Stratification is a measurable by product of fertilizer placement, residue management and tillage. Crop production is not affected under normal conditions due to the high percentage of the root system which is found in the upper 4 inches of the soil system. There can be fertilizer efficiency advantages to placement of fertilizer under the soil surface. Band place fertilizer is less subject to fixation especially under low soil test conditions for both P and K. Deep banding of P has not shown yield increase but K deep banding has resulted in yield increases in corn and soybeans.
Higher concentrations of phosphorus at the soil surface can result in elevated P levels in water leaving field sites. The surface placement of P results in increased surface concentrations. Placement of added P nutrient below the soil surface by incorporation or use of banded applications has shown improvements in water quality. If tillage is planned apply nutrients prior to tillage. Under no-tillage conditions using low disturbance banded application or planter placed may be beneficial from both a yield and water quality standpoint. Increases in tillage and plowing over current practices has the potential to result in increased risk of erosion and sediment losses which can be counterproductive to water quality thus further evaluation of cost benefits need to be measured.
Farmers may want to identify a few select fields to do a stratified soil sample see what the surface compared to deep accounting of nutrients are under their management system.
Where phosphorus fertilizer is placed in relationship to the soil surface can have consequences for both crop production and water quality. Nutrient stratification is a term used to describe nutrient concentrations changes seen at different soil profile depth increments. For non-mobile nutrients like phosphorus (P) and potassium (K), or soil characteristics like pH, soil test levels found on the surface (0-2 inch) can be different than deeper (8-12 inch) in the profile. Soil samples from the Sandusky watershed showed P levels of 59 ppm at 0-2 inch depth and 35ppm for the 2-8 inch depth. (Baker, 2017)
Reduced tillage systems, surface applications of nutrients, and nutrient released from decomposing residue increase the concentrations of P and K on the soil surface. Subsurface placement nutrients can reduce the tempory elevation of soluable nutrient on the soil surface where it is exposed to runoff or preferential flow through the soil profile elevating P losses. Subsurface placement exposes the soluable nutrient to more soil surface area stabilizing nutrient in the soil profile. Other nutrient factors are influenced by soil profile depth. For example soil pH is influenced by soil formation and subsoil materials with western Ohio soils formed on a more basic (higher pH) limestone base while eastern Ohio typically has more acidic (lower pH) shale underlying the surface. Soil pH affects influence nutrient availability. stratification pose for nutrient placement in crop production and water quality?
Options for avoiding surface application include fertilizer applications just prior to planned tillage. In a no-till system banded placed nutrient either in the row as starter or low disturbace banded applicators or strip tillage.
Crop Production Consequences
From a general crop production standpoint plant root systems are able to obtain nutrients throughout the soil profile to about 12”. Plants generally have a greater percentage of their root systems located near the soil surface. In field studies which measured root system percentage at different soil profile depths, corn was found to have 50% of the root system in the 0-2 inch depth and 28% in the 2-4 inch depth. (Fernandez, 2012).
Though plant roots are flexible in obtaining nutrients even from shallow depths other advantages may occur with subsurface placement.
- Under dry weather conditions plant roots in shallow soil depth may dry out resulting in temporary nutrient deficiency symptoms. Under dry conditions, temporary potassium deficiencies may be observed. There have also been observations of potassium deficiencies where soil test indicate adequate potassium. One useful piece of diagnostics might be an incremental soil test to determine nutrient status at different soil depths.
- Banded fertilizer has a benefit with low soil test conditions. At less than critical soil test levels banded fertilizer applications have a benefit of reducing the fertilizer expose to soil fixation. Fertilizer in the band maintains increased solubility due to reductions in fixation. For example phosphorus availability was measured at 3.4 to 17.1 times the soil background level in the fertilizer band. (Stecker, 2001)
- Fertilizer rates may be reduced with banding. Banded fertilizer rates may be reduced by 25-50%. The goal in fertilizer addition is to maintain soluble soil nutrient for plant uptake. Banded applications have the characteristic described above (Stecker, 2001) of less fixation of nutrient thus increased availability even 18 months after application.
- Fertilizer deep placement >4 inches. Deep banded placement is associated with strip tillage. These systems place nutrients 4-6 inch deep. Two studies in Iowa and Illinois provide yield results for deep placed P & K studies. Corn yields increased 5 bushel per acre in the Iowa studies (Mallarino, 2000). Corn increased 7% (Fernandez, 2012) and soybean yields increased 5 bu per acre (Farmaha, 2011) in Illinois. The effect was attributed to potassium. The Illinois soybean study also noted a concern of salting injury under certain conditions. Generally no yield effects were measured from added P.
Note there are limits to nitrogen and potassium total rates depending on how close the fertilizer is placed to the seed due to the potential of injury from fertilizer salt. For corn popup (placed in the row), the limit of N+K2O is based on soil cation exchange capacity (CEC). Soils with a CEC of 7meq/100g or less should limit pop-up fertilizer to 5 pounds of N+K2O , and soils with a CEC of 8meq/1000g or greater should apply no more than 8 pounds N + K2O. In a 2 by 2 placement do not exceed 100 pounds total N+K2O for corn or 70 pounds N+K2O for soybeans.
Water Quality Consequences
The higher concentration of phosphorus near the surface does have consequences for water quality. Higher concentrations of P measured in soil test results in higher levels of soluble nutrients. Water moving across the soil surface to surface drains or directly to ditches or stream will have a higher concentration of P. Preferential flow from macropores is the result of “biological activity (eg., root channels, worm holes, etc), geological forces (eg., subsurface erosion, desiccation and synaerisis cracks and fractures) or agrotechnical practices (e.g., plowing, bores, and wells). Surface cracks and channels that bypass the root zone are also responsible for rapid transport of moisture and chemicals through the unsaturated zone.” (from Cornell University) Preferential flow results in rapid movement of surface water through the soil profile to the tile system. Surface application of phosphorus only increases the surface P concentration from stratification, if present, and can result in increased P losses.
Observation in Edge of Field monitoring studies in Ohio have noted a 4 times greater nutrient loss from surface applied verses incorporated nutrients with a Mono Ammonium Phosphate (MAP) application of 175 pounds. Results are shown in Figure 2. The loss amounts to 1% of the applied P. A rainfall simulator study (Smith, 2016) indicated that loss as high as 17% of the application rate might be possible.
Cost which need to be considered include capital cost of new equipment and logistics cost of fertilizer application. Newer banded equipment is design to travel at higher speeds (up to 10 MPH) but swath width and other logistics may produce a higher per acre cost of application.
The potential cost tradeoff is that if banded fertilizer applications are used, application rates may be reduced up to 50% over surface applications rates due to the reduced fixation that occurs in the band that extends even 18 months after application. Long term strategies of soil sampling may need to be adjusted if banded fertilizer is consistently placed in the same row space. Inter row and between row spaces may need to be sampled separately. If more random banded application occurs this will be less of a concern. Regardless the number of cores collected in soil sampling should be increased to 20-25 cores versus 10-15 if no banding has occurred.
If starter fertilizer is used limits to nitrogen and potassium total rates need to be observed depending on how close the fertilizer is placed to the seed due to the potential of fertilizer salt injury. For corn popup (placed in the row), the limit of N+K2O is based on soil cation exchange capacity (CEC). Soils with a CEC of 7meq/100g or less should limit pop-up fertilizer to 5 pounds of N+K2O , and soils with a CEC of 8meq/1000g or greater should apply no more than 8 pounds N + K2O. In a 2 by 2 placement do not exceed 100 pounds total N+K2O for corn or 70 pounds N+K2O for soybeans.
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References used for this article:
Baker, DB, Johnson, LT, Confesor, RB and Crumrine, JP. Vertical Stratification of Soil Phosphorus as a Concern for Dissolved Phosphorus Runoff in the Lake Erie Basin. 2017. J. Environ. Qual. 46, doi:10.2134/jeq2016.09.03377.
Cornell University. Different types of preferential flow. http://soilandwater.bee.cornell.edu/research/pfweb/educators/intro/macro... [Verified 13-Nov 17].
Farmaha, BS, Fernández, FG and Nafziger, ED. No-Till and Strip-Till Soybean Production with Surface and Subsurface Phosphorus and Potassium Fertilization. 2011. Agronomy Journal 103: 6: 1862-1869.
Fernández, FG and White, C. No-Till and Strip-Till Corn Production with Broadcast and Subsurface-Band Phosphorus and Potassium. 2012. Agronomy Journal 104: 4: 996-1005.
Mallarino, A, Corn and Soybean Response to Deep Banded Phosphorus and Potassium. 2000. https://crops.extension.iastate.edu/corn-and-soybean-responses-deep-band... [verified 13-Nov 17].
Smith, DR, Harmel, RD, Williams, M, Haney, R and King, KW. Managing Acute Phosphorus Loss with Fertilizer Source and Placement: Proof of Concept. 2016. Agricultural & Environmental Letters 2016 1: 1:, doi:10.2134/ael2015.12.0015.
Stecker, JA, Brown, JR and Kitchen, NR. Residual Phosphorus Distribution and Sorption in Starter Fertilizer Bands Applied in No-Till Culture. 2001. Soil Science Society of America Journal 65: 4: 1173-1183.