Monopotassium phosphate

Monopotassium phosphate (also potassium dihydrogen phosphate, KDP, or monobasic potassium phosphate, MKP) -- 24 -- is a soluble salt which is used as a fertilizer, a food additive and a fungicide. It is a source of phosphorus and potassium, and is a buffering agent. When used in fertilizer mixtures with urea and ammonium phosphates, it minimizes escape of ammonia by keeping the pH at a relatively low level.

Fertilizer grade MKP contains 52% P2O5 and 34% K2O, and is labeled 0-52-34. It is often used as a nutrient source in the greenhouse trade and in hydroponics.

It is one of the components of Gatorade and is used as an additive in cigarettes.

At 400°C it decomposes, by loss of water, to potassium metaphosphate (KPO3)

Nonlinear optics use
As a crystal, it is noted for its non-linear optical properties. Used in optical modulators and for non-linear optics such as SHG (second harmonic generation).

Also to be noted is KD*P, Potassium DiDeuterium Phosphate, with slightly different properties. Deuterated KDP is almost always used in nonlinear frequency conversion of laser light instead of protonated (regular) KDP due to the fact that the replacement of protons with deuterons in the crystal shifts the third overtone of the strong OH molecular stretch to longer wavelengths, moving it mostly out of the range of the fundamental line at ~1,064nm of neodymium based lasers. Regular KDP has absorbances at this wavelength of around 5-6%/cm of thickness while highly deuterated KDP has absorbances of typically less than 1%/cm.