Module

Understanding Nutrient Imbalances

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Through diverse cropping and other agroecological management practices, soil is protected and replenished, which increases the chances that humans will have a nutritionally rich and diverse diet if they are given access to those foods. Alternatively, as you’ve learned, in the absence of these nutrients humans can become malnourished.

Just as humans can be undernourished, when macronutrients are not available in the soil in sufficient amounts plants cannot properly grow, develop or reproduce. In this module, we explore how both excess and deficiencies effect plant growth.

In natural ecosystems, plants are limited by nutrient availability within their ecosystem. In agroecosystems, management of macronutrients affects the ability of plants to grow, and affects the size of the crop produced. As with human malnutrition, malnourished plants have stunted growth, are more prone to damage by pests and diseases, and produce less. In general, healthy, well-nourished plants resist or tolerate diseases better than weak, malnourished plants. (Schumann, Spann, Mann, Obreza, Zekri, 2010).

By identifying nutrient disorders in plants, we can gauge the health of our soils. This chart shows some of the symptoms of nutrients plants may show, with the correlating nutrient they might be lacking. Below, we break down each soil macronutrient: what a deficiency looks like and what an excess looks like.


Nitrogen Deficiency

Leaves turn pale green to yellow. Oldest leaves are affected first, but in severe cases the whole plant can turn yellow. Growth is stunted.

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Nitrogen Excess

Nitrogen excess can occur with the overuse of synthetic fertilizers. The result is excessive vegetative growth that may appear healthy but is actually weak, spindly and sappy making the plant vulnerable to pest and disease. The plant will often have poor fruit growth and/or bitter fruits. In some cases, there may be leaf burn, or yellowing of the edges of the the leaf.


Phosphorus Deficiency

Growth is stunted in young plants. Leaves may appear reddish-purple, with the oldest leaves affected first.

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Phosphorus Excess

High rates of phosphorus fertilizer application may induce zinc, manganese and/or iron deficiency because the phosphorous, with its positive charge, competes with these micronutrients for plant uptake.  This can result in leaf chlorosis, wherein the leaf turns yellow but the vein stays green.


Potassium Deficiency

Leaves develop grey or tan areas near the margins that appear as ‘scorching’. Older leaves show these symptoms first. Occurs on sandy soils and soils low in potassium.

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Potassium Excess

Excess potassium, as a result of over-application of potassium fertilizer, can cause salt burn. Soils with high potassium levels can increase the likelihood of magnesium deficiency.


Calcium Deficiency

Growing tips of plants may die; younger leaves are affected. Root growth is slow, and root tips die. Tipburn of cabbage, cauliflower, lettuce; blackheart of celery; and blossom end rot of tomatoes are due to localized calcium deficiency within the plant. These disorders may occur on high calcium soils. Calcium deficiency may occur on acid and/or dry soils.

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Blossom end rot (caused by a calcium deficiency):

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Magnesium Deficiency

Oldest leaves turn yellow between the veins. In severe cases, younger leaves may be affected and older leaves may drop off. May occur on acid soils, sandy soils, or soils with high potassium levels.

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Wedge-shaped symptoms of magnesium deficiency, but the leaves may not always show the reddish-orange tones on the left.

 

 


Sulfur Deficiency

Symptoms of sulfur deficiency are similar to nitrogen deficiency except that youngest leaves are usually affected first. Can occur on sandy soils low in organic matter.

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Sulphur Excess

Excess—Rare, usually associated with saline condition


Chart 1: Deficiency Chart of Micronutrients

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Article 1: Effects of mineral nutrition on health & performance of citrus trees

by Schumann, Arnold; Spann, Tim; Mann, Tim; Obreza, Tom and Zekri, Mongi | Citrus Industry, July 2010.

 

FORUM QUESTION: NUTRIENT DEFICIENCIES

 

 

Reflection Questions