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Symptoms develop first in older leaves and move gradually up to the younger ones. In mild cases, the older mature leaves turn pale green. If not amended, over time a widespread chlorosis develop on those leaves together with a light red discoloration of veins and petioles. Petiole analysis actually allows growers to detect the outset of a nitrogen deficiency in the crop. As the deficiency progresses, these leaves eventually turn yellowish-white (veins included) and may curl or grow deformed. Young leaves remain pale green but grow much smaller than usual. Plant have a spindly aspect due to the reduced branching but their height is usually normal. Plants become more susceptible to water stress and the wilting of leaves is common. Premature death and shedding may happen, which results in considerably lowered yields. Recovery after application of nitrogen in the form of fertilizer becomes evident after a few days.
High rates of nitrogen are important during the vegetative growth of the plant. In periods of favorable weather, it is important to provide the fast growing crops with a good nitrogen supply, so that they can reach their maximum vegetative and fruit/grain production potential. Nitrogen deficiencies can be observed in sandy, well-drained soils with little organic matter as they are prone to the leaching of nutrients. Frequent rainfalls, flooding or heavy irrigation wash down nitrogen into the soil and can also lead to deficiencies. Periods of drought stress hinder the absorption of water and nutrients, resulting in unbalanced nutrient supply. Finally, the soil pH also plays a role in the availability of nitrogen to the plant. Both low or high soil pH negatively affect the absorption of nitrogen by the plant.
High levels of organic matter in soils can enhance soil structure and improve the capacity of the soil to retain water and nutrients. Organic matter can be added to soils as manure, compost, peat, or simply with the addition of nettle slag, guano, horn meal or nitrolime. Nettle slag can be sprayed directly on the leaves.
Amendments can be done using products based on urea, ammonium or nitrate. Nitrogen is often provide as part of base fertilization products containing different rates of this nutrient together with phosphorus and potassium (the so-called N-P-K base trio). Several products are available on the market and split applications are recommended. The grade depends on the soil type and the growth stage of the plant.