Agriculture: Fertilisers

Michael is out gardening, but his vegetables are not doing very well. I don’t understand! I planted vegetables on this patch last year and they grew great! This year they are so weak and pale! They have enough water and sunlight, so what’s wrong?

Hm, maybe there is something else that’s missing...? Like what? Apart from water and sunlight, plants need specific chemical substances to be able to grow, develop, and reproduce. These substances are called nutrients. The nutrients that plants need most are: carbon, hydrogen, oxygen, nitrogen, phosphorus, and potassium.

These are primary macronutrients. Then there are nutrients that plants need in moderate amounts: calcium, magnesium, and sulphur. These are secondary macronutrients. Lastly, nutrients that plants require tiny amounts of are, for instance, copper, chlorine, iron, and zinc. These are called micronutrients.

Where do plants get all these nutrients from? Carbon, hydrogen, and oxygen are non-mineral nutrients, and plants get these mostly from air and water. All the other nutrients are mineral nutrients and they come from the soil. Plants absorb mineral nutrients through their roots, and use mineral nutrients for different purposes — to grow, produce fruit and seeds, and to fight diseases or pests. Once in the plant, nutrients stay there, which means there are fewer nutrients in the soil.

And the more plants you grow, the more nutrients they absorb from the soil. So if you grow plants on the same patch of land over and over, the soil becomes depleted of nutrients. It is often not possible for the soil to replenish nutrients quickly on its own. As a result, it becomes more and more difficult for plants to grow. Farmers need to make sure that there are enough nutrients in soil at all times, so that crops grow healthy and strong, and produce high yield, year after year.

But how? This can be done by adding substances to the soil that are rich in nutrients. These are called fertilisers. There are two main types of fertilisers: organic and mineral. Organic fertilisers come from natural sources, such as plant and animal waste, peat or sewage.

Organic fertilisers contain carbon. Examples of organic fertilisers include manure from animal poop; and compost, which consists of decayed plants and other organic material. Organic fertilisers release nutrients slowly, over time, and increase soil quality in the long run. It might take a while to see the results, and it’s often difficult to estimate how much of each type of nutrient an organic fertiliser contains. Mineral fertilisers, often called chemical or synthetic fertilisers, are man-made.

They do not contain carbon. They are a mix of various compounds, obtained through different processes, from materials such as petroleum products or rocks. Very common mineral fertilisers are NPK fertilisers. These provide high amounts of nitrogen, phosphorus, and potassium - the three primary mineral macronutrients. Mineral fertilisers give quick results, because minerals in this form are easily absorbed by plants.

But this makes it easy to overfertilise plants. Nutrients from mineral fertilisers don’t stay in the soil for long, which means you might need to keep adding them. They are also easily dissolved in water and might cause water pollution. All plants need nutrients to survive and grow. Fertilisers make it possible to easily add necessary nutrients to the soil, so that farmers and gardeners can grow strong, healthy plants, and get high yields.

I will use rabbit poop and add it to the soil! My vegetables will get all the nutrients they need!