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When Less is More: Understanding Fertilizer and Solubility

The amphiphilic nature of humic substances allows them to work in water and hydrophobic environments, providing the critical conditions necessary for biological processes when they are closely associated with clays.

Make the gesture “just a little bit” by squeezing your thumb and index fingers as tight as you can; tighter, tighter — the amount of fertilizer you could hold between your fingers is about the amount dissolved in soil solution … per acre! That’s right; there is very little if any dissolved “plant food” in the water of a typical soil.

The amount of plant nutrients dissolved in soil solutions is so small that it is expressed as parts per million (ppm), not hundreds of pounds or tons per acre. While synthetic fertilizers are sold primarily on the basis of their water (aqueous) solubility, the emphasis on aqueous solubility is generally misunderstood and somewhat misguided.

It is generally known that over-application of extremely soluble synthetic fertilizers has been responsible for disrupting ecosystems and numerous environmental problems. What is not generally known is that all highly soluble soil inputs, including sulfates, chlorides and fluorides, disrupt the structure of water molecules, impeding the biochemical energy flows that affect the metabolism of plants, making them more susceptible to insect pressure and diseases and decreased water use efficiency.

It is also a well established fact that highly soluble phosphate fertilizers become “tied-up” soon after application. When there is an overabundance of dissolved phosphates in soil water, the soil system responds chemically by forming more stable forms of phosphorus, usually by chemically combining with calcium cations and complexing with lanthanides (rare earths) and organic matter. All of these materials can release phosphorus as plant nutrients through microbial activity.

Although water is critical to all life forms, there are numerous metabolic pathways in biological systems where it gets in the way and must be pushed aside; it’s called the hydrophobic effect.

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