Fertilizers for Foliar Nutrition: the Main Questions of Choice

Why microelements

The use of microelements in agricultural production is not a new topic. Scientists have worked on it for more than a century. Nutrient mixtures for various media, those of Knop, Timiryazev, Pryanishnikov and others, are proof of this. Microelements are gaining ever greater importance in crop production because of:

• growing demands from high-yielding crops grown with intensive technologies. For example, the table shows microelement removal with the yields of leading crops

  Crop - yield, t/ha	Mn	Zn	Cu	B	Mo
  Wheat - 8	700	400	70	60	5
  Corn - 14	2400	350	100	500	9
  Sugar beet - 50	600	300	80	500	5
  Sunflower - 3.5	400	350	60	400	-

• farming on lands with low microelement content. For example, possible microelement deficiency depending on soils:

  Soil conditions	Possible deficiency
  Light texture, alkaline reaction	B, Fe, Cu, Mn, Zn
  Peat soils	Cu, Mn, Zn
  Acidic soils	Cu, Zn, Mo
  Heavy texture	Cu, Mn, Zn

• more intensive use of high-quality (concentrated) fertilizers that contain almost no microelements;
• reduced use of organic fertilizers;
• depletion of soil reserves of microelements;

The role of microelements in plant life is quite powerful and varied, but first of all their action shows in the synthesis of enzymes and activation of biochemical processes in the plant, strengthening plant resistance to unfavorable conditions and disease agents, and improving harvest quality indicators. Producers in protected ground and intensive vegetable and sugar beet growing have used microelements since the middle of the last century, by applying industrial waste and superphosphates enriched with individual microelements to the soil, but over the last ten years there has been an avalanche-like growth both of information (mostly of an advertising nature) about the importance of microelements in plant mineral nutrition and of the number of foliar feeding fertilizers. During this period the number of registered trademarks and compositions grew in arithmetic progression. The consequence is quite a heavy saturation of the market with a large number of microfertilizers not always of the quality declared. Therefore it is often hard for the consumer to understand all the features of the fertilizers offered, let alone link them with the main means of production, the soil, and the optimal application dates and doses.

Why foliar feeding with microelements

1. A high level of uptake, several times higher than from the soil.

2. A high uptake speed, from several hours to several days

   Element	Time needed for uptake of 50% of the nutrient
   	Hours	Days
   Nitrogen	1-6	-
   Phosphorus	-	1-5
   Potassium	10-24	-
   Calcium	-	1-2
   Magnesium	2-5	-
   Iron	-	1 (8% utilization coefficient)
   Manganese	-	1-2
   Zinc	-	1-2

3. The biological features of crops are linked with the highest sensitivity to microelement nutrition conditions depending on development stages, such as:

   • tillering and the start of heading in small-grain cereals
   • 3–5 and 8–10 leaves in corn
   • 6–8 true leaves in sunflower
   • 7–8 leaves and budding in rapeseed
   • 4–6 and 8–10 true leaves of sugar beet
   • 3–5 leaves, budding to the start of pod formation in soybean and peas
   • bud burst, pink bud and fruit fill on fruit crops
   • 5–8 leaves, end of flowering and laying down of grape clusters

Which microelements

The market has a multitude of compositions developed «taking into account the biological needs of plants», although in fact this concept cannot be considered entirely correct. After all, the main route of nutrient supply from the soil is through the roots. The content of macro- and microelements is largely determined by the genetic features of soils, and for some elements also by human economic activity. At the same time the soil itself is often characterized by heterogeneous composition within a single field, not to mention the difference in soil properties in natural and climatic zones and soil provinces formed on radically different parent rocks. All this influences its fertility indicators and the formation and activity of microflora, the source of nutrient availability. Therefore crop needs for microelements for the Polissia zone with soils of light texture and organic origin will differ from those on soils of heavy texture, for example in the Left-Bank Forest-Steppe. Advertising materials of foliar fertilizer sellers contain a mass of different «fashionable» words that do not correspond to the essence, and there are also attempts to advertise in fertilizers a number of chemical elements whose role in the plant is not clearly established, for which there are no standards and for which in Ukraine there is no laboratory able to determine their content in any object (soil, plant, water, etc.). The availability to plants and the degree of uptake of nutrients are closely linked with the form of the compounds in foliar feeding fertilizers. Unfortunately many producers do not always have a clear understanding of these questions. In scientific chemical terminology the following forms of metal ions in a liquid medium are distinguished:

1. Hydrated ions (a solution of salts in water) enter readily into all possible chemical reactions and, as a result, only an insignificant share of them reaches the plant.
2. Complex compounds: the metal ion is surrounded not by water molecules but by other complexing molecules. Organic compounds containing amino (-NH2) or carboxyl (-COOH) groups are fairly good complexing agents. The mobility and degree of microelement uptake by plants from such complexes increase compared with hydrated ions.
3. Chelates are intracomplex compounds in which the coordination bonds of the complexing agent with the metal ion form a relatively stable cyclic structure. The term «chelate» comes from the Greek for «claw». The chelating molecule surrounds the metal and holds the microelement inside the chelate ring, keeping it in a form available to plants.

Why chelates

The effectiveness of a complex compound or microelement chelate is mainly characterized by its stability constant, a measure of the compound's ability to hold the metal as the actual complex against competing reactions. For comparison, the stability constant of Mg citrate is 3.2, while for EDTA-chelated Mg it is 13.6, that is, the latter is 10¹⁰ times more stable. Interesting data were summarized by the IFA (International Fertilizer Industry Association) (fertilizer.org). They prove that the synthetic chelating agent EDTA (ethylenediaminetetraacetic acid) has the best chelating properties in terms of stability and safety. Compounds with citric acid, humates, lignosulfonates and so on are often called natural chelates, but there is no convincing evidence that these compounds form true chelate structures. Moreover, the chemical structure of such complex organic complexes is sometimes undefined and may vary considerably depending on the processes used in their manufacture. The statement that organic complexes are more effective than inorganic sources has not been independently confirmed. Scientific research has shown that true EDTA chelates can be 3 to 5 times more effective than organic complexes. The degree of microelement uptake then reaches over 80%, compared with 20–40% for organo-mineral complexes. Another important question is the content of microelements in chelated form. For each microelement there is a chelation limit. For the chelating agent EDTA the maximum possible content of microelements is as follows (in weight percent):

In addition, the degree of chelation is an important factor. In some cases, to cut costs, not 100% of the microelement is chelated but, for example, 80%. In that case the microelement content in the fertilizer will be higher than stated above, but the uptake level of the non-chelated part of the microelement will be considerably lower.

In general the following conclusions can be drawn:

1. Using special fertilizers in modern conditions, especially when growing intensive varieties and hybrids, is one of the main factors in obtaining a yield close to the genetic potential of the crop.
2. Using microelements for foliar feeding significantly affects not only yield but also the main harvest quality indicators.
3. To obtain quality crop feeding, it is better to use microelements in the form of chelates. Chelate forms of microelements are much more effective than other forms of microelements, but they of course also cost more. Yet in most cases using the EDTA-chelated form of microelements becomes economically advantageous, unlike other forms of microelements which, though cheaper, are also much less effective. Alternatives to EDTA exist. Currently various chelating agents are used (IDS, EDDS, MGDA, GLDA and others), but the problems of stability, lower degree of chelation and, above all, higher cost of these compounds remain unsolved.