Corn and Soybean Phosphorus

 

Corn and soybeans require a steady supply of phosphorus (P) for early seedling vigor, root development, and stalk growth. P is used by plants in the lipid layer of the cell wall, as the backbone to genes (DNA and RNA) and for energy transfer (ATP). While P is an essential element (ranked 8th in the amount needed) it is one of the big 3 nutrients (N-P-K) because it yields more bushels per acre. Recommended corn and soybean soil test are between 20-40 ppm Mehlich.

In corn, most rapid P uptake occurs after the V6 stage (six leaf collars). About 80% of absorbed P ends up in the grain. Post-tasseling P availability is critical to maximize yields. In early vegetative planting (Planting to V6), P is essential for root, shoot, and initial ear structure development. Severe P limitations early can permanently reduce yield potential. The most rapid uptake of corn P is V6 (vegetative stage) to R1 (reproductive stage). Demand spikes as biomass multiplies after V6 (4 to 6 weeks after planting). About 50% of total P is acquired by the plant post tasseling onwards. Cold wet, compacted soils severely limit P availability. During reproductive grain fill stages, P is mobilized to kernels from roots, shoots, and leaves.

Corn P deficiency symptoms are stunted plants with purple-tinted leaves on the lower stalks triggered by cold, wet soils or root compaction; limiting plant growth. Herbicide stress may also cause corn plants to turn purple. In late season, leaf margins may yellow or die on older leaves. Stalks may remain thin, and maturity can be delayed. P is highly soil immobile but highly mobile inside plants. P deficiency symptoms show up first on older lower leaves. Corn plants can move mobile P nutrients to younger developing leaves.

To manage P levels, soil test first to determine Corn P needs. Melich P soil test levels of 20-40 PPM are considered adequate with no additional P fertilizer needed. Starter fertilizers near the seed are effective for early-season uptake but may not be needed if P soil tests are high. Corn will absorb excess P beyond (25% -80% beyond) what is needed to reach maximum yield (known as luxury consumption). Harvested grain removes significant amounts of P (approx. 0.37 lbs of phosphate per bushel of yield). Phosphate fertilizer (P2O5) * .44 = lbs of actual P/acre. For 200 bushels corn, 74 lbs P2O5 or 32.6 lbs P are removed; 250 bu. corn – 93 lbs P2O5 or 41 lbs P; 300 bu. corn – 111 lbs P2O5 or 49 lbs P.

Soybeans need only 1 lb of P compared to 15 lbs of P needed for wheat production in the first 30 days of growth. However, soybeans require significant amounts of phosphorus (P) for energy transfer, root development, and pod set. At harvest, roughly 70%-80% of the plant's accumulated P ends up in the grain, leading to high removal rates. Soybean removal rates are 0.75 to 0.80 pounds of P2O5 per bushel harvested. P₂O5 *.44 = lbs of actual P. Actual P lbs * 2.29 = P₂O5 lbs.

For soybeans, vegetative growth (V-Stages) uptake of P is initially slow. The plant directs energy toward building early shoot and root systems, which rely on symbiosis with beneficial arbuscular mycorrhizal fungi (AMF) to scavenge for available P in the soil. Peak demand for soybean P occurs at the R4 (full pod) growth stage, taking up approximately 0.45 lb of P per acre daily. Nearly 50% of total P is acquired during the reproductive phases. During late reproductive stages (R5.5 to R6.5), P uptake peaks and P stored in the leaves, stems, and petioles begins to rapidly remobilize directly to developing seeds.

A soybean crop yielding 50 bu./acre removes 40 lbs of P₂O₅ in the grain (17 lbs actual P), 80 bushel removes 64 lbs P₂O₅ (28 lbs actual P). P deficiency in soybeans is not always purple. Instead, slightly stunted plants, dark green to bluish-tinted foliage, and overall reduced pod counts are seen. P Deficiency is inhibited by acidic soils or cool, wet conditions early in the season. In No-till soybeans, often soybean P fertilizer is applied the year before in one application to a Corn-Soybean crop rotation. Soybeans glean the leftover P.

Phosphorus (P) attaches to clay (particulate P); to SOM, inside microbes, plant residues (organic P), to soil ions (aluminum, iron, calcium), and to various soil minerals (ex. Apatite). Plant available forms of P are H2PO4- and HPO42- also called orthophosphate. DAP (18-46-0) is orthophosphate while MAP (11-52-0) is a poly or multi-phosphate chains. P tied up in microbes is not plant available until the cells are consumed or die and leak out P.