Plants have a need for all nutrients. That means that healthy growth without algae is only possible if all conditions are complied with: light-CO2-N-P-K etc. If there are elements missing, then the growth will start to falter or can even stop.
Algae and leakage of sugars
If the growth falters, processes are not finalized and plants will start to 'leak' sugars. It is just these sugars which activate algae spores and algae to grow. Algae are much less sensitive to the deficit of or the lack of a nutrient and therefore the matter is to offer all elements from the Golden Triangle to prevent (algae) problems.
Redfield-ratio: NO3 versus PO4
New views teach us that nitrate and phosphate are not a reason for algae formation. It is precisely a lack of these macro elements which lead to algae formation, again by the leaking of sugars from the plant. Of course a very high content of phosphate (>2,0 mg/l) does give algae problems, but a value of 0,5 - 2,0 mg/l PO4 is acceptable and not a reason for algae formation.
Normally the plant is provided with N and P by the feeding of fish or waste. However, with little fish, scant feeding or many plants and light, precisely a deficit of these macro elements can occur. Many people direct the content of NO3-PO4 through the Redfield-ratio (try Googling on Redfield-ratio!) to neutralize a deficit. The Redfield-ration is the ratio between C-N-P, in the ratio 105-16:1. In practice we use the ratio between N and P (16 : 1) and convert that roughly to the water values nitrate and phosphate.
With introduction of additional nitrate and phosphate the correct ratio can be achieved and a deficit is prevented.
Carbon / CO2
A deficit in carbon (usually in the form of CO2) is also guilty of algae formation and bad plant growth. Then also the assimilation process falters and sugars and other substances are again formed, which are released and activate algae. So it is important to ensure sufficient carbon. Certainly in a well lighted aquarium with fast growing plants the need can be large. CO2 from a gas cylinder or liquid carbon from EasyCarbo is usually required to prevent problems.
Light: determines the nutrition advice!
Light is the impelling force of the assimilation and determines
b) absorption speed.
Especially the influence of light on the absorption speed is important: With a lot of light the plant will want to grow more and especially absorb more nutrients. The supply of nutrients can go too slow and the internal buffers will get empty. In this crucial moment nutrients are absorbed more and more out of the water and the demand increases strongly.