Potassium Dynamics in Soils Introduction The Potassium Cycle Forms of Potassium in soil Availability of Potassium in soil Potassium Management of soils
Introduction • K is the third most likely element after nitrogen and phosphorus to limit plant productivity. • Therefore it is commonly applied as fertilizer • It is present in soil solution as positively charged cation, K+ • It does not form any gases that is lost to air
Introduction • Its behavior in soil is influenced more by cation exchange reactions than by microbiological processes • It does not cause off site environmental problems • It is not toxic and does not cause eutrophication problems
-Potassium in Plant and Animal Nutrition • Potassium activates many enzymes in plants and animals that are responsible for energy metabolism, photosynthesis and other processes • As a component of the cytoplasm of plant cells, it helps to lower cellular osmotic water potential thereby increasing the ability of root cells to take up water.
-Potassium in Plant and Animal Nutrition • K is important for nitrogen fixation in legumes • Good K nutrition has been shown to help plants adapt to environmental stress • In animals including humans, K is important in regulating the nervous system and maintenance of good blood vessels.
-Deficiency Symptoms of Potassium • Reduced ability to adapt to environmental stress, e.g., drought, lodging, etc • Tips and edges of leaves become yellow (chlorosis) and then die (necrosis). • In some crops, K deficiency produces white necrotic spots that looks like insect damage.
The Potassium Cycle • Original source of K is primary minerals • Micas (biotite and muscuvite) • Potassium feldspar (orthoclase and microcline) • Potassium is released from the minerals during the weathering process • When available for plant uptake, it is taken up by plants in large quantities
The Potassium Cycle • In natural ecosystems, K is returned to the soil by rainwater leaching of foliage and by plant residue or as waste from animals that feed on plant • Potassium is lost by soil erosion and runoff, and leaching to the groundwater • Most potassium in agricultural ecosystems are lost through removal of crops and crop residues from soil
Forms of Potassium in Soil • The four forms of K are as follows: • K in primary mineral structure • K in nonexchangeable positions of secondary minerals • K in exchangeable form on soil colloid surfaces • K ions soluble in water • Total K in soil and its distribution as shown in the K-cycle is a function of the clay minerals present in soil
K–Fixation by Soil Colloids Nonexchangeable-K Exchangeable-K Soil Solution-K
Factors Affecting K Availability • Types of soil colloids • Ability of various colloids to fix K varies (2:1) • Wetting and drying of soils • Physically affects the structure of colloids • Freezing and thawing • Physically affects the structure of colloids • Soil Acidity (pH) • High pH increases the fixation of K because of less H+ and Al3+ at exchange sites.
Potassium Management • The main problem with managing soil K is that of converting the unavailable forms of the element to available forms • Available K is usually supplemented by fertilization • Another problem is removing a lot of crops from the soil without returning the crop residues • Attempts should be made to return as much residue as possible for the natural plant-soil cycling of K to continue
Potassium Management • Growing high K content plants places demand on the soil supply of potassium • To have high yields of such crops, e.g., alfalfa, it needs to planned in advance to supply soil with enough K to last the cropping season –fertilization • Light and frequent application of fertilizer is better than heavy and infrequent application to reduce luxury consumption • Treating soils with lime have been found to increase K retention in soils.