Water shortage: Shortage of water will result in a variety of behaviours, because plants have different mechanisms for handling water stress. Many Australian natives are particularly effective in being able to manage a shortage of water, with a whole variety of strategies such as waxy or specially shaped leaves.
One particular mechanism is to slow their reaction rate by cutting back transpiration. These plants do not appear to suffer stress; they simply shut up shop and stop growing, remaining in a state of suspension until water is available. They may even sacrifice entire limbs to survive.
Another survival mechanism employed by some plants is to put down very deep roots which will only operate if the upper soil is dry. The infiltration rate of air through dry soils is relatively high, so under dry conditions deep roots have adequate access to air. Because of their depth, or operating head, these deep roots do not provide for good growth. They simply support the plant. When the soil becomes wet, these deep roots will become starved of air and stop supplying water and nutrients. The more effective shallow roots will take over, giving much higher growth rates.
Grape vines are typical of the sort of crop which have the capability of putting down roots to considerable depths (up to 40 metres) in order to survive. However, under these conditions the whole plant will be in survival mode and will avoid making fruit or unnecessary growth until more favourable conditions occur.
Other plants, such as vegetables, do not have these survival mechanisms. When there is no water available, they die.
Excess water, porosity, air and compaction: Roots expel carbon dioxide into the soil and absorb oxygen, which means air and porosity are a crucial part of the plant’s environment. A few highly specialised plants, such as mangroves, have the ability to feed air through to the roots. Most plants, however, will stop growing and eventually die if air is excluded from the roots by waterlogging or compaction.
Soil compaction can arise from excessive application of water, as occurs in flood irrigation, or mechanical action, as from large sprinkler drops. Plant growth is reduced by the lack of air in the root zone.
Nutrients: Nutrients are obtained by adding fertiliser, by the decomposition of organic matter, and by biological action.
Many soils, particularly clays, can be rich in nutrients, but these are locked into the soil and are not available to the plant. Micro-biological action can release these nutrients.
Water shortage and nutrients: If there is an excess of nutrients - for example, the plant is over fertilised when water is short - the solution in the soil will be stronger than in the plant. The plant will be killed by osmotic pressure sucking water out of the plant.
Excess water and nutrients: Excess water in the soil with insufficient nutrients creates weak solutions and a high osmotic gradient. The plant will be able to draw in and evaporate large amounts of water but will not be able to extract sufficient food, resulting in reduced growth.
Obtaining the right balance of water, air and nutrients is vital to healthy growth.