How soil and water temperature affects seed germination

Have you ever wondered. "Why did my seed not grow? "

You just assumed the seed was bad.  Right? :) 

Last spring I planted carrots, radish, Okra, Marigolds and a few other things that never showed any sign of life. March thew June we had excessive amounts of rain. And July was one of  the coolest temperature in history. My seed drowned. Some plants came up and just sat there. We did not have enough sun till August. Then I had the marigolds start to bloom. Okra started to look like 1 month old plants at age of 3 months.  I was very lucky my red Pontiac potatoes and Tomatoes grew well. Yukon gold did not like all the water. Corn was too short and did not have many kernels. 

 So this tells me now.  

Temperature was too low in the soil and they drown as seeds.
Or, they sprouted and temp dropped enough to kill them.
The ones that never matured but grew late. They had there growth stunted by low temps.

When you want decide that it is time to start your seed for your growing area. The temperature of the soil is the first thing you need to know. If you are starting them indoors. You have a better chance to make sure the seeds will make it to the plant stage of growth. Some seed needs to be planted directly in the soil out side.
Somethings like tomatoes need to be planted in doors in my area. Because we are zone 5b. And do not have a long growing season, like the southern states do.

So right now, I need to plant some of the cool session crops like broccoli, cabbage, cauliflower.

Some people buy grow mats. But if your soil maintains the temperature of the room. There is no need to add heat to the bottom of the tray. As long as the room temperature is within the range needed to germinate the seed. My room is 68-74f and I took the temp of the soil and it is the same temp.

 Example germination temps

Cool season :   germinates at temps of (40°F to 90°F)

 80°F optimum

Warm Season: germinates at temps of50°F to 100°F 80°F optimum

What I have done is buy a cooking thermometer.  I can insert the metal probe in the soil to see the soil maintains at or near room temperature.

I have found Using smaller containers the soil temperature can change faster then using 1-5 gallon  grow bags.Were as outside ground soil is a extreme mass and will not increase as fast as the air temperature.

Another thing most people do not think about is the temperature of the water you are pouring on or under the soil. Tap water is very cold at this time of year. I found it best to take our what I may need for tomorrow and let it  become room temperature.  If not your could shock your plants.  Say they are happy at 70f and you poor 45 f over them. It will take a while for them to come back up to room temp.
Notes:
Starting seed outside/ Direct sowing

In the spring, cooler soil temperatures can limit plant growth.
In mid-summer, hot soil temperatures may prohibit seed germination.

 Growth temperatures
Cool Season crops
Daytime: 65F° - 80°F preferred 40°F minimum 
Nighttime greater than 32°F for tender transplants
down to mid-20s°F for established plants

Warm Season crops
Daytime:  86°F optimum 60°F minimum
They become week at temps below 55°F. This will stunt the plant, reducing yields.  Nighttime:above 32°F