ESSENTIALITY AND FUNCTION OF NICKEL IN PLANTS

The first evidence of a response of a field crop to application of a nickel fertilizer was demonstrated in 1945 for potato (Solanum tuberosum L.), wheat (Triticumaestivum L.), and bean (Phaseolus vulgaris L.) crops .

In these crops, the application of a dilute nickel spray resulted in a significant increase in yield.

These experiments were conducted on the ‘Romney Marshes’of England, a region that is well known for its trace mineral deficiencies, particularly of manganese and zinc.

 These experiments were conducted very carefully and excluded the possibility that the nickel applied was merely substituting for manganese, zinc, iron, copper, or boron, suggesting that the growth response was indeed due to the application of nickel.

 Interestingly, the soils of this region may be low in nickel since the conditions that limit manganese and zinc availability in these soils (acid sands of low mineral content) would also limit nickel availability to crops, and the concentrations of nickel provided were appropriate based on the current knowledge of nickel demand.

These same soil types also dominate the region of southeast United States where Ni deficiency is now known to occur.  

 Mishra and Kar  and Welch  reviewed the evidence of the role of nickel in biological systems and cited many examples of yield increases in field-grown crops in response to the application of nickel to the crop or to the soil.

The significance of these purported benefits of field applications of nickel is difficult to  interpret since the majority of the reported experiments used very high nickel application rates.

 None of these reports considered the possibility that nickel  influenced plant yield through its effect on disease suppression, nor was the nickel concentration in the crops determined. Indeed, prior to the availability of graphite-furnace atomic absorption spectrophotometers and inductively coupled plasma mass spectrometers (in the mid-1970s), it was exceedingly difficult to measure nickel at the concentrations ( 0.1 mg Ni kg  1  dry weight) later shown to be critical for normal plant growth.

 In the absence of information on tissue-nickel concentrations, it is impossible to conclude that the observed yield increases were the result of a correction of a nickel deficiency in the plant.

SYMPTOMS OF DEFICIENCY AND TOXICITY

 In legumes and other dicotyledonous plants, nickel deficiency results in decreased activity of urease and subsequently in urea toxicity, exhibited as leaflet tip necrosis .    

 With nitrogen-fixing plants or with plants grown on nitrate and ammonium, nickel deficiency results in a general suppression in plant growth with development of leaf tip necrosis on typically pale green leaves  (Figure 14.1 and Figure 14.2). These symptoms were attributed to the accumulation of toxic levels of urea in the leaf tissues.

NICKEL FERTILIZERS 

 Essentially under all normal field conditions, it is unlikely that application of nickel fertilizer will be required. Exceptions to this concept occur when urea is the  primary source of nitrogen supply, in species in which ureides play an important physiological role (2), when excessive applications of   Handbook of Plant Nutrition   Zn, Cu, Mn, Fe, Ca, or Mg have been made over many years  and perhaps also in nitrogen-fixing crops grown on mineral-poor or highly nickel-fixing (high pH, high lime)soils.

 In experiments utilizing highly purified nutrient solutions or tissue-culture media, supplemental nickel may also be beneficial. In all of these cases, the  nickel demand is quite low and can be satisfied easily with NiSO4  or other soluble nickel sources including Ni-organic complexes (Bruce Wood, personal communication).

In solution-grown plants and as a supplement to foliar urea applications, a nickel supply of 0.5 to 1 μM is sufficient.