Responses of Crop to Micro-Nutrients

After reading this article you will learn about the responses of crops to micro-nutrients.

In India most soils are widely deficient with zinc, boron and molybdenum to the level of about 40-50; 60 and 60-70% respectively out of other lesser magnitude of deficient micro-nutrients like Fe, Mn and Cu. Therefore, an emphasis is to be given to manage all these deficient micro-nutrients for their corrections through fertilization programme considering right sources, rates and methods of applications.

It has been reported that the response of cereals, millets, pulses oilseeds, vegetables and cash crops ranged from 4.5-8.9; 3.0-6.8; 3.4-5.8; 1.6-5.5; 2.0-15.3 and 3.9-96.8 q/ha respectively to the application of iron. The average response of wheat and rice was almost of equal magnitude.

The average response of groundnut, onion, sunflower, tomato and sugarcane to the application of manganese was 1.1; 39.6; 5.5; 6.0 and 17.8 q/ha respectively.

In case of Cu, both organic and inorganic Cu-fertilizers are used to control Cu deficiency and their efficiency in relation to crop response may vary according to manure and type of soils as well as type of crops. The application of Cu as soil and foliar shows good response to different crops including flowers especially marigold (Das, 2007).

In India, various methods of Zn application to different soils and crops have been evaluated to correct its deficiency under field conditions through All India Co-ordinated Research Project on Micro-nutrients, ICAR, New Delhi and other research institutions and State Agricultural Universities.

Most results have showed that the response of rice growing under submerged puddle conditions exhibited a greater response compared to other crops like, wheat, maize, pulses, oilseeds, vegetables etc. to the application of Zn either applied as foliar or soil.

Response to Zn varied greatly among the crops in each state as well as among the states for each crop because of wide Variation in the sensitivity of crops to Zn and magnitude of its deficiency in soils (Das, 2007).

However, responses as large as 47, 48, 20, 12 and 8 q/ha of wheat, rice, maize, barley and oats respectively were recorded to the application of Zn in most deficient soils. Besides, Boron and Mo exhibited a greater response towards the increase in yield and quality of wheat, oilseeds and high valued vegetables.

The best practice has been to broadcast and incorporate Zn as a pre-plant application. This should provide several years effectiveness. Chelates are foliar applied to correct Zn deficiency during the growing season but have little residual value. Oxide forms of Zn may have limited effectiveness in the year of application, but may be used to provide residual effect.

Oxy-sulphate forms may provide some immediate plant need as well as a residual effect. The higher the percentage of sulphate (soluble and plant available) fraction, the more Zn will be in the plant available form. Corn may respond to zinc fertilizer at a low level of zinc, but alfalfa will not. The soil test for zinc is the best guide to use in determining the need for a zinc fertilizer.

In general, zinc may be needed for sensitive crops where:

i. The soil is calcareous (pH is higher than 7.3 because of excess lime).

ii. The topsoil has been removed either through erosion, land leveling or terrace building.

iii. Soils are very sandy with a low organic matter content.

There have been some observations that applying high rates of phosphorus without zinc on calcareous soils with a low zinc level can reduce corn yields. If zinc is included with phosphorus in these situations, yields are improved.

These can be divided into four groups:

(1) Organic chelates;

(2) Organic non-chelates;

(3) Soluble inorganics; and

(4) Insoluble inorganics.

The chemical characteristics of zinc sources in one group are completely different from those in another. From the standpoint of fertilizer use, the mobility of the various carriers (sources) is very important. The zinc chelates are mobile in soils and will move with the soil water.

The insoluble inorganic zinc carriers (zinc oxide, zinc carbonate) are not mobile, and must be applied as small, finely divided particles, broadcast and thoroughly incorporated so the plant roots will come in contact with the zinc fertilizer. Organic non- chelated zinc carriers and the soluble inorganic carriers are soluble but not very mobile in the soil. These carriers need to be placed in the root zone to assure root-zinc contact.

Placement of Zinc Fertilizers:

The mobility of the various carriers has a strong influence on the way in which zinc fertilizers are applied for crop production. When the dry sources of zinc are considered, all materials are equally effective for crop production except for the granular formulations of the insoluble inorganic compounds.

For the most part, zinc fertilizers can be either broadcast and incorporated into the soil or applied in a band at planting. In general, the rate of zinc suggested for a broadcast application can be reduced by half when the zinc fertilizer is placed in a band near the seed. Plant nutrients supplied in a fertilizer are usually applied at rates sufficient to grow the current crop.

With zinc, however, it may be more practical to raise the zinc level of the soil, thus assuring an adequate supply for several years. This approach requires that relatively high rates of zinc be broadcast and incorporated. Granular zinc sulfate or finely ground zinc oxide can be used for this purpose.

Zinc may also be applied with irrigation water through fertigation and in that case, Zn must be freely mobile. The chelated materials are the mobile sources of zinc, and are thus appropriate sources for this practice. Effectiveness ratios, such as 10: 1 or 5: 1, are often used for some zinc sources.