Phosphate and Nitrate Fertilizers
Useful nutrient chemistry requires matching chemical form, dose, timing, placement, and safety to measured need.
1. Chemical forms and biological roles
Nitrogen is needed for amino acids, proteins, nucleic acids, and chlorophyll. Roots take up nitrogen mainly as nitrate, $NO_3^-$, and ammonium, $NH_4^+$. Phosphorus is needed for ATP, nucleic acids, phospholipids, and phosphorylation. Roots mainly absorb $H_2PO_4^-$ and $HPO_4^{2-}$, with their proportions controlled by pH.
Microbial nitrification oxidizes $NH_4^+$ through nitrite to nitrate under suitable aerobic conditions. Nitrate is mobile and can move below roots with water. Phosphate is less mobile because it adsorbs to mineral surfaces or forms sparingly soluble compounds; strong fixation can reduce short-term availability.
2. Quantitative fertilizer chemistry
A label or formula must be converted to actual nutrient mass. For a pure compound, nutrient mass fraction is $w=\frac{\text{mass of nutrient atoms in one mole}}{\text{molar mass of compound}}$. The nutrient delivered is $m_{nutrient}=w\,m_{fertilizer}$. Distinguish elemental P from a reported phosphate-oxide equivalent and elemental N from the mass of nitrate ion; they are not interchangeable.
- Write the fertilizer formula and determine molar mass.