This is the second article in a series of blog articles that will focus on some fundamental information on UT fertilizer recommendations for corn, with a different nutrient featured in each article. Commonly recommended nutrients for use in corn production in TN include nitrogen (N), phosphorus (P), potassium (K), sulfur (S), and zinc (Zn). Each nutrient will be discussed in terms of the relevant soil test that maybe used or used in recommendation; recommended preplant, starter, sidedress, and foliar fertilizer applications; and plant/tissue analysis. Today’s focus will be on phosphorus (P).

UT phosphorus (P) fertilizer recommendations for optimal corn productivity

Today’s blog will focus on UT phosphorus recommendations for optimal corn productivity under conditions in TN. Phosphorus plays an important role in plant reproduction especially pollination and kernel setting. In adequate P can reduce stalk strength, delay crop maturity,  poor kernel set and lead to yield loss.

Soil test for phosphorus

UT Publication PB 1645, Best Management Practices for Phosphorus in the Environment provides an excellent review on phosphorus. Phosphorus fertilizer application rate should be based on soil test. In TN, P fertilizer recommendations are based on Mehlich I extraction procedure because it correlates well with the soils in Tennessee. However, a calibration for Mehlich III has been established for west TN soils is described in the UT Publication SP763, UT fertility recommendations for Tennessee row crops. Detailed information on how UT recommendation was developed is addressed in UT Publication W795, University of Tennessee Fertilizer Recommendation Development. Row crop sustainability. Extension Publication, SP763.

Preplant phosphorus fertilizer application

Phosphorus is typically applied near or at planting in corn in the spring. Source: Phosphorus fertilizer sources are equal in their ability to supply P if correctly applied, even though studies have found polyphosphate promotes more root growth than orthophosphate. The International Plant and Nutrition Institute (IPNI) developed one-page fact sheets on the various granular and liquid P fertilizer sources and gave a review on their use. Placement: For soils testing medium or higher, either banding or broadcasting is an effective method of application. However, banding phosphate has been found to be more effective than broadcasting on soils testing low in phosphorus. Rate: Application rates should be based on soil test and can be modified based on crop production history.

Starter phosphorus fertilizer application – Source, Placement, and Rate

The additive effect of most starter fertilizers on corn yield, regardless of placement method is not very consistent, and the discrepancies may partly be attributed to prevailing conditions at planting, management decisions, and tillage practices. Generally, starter fertilizer application tends to increase grain yield when the crop is planted way earlier than the recommended planting date or under poor growing conditions such as low temperature and excessive rainfall. More often, the beneficial effect of starter fertilizers on grain yield are more prominent in soils with conservation tillage practices such as no-till.

Source: Research has shown that a combination of ammonium-N and phosphate for optimum effectiveness of starter fertilizer, the (except for high testing soils). In addition, placement methods influence the choice of starter materials. Ammonium polyphosphate (10-34-0) and monoammonium phosphate (11-52-0) based materials are excellent for in-furrow or 2×2 starter fertilizers. DAP is not recommended for in-furrow or 2×2 because of possible injury to germinating seeds and root inhibition. Placement: Currently, “2×2” is the most common starter placement method, where fertilizer is placed 2 inches to the side of and 2 inches below the corn kernel at planting (Figure 1A). Banding phosphate two inches to the side and below the seed level may result in increased yields on soils testing low in phosphorus. An alternate placement method that has gained traction among corn producers is in-furrow or “pop up”, where fertilizer is applied in furrow or placed in contact with the seed (Figure 1B). Other starter placement options that can provide excellent results depending on soil types and environmental conditions include surface dribble or broadcast (Figure 1C).Figure 1. A 2×2, B. in-furrow and C. broadcast starter fertilizer placement methods

Sidedress phosphorus fertilizer application – Source, Placement, and Rate

In TN sidedress application of phosphorus for corn is not recommended.

Foliar phosphorus fertilizer applications

Foliar P fertilizers are not recommended for corn production in TN since unbiased field studies conducted in TN show poor performance of foliar products under our growing conditions.

Plant/tissue analysis for phosphorus

Plant analysis helps to monitor the P status as well as diagnose deficiency in corn before deficiency symptoms become visible. Accurate interpretation of plant analysis depends on recommended plant parts and growth states. For seedling plants (< 4 inches in height), the whole plant about 1 inch above the soil surface is recommended. For corn at early growth stage (> 4 inches in height to tasseling), the most recently matured leaf (with visible collar) from the tops of 15-20 plants is recommended. For corn plants at tasseling, the earleaf (leaf adjacent to the uppermost developing ear) of 15-20 plants is recommended.

Visual deficiency symptoms

There are two key information that will accurately help you recognize deficiency symptoms, the form and location of symptoms on the affected crop will help guide the diagnosis. Location is where the symptomology occurs on the plant and this largely depends on the mobility of the nutrient within the plant. Nutrients can be classified as mobile or immobile depending on their mobility within the plant. However, for Mobile nutrients can redistributed in the plant. So for mobile nutrients, when there is insufficient supply of nutrients, nutrients in the plant is translocated to newer and growing part of the plants so deficiency symptoms tend to show on older or lower leaves. On the other hand, immobile nutrients have limited mobility within plants and deficiency symptoms occur in younger, upper leaves. Forms associated with deficiency may take several forms including chlorosis, necrosis, and abnormal growth. Chlorosis occurs when the production of chlorophyll is reduced which results in a yellow to pale green leaf color. Necrosis occurs when the plant tissue dies, indicated by the browning of the affect plant part. Abnormal growth occurs when the inadequate amounts of a nutrient in the plant restrict cell elongation and replication. This results in stunted growth, deformation, or crinkled leaves.

Phosphorus is a mobile nutrient as such visual deficiency symptoms occur first on older leaves and the commonly associated forms are chlorosis and abnormal growth.

Plant: Stunted dark green plants.

Leaf: Appear first on older (lower) leaves. Reddish-purplish leaf tips and margins.

Ear: Incomplete kernel set.

Figure 2: Corn plants with P deficient leaves. Phosphorus deficient leaves with a purplish leaf tips with margins. Visual deficiency symptoms appear first on older (lower) leaves. Photo from Moe’s Diagnostic Centre.