fertilisers Flashcards
why do we need fertilisers? (5)
nitrates and phosphates are two very important mineral ions that are often limiting factors in plant and algal growth.
intensive food production results in mineral ions continually being lost from the soil as crops are harvested or used to feed livestock (e.g. cattle and sheep) which are also removed. plants are not left to decompose and so minerals are not recycled.
it is therefore necessary for farmers to use fertilisers to replace the lost mineral ions and maintain crop productivity.
the most important inorganic ions found in fertilisers are nitrates, phosphates and potassium, containing the elements nitrogen (N), phosphorous (P) and potassium (K) respectively. these are needed in relatively large amounts by plants. other elements are only required in small amounts, e.g. iron and magnesium.
different species of plant require different proportions of each of these elements.
what do these different biological molecules require?
- proteins
- nucleic acids
- phospholipids
- ATP
- proteins: nitrates
- nucleic acids : nitrates and phosphates
- phospholipids :phosphates
- ATP: phosphates (and nitrates)
what are the two types of fertilisers and what do they provide?
there are 2 types of fertilisers (natural and artifical), both of which provide the chemical elements required for healthy plant growth.
describe the two types of fertilisers
natural fertilisers consist of organic plant or animal matter, containing complex organic compounds, e.g. urea, which releases mineral ions as it decays.
artificial fertilisers are manufactured and contain concentrated mineral ions (especially NPK compounds) in an inorganic, soluble and ready-to-use form.
what are the advantages and disadvantages of the two types of fertiliser?
NATURAL/ORGANIC EG FARMLAND MANURE
ADVANTAGES
- slow release of nutrients
- adds humus to soil – improves structure & drainage
- nutrients not readily leached
- useful, cheap means of disposal of farm waste
DISADVANTAGES
- may be difficult to spread
- minerals only released slowly
- unknown quantities / concentration of nutrients
ARTIFICIAL/INORGANIC
ADVANTAGES
- nutrients in concentrated form
- can be applied in smaller amounts (cost effective)
- nutrients released rapidly into soil
- clean chemicals that lack odour
DISADVANTAGES
- readily leached from soil, so can damage the ecosystem of ponds/lakes
- risk of fertiliser spray spreading to other areas
- expensive to buy/manufacture
why may crop yield fall even as amount of fertiliser is increased?
higher concentration of ions may reduce the water potential of the soil
plants may lose water through their roots by osmosis
high concentrations may be toxic to roots
fertilisers in excess can have widespread environmental effects, such as …
fertilisers in excess can have widespread environmental effects, such as reduced species diversity, leaching and eutrophication
how can fertilisers cause reduced species diversity?
fertilisers increase growth of species such as grasses and nettles. these rapidly growing species outcompete many other species, which die as a result. they will compete for light, mineral ions and water.
farmland used in intensive farming tends to have far fewer plant species than they would have had in the past, which leads to fewer species of animals due to a reduced variety of food and habitats.
how can fertilisers cause leaching?
inorganic fertilisers are very soluble in water. leaching is the process by which nutrients such as nitrates are removed from the soil. rain water will dissolve any soluble nutrients, such as nitrates, and wash them out of the soil. the leached nitrates then find their way into watercourses, such as streams and rivers, which in turn may drain into freshwater lakes. this can then cause eutrophication.
how can fertilisers cause eutrophication?
in most lakes and rivers there is naturally very little nitrate meaning nitrate is a limiting factor for plant and algal growth. once nitrate leaches into the watercourse, it ceases to be a limiting factor and plants and algae grow exponentially, resulting in the following course of events:
- an ‘algal bloom’ occurs (rapid algal growth)
- a dense layer of algae forms on the surface of the water, absorbing light and preventing it from penetrating to lower depths, so plants below the surface cannot photosynthesise and therefore die
- increased competition between algae results in death of algae too.
- when the algae and plants die, they are decomposed by bacteria and other saprobiotic organisms, whose populations suddenly increase.
- the increase in aerobic respiration by bacteria uses up oxygen.
- the dissolved oxygen in the water becomes depleted, causing many species of invertebrates and fish to die
pollution of a river with sewage can also cause eutrophication as the organic material in sewage is fed upon by bacteria, which will respire aerobically, using up the oxygen in the river. if detergents are present, these contain phosphates which will encourage algal growth.
