1. © SSER Ltd.

2. Green plants manufacture organic compounds such as carbohydrates, proteins and lipids by the process of photosynthesis The manufacture of these organic compounds requires a supply of inorganic mineral elements that agricultural crops obtain from the soil The most important of these inorganic elements are nitrogen, phosphorus and potassium (NPK); these are commonly described as macronutrients as they are needed in relatively large amounts by the plants Other nutrients, known as micronutrients, are required for the healthy growth of plants and these include trace elements such as iron and magnesium; these elements are only required in small amounts NITROGEN – absorbed by plants principally as nitrate ions. Needed for the manufacture of amino acids, proteins and nucleic acids PHOPHORUS – absorbed by plants as phosphate ions. Needed for the manufacture of nucleic acids and ATP POTASSIUM IONS – required for the efficient functioning of certain enzymes involved in photosynthesis and respiration

3. Under natural conditions, organic compounds are passed to the decomposers whose activity converts these compounds into inorganic nutrients for recycling; this process is illustrated below for the recycling of nitrogen

4. When plants are grown as crops for use by humans, they are harvested by the farmer and removed Harvesting this sugar cane crop at the end of the growing season means that nutrients, locked up in the crop, are NOT returned to the soil

5. In this field, the grass is harvested for use as winter hay and nutrients are not returned to the soil for recycling

6. When crops are grown and harvested year after year, there is a need to replace the lost nutrients if soil fertility is to be maintained Fertilisers are used to replace the nutrients that are lost from the soil Fertilisers may be: Organic, e.g. farmyard manure Inorganic, e.g. manufactured fertilisers such as NPK fertiliser that supplies nitrogen, phosphorus and potassium

7. Inorganic fertilisers are composed of concentrated sources of macronutrients that can be spread evenly over the crop

8. Manure is a form of organic fertiliser composed of the faeces of farm animals mixed with straw

9. This farm manure is being applied as a slurry and is a useful method for the farmer for disposing of animal waste

10. Inorganic fertilisers are very soluble in water and, when present in excess, are readily leached from the soil and find their way into water courses The enrichment of aquatic environments by nutrients such nitrates and phosphates is known as EUTROPHICATION Levels of eutrophication have significantly increased as a result intensive farming and these levels are having a damaging effect on the environment

11. Nitrates and Phosphates from excess fertiliser leach into waterways Algae and water plant populations increase rapidly

12. When the algae and plants die, they are decomposed by aerobic bacteria whose populations subsequently increase Bacterial population increases Dissolved oxygen in the water becomes depleted as the increased bacterial population respires The capacity of the aquatic environment to support organisms such as invertebrates and fish is dramatically reduced and many die

13. The use of fertilisers in the agricultural fields beyond the trees in this photograph has resulted in leaching of nutrients into the river Excess nutrients have caused the overgrowth of algae in this river and formed an algal bloom