Potassium (k) occurs in nature as a potassium salt in the form of the mineral sylvite (KCl-NaCl) which is deposited as a result of geological activity and the evaporation of sea water, or as a constituent of the earth crust in the mica and feldspar minerals.
Functions of potassium in plants
Potassium is basic to plants life, since it is vital to many plant processes. While it does not become a part of the chemical structure of plants, it performs many essential functions.
Enzyme act as catalysts for chemical reactions; they bring together other molecules in such a way that the chemical reaction can take place. Potassium is required to activate at least 60 different l enzymes involved in plant growth.
When the sun energy acts to combine Co2 and water to form sugars, the initial high energy product is adenosine triphosphate (ATP) the ATP is then used as energy sources for many other chemical reactions.
The electrical charge balance at the site of ATP production is maintained with K ions. When plants are deficient in K the rate of photosynthesis and the rate of ATP production is reduced, and all the process dependent on ATP is inhibited.
- Protein and starch synthesis
- Transport of amino acids in the sites of protein synthesis, enzyme activation, and balancing of electrical charges are among the key roles of K. potassium is thus required for every major step of protein synthesis. The enzyme responsible for the synthesis of starch in leaves is activated by K.
The accumulation of K in plant roots produces a gradient of osmotic pressure that draws water into the roots. Plants deficient in K are thus less able to absorb water and more subject to stress when water is in short supply.
When water supply is short, K is pumped out of guard cells and the pores close tightly to prevent loss of water. If the supply of K is inadequate, the stomata becomes sluggish and water is lost as a result, plants with an adequate supply of K are less susceptible to water stress.
- Water and nutrient transport
When K supply is reduced the transportation of nitrates, phosphates, calcium, magnesium and amino acids is depressed. As in case of phloem transport systems, the role of xylem transport is in conjunction with specific enzymes and plant growth hormones. An ample supply of K is thus essential operation of these systems.
- Crop quality
Potassium is known as a quality element. High levels of available K improves the physical quality and nutritional value of grain, forage, and crops used for human consumption. Since quality is becoming an increasingly important market factor, especially for export, adequate k supply will become more critical.
- Stress resistance
Potassium is known to develop resistance of crop plants to pests, disease, salinity, excesses of temperature and drought.
Potassium deficiency symptoms and luxury consumption
- It is important to remember that long before symptoms of K-deficiency become visible, severe losses in both crop yield and quality can occur. The first sign of deficiency is reduction in growth rate. Plants becomes stunted and usually leaf color becomes dark-green. At a more advanced stage, specific deficiency symptoms begin to appear.
- Decreased drought resistance
- Appearance of white, yellow or orange chlorotic spots or stripes on older leaves, usually starting from leaf tips and margins. In some species irregularly distributed chlorotic spots appear, but in all cases symptoms starts from leaf tips. The base of the leaf usually remains dark green.
- The chlorotic areas becomes necrotic, the tissue die, and the leaves dry up.
- The symptoms spread to younger leaves and finally the entire plant may be die.
- Roots of K deficient plants are poorly developed and often effected by rot.
- Disease incidence is increased and crop quality is severely reduced, especially in the case of vegetables, fruits, tobacco and fiber crops.
When there is an abundant supply of K, plants can absorb K in excess of quantities required for normal functioning this is called luxury consumption. In this case the application of K fertilizer does not result in increased yield that may lead to depression in yield.
Soil potassium status
Most Pakistani soils are rich in available K ammonium acetate extractable K (NH4OAc-K). This availability however depends upon many factors: parent material, weathering condition and type of clay. Most of the soils have adequate K less than 150mg/kg.
The critical limit Pakistan for soil K deficiency is considered to be 150mg/kg NH4OAc-extractable K2O. Thus below this K level response to added K fertilizer is expected and above this level it is normally not expected.