NPK AND ZINC IN PLANTS

NPK AND ZINC IN PLANTS

1.0 Zinc in plants

Zinc (Zn) is one of the eight essential micronutrients. It is needed by plants in small amounts, but yet crucial to plant development.

In plants, zinc is a key constituent of many enzymes and proteins. It plays an important role in a wide range of processes, such as growth hormone production and internode elongation.

Zinc Deficiency

Zinc deficiency is probably the most common micronutrient deficiency in crops worldwide, resulting in substantial losses in crop yields and human nutritional health problems.

Deficiency in zinc might result in significant reduction in crop yields and quality. In fact, yield can even be reduced by over 20% before any visual symptoms of the deficiency occur!

The cost to the farmer, associated with loss of production, is by far higher than the cost of testing the soil and plant tissue and applying zinc fertilizers.

The mobility of zinc in plants varies, depending on its availability in the soil or growing media. When zinc availability is adequate, it is easily translocated from older to younger leaves, while when zinc is deficient, movement of zinc from older leaves to younger ones is delayed.

Therefore, zinc deficiency will initially appear in middle leaves.

Symptoms of zinc deficiency include one or some of the following:

•   Stunting - reduced height
•   Interveinal chlorosis
•   Brown spots on upper leaves
•   Distorted leaves

As mentioned above, the visual symptoms usually appear in severely affected plants.  When the deficiency is marginal, crop yields can be reduced by 20% or more without any visible symptoms.

      

                       Zinc deficiency in corn                                           Zinc deficiency in cotton

In order to identify a zinc-deficient soil, the soil and the plant should be tested and diagnosed. Without such tests, the soil might remain deficient in zinc for many years, without the farmer identifying the hidden deficiency, as visual symptoms may not occur.

Zinc deficiency is common in many crops and on a wide range of soil types. It affects the main cereal crops: rice, wheat and maize as well as different fruit crops, vegetables and other types of crops.

Soil conditions that can result in zinc deficiency include:

•   Low total zinc level in the soil (available + unavailable zinc)
•   Low organic matter content or too high organic matter content (e.g. peat soils)
•   Restricted root growth (e.g. due to hardpan, high water table etc.)
•   High soil pH
•   Calcareous soils or limed soils
•   Low soil temperature
•   Anaerobic, waterlogged conditions
•   High phosphorus level in the soil

  

Diagnosing Zinc Disorders

Visual observation can be a quick diagnostic tool to identify zinc deficiencies. However, it requires knowledge and expertise, as symptoms may be confusing. In addition, once visual symptoms appear, yield loss has already occurred.

Regular soil or plant testing is the best practice to determine if zinc application is required and to ensure that zinc does not accumulate in the soil to undesirable high levels.

DTPA-extraction is the most commonly used soil test to determine available zinc levels in soils.

Zinc toxicity is quite rare and under normal conditions, most soils will have either normal or deficient level of zinc.

Crop	Plant Part	Sampling Time	Low	Adequate	High	Excessive
			-----------------ppm---------------------
Alfalfa	Top 6 inches	1/10 loom	<20	20-70	71-120	>120
Corn	Earleaf	Silking	<20	20-75	76-150	>150
Wheat	Flag leaves	Maturity of flag leaves	<18	19-70	71-100	>100
Rice	Youngest fully developed leaves	Mid-tillering	<20	21-160	161-250	>250
Cotton	Youngest mature leaves	Late bloom / maturity	<50	51-300	300-400	>400

Interpretation of zinc levels in plant tissue of various crops

Correcting zinc deficiency

Zinc fertilizers can be applied to zinc-deficient soils, once deficiency is identified. The most common fertilizer sources of Zinc are Zinc chelates (contain approximately 14% zinc), Zinc Sulfate (25-36% zinc) and zinc oxide (70-80% Zinc), where Zinc Sulfate is the most commonly used source of zinc.

Foliar zinc applications – foliar applications of zinc are not as effective as soil-applied zinc. The foliar application can overcome visual symptoms but it is less effective in increasing the yield.

2.0 Potassium in Plants

Potassium is an essential plant nutrient and is required in large amounts for proper growth and reproduction of plants. Potassium is considered second only to nitrogen, when it comes to nutrients needed by plants, and is commonly considered as the “quality nutrient.”
It affects the plant shape, size, color, taste and other measurements attributed to healthy produce. 
Plants absorb potassium in its ionic form, K+.

Roles of Potassium in Plants

Potassium has many different roles in plants:

• In Photosynthesis, potassium regulates the opening and closing of stomata, and therefore regulates CO2 uptake.
• Potassium triggers activation of enzymes and is essential for production of Adenosine Triphosphate (ATP). ATP is an important energy source for many chemical processes taking place in plant issues.
• Potassium plays a major role in the regulation of water in plants (osmo-regulation). Both uptake of water through plant roots and its loss through the stomata are affected by potassium. 
• Known to improve drought resistance.
• Protein and starch synthesis in plants require potassium as well. Potassium is essential at almost every step of the protein synthesis. In starch synthesis, the enzyme responsible for the process is activated by potassium.
• Activation of enzymes – potassium has an important role in the activation of many growth related enzymes in plants.

Potasssium deficiency in plants

Potassium deficiency might cause abnormalities in plants, usually the symptoms are growth related.

Potassium deficiency symptoms

      

Potassium deficiency                Potassium deficiency         Potassium deficiency
           in banana                                  in citrus                             in potato
Cholrosis – scorching of plant leaves, with yellowing of the margins of the leaf. This is one of the first symptoms of Potassium deficiency. Symptoms appear on middle and lower leaves.
Slow or Stunted growth – as potassium is an important growth catalyst in plants, potassium deficient plants will have slower or stunted growth.
Poor resistance to temperature changes and to drought – Poor potassium uptake will result in less water circulation in the plant. This will make the plant more susceptible to drought and temperature changes.
Defoliation - left unattended, potassium deficiency in plants results in plants losing their leaves sooner than they should. This process might become even faster if the plant is exposed to drought or high temperatures. Leaves turn yellow, then brown and eventually fall off one by one.

Other symptoms of Potassium deficiency:

•   Poor resistance to pests
•   Weak and unhealthy roots
•   Uneven ripening of

           3.0Nitrogen Management

Nitrogen is one of the most important essential elements for plants and is required in comparatively large amounts.

Successful nitrogen management can optimize crop yields and increase profitability while minimizing nitrogen losses to the environment. However, nitrogen management is unique and might be a complex task. 

  

Nitrogen disorders in plants

Nitrogen deficiencies might result in stunted growth, chlorotic leaves and significantly reduced yield.

Excess of nitrogen might result in poor root system, soft tissue, delay in harvestable products, low quality yield and higher susceptibility to disease and pests.

Nitrogen is mobile within the plant and, therefore, deficiency symptoms are expressed on older leaves.

  

Nitrogen deficiency symptoms

Nitrogen sources and available forms

Nitrogen behavior is complex and it is determined by many physical, chemical and biological processes. These processes are very much affected by environmental factors.

Natural nitrogen is present mainly in air and soil.

Atmospheric nitrogen - The atmospheric nitrogen is a major reservoir of nitrogen, but it is unavailable to most plants. Only legume plants can use atmospheric nitrogen in biological processes that involve bacteria. Small amounts of nitrogen are deposited by rain.

Soil nitrogen – most of the nitrogen in soil is contained in organic matter. The organic matter is relatively stable and it is not directly available to plants.

Plants can absorb nitrogen only in its inorganic forms, NO3 (nitrate) and NH4 (ammonium). Only about 2-3% of the nitrogen in the organic matter becomes available to plants per year, in a process called "mineralization".

This process involves bacteria that convert organic nitrogen to mineral nitrogen, which is available to plants.  The mineralization process is influenced by environmental factors, such as temperature, moisture, aeration, and soil pH.  

For example, excess moisture limits the availability of nitrogen and slows down the mineralization. Mineralization is optimal at 30C and at neutral to slightly acidic pH.

Nitrogen Losses

Nitrogen might be lost from the soil and, therefore, become unavailable for plants, in several ways:

•   Leaching – nitrate (NO3) easily moves downward along with water, as it is not held by soil. As a result it might be washed out below the root zone, with the flow of water.

•   Volatilization – nitrogen is lost as an ammonia (NH3) gas. This might happen when fertilizers containing urea are surface-applied.

•   Denitrification – nitrate-nitrogen (N-NO3) is converted back, by bacteria, into nitrogen gas, that is lost into the air. This process occurs when the soil is saturated or very wet.

Nitrogen Management

Successful nitrogen management can optimize crop yields and increase profitability while minimizing nitrogen losses to the environment.

Timing - One of the main challenges in deciding on a nitrogen fertility program is the timing of the application. In fertigation systems, the best practice would be to apply frequent small applications, at rates that meet the crop requirements.

In less intensive crops, like cereals and grains, where only a few fertilizer applications are made, timing of nitrogen application is critical.

Applying nitrogen too early holds the risk of losing it through leaching, before the crop takes it up, especially if rains are to come. The common approach, in such cases is to split the nitrogen application, where most of the nitrogen fertilizer just before the crop's maximum demand for nitrogen.

However, there is a risk of applying the nitrogen fertilizer "too late", if logistic or weather conditions do not allow applying it when planned.  

Determining nitrogen application rates - Nitrogen goes through of the quick and constant changes between its different forms and it is highly mobile in the soil. As a result, testing soil nitrogen gives a reading that is valid only the same moment of testing, and might lead to erroneous recommendations for nitrogen application.  

Therefore, the common approach is to give nitrogen recommendations based on yield goal and the nitrogen uptake of the crop.

Nitrogen credits, due to organic matter in the soil and residues of previous crops, should be also taken into account when making nitrogen fertilizer rate recommendations.

New methodologies and approaches for testing soil nitrogen are currently being developed and evaluated.

4.0  Phosphorus and Plants in Soil

Phosphorus is an essential macro-element, required for plant nutrition. It participates in metabolic processes such as photosynthesis, energy transfer and synthesis and breakdown of carbohydrates.

Phosphorus is found in the soil in organic compounds and in minerals. Nevertheless, the amount of readily available phosphorus is very low compared with the total amount of phosphorus in the soil. Therefore, in many cases phosphorus fertilizers should be applied in order to meet crop requirements.

The reactions of phosphorus in soil

Phosphorus is found in soils both in an organic form and an un-organic (mineral) form and its solubility in soil is low. There is equilibrium between solid phase phosphorus in soil and the phosphorus in the soil solution.

Plants can only take up phosphorus dissolved in the soil solution, and since most of the soil phosphorus exists in stable chemical compounds, only a small amount of phosphorus is available to the plant at any given time.

When plant roots remove phosphorus from the soil solution, some of the phosphorus adsorbed to the solid phase is released into the soil solution in order to maintain equilibrium.

The types of phosphorus compounds that exist in the soil are mostly determined by soil pH and by the type and amount of minerals in the soil. Mineral compounds of phosphorus usually contain aluminum, iron, manganese and calcium.

In acidic soils phosphorus tends to react with aluminum, iron and manganese, while in alkaline soils the dominant fixation is with calcium. The optimal pH range for maximum phosphorus availability is 6.0-7.0.

In many soils decomposition of organic material and crop residue contributes to available phosphorus in the soil.

Phosphorus uptake by plants

Plants take up phosphorus from the soil solution as orthophosphate ion: either HPO4-2 or H2PO4-. The proportion in which these two forms are absorbed is determined by the soil pH, when at higher soil pH more HPO4-2 is taken up.

The mobility of phosphorus in soil is very limited and therefore, plant roots can take up phosphorus only from their immediate surroundings.

Since concentration of phosphorus in the soil solution is low, plants use mostly active uptake against the concentration gradient (i.e. concentration of phosphorus is higher in the roots compared with the soil solution).

Active uptake is an energy consuming process, so conditions that inhibit root activity, such as low temperatures, excess of water etc., inhibit phosphorus uptake as well.

Phosphorus deficiency

Symptoms of phosphorus deficiency include stunted growth and dark purple color of older leaves, inhibition of flowering and root system development. In most plants these symptoms will appear when phosphorus concentration in the leaves is below 0.2%.

Phosphorus in excess

Excess of phosphorus mostly interferes with uptake of other elements, such as iron, manganese and zinc. Over-fertilization with phosphorus is common and many growers apply unnecessarily high amounts of phosphorus fertilizers, especially when compound NPK fertilizers are used or when irrigation water is acidified using phosphoric acid.

Phosphorus in nutrient solutions and soilless media

The acceptable concentration of phosphorus in nutrient solutions is 30-50 ppm, even though it was found that it can be reduced to 10-20 ppm. In nutrient solutions that flow continuously the concentration can be as low as 1-2 ppm.

In soilless media, much like in soil, phosphorus accumulates with each phosphorus addition, and minerals of phosphorus and calcium or magnesium start to precipitate. The types of minerals that are formed depend on the pH of the media.

Testing Soil Phosphorus

Phosphorus soil test level gives a measure of the capacity of the soil to supply phosphorus to the soil solution. The soil test does not measure the total amount of phosphorus in the soil, because the available amount of phosphorus is much less than the total amount.

It also does not measure the phosphorus in the soil solution, because the amount of phosphorus in the soil solution is usually very low and does not represent appropriately the amount of phosphorus that plants can potentially absorb during the growing season.

Phosphorus soil test is actually an index that helps predict the fertilizer requirement of the crop. The recommendations for fertilizer application are determined based on many field tests in many soils and crops.

Different testing methods result in different values, which have to be interpreted accordingly. For example, a result of 25 ppm phosphorus obtained with "Olsen" testing method, may have a different interpretation than the same result obtained with "Bray" testing method.

But confusion doesn't end here - different labs that use the same testing method can determine different interpretations for the same values.

Taking the soil sample correctly is very important for reaching results that truly represent the level of available phosphorus. For example:

• Soil sampling depth - Since phosphorus is not mobile in soil, samples that are taken from the topsoil will usually indicate higher amount of phosphorus than samples that are taken from the subsoil.

• Fertilizer application methods - Most of the phosphorus applied to soils remains within 1 or 2 inches from the point of application. Therefore, the exact location from which samples are taken can affect the result considerably.