March 26, 2003

David Elison - Agronomist M-C District

Phosphorus crop use requirement insights

With the listing in our grower guide book this year of the new CFEP generated guidelines for Phosphorus crop use requirement in sugarbeets, I thought it might lend support and broader understanding of these guide lines if some additional information regarding phosphorus were given.

Phosphorus is one of the three major plant nutrients required for all plant growth. It is an inherent part of all plant constituents because it is first and foremost a component part of the energy packets ATP (adenosintriphosphate) and ADP (adenosindiphosphate) which are the energy source form for all cellular metabolism. It has to be there for plant growth, maintenance and existence.

Phosphorus exists in the soil usually in two forms. Inorganic or mineral form and as organic or a "combined form" as it exists in soil humus; which is the decomposing parts of former plant tissues.

Plants utilize phosphorus in the orthophosphate forms of H2PO4 -1 or HPO4 -2, which are also the forms in which it exists in soil solution. The concentration of these ions in the soil solution, (the liquid faction of soil), and the maintenance of this concentration are of great importance to plant growth. This is because plants absorb phosphorus from solutions in proportion to the concentration of phosphate ions in the solution. This in turn, if other factors are not limiting, will evidence itself by increased growth; proportional to the greater amounts of phosphorus absorbed by the plant; when concentrations in soil solution are higher.

Therefore, the rate at which the phosphorus concentration in the soil solution is renewed becomes very important, because plant roots do not absorb phosphorus uniformly from the soil mass. .Movement of soil phosphorous to plant roots does not occur beyond a shell extending more than a few fractions of an inch from the root in the soil

The most active site for phosphorus uptake by the root is at the root tip. The rate is slower over the rest of the root surface. The replenishment of the solution-phase phosphorus then is taking place at a faster rate in the small areas near the root tips. These factors become very important in making management decisions about placement and timing of phosphate fertilizers.

Phosphate fertilizers are applied as mono or dicalcium phosphate which when applied to soil, makes contact with soil and soil moisture, and begins to react with various soil constituents either positively or not so positively relevant to plant needs.

Phosphorus is a negatively charged ion or molecule and even though it is negatively charged it adheres to soil particles very readily through means of a chemical bond called covalent bonding which is a sharing of its electrons with that of soil particles.

Because it adheres to soil particles and/or reacts and combines with other soil elements so readily, it does not move very far in the soil from where it is physically placed.

Even though a farmer may apply, for example, 150 lbs. of phosphate fertilizer per acre to his field in preparation for crop nutrient needs for the year, at any one time there is very little phosphorus in soil solution. Usually only about 1 % or less of that which is present in the soil profile. The H2PO4 -1 and HPO4-2 forms which plants absorb are in soil solution in very small quantities at any one time, generally only a few ppm and often less than 1 ppm.

This concentration is replenished on a continuous basis by various chemical processes which are on-going in the soil. Here again, the rate at which phosphorus moves into soil solution and at what concentration, is influenced by many factors, some which will be discussed. These factors can directly affect the health and the growth of the crop.

Some basic information about sugar beet growth pattern is useful when considering the role of phosphorus in its nutritional needs. About half of the phosphorus in plant tissues is in the top and half in the roots. This makes sense when one considers the pervasiveness of phosphorus in nearly all plant tissues and plant constituents. Its concentration however, is greatest in young seedlings and subsequent young plant tissues formed in early growth. It then starts to decrease about August until the crop is harvested. In well fertilized soils, the beet crop can take up an average of 3 lbs. of phosphate per ton of beets produced in a season. The range of uptake is somewhere between 50 - 89 lbs. / acre with the average being about 70 lbs. / acre.

These facts would suggest then, that one of the more critical growth segments for availability of phosphorus and its uptake would be early establishment and growth of the crop. It would follow then that this is also a critical time, as previously explained, for the availability of phosphorus for plant uptake in the soil. This availability then is dependent upon the concentration of phosphorus in the soil solution in the immediate proximity of the developing beet roots.

There are factors which affect the availability and concentration of phosphorus in soil solution and hence its uptake by the crop. The proximity or closeness , ( or the lack thereof), of the placement of phosphate fertilizers can affect the crops growth. Placement of fertilizer in a band close to developing seedlings should facilitate early growth needs, etc. Adequate phosphorus in the rest of the rooting profile should provide for later season foraging by the now expanded root system. Continuous top dressing of fertilizer on the surface soil with shallow incorporation in a minimum tillage program without ever deep plowing or ripping to physically place the fertilizer lower in the soil profile can cause challenges for a deep rooting crop such as sugar beets as the growth season progresses.

It takes time for fertilizers to convert from the form in which they are delivered to the soil into a plant usable form. This conversion takes time as well as heat and microbial action working on them. In short, it takes time and energy for all plant- soil relationships to occur. Cold, wet soils in a given spring make for slower conversion of phosphate fertilizers to plant usable phosphorus forms because the microbial machinery and the chemical processes in the soil are slowed under such conditions.

Soil types and constituents have a decided affect on the availability of phosphorus. Clays and composite clay soils adsorb more phosphorus than sandy soils. They also usually have a higher concentration of calcium ions (calcium carbonate) which also adsorbs more phosphorus. Heavier soils also are more prone to compaction which in turn disrupts nearly all of the above mentioned processes which aid in making phosphorus available for plant use. In addition it restricts root expansion physically which hinders a plant from expanding its base for uptake of nutrients from the soil.

Soil pH , (how acidic or alkaline a soil is), and the percent of free lime is of tremendous import to availability of phosphorus in the soil. Phosphorus is most available in soils with pH of 6 to 6.5. Above 7 the tie up with free lime (calcium carbonate), becomes great. This in turn affects its availability to the crop. As the amount of free lime increases in soil, then more phosphorus is tied up or undergoes a reaction with it to form a calcium -phosphate precipitate which makes the phosphorus unavailable for crop use. In addition, metallic ions or molecules of many trace elements such as iron, zinc, and magnesium are tied up also in this same precipitate complex of calcium-phosphate.

These phosphates as well as the other elements involved, can only become available to plant use when they are reacted with by acids from other fertilizer sources or by the acids produced through microbial action in the soil.

The amount of organic matter in a soil is a gage of its potential for conversion of fertilizers, crop residues, and other bound up elements, into plant nutrient forms. The decomposition of soil organic matter releases CO2 as a by product of microbial metabolism which in turn forms carbonic acid. Acetic acid is another by product of the same. These acids then, react with various bound or adsorbed phosphates to free them up and facilitate their conversion and movement into the soil solution where plants can then utilize them. This is a continuous process and as such acts as the pump to move phosphorus into availability. The greater the amount of organic matter present in a soil, the greater the pump. As a general statement, this is one of the reasons why we experience greater fertility in higher organic matter soils.

Plants require energy to take up phosphorus, as well as all other nutrients and moisture. When plants are small and temperatures are cold, these processes slow down. We often see a lag of growth in beets when we get extended periods of cold and wet in the spring. The color of the young crop may appear lighter than desired and is usually due to poor nitrogen uptake as well as phosphorus, etc. because the nutrient "pump" has been slowed by the cold and sometimes wet conditions.

To summarize what has been presented in this information it could be said that when conditions such as high soil pH and high free lime % exist in soils, as well as low organic matter, then higher rates of soil phosphorus fertility should be maintained to compensate for the tying up of a greater percentage of the applied phosphate fertilizers.

The relating of free lime percent to the amount of phosphorus ppm on a soil sample and the resulting recommendation for phosphate to be applied has its basis in the soil processes which have been just been discussed. The University of Idaho recommendations for phosphate as we have included them in the grower guide book for 2003, have came about through a combined effort from all parties involved in the industry to do field strip trials, assessing the response to rates and given free lime levels under field conditions and evaluate their yield and quality response. This is a very good addition to our bank of understanding of crop nutrient needs and as such should be carefully considered when assessing phosphate application needs of each field which is to be planted into sugar beets.

Not only should high free lime as stated on a soil sample be considered when making a decision on phosphate fertilization, but also a high free lime or calcium carbonate content in ones deep well irrigation water should also be considered and compensated for if particularly high amounts are present.

Banding of phosphate fertilizer may be superior if you have low soil test values or cold and wet soil conditions early in the year in conjunction with excess free lime percent and / or high calcium carbonate in the irrigation water. Banding may be adequate, particularly following a high phosphate fertilized crop such as potatoes.

Broadcasting of phosphorus may be necessary following a shallow rooted crop such as grain which has been grown under minimum till for a couple of years and soil test values for the first foot of soil show low, (i.e. low teens ppm or lower). Equally important would be to plow down fertilizer phosphorus following these conditions. Placement of needed phosphate relevant to crop rooting characteristics ( deep in the case of sugar beets) , is basic to its availability to the crop.

Compacted soils, or soils too loose or "open to dry down", in the early stages of plant growth are also a concern. These factors affect the movement of "P" into soil solution as well as the very existence of soil solution.

Work done by researchers Albert L. Sims and Larry J. Smith, University of Minnesota, Crookston, and reported in NDSU 2000 Sugarbeet Research and Extension Reports, shows some interesting data concerning phosphate fertility and its potential effects on the growth of the crop. They conducted a study to see if using starter phosphate fertilizers could reduce the need for broadcast applications of the same. Their data indicates that starter fertilizer ( 10-34-0) in conjunction with normal broadcast P (0-46-0), or larger amounts of broadcast P fertilizer alone, "may reduce a sever P deficiency early in the growing season, allowing greater accumulation of root dry matter during the latter part of the growing season. "Maximum yields occurred with the broadcast rate of 45 lbs. P2 05 / acre, which was the highest rate applied. Similar results were achieved with 3 gallons of 10-34-0 banded over the row near the seed or to the side. Their suggestion was that " P fertilizer utilization efficiency is enhanced with starter fertilizer application with or near the seed compared to broadcast P fertilizer applications because of better P placement relative to the root system. "

Work conducted just last year by U of I soil scientists Dr. Bryan Hopkins and Dr. Jason Ellsworth , addressed similar queries regarding the effectiveness of starter-band applications of phosphorus to sugarbeet in alkaline, calcareous soil. They compared ammonium polyphosphate to phosphoric acid in a band over top of the row, two inches below the row and 6 inches below the plant row. The phosphoric acid performed no better than the control which received no fertilization. The ammonium polyphosphate, either at surface or 2 inches below gave a two ton per acre yield increase. The 6 inches below surface treatment was even better. All of the deep banded treatments were ammonium polyphosphate and produced more yield whether they were used in combination with either the same or phosphoric acid surface bands, or with broadcast P. The question was posed as to why the APP bands increased yield and the phosphoric acid did not. The answer ? "The possibility is that the banded ammonium and / or a combination of banded ammonium with phosphorus are responsible for yield increase.