If applying some fertilizer is good, then more is better, right? Wrong!

How much fertilizer should I apply? Some take the approach that if a little of something is good – more is better. This is rarely a good approach, particularly with fertilizer applications.

There are 17 elements necessary for plant growth (see table). Three come primarily from air and water (C, H and O) entering through leaves or roots. The rest are obtained from the soil, entering the plant through roots. Maximum yield is directly related to the most limiting element in the system. Soil-obtained nutrients are separated into macro and micro nutrients depending on how much is needed. This is not to say macros are more important. All 17 are necessary. The most important at a particular time is the one most limiting. When levels of that one become adequate then another takes its place. 

The 17 elements essential for plant growth and their absorbed form.

Taken up as gas or water

Carbon (C)

CO2

Oxygen (O)

CO2

Hydrogen (H)

H2O

Taken up from the soil solution

Macronutrients

Nitrogen (N)

NO3- and NH4- -

Phosphorus (P)

H2PO4- and HPO4- -

Potassium (K)

K+

Sulfur (S)

SO4- -

Calcium (Ca)

Ca++

Magnesium (Mg)

Mg++

Micronutrients

Iron (Fe)

Fe++ and Fe3+

Chlorine (Cl)

Cl-

Manganese (Mn)

Mn++

Boron (B)

B(OH)3

Zinc (Zn)

Zn++

Copper (Cu)

Cu++

Nickel (Ni)

Ni++

Molybdenum (Mo)

MoO4- -

When it comes to applying nutrients, growers can’t assume they know what and how much is needed. If growers apply nutrients in willy-nilly fashion, they are potentially headed for some serious issues. Too much can lead to toxicity, excessive growth that reduces fruiting potential, or shortages of other nutrients. How responsive plants are to certain nutrients to some extent can be species dependent. What that means is that nutrient levels helpful to one plant may be harmful to another.

Along with phosphorous (P) and potassium (K), nitrogen (N) is considered a primary macronutrient. Most plants are responsive to N in such a way that as N increases so does plant mass until the response curve plateaus and then declines as higher levels become toxic. (This type of response curve is true for any of the soil-obtained nutrients.) However, increasing plant mass is not always a good thing. When excess N is applied to tomatoes and most vine crops, their response is to grow more foliage (increase plant mass), but not produce more fruit (the economic unit). So excess N, although producing a larger plant, can delay and reduce fruit yield.

Boron (B) is an essential micronutrient often limiting in sandy soils due to its high solubility and mobility through soil. Adequate B levels for most crops are in the 2 to 4 pounds actual B per acre range. Many sandy soils through leaching move toward one pound per acre if no additional B is applied. So when 4 pounds per acre is added, it brings the level to five which is still in the safe range. However, if 4 pounds is added to a soil having adequate B levels, it can raise it to potentially toxic levels.

Some nutrients can also be antagonists for other elements. They essentially compete with each other for uptake by the plant. If excess calcium (Ca) is added to the system, it can potentially influence the uptake of K, Magnesium (Mg), iron (Fe), B and other nutrients. If too much Mg is added, it can affect the uptake of Ca and K. There are many other interactions that need to be considered when nutrients are added. Therefore, when designing a nutrient management program, careful consideration based on a soil test is recommended.

Do not assume crops need certain nutrients and certainly don’t have the mindset that adding more is better. Yearly soil testing followed by tissue analysis if deficiency or toxicity is suspected is the way to go, at least until you have established a good history for your site. For example, I take yearly soil samples for the standard test, but have the B test conducted every four years or so. Since I have enough history with the soil, I know it will be around 2 pounds per acre on my sandy soil, so I feel safe in routinely adding 2 pounds per acre B every year, but I periodically check B levels to be sure they stay in line.

Soil samples can be taken any time of the year as long as the soil is not frozen. Late fall and early spring are the most popular times. For tips on soil sampling and submission, go to Michigan State University’s Soil and Plant Nutrient Laboratory website.

If you have further questions concerning nutrient application or other commercial vegetable production issues, contact your local Michigan State University Extension office or Ron Goldy at .(JavaScript must be enabled to view this email address).

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