soil nutrition - potassium

potassium plant and animal nutrition

remains in ionic form (K+) in solution in the cell regulating cellular osmotic water potential. acts as an activator for cellular enzymes. essential for photosynthesis, for protein synthesis, for nitrogen fixation in legumes, for starch formation, and for the translocation of sugars. helps plants adapt to environmental stresses.

potassium content in healthy leaf tissues

1-4%

potassium environmental issues

no serious off-site environmental problems. not toxic and does not cause eutrophication in aquatic systems. however, overuse: can reduce plant uptake of calcium and other essential cations, can negatively impact soil physical quality in a manner similar to sodium

sources of potassium

supply to the soil solution mainly by mineral weathering and CEC from colloid surfaces.  mineralization of soil organic matter (little effect-not a structural component of organic molecules).

plant uptake potassium as

ion form (K+)

potassium deficiency

reduced drought tolerance, increased lodging, growth is slowed, tips and edges of the oldest leaves begin to yellow (chlorosis) and then die (necrosis)

K problem in soil fertility

easily exchangeable K usually accounts for only a small fraction of the total soil K, much potassium is locked up in the structural framework of soil minerals. because of the complimentary ion effect, leaching losses can be reduced by liming an acid soil. some plants may take up potassium beyond the amount needed for optimum growth (luxury consumption)

because of the complimentary ion effect, leaching losses can be reduced by _____ an acid soil. 

liming

potassium in primary mineral (micas, feldspars) structures is: 

unavailable

potassium in non-exchangable positions in secondary minerals (vermiculite, colloidal sized mica) is only ______. this includes what is termed ______

slowly available, fixed K

potassium on the cation exchange sites of soil colloids is:

readily available to plants

potassium ______ is readily available and the immediate source of ions taken up by plants. 

soluble in water

factors affecting potassium fixation

nature of soil colloids. wetting and drying cycles which cause the clay interlayers to become more exposed, both for the fixation and the release of K. freezing and thawing cycles, which cause effects similar to those of wetting and drying. the presence of excess lime (decreased the ratio of K to other cations—especially Ca and Mg—on the cation exchange complex—K can leach easily)

potassium is supplied to the soil solution mainly by:

mineral weathering and cation exchange from colloid surfaces.

organic matter mineralization has little effect on potassium because

potassium readily leaches out of plant residues and so is not a structural component of soil humus

certain 2:1 type clays, especially vermiculite can fix potassium ions in interlayer positions. this K fixation is similar to that of ammonium in that:

both become inaccessible to normal cation exchange and to root uptake

in some soils, ________ can supply potassium fast enough to maintain a sufficient supply of soluble and exchangeable K

mineral weathering

potassium is not lost from soil as:

gaseous forms

for potassium, both leaching and erosion losses can be:

substantial

practical aspects of potassium management

add K fertilizers where crops removes readily available forms of potassium more rapidly than mineral weathering. should be applied close enough to the time of plant need, especially on soils with high potassium fixing tendencies, or very coarse textured soils with high potassium leaching potential. return of animal manures and plants residues to the full extent possible can greatly reduce the need for fertilizer additions