant available K by trapping soil K+ between 2 clay units

K form in nature and use by plants

Plant available = K+

K+ exists in plants as free K+ (not chemically bound in plant)

Used for the activation of 60 enzymes, cell energetic,metabolism, and water movement in cells

K movement in soil

Mainly diffusion, some root interception

K+ adsorbs onto clay and Organic matter surfaces and gets fixed between clay mineral units

What factors affect soil K availability to plants?

• Clay Quantity: soil K retention increases as clay quantity increases, solution K (available to plants) increases as CEC increases

• Clay minerals: Mica and feldspar weather and release K+, Illite and Vermiculite fix K and lower plant availability

• Tillage influences: K diffusion is slow in cold, dry soils. Conventional tillage heats soil faster, strip till is a good balance between conventional and no-till

Minerals and their availability mechanisms that influence soil K availability

Mica and Feldspar chemically weather and release K+ and increase available K levels

Illite: 2:1 non-expanding clay

Vermiculite: 2:1 expanding clay

Both fix K and lower soil K availability. Expanding clay traps K ions as it shrinks and swells.

Issues regarding tillage practices which influence soil K availability

Conventional: heats topsoil faster, encourages higher K diffusion rates sooner, dries soil out a bit

No-till: keeps soil covered and cooler, much slower K diffusion rate

Strip-till: good balance between the two

Popular fertilizer K sources and forms

Soluble chemical form: K+, fertilizer analysis expressed as K2O

• Potash (0-0-60) - most popular, chemical formula is KCl dry material, is made from processed rock KCl

• KCl Solution (liquid, 0-0-15) -is potash dissolved in water

Effective practices that enhance K availability to crops

• Always use soil test results to formulate recommendations

• Most common is to broadcast apply and incorporate with tillage

• Safer option is to apply 2×2 starter bands to prevent high salt concentrations damage

• No-till fields apply in strip till bands for wide row grain crops

chemical changes that influence exchangeable K

Changes in CEC and soil pH affect how tightly K+ is held by clay and OM. Faster processes.

chemical changes that influence fixed K

Clay tightly holds K+ through shrinking/swelling cycles. K+ ions are released when primary minerals are weathered. Slow process.