WHAT IS AN INOCULANT?
For my beans and peas (legumes)
Legumes are a wonderful group of plants comprised of beans and peas. If you are not sure if it's a legume take a look at the flowers. All legumes have the same kinds of flowers as peas. Some other plants have pea like flowers but none are the same as the true peas.
The special thing about legumes is that they are able to fix nitrogen from the atmosphere and make is useable to themselves and other plants. They do this with nodules on their roots which contain a special kind of bacteria - a good bacteria, not all bacteria are bad. These bacteria form a symbiotic relationship with the legume plants. This means that the plants give them something they want in this case sugars and starches as food, and the bacteria give the plant something it wants, nitrogen in a form it can use.
In order for this to work the bacteria must be present in the soil around the plant which then creates some protective tissure around it in the form of the root nodules and the process of making the nitrogen begins. In some cases these bacteria are present in the soil around the plant and easily make a lasting relationship with it. However in many cases the bacteria is not present. If this is the case the plant cannot make nitrogen and it and the plants around it suffer.
These plants and bacteria are fussy. Each plant needs a specific type of bacteria for this to work. If this specific type of bacteria is not present still no nitrogen. Nodules may still form on the roots of the plant, but they don't function. Functioning nodules are pink or reddish when cut open, while non fuctioning ones are tan while green ones mean they are dying.
Because it's such a hit and miss case if the right bacteria are present many legume seeds come coated with an inoculum so when the seed starts to grow the bacteria it needs is right there with it. Bear in mind that the bacteria have to be within 2 mm (0.078") from the root of the plant to be of any use to it. So you need to have a LOT of bacteria in the soil if you don't have a coated seed.
It is very important to have inoculated seed if you are sowing in a new area, an area where the conditions are harsh such as poor soil, salty areas or areas with either too much or too little water. It's also important in areas where the land has been used for one type of plant for a long time. This could be a field where the farmer has only grown corn or soy beans or it could be your lawn where you have only grown grass.
Monocultures are really bad for diversity. If you grow the same thing in the same place all the time it depletes the soil of the nutrients that particular plant needs, but it also starves any other bacteria, fungi or soil micro organisms that would normally live in healthy soil. There is nothing there for them to survive on. So if you suddenly introduce another plant - such as a legume - there is no natural beneficial bacteria or any other kinds of soil micro organism that would help that plant along. So you need to introduce them with your seed in order to make the plant grow better and help make your new plant environment a healthier place.
If you have a diverse healthy garden with good rich soil you have been working on to build up and rotate a lot of your plants around, then most likely you will have some of the right inoculant bacteria near your plants.
Still confused? Try this analogy.
You have a symbiotic relationship with your pizza parlor. You want their pizza and they want your money. You give them money they give you pizza. Neither of you can survive without the other one - well maybe some people could but a lot of students couldnt.
Now maybe you only like a specific type of pizza say, Pizza Hut Pizza, and hate Dominos. (the opposite could also be true, pick your favorite and worse pizza and plug into the name slots). You would go to any lengths to get Pizza Hut pizza and not eat Dominos if it was the last pizza on earth. If there is no Pizza Hut near you you are going to be unhappy and not eat pizza as often as you would if there was one on the corner of your block. You have to go to enormous lengths to get the pizza you want which makes life much harder for you than so you don't survive as well.
Now what would happen if you suddenly got moved to a new area where there were NO pizza parlors? (your lawn or old monoculture field). You don't have a car so you can't drive to a pizza parlor, which by this reckoning is over 50 miles away! You need someone to come along and open a pizza parlor in your neighborhood so you can be happy and thrive again.
So make your plants happy. Make sure you don't plant monocultures and ensure that if you plant new legumes you give them the right inoculant or have inoculated seed.