A Detailed Explanation of the Relationship Between Cotton Nutrition and Nitrogen Fertilizer

I. Nitrogen in the Cotton Plant: The average nitrogen content in plants accounts for about 1.5% of dry weight (range 0.3-5.0%). Cotton seeds contain 2.8-3.5% nitrogen, fiber contains 0.28-0.33%, and stems contain 1.2-1.8%, making cotton a crop with relatively high nitrogen content. According to experiments, with a lint yield of 62.7-94.7 kg per mu, nitrogen from emergence to squaring accounts for about 4.5% of the total accumulation over the life cycle, from squaring to flowering accounts for 27.8-30.4% of the total accumulation over the life cycle, from flowering to boll opening accounts for 59.8-62.4%, and from boll opening to harvest accounts for 2.7-7.8%. It can be seen that the period when cotton accumulates the most nitrogen is from flowering to boll opening. The forms in which nitrogen exists in plants fall into the following two major categories:

1. Non-protein nitrogen: Also called soluble nitrogen, referring to nitrogen that is soluble in water or certain solvents, including nitrate nitrogen, ammonium nitrogen, soluble amino acids and other soluble nitrogen-containing compounds.

2. Protein nitrogen: Refers to macromolecular nitrogen-containing compounds that are insoluble in common solvents, such as proteins. Non-protein nitrogen and protein nitrogen in plants are constantly changing and interconverting. In old leaves and old organs, decomposition predominates, and the decomposed nitrogen-containing substances are transferred to young organs for re-synthesis; in young organs, synthesis predominates. By the time seeds form, nitrogenous compounds in all organs can decompose and be transferred to the seeds for re-synthesis and utilization. Generally, during the vegetative growth stage of plants, the proportion of non-protein nitrogen is relatively high; while during the reproductive growth stage, especially after entering maturity, protein nitrogen is absolutely dominant, otherwise phenomena such as excessive vegetative growth, delayed maturity with excessive greenness, and spread of diseases will occur.

Soil likewise contains various forms of non-protein nitrogen and protein nitrogen. The former is called hydrolyzable nitrogen, which is easily absorbed and utilized by crops, but what crops frequently absorb in large amounts is nitrate nitrogen and ammonium nitrogen.

The distribution of nitrogen in plants is concentrated in the parts with the most active life activities; therefore, whether nitrogen supply is sufficient or not greatly affects the growth and development of plants. The bud and boll stage of cotton is a particularly important stage for nitrogen nutrition. Ensuring normal nitrogen nutrition at this stage can promote growth and development and increase yield. Nitrogen entering the crop may also be lost through secretion of soluble nitrogen and volatilization of ammonia, mainly occurring at the top of the crop, especially from flowering to maturity. According to estimates, the loss of nitrogen absorbed by crops can reach 5 kg per mu per year.

In production, situations of insufficient or excessive nitrogen nutrition are often encountered. Insufficient nitrogen generally manifests as short and slender cotton plants, reduced branches and squaring and flowering, fewer and smaller bolls, yellow-green leaves, gradual drying of basal leaves, significant premature senescence and early maturity; excessive nitrogen nutrition manifests as excessively luxuriant growth, large leaves, long petioles, light green leaf color, continuous emergence of axillary buds, high shedding of buds and bolls, late maturity, excessive non-protein nitrogen in the plant, susceptibility to pests and diseases, increased cotton boll rot, thick boll husks, and reduced fiber quality.

II. Physiological Functions of Nitrogen: Nitrogen is a constituent component of many important organic compounds in crops, such as proteins, nucleic acids, chlorophyll, enzymes, vitamins, hormones, etc., all of which contain nitrogen. Protein nitrogen can account for 80-85% of the total nitrogen in the plant. Proteins contain about 16% nitrogen on average. Nitrogen deficiency in plants reduces protein synthesis and impedes the formation of new cells, leading to slow or even stalled plant growth. Without nitrogen there is no life, so nitrogen is called the element of life.

Chlorophyll is the site where plants carry out photosynthesis. When nitrogen is deficient, the chlorophyll content in plants decreases, leaves turn yellow, photosynthesis weakens, and crop yield decreases significantly.

Enzymes themselves are proteins and are biological catalysts for metabolic processes in plants.

III. Crop Absorption and Utilization of Nitrogen: Crops have the ability to absorb and assimilate inorganic nitrogen compounds. The nitrogen absorbed from soil is mainly ammonium salts and nitrates. The ammonium nitrogen absorbed into the plant can directly combine with photosynthetic products to form amino acids, and then form other nitrogen-containing organic substances. Nitrate nitrogen, however, must be reduced to ammonium nitrogen in the plant before it can be absorbed and utilized. Plants can directly absorb urea and certain ammonium salts as nitrogen sources through leaves and roots, but the assimilation process of urea in crops has not yet been fully clarified. It is generally believed that urea is decomposed into ammonium nitrogen under the action of urease in crops before being utilized.

IV. Relationship Between Cotton Nutrient Absorption and Yield: According to experiments, the amount of nutrients absorbed by cotton per unit area increases with increasing yield, but not proportionally. Rather, the amount of nutrients absorbed per 50 kg of lint produced decreases as yield increases, and nutrient use efficiency increases as yield increases. The absorption and utilization of nitrogen also follows this pattern: