Crops do not use a large quantity of phosphorus, but what they do use is most important and the supply must be good. Crops use two to four times as much nitrogen and potash as phosphorus, but phosphorus is held so firmly by the soil that crops seldom get enough and farmers have had to spend annually about one hundred million dollars for this element as a fertilizer to add it in an available form to the soil. The phosphorus is chemically complex. It has many chemical handles with which to hang onto soil surfaces, and so does not move readily in the soil solution. I have compared phosphorus in the soil to a monkey in the jungle. One could not throw a monkey far through a thick jungle growth because arms, legs, and tail would soon catch a limb or vine to hang onto. Phosphorus likewise in the soil gets tangled up in many similar ways and tends to be hard to move and to be kept soluble and available.
Complex behavior – To understand this complex chemical behavior, let us pretend that phosphorus is acting in a play as Miss Ortho Phosphate, highly attractive to Mr. Root Hair of the Plant Family. The stage is the soil and the scene of our first act is in a very acid room (soil at ph between 4.8 and 5.3). Along the walls of the room (colloidal surfaces) are numerous three-handed boys (trivalent, iron and aluminum oxides and hydrates) that have a great attraction for Miss Phosphate, who also has three hands. As long as these Iron and Aluminum boys have Miss Phosphate in this acid ballroom they can really strut their stuff and at once they are engaged and married. The marriage is made secure (insoluble) in the triple bonds of these hands. The knot is too secure (precipitates of insoluble iron and aluminum phosphates) for Mr. Root Hair to have much of a chance with his courting. He gets jilted and his poor mother, the Plant, starves because her son could not compete with this aggressive competition in such an unfavorable environment.
Addition of lime – In the second scene, the room has been brightened up a bit with lime. There is just the faint trace of lime in the decorations (light liming to ph 5.3 to 6.0). But this change has proven allergic to the Iron and Aluminum boys for now they have lost a lot of their pep. They are, in fact, almost put out of commission (become insoluble) and the only time they can embrace Miss Phosphate is when she bumps occasionally into them as they sulk along the walls (absorption of phosphates by iron and aluminum in the clay particles). But once she touches the grabby hands of these sluggish, immobile Iron or Aluminum fellows, she is really made a prisoner, held in a clumsy embrace (absorption) but not nearly so snugly as when she was held by these same individuals when they danced about (soluble and mobile) in the first scene.
We notice in the second scene a big, dark, spongy fellow that in many physical respects resembles Mr. Clay Particle in which Iron and Aluminum are sandwiched. This dark chap answers to the name of Mr. Lignin Humate from the Family of Organic Matter. Strangely enough, he tends to get into the hair of the Iron and Aluminum boys by getting in front of them to such an extent that these boys, who love Miss Phosphate so much, cannot touch her or hold her hand (protective colloid reaction). In fact, there is a rumor that this Humate fellow actually replaces Miss Phosphate at times by crowding her out of the embrace of these three armed bandits (anionic exchange). Mr. Root Hair enjoys having Mr. Humate around because he has discovered that Miss Phosphate is more free to give him some attention. In fact, in such situations, Miss Phosphate is very likely to become annexed to the Plant Family.
The Happy Ending – The stage setting becomes a happy affair in scene three for here everything is sweetened up just right with lime (about ph 6.5). We find that the Iron and Aluminum boys are in jail (insoluble), and Miss Phosphate is keeping steady company with Calcium and Magnesium, the two sons of Old Man Dolomitic Lime. She is rather free to go to the Plant Family at any time, for whenever the door is open to the attractive boys, Calcium and Magnesium, they usually drag Miss Phosphate in, also (mono-calcium and mono-magnesium phosphates, slightly water-soluble).
It is interesting to note what happens if the stage becomes overdecorated with lime (overliming, ph above 7.0). Then Miss Phosphate and the Lime sons, especially with Mr. Calcium, go into such an embrace that they can hardly be moved (insoluble tri-calcium phosphate). Only a few of the relatives of the Plant Family such as the Legumes are able to get them to move. The Legumes seem to have a better broom for sweeping them off their feet (root contact and action of excreted carbonic acid) than do their weaker relatives the Grasses.
So, we come away from our play with the conclusion we liked the third scene best and that our hero, Mr. Root Hair, is further favored by having Mr. Humate around acting as the Jailer for the Iron and Aluminum boys and being otherwise generally helpful.
Method of Application – The behavior of phosphorus in the soil is extremely complex and this story is intended to illustrate only some of the chief characteristics. The practical advantages of correct liming, the addition of organic matter, and the use of legumes is implied. Phosphates do not move to any appreciable extent in the soil and are rapidly converted into unavailable forms when mixed thoroughly in the soil. For this reason they should be localized or placed with as little mixing with the soil as possible in bands. Top-dressing or high placement of phosphates for cultivated crops is not effective, on pastures where roots are near the surface, top-dressing does satisfactorily. Of course, it needs to work in cooperation with the other nutrient elements because the plants will not respond well to additions of phosphates if one or more other nutrient elements are limiting.
Originally published in Northern Nut Growers Nutshell 1947 by George Scarseth
Subsequently edited and published in "Man and His Earth" 1962.