Question
how does  fertilize nitrogen phosphours potassium help a plant/grass grow.what does each one do?

Answer
Plants require sixteen essential nutrients for proper growth and development. You can consider the essential minerals as "building materials" which the plants need to grow.

As opposed to humans who get energy from eating energy (such as potatos, meat, rice, etc), plants need to produce their own foods. They take mineral nutrients (chemical elements) and fuse them/combine them and produce their own carbohydrates, proteins, etc.

Each essential nutrient is equally important to the plant, yet each is required in vastly different amounts (some are require in several pounds whereas others in 1/2000 of a gram)

The differences in volume between have led to the grouping of these essential elements into three categories; primary (macro) nutrients, secondary nutrients, and micronutrients.

The macro nutrients are: nitrogen, phosphorous and potassium
The secondary nutrients are: calcium, magnesium, sulphur
The micro nutrients are: boron, chloride, copper, iron, manganese, molybdenum, and zinc.

Generally you could compare primary and secondary nutrients to "raw materials" used in the production of plant food (by the plants themselves) and compare micronutrients to our vitamins (just very broadly speaking).

In most cases soil contain sufficient nutrients to support plant growth. However, gardeners found that some nutrients were removed from the soil quickly (this was most often the primary nutrients) and therefore fertilizers were created which contained these primary (macro) nutrients. It was not necerssary to resupply secondary and micro nutrient every time as these are needed in smaller amounts

Therefore, the reason we fertilize with nitrogen, phosphorous and potassium is to resupply nutrients which is removed from the soil by plants faster than nature can resupply on its own.

If nutrients were resupplied by nature (through manure from animals or fallen leaves and fruit being left to rot in place) then there would be no reason to fertilize. However, today we rake leaves off the lawns, we collect fruit, we keep animals out of the suburbs and cities. This then require us to find another way of bringing nutrients back to the soil, and we can do this through fertilization.

Fertilizer is simply a manmade way of resupplying the nutrients which the plants consume during growth.

All nutrients have several purposes and you can not say that one nutrient is responsible for one thing, but overall there are some broad guidelines which can be used:

PRIMARY (MACRO) NUTRIENTS


NITROGEN

?Necessary for formation of protein
?Essential for plant cell division (e.g. plant growth)
?Directly involved in photosynthesis
?Necessary component of vitamins
?Aids in production and use of carbohydrates
?Affects energy reactions in the plant

PHOSPHORUS
?Involved in photosynthesis, respiration, energy storage and transfer, cell division, and enlargement
?Promotes early root formation and growth
?Improves quality of fruits, vegetables, and grains
?Vital to seed formation
?Helps plants survive harsh winter conditions
?Increases water-use efficiency
?Hastens maturity

POTASSIUM
 
?break down and translocation of starches (plant energy)
?Increases photosynthesis
?Increases water-use efficiency
?Essential to protein synthesis
?Important in fruit formation
?Activates enzymes and controls their reaction rates
?Improves quality of seeds and fruit
?Improves winter hardiness
?Increases disease resistance

SECONDARY NUTRIENTS
 
The secondary nutrients are calcium, magnesium, and sulphur. For most crops, these three are needed in lesser amounts that the primary nutrients.
CALCIUM

 
?Utilized for Continuous cell division and formation
?Involved in nitrogen metabolism
?Reduces plant respiration
?translocation of photosynthesis from leaves to fruit
?Increases fruit set
?Essential for nut development in peanuts
?Stimulates microbial activity

MAGNESIUM
 
?Key element of chlorophyll production
?Improves utilization and mobility of phosphorus
?Activator and component of many plant enzymes
?Directly related to grass tetany
?Increases iron utilization in plants
?Influences earliness and uniformity of maturity

SULPHUR

?Integral part of amino acids
?Helps develop enzymes and vitamins
?Promotes nodule formation on legumes
?Aids in seed production
?Necessary in chlorophyll formation

MICRONUTRIENTS
The micronutrients are boron, chlorine, cooper, iron, manganese, molybdenum, and zinc. These plant food elements are used in very small amounts, but they are just as important to plant development and profitable crop production as the major nutrients. Especially, they work "behind the scene" as activators of many plant functions.

BORON

?Essential of germination of pollon grains and growth of pollen tubes
?Essential for seed and cell wall formation
?Promotes maturity
?Necessary for sugar translocation
?Affects nitrogen and carbohydrate
CHLORINE
?Not much information about its functions
?Interferes with P uptake
?Enhances maturity of small grains on some soils
COPPER
?Catalyzes several plant processes
?Major function in photosynthesis
?Major function in reproductive stages
?Indirect role in chlorophyll production
?Increases sugar content
?Intensifies color
?Improves flavor of fruits and vegetables
IRON
?Promotes formation of chlorophyll
?Acts as an oxygen carrier
?Reactions involving cell division and growth
MAGANESE
?Functions as a part of certain enzyme systems
?Aids in chlorophyll synthesis
?Increases the availability of P and CA
MOLYBDENUM
?Required to form the enzyme "nitrate reductas" which reduces nitrates to ammonium in plant
?Aids in the formation of legume nodules
?Needed to convert inorganic phosphates to organic forms in the plant
ZINC
?Aids plant growth hormones and enzyme system
?Necessary for chlorophyll production
?Necessary for carbohydrate formation
?Necessary for starch formation
?Aids in seed formation