The Oil Drum

I guess these CSIRO guys don't know what they're doing, then.

Nitrogen fertilisers are used by the sugarcane industry to promote plant growth and high sugar yields. Whilst much of the nitrogen applied to fields is taken up by the sugarcane crop, a percentage leaches through the soil into groundwater tables or nearby waterways.

[...]

To support long-term sustainability, the sugar industry is seeking new ways to maintain productivity while minimising the environmental impacts of nitrogen fertiliser use.

Stupid scientists. There they are thinking that sugarcane requires nitrogen fertiliser, when according to Pholostan it actually adds nitrogen to the ground. They should listen to the random internet guy!

No commonly-used crop adds nitrogen to the soil. Legumes and some leafy vegetables have a fungus/bacteria on their roots which adds nitrogen; this is the reason for alternating wheat with beans in cropping, or introducing alfalfa and other manure crops.

Turning sugar cane into ethanol leaves behind a slurry which is typically disposed of in landfills. Bagasse ashes contain very little nitrogen, however are useful as potash (potassium, with nitrogen and phosophorous one of the three main artificial fertilisers in use around the world).

It's pretty simple, really. If you take something out of the soil, you need to put something back in, or there's less next year. Nowadays we decide to maximise production, taking a lot out, and putting it back in in the form of ammonia and the like. If those artificial inputs are absent, we'll have to put more back in by putting stalks back in the soil, manure crops and the like.

Using the entire crops for biofuels will mean we're taking more out than we put in. So either we use artificial fertilisers to make up for that, or we degrade the land and have to get new agricultural land by deforestation.