Farmers work hard to ensure that there are sufficient nutrients in their soil for crops and plants to grow successfully. They may do this by adding an NPK fertiliser. This stands for nitrogen, phosphorous and potassium. The fertiliser is graded according to the proportions of the three elements by mass in the fertiliser. A popular choice is NPK 25.5.5 which means 25% by mass nitrogen, 5% phosphorous and 5% potassium.
Farmers can also add nutrients to their soil by spreading muck or slurry (manure). This is a mixture of animal faeces and bedding straw that has been piled up and left to rot for several months. It breaks down in the soil and releases essential nutrients reducing the need for artificial fertiliser which is expensive.
Careful crop rotation can also ensure that nitrogen is replenished in the soil. Plants like clover actually add nitrogen to the soil because their roots have nodules that play host to bacteria capable of extracting nitrogen from the air and fixing it into the soil as nitrate. By alternating between nitrogen-using and nitrogen-replacing crops farmers can reduce their dependency upon artificial fertiliser. Nitrate has such a massive impact on crop yields that most farmers will use some fertiliser at some point.
Nitrate can also get into the soil after thunderstorms. Lightning can cause the usually unreactive nitrogen to oxidise and form very dilute nitric acid when it dissolves into rainwater. This adds to the natural acidity in rainwater and is part of the nitrogen cycle that is so important for all living things on Earth.
Modern farmers may well map the yields over their entire farm using SatNav then when it comes time to spread fertiliser they can spread it more thickly where the yields have been low in previous years. This can be done with great precision with computers logging the tractor’s position via satellite and automatically controlling the flow of fertiliser to the ground.
Questions…
- Suggest three ways that nitrates can get into the soil.
- What do plants need nitrate for?
- Name a plant nutrient that has not been mentioned in the post above. How does the plant use it?
- NPK 25.5.5 fertiliser is 25% nitrogen, 5% phosphorous and 5% potassium by mass. What makes up the other 65% of the mass of the fertiliser?
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