Métodos de prueba

This chapter presents information on (1) the functions of nutrients in plants, (2) the effects of nutrients on plant growth and quality, (3) recognition of symptoms of deficiencies of nutrients, and (4) how to supply nutrients to plants. For most of the nutrients, specific metabolic functions in plants have been identified. Participation in these metabolic roles is a factor that makes an element essential. Because of the met- abolic disorders associated with shortages of a nutrient, limitations in supply of any nutrient may restrict plant growth, development, and yields and cause appearance of symptoms of deficiency. Often deficiencies of nutrients are expressed in lower qual- ity of produce. Increasing the supply of the nutrient will enhance growth and yields within limits and also will have effects on crop quality, for example, developing green color in a leafy vegetable crop. However, supply of nutrients in excess of the needs of a crop may have an adverse effect on crop quality, often lowering quality or suppressing harvest yields.

Severe shortages of nutrients usually lead to development of symptoms of defi- ciency. Recognition of these symptoms is a useful way of identifying nutritional disorders in a crop. If the deficiency is detected in time, fertilization may restore crop productivity. If the deficiency is recognized too late for correction in the current crop, the grower is alerted that remedies need to be taken for the next season.

Fertilizers are materials that carry plant nutrients to the soil. This chapter will present and evaluate organic and chemical fertilizers for each of the plant nutrients and will discuss practices that increase the nutrient-supplying capacity of soil.

nitrogen FunCtiOns

The discovery of the essentiality of nitrogen is attributed to Theodore de Saussure, who in 1804 published his research that showed that normal growth of plants was not possible without the absorption of nitrates and other minerals from the soil. Nitrogen has many functions in plants. It is a component of proteins, genetic material (the nucleic acids DNA and RNA), chlorophyll, and many other compounds that are vital in plant metabolism. Proteins are nitrogen-rich compounds and are major nitrog- enous constituents of plants. By weight, about one part in six of the average protein is nitrogen. About 85% or more of the nitrogen in plants is in protein. Another 10% of the total nitrogen in plants is in soluble nitrogenous compounds, such as uncombined

amino acids and unassimilated nitrate and ammonium. The remaining 5% or less of the total nitrogen is in the genetic material, chlorophyll, coenzymes of metabolism (vitamins and the like), and lipids, among other compounds.

eFFeCtsOF nitrOgenOn plant grOwthand Quality

Nitrogen is a potent nutrient, the deficiency of which can severely limit crop production. Application of nitrogen fertilizers must be monitored closely to avoid underfertilization or overfertilization. Nitrogen is deficient in about 70% of crop land. Recovery of growth and yield potential from nitrogen deficiency during resupply of the nutrient can be rapid; however, over-application of nitrogen may have adverse effects on growth and quality. Some of the effects of limited, optimum, and excessive nitrogen fertilization follow.

vegetative growth

Applications of nitrogen fertilizers to plants promote vegetative growth (leaves, stems, roots) and reproductive growth (flowers, fruits, seeds), but stimulation of veg- etative growth generally exceeds that of reproductive growth Also, shoot growth is enhanced more than root growth. Suppressions of root growth and reproductive growth by excessive fertilization occur at lower applications of nitrogen than with stem and leaf growth. These responses are due to the effects that nitrogen has on the hormonal balance of plants. Nitrogen promotes the biosynthesis of growth-regulat- ing compounds that stimulate shoot growth and that inhibit root and reproductive growth.

With crops that are grown for their vegetation, such as spinach, lettuce, and cel- ery, the promotion of vegetative growth by nitrogen is a favorable response. However, with root crops, for example, beets and carrots, stimulation of shoot growth may be so strong that yields of roots are diminished by application of nitrogen. With fruit- types of vegetables, tomatoes, for example, stimulation of growth of stems and leaves by overly generous applications of nitrogen fertilizer may dominate and persist for so long that yields of fruits are reduced relative to those obtained with optimum fer- tilization. No advantage is gained by pruning of the vegetative growth to try to bring

Increasing Nitrogen Application

R V

Relative Plant Grow

th

Reproductive (Flowers, Fruits, Seeds)

Vegetative (Leaves, Stems, Roots)

Figure 3.1 Relative growth of plants in response to nitrogen fertilization. Upright lines

indicate relative levels of fertilization associated with depression of reproductive growth (R) and vegetative growth (V).

production of vegetation in balance with flowering and fruiting. Vegetative growth is still likely to dominate after pruning so that the first new growth after pruning is vegetative. Pruning in this case weakens the plant and further retards reproductive growth.

effects on succulence

Nitrogen fertilization increases the proportion of water in plants and increases the succulence or juiciness of vegetative parts. Succulence is a desirable feature in many vegetables—lettuce, spinach, radish, celery, and others for which the vegetative por- tion is edible portion. Possibly, the succulence of fruits can be raised by nitrogen fertilization, but the effects will be smaller than those on the vegetation.

effects on cell walls

Cell walls of plants that are well nourished with nitrogen are thinner than those receiv- ing lesser amounts of nitrogen. Nitrogen in plants promotes protein synthesis at the expense of carbohydrate synthesis and accumulation. Cell walls are made of carbohy- drates or of materials derived from carbohydrates. The diversion of carbohydrate to protein lessens the amount of material available for synthesis of constituents (cellulose, lignin, for example) of cell walls; consequently, cell walls do not thicken as much with an abundant supply of nitrogen as they would under situations of lesser abundance of nitrogen (Figure 3.2). In addition, protein synthesis increases the protoplasmic portion of cells and increases the amounts of water in cells. The pressure caused by the force (called turgor pressure) of the increased water on the cell walls in interaction with other growth factors causes the walls to stretch and to become thinner.

The thin cell walls and high water content give characteristics of succulence and crispness to vegetables. These vegetables are not fibrous or tough. All vegetative portions of plants may be affected in this way. The stretched-out, thin-walled cells of stems are weak. Consequently, the stems are weak and may not be able to support plants in an upright position.

High Nitrogen Nutrition

Low Nitrogen Nutrition

Figure 3.2 Plant cells growing with a high level or with a low level of nitrogen nutrition.

Cell walls of plants receiving high amounts of nitrogen are thinner than those receiving low amounts of nitrogen fertilization.

The displacement of a plant from its upright position is called lodging (Figure 3.3). Lodging of plants can cause losses of produce by the material falling on the ground and becoming dirty, rotting, or being below the level of harvesting equipment. Lodging is a particular problem with small grains—the cereals, wheat, oats, bar- ley, and rye. Fairly recent developments of dwarf wheat and rice varieties that are stiff-stemmed and that respond without lodging to fertilization with nitrogen have allowed for raising of productivity of cereal grains. The development of these grains that were responsive to high amounts of nitrogen fertilization was a part of the Green Revolution, introducing an intensive form of agriculture that raised world-wide pro- ductivity of crops.

Plant organs that contain these thin-walled cells in addition to being structur- ally weak are bruised easily. Considerable damage by breakage of leaves and stems may occur during harvest. The high water and protein contents and thin-walled nature of cells makes plant organs susceptible to attack by insects and diseases. These cells offer little resistance to attack and offer excellent nutrition for insects and diseases.

(a) Shoot Growth

(b) Cellular Structure in Stems 1. Deficiency 2. Optimum

Nutrition 3. Lodging

Figure 3.3 Plant shoot growth and cellular structure in stems of (1) unfertilized plants,

(2) optimally fertilized plants, and (3) overfertilized plants (with lodging). Note that cell walls progressively become thinner as nitrogen fertilization increases.

effects on Plant Maturation

The maturation of a crop is slowed by nitrogen fertilization. Nitrogen fertilization must be appropriate for the kind of crop that is being grown, that is, whether the crop is grown for its vegetation or fruits or seeds and whether the crop has a short or long season for maturation. Through its effect on growth regulators (hormones), nitro- gen fertilization enhances and prolongs the vegetative stage of plant development. Flowering and fruiting may be delayed by nitrogen fertilization.

Ample nitrogen fertilization is needed for development of a good vegetative frame. Plants that are undernourished will be weak and spindly and will be unable to support their flowers and fruit. Undernourished plants will have a short period of productivity, and yields will be curtailed by senescence or aging of the plants prematurely. Nitrogen fertilization will permit development of strong plants that can support a high level of flowering and fruiting and maintain a long period of produc- tivity. With varieties of tomato and cucumber that flower and fruit over a period of time, nitrogen fertilization will prolong this period of productivity so that yields are enhanced above those of underfertilized crops. However, overabundance of nitro- gen may delay reproductive growth so that it occurs so late in the season that little productivity is obtained before cold weather or frost. In extreme cases, reproductive growth is delayed so long that none occurs before frost or that which occurs is insuf- ficiently mature to give a product that can be harvested.

symptOmsOF nitrOgen deFiCienCyin CrOps

Nitrogen is essential for syntheses of proteins and chlorophyll. With shortages of nitrogen, syntheses of these compounds will be restricted. These restrictions in plant metabolism will appear in the development of symptoms of nitrogen deficiency (Figure 3.4). Nitrogen-deficient plants are spindly, meaning slow-growing, weak- growing, or stunted, and off color. If nitrogen was limited from the initiation of growth soon past the seedling stage, the entire plant will be light-green or yellow- green. If the supply of nitrogen is exhausted after considerable growth is made, the lower leaves will have a light-green or yellow-green coloration. Eventually, the lower leaves will turn brown and drop off, unless the supply of nitrogen is restored.

Nitrogen is a mobile element in plants, meaning that it can be transported readily from one part of a plant to another part. Nitrogen can be mobilized or transported from the old leaves of the plant to the young growing regions or reproductive organs of plants. The resulting transport of nitrogen from the old leaves to the growing regions and reproductive organs results in depletion of nitrogen and in the appear- ance of nitrogen deficiency symptoms in the old leaves. The yellowing and stunting of plants are identifying characteristics of nitrogen deficiency. Often, during nitro- gen deficiency, lower leaves may become reddened, particularly along the veins. Sometimes, this symptom is confused with that of phosphorus deficiency, which also produces reddening of lower leaves, and experience is needed to learn how to differentiate nitrogen deficiency from phosphorus deficiency. The nitrogen-defi- cient foliage will have an overall lighter color than the phosphorus-deficient foliage. Reddening of phosphorus-deficient foliage will be spread across the leaf blades and

will be strongly evident on the underside of the leaf, whereas the reddening during nitrogen deficiency will be concentrated along the veins and will be equally notice- able on the top or bottom of the leaves.

Nitrogen deficiency during vegetative growth will force plants to mature earlier with significant losses of quality and yields. Number and size of flowers and fruits will be smaller in a nitrogen-deficient crop than in a nitrogen-sufficient crop. Root growth will be restricted greatly by nitrogen deficiency, for roots will not grow in soil zones that are nitrogen deficient. Although nitrogen has differential effects on growth of different plant parts, it is needed for growth of all plant parts.

Readily available nitrogen applied to the soil can be absorbed rapidly by a plant. Deficiencies that appear during the early stages of vegetative growth may be corrected with only a small loss of yield potential. Deficiencies that appear in late stages of veg- etative growth can be corrected, but yields will be limited substantially by the defi- ciencies. Deficiencies that appear near or at the time of flowering or fruiting are not likely to be corrected during the current season and can limit yields substantially.

Often as plants mature, nitrogen is transported from the vegetative organs to the reproductive organs. This process is a natural one in which nitrogen accumulated during vegetative growth is used later in reproductive growth. Therefore, appearance

Overall growth of plant is stunted and spindly Upper leaves may remain green Lower leaves turn yellow or brown with N deficiency With extreme N deficiency, lower leaves abscise and fall to ground N-Nitrogen Exhaustion

Figure 3.4 Expression of nitrogen deficiency symptoms on shoots of plants growing in

of symptoms of nitrogen deficiency on foliage during crop maturation is not always an indication that nitrogen supply in the soil was insufficient.

amOuntsOF nitrOgen reQuiredBy CrOps

Crops differ in the amounts of nitrogen that they remove from the soil in plant growth and development. The amount of nitrogen removed is a function of the amount of dry matter that a crop produces and of the nitrogen concentration in the dry matter. Well- nourished plants have from 1% to 4% nitrogen or more in their foliage, with variation occurring among species and with plant age. Nitrogen-deficient plants may have less than 1% nitrogen in their foliage. High-yielding crops—those producing large amounts of vegetative growth, fruits, or seeds—have heavy requirements for nitro- gen. A fast rate of growth in conjunction with a large amount of growth demands a rapidly and abundantly available supply of soil nitrogen. Crops that are grown at high densities remove large amounts of nitrogen from the soil because of the high productivity of crops grown in dense plantings. High-yielding crops that have high protein contents have high demands for soil nitrogen. Some examples of crops with high demands for nitrogen from the soil are as follows.

High demand for nitrogen

More than 120 lb N per acre (3 lb N per 1000 sq ft) removed in one season. Corn

Potatoes Tomatoes Celery

Forages (hay, pasture)

Crops with moderate demands for nitrogen produce less dry matter than those with the high demands for nitrogen. These plants may be inherently low yielding or may be grown at lower densities of planting than those with high demands for nitrogen. Most vegetable crops fall into this category. Some crops that have moderate demands for soil nitrogen are listed as follows.

Moderate demand for nitrogen

Between 50 and 120 lb N per acre (1.25 to 3 lb N per 1000 sq ft) removed in one season. Some crops with moderate nitrogen requirements include

Most vegetable crops

Short-stemmed cereals (wheat, oats, barley, rye) Garden legumes (peas, beans)

Turfgrasses

Crops with low demands for nitrogen from the soil include those that are slow grow- ing and those that have a very short growing season. Many root crops have low requirements for nitrogen. Overfertilization of root crops with nitrogen may promote

shoot growth and inhibit root growth with the result that root yields are consider- ably lower than those produced with optimum fertilization. Crops that are grown for their sugar contents remove low amounts of nitrogen in proportion to their yields. Generous nitrogen fertilization of these crops will enhance dry matter production but will lower the sugar yields because of the diversion of carbohydrate to protein. Some crops that remove small amounts of nitrogen from the soil follow.

low demand for nitrogen

Less than 50 lb N per acre (1.25 lb N per 1000 sq ft) removed in one season. Tobacco

Root crops (carrots, turnips, sweet potatoes) Radish

Tall-stemmed cereals Orchards

The amounts of fertilizer applied to crop land also vary with soil types, weather, production goals, management practices of growers, varieties of crops, and other fac- tors. Applications of nitrogen from fertilizer may be 50% to 100% greater than the amount that is removed by a crop to compensate for losses and failures of plants to recover nitrogen that is applied. Some of the applied nitrogen may be lost by leaching into the ground, volatilization into the air, denitrification (biological conversion of nitrate to gaseous forms of nitrogen), competition from weeds and microorganisms; some of the nitrogen may be placed out of reach of plant roots, as in between rows of widely spaced plantings. Plants typically recover 50% to 70% of the nitrogen applied in fertilizers, although the recovery may be much less than 50% if applications are high, weeds are competitive with the crops, and conditions for loss are favorable. Losses of nitrogen can be limited by conservation practices, such as supplying nitro- gen in several applications, called split applications, during the season and place- ment of fertilizer in bands along rows of crops rather than broadcasting it between rows (Figure 3.5).

Split applications of nitrogen help to supply nitrogen to crops as the need for nitrogen arises. One-time applications at the beginning of the season are subjected to losses by leaching and other environmental factors. Costs of multiple applications are deterrents to use of this practice relative to applying all of the fertilizer at plant- ing. Placing the fertilizers in bands locates the nutrients in close proximity to the roots and limits the losses to leaching and to weeds. Banded nitrogen may be applied all at planting or during the growing season by sidedressing.

Growers using organic fertilizers that release nitrogen slowly should apply nitro- gen in amounts that are related inversely to the rate of release of nitrogen; that is, fer- tilizers that release nitrogen slowly usually are applied in large quantities. Nitrogen from these slow-release fertilizers may carry over to the next growing season, and a downward adjustment, based on experience or expert advice, can be made in the second and subsequent years of fertilization. The buildup of available nitrogen from slow-release organic fertilizers is slow, and therefore many people tend to overvalue the benefits of the residual effects of fertilizers. An indication of the residual value

of nitrogen from a single application of farm manure from large animals is shown in Table 3.1.

Utilizing the data in Table 3.1, if a single application of farm manure delivering 200 lb of N per acre (20 tons of manure per acre) were made, about 100 lb N would be available during the current crop year, with about 16 lb being available in the first following year, 10 lb in the second following year, 4 lb in the third following year,