Before sowing, certain seeds first require a treatment prior to the sowing process. This treatment may be seed scarification, stratification, seed soaking or seed cleaning with cold (or medium hot) water. Seed soaking is generally done by placing seeds in medium hot water for at least 24 to up to 48 hours. Seed cleaning is done especially with fruit (as the flesh of the fruit around the seed can quickly become prone to attack from insects or plagues. To clean the seed, usually seed rubbings with cloth/paper is performed, sometimes assisted with a seed washing. Seed washing is generally done by submerging cleansed seeds 20 minutes in 50 degree Celsius water. This (rather hot than moderately hot) water kills any organisms that may have survived on the skin of a seed. Especially with easily infected tropical fruit such as lychees and rambutans, seed washing with high temperature water is vital.
In addition to the mentioned seed pretreatments, seed germination is also assisted when disease-free soil is used. Especially when trying to germinate difficult seed (e.g. certain tropical fruit), prior treatment of the soil (along with the usage of the most suitable soil; e.g. potting soil, prepared soil or other substrates) is vital. The two most used soil treatments are pasteurisation and sterilisation. Depending on the necessity, pasteurisation is to be preferred as this does not kill all organisms. Sterilisation can be done when trying to grow truly difficult crops. To pasteurise the soil, the soil is heated for 15 minutes in an oven of 120 °C.
Source : http://en.wikipedia.org/wiki/Sowing
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