POSTPLANTING FERTILAZATION
By Roger C. Styer, Ph. D., and David S. Koranski, Ph. D.

INDIVIDUAL NUTRIENTS
Nitrogen (N) is used by the plant for amino acids, proteins, enzymes, and chlorophyll. There are three forms of N in fertilizers: Nitrate nitrogen (NO₃), ammoniacal nitrogen (NH₄⁺); and urea NH₂-CO-NH₂). Urea is considered to be the same as NH₄, since urea needs to be converted to NH₄ to be assimilated by plants. The NO₃form is readily taken up by the roots, is very mobile within the plant, and can be stored within the plant at high levels with not toxicity... Some NH₄ can be taken up directly by roots, but most of it needs to be changed to NO₃ by bacteria in the soil before root uptake can occur. This bacterial conversion of NH₄ to NO₃ is dependent on soil temperature and soil pH. Cool temperatures (less than 60F, 15C) and low soil pH (less than 5.5) significantly reduce the conversion, and NH₄ toxicity can occur. Most of the NH₄ taken up directly by roots has to be incoprorated into organic compounds in the roots. Plants can store NH₄ only at low levels without suffering from toxicity.

For a given species, the proportion of NO₃ reduced in the roots increases with temperature and plant age. The uptake rate of the accompanying cation also affects this proportion. With K as the accompanying cation, translocation of both K and NO₃ to the shoots is rapid; correspondingly, NO₃ reduction in the roots is relatively low. In contrast, when Ca is the accompanying cation, NO₃ reduction in the roots is considerably higher. This means that when Ca accompanies NO₃, the roots get more of its use to themselves, resulting in greater root dry weight. When K accompanies NO₃, the shoots get more to use than the roots, resulting in greater shoot dry weight.

When total N content increases, storage carbohydrates decrease, but the content of lignin (a component of cell walls) increases. High N levels compete with K (2N:1K), which is important in sucrose loading in the phloem of plants. High NH₄ levels (greater than 10ppm) will inhibit uptake of Ca and copper (Cu) (Table 11.1). The plant will correspondingly have faster shoot growth than root growth, thinner leaves and stems, and dark green color when NH₄ inhibits Ca uptake.

Nitrogen deficiency is not difficult to detect. The green chlorophyll pigmentation fades to a lighter green and eventually to a yellow color (chlorosis). The telltale sign is the development of chlorosis in the older leaves because N is a mobile element. Some bedding plants that normally develop red or bronze pigmentation, such as amaranthus, celosia, colues, and begonia, develop a light red color, even pink, rather than their normal deep red color or bronze color when N is deficient. Marigolds may develop an abnormal red color or red spots across the lower leaves, and the overall plant is stunted.

The symptoms of NH₄ toxicity for petunias consists of chlorosis along the margin of the older leaves as well as the rolling up of the margins of these leaves. Geraniums exhibit intervenial chlorosis of older leaves, with a red discoloration developing within the chlorotic area. For crops in general, the sequence of the NH₄ toxicity symptoms starts with wilted appearance of older leaves, even thought these leaves remain turgid (Not wilted). Affected leaves become thickened and leathery. Chlorosis develos in older leaves. In some species, it can be interveinal, but in many it is very irregular and does not fit a pattern. Necrosis soon follows chlorosis on older leaves. All symptoms progress up the plant from older to younger leaves. Root tips also are killed from high levels of NH₄. Often, reddish brown color goes along with the death of the root tips. Coleus, cosmos, geranium, pansy, petunia, salvia and zinnia are bedding plants that are particularly susceptible to NH₄ toxicity.

To correct problems with N deficiency, fertilize with a N fertilizer on a regular basis, depending on the crop and growth stage. Constant feed (with each watering) or and increase in concentration (ppm) may be needed to get plants going again. Keep media soluble salts between 0.5 and 1.2 mmhos (2:1 dilution). High light (more than 3000 fc, 32,280 lux) and high temperatures (more than 75^oF, 24^oC) will decrease available N levels quickly. Make sure the roots are growing and healthy looking for N to be taken up in to the plant. To correct problems with NH₄ toxicity, keep soil temperature greater than 65^oF (18^oC) when using fertilizers with NH₄ or urea in them. Keep soil pH levels above 5.5 for NH₄ conversion to NO₃ to occur. Reduce the amount of NH₄ in the fertilizer program by substituting the NO₃ form, reducing concentration and frequency, and leaching. Ammonium toxicity is a greater problem in winter than in summer, and more so in northern regions than in southern regions.

Potassuim (K) The most versatile and necessary plant nutrient is potassium. It is important in cell elongation, protein snythesis, enzyme activation, and photosynthesis. Potassium functions as a carrier across cell membranes for other nutrients and carbohydrates. as a carrier, it is responsibile for sucrose loading in the phloem of plants, whereby photosynthates are transported from leaves to flowers, seed, fruits, and roots. Potassium play a major role in contributing to osmotic potential of cells and tissues (turgor). It functions in the opening and closing of leaf stomates due to changes in turgor pressure (accompanying photosynthesis). Plants withstand drought stress with less decrease in photosynthesis at high K levels.

Uptake of K is highly selective and closely coupled to the plant's metabolic activity. Potassium is highly mobile in plants at all levels. Potassium is needed in plants at the same level as N and Ca. Potassium deficient plants are more susceptible to frost damage and fungal attacks.

The first sign of K deficiency is the developement of chlorosis along the margins of older leaves. For some species, such as tomato, it may start as large yellow (chlorotic) spots across olders leaves. This chlorosis quickly gives way to necrosis. This transition accurs so rapidly that chlorosis is often unnoticed. For most species, the necrosis occurs along the leaf margin, particularly at the leaf points. For species that form chlorotic spots across the leaf, necrosis will show up in these same spots. These symptoms progress from leaf to leaf up the plant.

One problem with some bedding plants that form red pigmentation, such as coleus, begonia, and celosia, is that both N and K deficiencies cause an abnormal, pale red discoloration or pink color on these lower leaves. Marigolds and salvia exhibit yellow-orange spots on the lower leaves along the edge. Marigolds can also develop very short leaf petioles, and the lower leaves may curl down. A very bushy-looking marigold plant results from K deficiency. These distortions also resemble a 2,4-D injury; however, the symptoms start at the base of the plant for K deficiency, rather than at the top.

To correct K deficiency, make sure K levels are approximately the same as N and Ca by providing a balanced fertilizer program. Potassium is very leachable and must continually be replaced with the feed program. High levels of N (2N:1K) and Na (sodium) will inhibit K uptake (table 11.1). Check your water quality of Na levels (Keep under 40ppm). If K becomes excessive, it will inhibit uptake of N, Ca, and Mg. Under contions of cool, cloudy weather, K will be taken up more than Ca, and an imbalance can result.

Phosphorus (P) An adequate phosphorus supply is important for root system development, rapid shoot growth, and flower quality. Phosphorus is easily taken up by the roots and can be stored in the plant. The biggest role that P plays is the energy compund, such as ATP, that the plant needs for synthesis and degradation.

Generally, in greenhouse crops, growers tend to overfertilize with P. Phosphorus is only needed at one fifth to one tenth the quantity of N and K. On the other hand , P can be very leachable in soilless mixes. Marconi and Nelson showed that P does not leach in soil-based mixes, but will leach almost 40 percent more in peat-based soilless mixes. Much of the P can be supplied preplant by means of superphosphate is available as a powder, while treble superphosphate is a coure prill....

Uptake of P by the root system is greatly reduced under cool soil temperatures (less than 55F, 13C) or high media pH (greater than 6.5). Raising the soil temperature 5F (3C) or lowering the media pH to less than 6.5 will enable the plants to grow out of this condition more easily than will the addition of more P. An excessive P level may reduce the ability of a plant to take up and utilize several of the micronutrients (table 11.1).

The symptoms of P deficiency begin with darker green foliage color than normal and stunting of the top growth, which eventually becomes very severe. These symptoms make P deficiency difficult to detect unless there are normal plants nearby to compare with. The next symptom in many species, such as tomato, is purple pigmentation of older leaves, particularly on the undersides. A few species do not form this color. Chlorosis on leaves occurs next, followed after some time by necrosis of older leaves. These symptoms progress up the plant. Bedding plants that have red pigmentation tend to form more of the purple burn on older leaves. Marigolds develop red pigmentaion on the margins of the older leaves....

This is a reprint from Roger C. Styer, Ph.D., and David S. Koranski, Ph.D. 1997. PLUG & TRANSPLANT PRODUCTION: A Grower's Guide. Batavia, Illinois, Ball Publish.
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