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THE NITROGEN CYCLE: WHAT A GAS!! Although nitrogen gas makes up 79% of the Earth’s ___________________, it cannot be used in that form. Nitrogen is needed so cells can make ____________________ and genetic material like ____________________. For this to happen, the nitrogen gas must first be converted into ____________________, which plants can use. This process is known as ____________________ and can be performed by bacteria in the soil or by ____________________. The bacteria in the soil are found in lumps on roots called ____________________, in a special group of plants called ____________________, one example being ____________________. Denitrifying bacteria can break nitrates into ____________________ and then back into nitrogen gas. This process is known as ____________________ and removes the nitrogen from the soil. The bacteria that do this are called anaerobic because ____________________ will kill them. This is why people put holes in their lawns (a process called ____________________) which allows oxygen in to kill this bad bacteria. If a farmer continues to grow the same crop of corn in a field for too long, all the nutrients will get used up. As a result, he will leave a field empty (called ____________________) and allow clover or some other useful plant to grow, returning the nitrogen to the soil. If the soil is poor, he may add manure or some chemical fertilizer to return the ____________________ back to the soil, so that more crops can be grown. Word List: nitrates nodules denitrification protein DNA nitrites nutrients atmosphere lightning fallow soybeans nitrogen fixation oxygen aeration legumes