The reason Urea CO(NH2)2 isn't used for spring green up is soil temperature. Urea needs microbial activity to break it down to NO3 to be up taken by the plant. The Beneficial Microbes that break down urea are inactive in colder soils.
My soil being a calcareous sandy has a natural pH of 9.5. Therefore I want all my fertilizer to be acid forming. Therefore I use Ammonium Sulfate year round and want a high percentage in any fertilizer blend I use whether I mix it myself or buy it. Being in South Florida my soil never gets cold enough to stop microbial activity and Urea can be used year round effectively unlike the North.
Part of our Calcareous sandy soil is the lack of CEC or Chemical holding power. Therefore our fertilizer must be spoon fed or it leached out very quickly.
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