Growers of all crops are bombarded with advertisements and salespeople selling a wide range of fertilizers and supplements. For specialty crop growers there is added uncertainty given the limited research specific to these crops on all aspects of fertilization. As a starting point, it is important to understand the different types of products, the purpose of each within a fertility program, and the actual value or cost-benefit of applying each product.

While definitions vary, a fertilizer is a product that provides one or more essential elements (nutrient) to the soil or to the plant directly. The essential elements are nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, boron, chlorine, iron, manganese, zinc, copper, molybdenum, and nickel. For the purpose of this article, a fertilizer is a product designed to substantially increase the availability of one or more essential nutrients to the crop to address the needs of the crop for that nutrient. Manures, composts and other natural products would also be considered fertilizers.

Definitions for a supplement vary even more than a fertilizer. For the purpose of this article I will go with the Canadian Food Inspection Agency definition, which is “any substance or mixture of substances, other than a fertilizer, that is manufactured, sold or represented for use in the improvement of the physical condition of soils or to aid plant growth or crop yields”. While a supplement may have essential elements in it, it has insufficient amounts of that element to directly and/or substantially address the needs of the crop for that element. A supplement, for example, may stimulate the crop to form more roots and thereby address nutrient deficiency issues, or it may help the plant tolerate stresses. A biostimulant is one main type of supplement that stimulates the crop to resist stress or grow faster without directly affecting plant nutrient status.

When deciding what type of product to apply to your crop, the first thing you need to consider is what the purpose of the product will be.

The time to use a fertilizer is:

1. You are looking at a soil test and note that a particular nutrient is low
2. You need to address the annual nutrient needs of the crop based on research or previous experience (e.g. annual nitrogen application, specific micronutrients known to be problematic for the crop)
3. You note a visual deficiency in the crop and need to correct it

The time to use a supplement or biostimulant is:

1. Research shows an added benefit for your crop that cannot be achieved with fertilization alone, or research shows benefits to other crops and you want to trial it on your crop.
2. Despite adequate fertility, your plants still do not look healthy or as good as they could be, and you want to try something new.
3. You note a particular stress on your crop, and you want to try a supplement known to address that stress.

A fertility program is aimed at meeting the annual nutrient needs of the crop. The nutrient needs of the crop do not change depending on the type of product you apply or whether you are fertilizing conventionally or organically. For example, if research shows that a crop needs 100 kg/ha of nitrogen for optimal yields, the crop will need to get 100 kg/ha of nitrogen from somewhere regardless of how you are growing the crop or what fertilizers or supplements you are applying. In fact, nutrient needs may increase if a supplement increases crop yields. While specific fertility needs of specialty crops may not be known, it is known that they would be in the same general range as other similar horticultural crops. A supplement could temporarily increase the availability of nutrients by increasing root growth, for example, and reduce the need for fertilization that year. However, that just depletes the nutrient in the soil and eventually it must be replaced, whether it is months or years later.

In organic production, the goal is to build up the nutrient content of the soil so annual fertilization may not be required or is only required in small amounts of approved organic amendments. This is done by fertilizing with manure or compost, using cover crops and green manures to recycle nutrients that may have been lost by leaching or erosion, using legume crops that fix nitrogen, and building up the microflora in the soil to make nutrients more available to the crop. However, this doesn’t change the amount of nutrients required by the crop, it just changes how those nutrients are applied and may delay the availability of those nutrients until months or years after the application date. There are resources available that can help you determine when and how much of each nutrient are released from incorporation of natural fertilizer products like manure, compost or legume cover crops. Consult OMAFRA Publication 611 Soil Fertility Handbook for more information.

The value of a fertilizer or supplement is a factor of the cost, concentration and rate of the product and the potential marketable yield benefit from its use. If the goal is to fertilize the crop, then the first thing to look at is the concentration of the nutrient you want to apply and the price per unit of the fertilizer. For example, if a fertilizer is 34-0-0, then it is 34% nitrogen. If the cost of the fertilizer is $10 per unit, then the cost per unit of nitrogen is determined by dividing the cost by the proportion of nitrogen (the percentage divided by 100). So, $10/0.34 = $29.40 per unit of nitrogen.

Let’s say you found a fertilizer this is much cheaper at $5 per unit. The product is 5-0-2. The cost per unit of nitrogen would be $5/0.05 = $100, hardly a bargain compared to the 34-0-0, although it also supplies a small amount of potassium at the same time. The potassium only adds value if it is deficient in the soil and it is replacing another fertilizer.

The next thing you need to determine is whether the product is truly a fertilizer or is a supplement in disguise. Some products appear to be fertilizers, but an examination of the suggested rate of application shows that they do not come close to providing enough nutrients to meet the needs of the crop.

I recently came across a product that was advertised as a fertilizer with a macronutrient ratio of 0.5-0.03-0.8. The suggested application rate was 5 kg/ha. A quick calculation (application rate x proportion of nutrient in fertilizer), shows that the product would provide 25 grams of nitrogen, 1.5 grams of phosphorus, and 40 grams of potassium per application.

The average crop requires 100 kilograms/hectare of nitrogen per year, which is 4,000 times more than what you would get from the “fertilizer”. Phosphorus and potassium requirements of the crop are also much higher than the “fertilizer” would supply. Application of these kinds of products above the suggested application rates on the label is not advised either because it could damage the crop and/or be too expensive to apply.  Damage may occur because the active ingredients in these products are often not simple nutrients, but natural or synthetic chemicals that do not normally occur on the leaf or in the root zone. This damage may be crop-specific, and without research, the potential damage on specialty crops may not be known. These kinds of “fertilizer” products may provide benefits to the crop at the suggested application rates, but it would be in ways other than as a direct fertility product.

While we know that crops need the essential nutrients, there is limited or no research on most supplements on specialty crops. The grower would have to conduct research on a small area first to determine if it is effective or ask the company for research results. The value of a supplement is determined by the potential yield increase multiplied by the selling price of your product. Compare this value to the cost of applying the supplement per area to determine if it is worthwhile applying it. Also consider whether the addition of a fertilizer, which is usually much cheaper, would provide similar yield benefits at a fraction of the cost of a supplement.

Failing to conduct thorough research when choosing products to apply the crop could result in prohibitive costs or damage to the crop. For specialty crop growers, it is usually best to use conventional or organic fertilizers to supply the crop with nutrients. When those needs have been met annually, then it may be time to experiment with supplement products to further boost your crop yields, based on deficiency symptoms or specific stresses you are facing. To find out how to determine the nutrient needs of specialty crops, consult OMAFRA’s Specialty Cropportunities site. Nitrogen needs are addressed within each crop profile on the site, and phosphorus and potassium needs are addressed in the Fertility Requirements for Specialty Crops section.