Is there a benefit to the beneficial elements?

Wednesday, August 1, 2012

The answer is 'yes,' but there is no compelling reason to include them in a fertilizer program for crops grown in Ontario

by KEITH REID

Most of you, I am sure, are familiar with the essential elements for plant growth, at least to the level of recognizing the macro-nutrients (nitrogen, phosphorus and potassium), the secondary nutrients (calcium, magnesium and sulphur), and at least knowing that there are essential micronutrients even though you may not be able to name them all. All of these elements must be available to a plant if it is to successfully grow and reproduce.  However, there is another group of elements that the plants do not need to have, but which can stimulate plant growth or partially replace the function of one of the essential elements. These are referred to as beneficial elements.

The three most commonly identified beneficial elements are sodium, silicon and cobalt.  There are other elements, like selenium, iodine or chromium, that are required for animal growth and so are desirable to have in feed crops, but the focus today is on the elements that have an impact on crop growth.

Sodium. We commonly think of the harmful impacts of sodium, since too much sodium can impair soil structure and also be an indicator of salt levels that are harmful to plants.  For salt-tolerant plants, however, sodium can replace a large part of the crop's requirement for potassium and even improve yields over supplying potassium alone.  This includes crops like sugar beets, turnips and, to a lesser extent, wheat. Crops like barley, potatoes and tomatoes do not show any increase in yield, but can substitute a small part of their potassium requirement with sodium. Corn, soybean and dry beans fall in the category of sodium-intolerant plants, and so cannot substitute sodium for potassium.  

The implications for management are that moderate levels of sodium on a soil test are not harmful and, since most of the major field crops cannot utilize sodium effectively, there is no justification for adding sodium to a fertilizer program.

Silicon. Silicon is the second most abundant element in the earth's crust, being the backbone of most sand and clay minerals, but the amount in soil solution is very small.  Some plants accumulate a lot of silica into their cell walls, where it helps promote the strength of the plant. You may be familiar with the glassy, brittle stems of field horsetail (Equisitum), which gets this consistency from the high silica content.  

Among crop species, wetland rice shows the greatest response to silica, where it improves stem strength and disease resistance. But silica is not yet to the point of being a normal part of the fertilizer program, since many soils provide enough silica to meet crop needs. Among the crops grown in Ontario, the cereals would have the highest requirement for silica, but still much lower than what the soil can provide naturally. Broadleaf crops have very low silicon requirements and actively exclude silicon uptake by the roots.  

Cobalt. The need for cobalt by animals, and particularly ruminants, has been long established, but it has never been shown to be an essential element for plant growth.  Where it does have a big impact, however, is on the activity of nodules on legume roots in N fixation. Fortunately, the incidence of cobalt deficiency in legumes is rare, limited to a few very poor sandy soils in Australia. The amount needed by plants is so small that most soils can provide adequate amounts to meet crop needs, so supplemental cobalt is not required.

The levels needed by livestock are much higher than the critical levels for nodulation of legumes. Since most livestock are receiving supplemental cobalt with their feed, and fields receiving livestock manure will be well supplied in any case it isn't necessary to add it to a fertilizer program.

So, in summary, the beneficial elements really do provide benefits in some cases, but there is no compelling reason to include them in a fertilizer program for crops grown in Ontario. BF

Keith Reid is Manager (Eastern Canada) – Soil Nutrient and GHG Management,
Agriculture and Agri-Food Canada, Guelph.