Nitrogen deficiency is much more common in soil than in hydroponics. Hydroponic nutrients are usually very high in nitrates, often to luxury levels. So nitrogen deficiency in hydroponics is rare unless the EC is much too low or if carbohydrate additives are used. In soil, however, the opposite is true. Nitrogen deficiency is one of the most common nutrient deficiencies in soil and outdoor gardening. Nitrates are very water soluble and leach out of the soil easily. Heavy rains or overwatering can quickly wash the nitrates away from the root zone, leading to a nitrogen deficiency. Microorganisms can also compete for nitrogen in the soil, especially if the carbon to nitrogen ratio is too high.
Nitrogen deficiency can be corrected by applying either organic or inorganic fertilisers, but nitrate or ammonium-based fertilisers work the most quickly. Any general-purpose “grow” formula will usually provide enough nitrogen to correct major deficiencies. But if you want greater precision and better results, nitrogen-specific fertilisers such as ammonium nitrate and ammonium sulfate are the most effective. Just make sure not to overdo it! Spoon feeding extra nitrogen a little at a time is much better than trying to force-feed the plant all at once. Remember, you can always add a little more nitrogen if it becomes necessary, but careless over fertilising can harm the plant.
Foliar feeding is another good option for correcting a nitrogen deficiency. CAUTION: never use nitrate-based fertilisers as foliar sprays! Nitrates can produce carcinogenic compounds if sprayed on leaves. Instead, only use ammonium-based nitrogen fertilisers as foliar sprays. Ammonium-nitrogen is easily absorbed through leaf tissue, and it is much safer. If you prefer a more organic approach, amino acids are a good alternative. Studies show that some amino acids are absorbed by leaves just as quickly as ammonium-nitrogen and may provide additional benefits to the plant.