Phosphorus is essential for plant life. It promotes early season growth as well as early maturity. But after nitrogen, phosphorus (P) is probably the second most widely deficient nutrient in soils. There are two reasons: First, P levels are low in parent materials from which soils are formed. Second, P is inclined to form compounds with calcium, magnesium and other common soil ions, making it unavailable to plants. That is why you have to apply phosphorus fertilizer.
But phosphorus has been the subject of environmental concern as it can contribute to algae growth in lakes. Algae can starve the lake of oxygen needed for all other life. This environmental concern has raised the issue of phosphorus fertilizer rates. Therefore, striking a balance between providing essential food to crops and protecting the environment has become a contemporary issue for both agricultural and urban users of P fertilizers.
Prairie soils contain three pools of P: inert, labile and soluble. Inert is by far the largest pool, but this phosphate is bound to soil ions and unavailable. Plants feed directly from the soluble P pool only, which is constantly replenished by the labile pool. Think of the labile pool as a warehouse and the soluble pool as outlets. As soon as the inventory drops in the outlets, more is brought from the warehouse to fill in the void.
Soluble P, which represents the mobile form of P and at the same time the only readily available P form, is so low (average in Prairie soils 0.06 pounds P per acre in the top six inches) that in order for the soil to supply a 40 bushel per acre crop of wheat, the labile pool will have to renew the soluble pool some 775 times! This then means that we have virtually no fertilizer P losses on the Prairies through leaching. So any losses of fertilizer P normally would occur either through plant removal or erosion (water or wind).
Fertilizer P added to the soil very quickly becomes part of the “labile” P pool with some ending up in the inert “very slowly available P” pool. Farmers want to increase the proportion of fertilizer P that is utilized by a crop. They have achieved this by choosing the right products, and placing them in the right position in relation to plant seedling roots at the right time.
Products to increase P availability face huge challenges
Recently, a number of chemical or biological products have been introduced in an effort to increase soil and/or fertilizer P solubility or to “protect” P from being immobilized in the soil. All these products face two major challenges: First, they cannot possibly get around all aspects of soil chemistry that lock up P. Second, plants need most of their P in a narrow window from one to six weeks after germination. Products have to be able to provide adequate soluble P in that window.
Consider the rate with which P enters plant roots. Young seedlings do not need any external P right away — the get what they need from inside the seed. But demand rises quickly once the seed supply runs out. Approximately 10 days to two weeks after germination the rate of P entering the roots is at maximum and quickly diminishes after that. Five to six weeks after germination, P only trickles into the plant roots. This is because the skin (epidermis) of the roots becomes hard and more difficult to penetrate. It is well known that 75 per cent of plant P requirements are met in the first quarter of a crop’s life.
Now consider a “more soluble P” form, such as the one afforded by 14-14-0-4.4 and compare it to a common source, such as 10-34-0, as Geza Racz did in the late ‘80s. The more soluble form resulted in higher P concentration in the soil solution to begin with (Table 2). But if this form was to be seed-placed with a crop, by time the crop would need P (about two weeks after seeding) this “more soluble” P form would be at a disadvantage as it ends up being immobilized faster.
In the May 29, 1989 issue of Grainews, you will find an article presenting a condensed version of some research I did showing that feeding P to wheat at tillering or at early boot stages increased P levels in the plants but had no impact on grain yields. So any P enhancing product (chemical or biological) will have to bring about solubility and availability of P at the right time, otherwise it will be ineffective. That’s why some farmers have positive experiences and others negative experiences using a product such as JumpStart. Success of these products may be greater for crops such as peas whose P uptake patterns match the release of P from these products.
Weather conditions can also be a factor in performance of these products as P solubility and availability is very much weather dependent. A cool spring that contributes to a pronounced “pop-up” effect may render these products unnecessary (not ineffective) as sufficient P is released from the soil.
A new product, Avail, has made inroads in the U.S. as a fertilizer P enhancer by “fixing” calcium around the P fertilizer granule in alkaline soils. This prevents calcium from binding to P. But Avail has yet to prove effective in Western Canada because extractable calcium levels in non-calcareous and calcareous soils are overwhelmingly high. They average 6,400 and 13,000 pounds per acre, respectively. You simply cannot afford to apply enough Avail to make a difference.
In the next article, we will examine application rates for fertilizer P that will give you maximum agronomic and economic yields in Western Canadian soils.
Source: February 25, 2008 issue of Grainews
- reprinted with permission