{"id":1508,"date":"2017-12-20T18:56:39","date_gmt":"2017-12-20T18:56:39","guid":{"rendered":"http:\/\/taurus.ag\/?p=1508"},"modified":"2018-03-21T20:17:57","modified_gmt":"2018-03-21T20:17:57","slug":"part-2-understanding-crop-available-phosphorus","status":"publish","type":"post","link":"https:\/\/taurus.ag\/part-2-understanding-crop-available-phosphorus\/","title":{"rendered":"AGRICULTURAL NUTRIENT PROFILE: PHOSPHORUS-PART 2"},"content":{"rendered":"

Understanding crop available phosphorus<\/h3>\n

The behavior of Phosphorus (P) in the soil has long puzzled scientists. Curiously, Phosphorus is not particularly mobile. In western Canadian soils, there is generally an extremely low probability of leaching.[1]<\/a> This poses a challenge, since this key nutrient must be absorbed through the plant\u2019s root system.<\/p>\n

Before we get into the conundrum around the behavior of Phosphorus, it is important to realize that Phosphorus exists in three states within the soil: as Soluble P <\/strong>(plant available), Slowly Soluble P<\/strong> and Insoluble P<\/strong>. As we\u2019ll see, as the soluble P is consumed, it is replenished from reserves of slowly soluble P.<\/p>\n

Why isn\u2019t phosphorus mobile in the soil?<\/h3>\n

Plants absorb ionic forms of Phosphorus that are made available in the soil solution. This includes H2<\/sub>PO4<\/sub>–<\/sup><\/strong> (a readily available form) and HPO4<\/sub>=<\/sup><\/strong>. These forms of Phosphorus are anions (which are negatively charged) and therefore are not attracted and retained by the negatively charged soil.<\/p>\n

So, by all logic, Phosphorus anions should <\/em>be mobile in the soil \u2013 as is the case with Nitrogen anions. In fact, the opposite is true.<\/p>\n

Researchers believe this is due to the fact there are extremely low levels of available phosphorus <\/strong>in the soil solution at a particular time \u2013 typically less than 0.00005 grams\/L. According to estimates, normal crop growth requires the phosphorus in the soil solution to be replaced twice per day <\/strong>(on an average). [2]<\/a> Meeting that need comes down to the ability to replenish the soil solution to keep pace with the plant\u2019s absorption of Phosphorus through its roots.<\/p>\n

This is achieved through an \u2018equilibrium reaction\u2019. Essentially, whatever Phosphorus is removed from the soil solution by the plant will be replaced by new P from the available phosphorus reservoir in the soil.<\/p>\n

Slowly Soluble\/Insoluble P\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Soluble\/Plant Available P<\/u><\/strong><\/p>\n

– P minerals & compounds\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 – Phosphorus in ionic form<\/p>\n

– Organic P<\/p>\n

RELATIVELY UNAVAILABLE \u2192\u2192\u2192\u2192\u2192\u2192\u2192<\/em><\/p>\n

\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u2190\u2190\u2190\u2190\u2190\u2190\u2190<\/em>SOIL SOLUTION P<\/em><\/p>\n

Factors affecting crop available phosphorus.<\/h3>\n

Mobility isn\u2019t the only challenge when it comes to getting available phosphorus to the plant. There are several factors at play.[3]<\/a><\/p>\n

Aeration:<\/strong> Oxygen is required for mineralization and the breakdown of organic matter, which is necessary for naturally occurring P to become plant available. Plants also require oxygen to absorb phosphorus and other nutrients.<\/p>\n

Compaction:<\/strong> This reduces aeration in the root zone, which affects absorption of P and other nutrients.<\/p>\n

Moisture: <\/strong>Phosphorus ions are carried in the soil solution. Water is necessary to increase availability of P in the soil (including P supplied through fertilizer).<\/p>\n

Soil Particle Size:<\/strong> Phosphorus becomes tied up in clays. It is somewhat more mobile in sandy soils.<\/p>\n

Temperature:<\/strong> In many types of soil, higher temperatures facilitate the breakdown of organic matter. Extreme high or low temperatures can reduce the plant\u2019s ability to absorb Phosphorus.<\/p>\n

Soil pH:<\/strong> The ideal pH range for maximum availability is 6.0 to 7.0, which is optimal for H2<\/sub>PO4<\/sub>–<\/sup> ions. In highly acidic or basic soils, chemical reactions can cause the phosphorus to become tied up in an insoluble phosphate.<\/p>\n

Other Nutrients:<\/strong> Other nutrients promote healthy root development, which aids in the absorption of P. Ammonium plays a role in P uptake.<\/p>\n

Arbuscular mycorrhizal fungi (AMF): AMF<\/strong> is a microbe that forms a symbiotic relationship with plants. Its hyphae extend the reach of the roots, allowing the plant to access Phosphorus that would otherwise sit beyond the reach of the root system. (Click here<\/a> to learn more about this fascinating and highly beneficial relationship).<\/p>\n

Micronutrient Deficiencies:<\/strong> Several micronutrients are necessary for plants to respond to phosphorus fertilizers.<\/p>\n

Placement:<\/strong> Because Phosphorus has limited mobility, fertilizers must be applied as near to the seed as possible \u2013 but far enough away to prevent injury. Banding applications are considered to be the most effective.<\/p>\n

Next: Building Phosphorus in the soil.<\/h3>\n

Achieving and maintaining adequate levels of plant-available phosphorus is vital to the success of all crops. There are several strategies for maximizing crop available phosphorus, which we will explore in Part 3 of this series.<\/p>\n

Want to find out more about improving soil quality?\u00a0<\/em><\/strong><\/p>\n

\"Learn<\/a><\/span>