What are the symptoms of nutrient deficiency and toxicity in walnut trees?
The key to scientific fertilization lies in accurately judging the surplus and deficiency of nutrient elements in the soil and in the tree's nutritional status. The basis for fertilization includes two aspects: morphological diagnosis and nutritional diagnosis.
Morphological diagnosis is based on the external morphological characteristics of the fruit tree to preliminarily judge the abundance or deficiency of nutrient elements and to guide fertilization. Conducting morphological diagnosis requires rich experience. Generally, if the leaves are large and numerous, thick and dark green, the branches are sturdy, the buds are plump, the fruiting is uniform, the quality is excellent, and the yield is high and stable, it indicates that the tree's nutrition is normal; otherwise, the cause should be identified and measures taken to improve it. The common symptoms of nutrient deficiency and toxicity in walnut trees are described below for reference.
When nitrogen is deficient, at the beginning of the growing season the leaf color is relatively pale and gradually turns yellow, the leaves are sparse and small, and premature leaf drop often occurs; the new shoot growth is reduced, and the tree vigor declines, with severe flower and fruit drop; in severe cases, the small branches at the top of the plant die, and the yield decreases noticeably. However, similar phenomena may also occur under drought and other adverse conditions. Excessive application of nitrogen fertilizer often causes excessive vegetative growth of branches and leaves, tissues that are not well developed, delayed fruit ripening, reduced storage tolerance, and aggravated physiological disorders.
When phosphorus is deficient, the tree is generally very weak, the leaves are sparse, the leaflets are slightly smaller than normal, the leaves show irregular yellowing and necrosis, and premature leaf drop occurs. When phosphorus is excessive, it affects the uptake of nitrogen, potassium, and calcium, and can also reduce the activity of iron in the plant and soil, causing the leaves to turn yellow and triggering little leaf disease caused by zinc deficiency.
Potassium deficiency symptoms mostly appear on the leaves in the middle part of the branches. At first the leaves turn grayish white (similar to nitrogen deficiency), then the leaf margins of the leaflets curl inward in a wavy manner, the underside of the leaves appears light gray (or bronze-colored), the growth of leaves and new shoots is reduced, and the nuts become smaller.
When calcium is deficient, the root system is short and thick, curved, and the tips soon turn brown and die. Above ground, it first appears on young leaves: the leaves are small, twisted, with deformed leaf margins, and spots or necrosis often appear; in severe cases the branches die, and physiological disorders of the fruit worsen. When calcium is excessive, iron, manganese, zinc, boron, and others are easily converted into unavailable forms, causing deficiency symptoms.
When iron is deficient, the young leaves lose their green color, the mesophyll appears yellow-green, and the veins remain green. When iron deficiency is severe, the leaves are small and thin, appearing pale yellow or creamy white, and may even develop a scorched appearance or fall off. Because iron is not easily mobile within the tree, the first sign of iron deficiency appears on the young leaves at the top of the new shoots.
When zinc is deficient, the bud break of the buds at the top of the branches is delayed, the leaves are small and yellow, forming a clustered appearance, known as "little leaf disease"; the new shoots are thin and the internodes are short; in severe cases, the leaves gradually fall from the base of the new shoot upward, the branches die, and the fruit becomes smaller.
When boron is deficient, the tree grows slowly, the branches are slender, the internodes become shorter, and the leaflets are irregular in shape, sometimes sepal-like; in severe cases the shoot tips die. Excessive boron can cause toxicity, causing tissue necrosis. The symptoms first appear at the leaf tip and gradually spread to the leaf margin; in severe cases the necrotic part expands to between the veins within the leaf, and the margins of the leaflets curl upward, appearing scorched.
Magnesium is a major constituent element of chlorophyll. When magnesium is deficient, chlorophyll cannot be formed, showing chlorosis. Yellowing first appears at the leaf tip and along both side margins, and gradually extends toward the leaf base, leaving a V-shaped green area; the yellowed part gradually dies and turns dark brown.
When manganese is deficient, there is a unique chlorotic symptom: the chlorosis develops from the main vein toward the leaf margin between the veins, the chlorotic parts appear rib-like, and the top leaves of the shoots remain green. In severe cases, the leaves become smaller and the yield decreases.
When copper is deficient, the leaves at the tips of the new shoots first lose their green color and turn yellow, then develop a scorched appearance; the branches shrink slightly, and there are small dark brown spots at the tips of the new shoots. The fruit turns slightly white, and the kernel shrinks severely.
Nutritional diagnosis is a method widely used abroad to determine and adjust fertilization for fruit trees. Nutritional diagnosis can promptly and accurately reflect the nutritional status of the tree, analyze the deficiency or excess of various nutrient elements, distinguish similar symptoms caused by different elements, and detect them early before symptoms appear. Therefore, scientific fertilization based on nutritional diagnosis can ensure the normal growth and development of fruit trees. Nutritional diagnosis includes leaf analysis and soil analysis. It determines the content of mineral elements in the leaves according to uniformly specified standard methods, and by comparing with the standard values of leaf analysis, determines the abundance or deficiency of that element. Then, based on the local soil nutrient status (that is, the results obtained through soil analysis), fertilizer efficiency indicators, and the interactions among mineral elements, a fertilization plan and fertilizer ratio are scientifically formulated to guide fertilization.