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Article 5-1 Plant Nutrition

By Dr. Tahir Mahmood

The success of any garden/crop begins with the soil. A fertile, biologically active soil provides plants with enough nutrients for good growth. Fertilizers supplement can renew these nutrients, but they should be added only when a soil test indicates the levels of available nutrients in the soil are inadequate. In the garden, whether you are growing annuals or perennials, vegetables or flowers, most of the crops have a few short months to grow and develop flowers and fruits. The soil must provide a steady, uninterrupted supply of readily available nutrients for maximum plant growth. Best growers recognize the need for timely nutrient application to promote vigorous plant growth in landscapes and gardens.

Plants contain practically all the (92) natural elements but need only 16 for good growth. Thirteen of these essential mineral nutrients, commonly abbreviated, though with less precision to nutrients and three are non minerals.

Essential mineral nutrients (13) required for growth (of equal importance physiologically) Macronutrients (six) of which the critical contents in plants are two-30 g/kg of dry matter Major nutrients (three), applied in fertilizers for almost all crops on most soils:

  • N = nitrogen (taken up as NO3- or NH4+)
  • P = phosphorus (taken up as H2PO4- etc.)
  • K = potassium (taken up as K+)

Secondary nutrients (three), applied in fertilizers mainly for certain crops on some soils

  • S = Sulphur (taken up as SO42-)
  • Ca = Calcium (taken up as Ca2+)
  • Mg = Magnesium (taken up as Mg2+)

Micronutrients (seven) of which the critical contents in plants are 0.3-50 mg/kg of dry matter Heavy metals (five)

  • Fe = Iron
  • Mn = Manganese
  • Zn = Zinc
  • Cu = Copper
  • Fe, Mn, Zn, Cu taken up as divalent cation or chelate
  • Mo = Molybdenum
    Taken up as Molybdate MoO42- Non-metals (two)
  • Cl = Chlorine, taken up as Cl-
  • B = Boron, taken up as H2BO3-, etc...
    Some beneficial nutrients useful for some plants
  • Na = Sodium
    (taken up as Na+; can partly replace K for some crops)
  • Si = Silicon
    (taken up as silicate, etc., e.g. for strengthening stems to resist lodging)
  • Co = Cobalt
    (mainly for N-fixation of legumes)
  • Cl = Chlorine
    (useful for some crops in greater than essential amounts, for osmotic regulation and improved resistance to some fungi)
  • Al = Aluminum
    (perhaps beneficial for some plants, e.g. tea?)

Non-Mineral Nutrients
These are Hydrogen (H), Oxygen (O), and Carbon (C). These nutrients are found in air and water and in a process called photosynthesis; plants use energy from the sun to change carbon dioxide (CO2-Carbon and Oxygen) and water into starches and sugars. These starches and sugars are the plant food.

These nutrients are essential for plant growth. Plants will grow normally until they run short of one nutrient. Then growth is limited by the availability of that nutrient. Occasionally two or more nutrients will run short at the same time. If the nutrients are deficient, or too abundant, then plants will be discolored or deformed. The deficiency symptoms will indicate which nutrient or nutrients are needed. However, it is much better to supply additional nutrients before deficiency symptoms appear. A soil test will tell which nutrients are low before growth is affected.

Amendments that are commonly added to soil are:

  • Sand or profile soil conditioner: to improve aeration and drainage.
  • Compost: to add organic matter, nutrients and to improve aeration and drainage.
  • Lime or sulfur: to raise or lower pH.
  • Fertilizers: to add specific nutrients.

All these are important for plants, but the most important of these are fertilizers, because they are the life line of plant, not only provide plant nutrients and if organic fertilizers will be used for soil application it also improved the soil structure (porosity/aeration, water retention etc).

What is fertilizer?
Fertilizer is any material that supplies one or more of the essential nutrients to plants. Fertilizers can be classified into one of two categories: organic or inorganic.

Organic fertilizers: The word organic applied to fertilizers simply means that the nutrient contained in the product are derived solely from the remains (or a by product) of once living organism. These materials include animal wastes, crop residues, compost and numerous other byproducts of living organisms. Old leaves, peanut hulls, bone meal, blood meal, kelp meal, fish meal, cottonseed meal and animal manures are just a few examples of organic fertilizer sources.

There are many advantages of using organic fertilizers. One advantage of organic materials is that they provide beneficial organic matter that can improve the soils water and nutrient holding capacity. This organic matter also creates an environment that encourages beneficial soil organisms such as earthworms. Another advantage is that organic materials take longer to breakdown and release nutrients. This creates a slow-release situation that provides nutrients over a longer period of time and can also help in reducing the loss of nutrients to leaching. A third advantage to organic materials is that you can sometimes find free or inexpensive sources, if you take the time to search for them. There are number of organic fertilizers available in the market like Growth Max (sea weed extract), fish fertilizers etc. There are some other organic foliar sprays available that are directly extracted from plant (seed, fruit or leaves).

Most plants in soils will grow better if additional nutrients are provided by fertilizing. Often, the soil doesn't hold enough of these nutrients in the quantities needed for desirable growth and production. The nutrients that are in the soil are often used up and need to be replaced. A soil test will give a complete and accurate measure of the nutrients in the soil. A general recommendation is that all soils need more nitrogen. Shallow rooted plants, such as grass and flowers, need more phosphorus and potassium. Acid loving plants, such as rhododendrons, azaleas, camellias, junipers and pin oaks, often need more iron. Sometimes sandy soils need micronutrients, but rarely clay soils. Certain micronutrients may be deficient in certain parts of the country. Many fertilizers are available to supply additional nutrients. The nutrients are identical whether they come from organic or synthetic sources, but the source will affect how fast the nutrients are available to plants. Ammonia sulfate and water soluble fertilizers release most of their nitrogen in a few days and may burn plants if too much is applied. Blood meal releases its nitrogen over a period of months. Organic fertilizers and specially treated synthetic fertilizers release slowly so they last longer and won't burn. Deeper rooting trees and shrubs can be fertilized once a year, but shallow rooted plants, such as grass and flowers, will need regular fertilizing throughout the growing season.

Fertilizer Recommendations
It is difficult to recommend a specific fertilizer type or amount of fertilizer for any given situation. All fertilizer recommendations should take into consideration soil pH, residual nutrients, and inherent soil fertility. Fertilizer recommendations based on soil analyses are the very best chance for getting the right amount of fertilizer without over- or under-fertilizing.

Fertilizer recommendations based on soil tests result in the most efficient use of lime and fertilizer materials. This efficiency can occur only when valid soil sampling procedures are used to collect the samples submitted for analyses. To be beneficial, a soil sample must reliably represent the field, lawn, garden or "management unit" from which it is taken.

Fertilizer Rate Calculation

The nutrient recommendations based on soil test results or general recommendations for landscapes and gardens are expressed in pounds per 1000 square feet. How much fertilizer (Organic/Inorganic) needed will depend on the nutrient recommendation, the concentration of the nutrient in the fertilizer selected, and the size of the area fertilized. By using a very simple formula one can calculate the amount of fertilizer required to meet the nutrient recommendation: Fertilizer Needed =

Where A is the recommended rate of any nutrient from the soil test report, B is the percent of any nutrient in any fertilizer and C is the area to be fertilized. Normally home garden and landscapes areas are usually different than 1000 sq. ft. One can estimate the size of the area by using a measuring tape to calculate the area by multiplying length by width.

For example if a home gardener need to apply 4 lbs of nitrogen per 1000 sq. ft of area and he is using a product of 15-15-15 (NPK) and the total area is 2300 sq. ft. How much of 15-15-15 he need to add 4 lbs of Nitrogen on 2300 sq. ft.? Here A is 4, B is 0.15 (15%) and C is 2300. Putting these values in the above equation the answer will be 61 lbs of 15-15-15 for the entire area.

Fertilizer Application Method
Fertilizers may be applied on or into the soil or directly to the plants. The aim is to apply them cheaply, uniformly and effectively. The method will depend on the type of material. Mineral fertilizers applied as nutrient sources:

  • Solid water-soluble fertilizers are evenly distributed on to the soil surface (penetration into the root zone then takes place during leaching after dissolution by water) or are placed directly into the root zone, e.g. beneath the seed at sowing time.
  • Solid water-insoluble fertilizers are distributed on to the soil surface and mechanically mixed into the arable layer (where this is impracticable, e.g. on grassland, there is a natural but slow penetration by soil organisms).
  • Liquid fertilizers are:
  • Either sprayed on to the soil surface in their original concentration and left to penetrate (only suitable if no gaseous losses occur);
  • or mixed into the soil immediately after application (to prevent gaseous losses);
  • or sprayed in diluted form directly on to the plants.

Although possessing the advantage of quick action, the amount of fertilizer that can be distributed in a foliar dressing is limited by the sensitivity of the leaves to osmotic agents such as dissolved salts or (somewhat less) to organic chemicals such as urea. With the exception of some N fertilizers (due to the rather high tolerance of urea), foliar application can supply only very limited amounts of the primary nutrients compared with requirements. The situation is somewhat better for the secondary nutrients, but the best results are obtained with micronutrients because a relatively large proportion of the total requirement can be supplied in a single spraying. In cases of marked deficiency, repeated spraying is essential. Spraying is most effective, and the risk of scorch is minimized if the spray droplets do not dry too rapidly, i.e. on cloudy days and in the early morning or late afternoon.

Bulky organic manures and amendments:
They should be evenly distributed on the soil surface to the extent possible and mixed into the arable layer, but some amendments need to be applied directly into the subsoil. Organic manures with a low nutrient content may be used as mulch (surface protection layer).

Again at the end I would say the best way to get an idea about how much nutrients are needed for a good growth and which must be added to the soil is to get your soil tested. This will not only serve the purpose, save the money but also be helpful for avoiding ground water pollution which is of great concern in modern world.

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