Choosing the Right Fertilizer for Flowering and Fruiting Crops

Let’s suppose that a grower wants to produce a flowering or fruiting crop, most realize that phosphorus is a very important nutrient in this part of the life cycle.  They go down to the local garden center and see a wide array of fertilizers, some are powders, some are liquids, some are natural and some are manmade.  Our grower decides to go with bone meal since it’s around 10% Phosphorus with additional Nitrogen and Calcium.  Is this the best choice?  The answer isn’t very straightforward.

Understanding Phosphorus: A Key Nutrient for Flowering and Fruiting

The nutrients contained in this type of fertilizer are insoluble in water and for nutrients to be available to the plant they have to be in a form that will dissolve, otherwise they won’t be absorbed into the roots.  While these nutrients are not available immediately that doesn’t mean that they never will be, they have to be broken down by soil microbes before they can be absorbed by the plants and that process could take years depending on soil conditions.

The Challenge of Insoluble Nutrients in Fertilizers

Phosphorus is among the most important nutrients and it is also one of the most problematic as far as lockouts go.  It readily reacts with such common ions as Calcium, Magnesium, Iron, and Aluminum to form insoluble salts.  That means that while a soluble form could be given to the crop it doesn’t mean that it will be available to the plant during the whole life cycle, as it will readily react with the aforementioned ions.  The way to get around this is to use a coated slow release product or to give multiple smaller doses throughout the life cycle. 


phosphorus cycle : soil processes


Iron in Fertilizers: Soluble vs. Insoluble Forms

The most frustrating nutrient for many of us who formulate fertilizer products is Iron.  There are several soluble forms, the sulfate form is the cheapest and is permitted in organic agriculture.  Here again it’s very unstable and converts to an insoluble form very quickly after application.  The chelated forms EDTA, DTPA, and EDDTA are far superior to the sulfate forms but also have limitations.  The EDTA form is the most common since it’s the cheapest but is best at lower pH levels.  DTPA and EDDTA are better for wider ranges of pH levels but are quite expensive.  For hydroponic growers, most have seen red brown material on the bottoms of their reservoirs.  This is literally rust, as Iron will react with oxygen from the atmosphere and fall out.  This is one of the reasons why changing nutrient solutions regularly is so important. 


*Did You Know*

In the primitive earth the oceans contained high levels of dissolved Iron Sulfate, giving them a blue – green colour.  When photosynthesis evolved algae started producing oxygen as a by-product.  This gave rise to the oxygen rich atmosphere we all know and love but also meant the end of the Iron rich oceans as the oxygen reacted with the Iron Sulfate producing rust which fell out of solution.  Evidence of this can be seen in geological structures around the world, the most spectacular formations being from Australia.


australian banded iron rock formation


The Role of Nitrogen in Plant Growth

Nitrogen is one of the most easily available nutrients…most of the time.  The nitrate and ammonium forms generally don’t precipitate out and will be readily available to the plants.  Urea is the most common form of Nitrogen used in agriculture today, and while it can be used by plants in a very limited capacity in that form most of it has to be converted into ammonia by soil microbes before the plant can readily use it.

Understanding Different Forms of Nitrogen in Fertilizers

Organic Nitrogen is usually in the form of proteins such as blood meal and fish emulsion, these are not forms that the plant can use and have to be broken down by soil microbes.  While these proteins are somewhat soluble the complex structure doesn't absorb through the roots.  Overall nitrogen becoming unavailable doesn’t usually happen with it becoming locked up in the soil but rather it being converted back into Nitrogen gas and escaping to the atmosphere.  

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