Edward B. Rayburn, Extension Specialist
Fertilizer recommendations returned from a soil testing laboratory are based on the analysis of a soil sample and an expected yield for the crop listed on the soil test form. Sometimes it is desirable to know the fertilizer needs of a crop if the yield is different from that used by the computer. Also the economics of fertilizer use changes as the cost of fertilizers change relative to the value of the crop. The work sheet provided with this fact sheet, outlines the calculations needed to determine the fertilization requirement of a forage crop at a specific yield level.
Before a site specific fertilizer recommendation can be made, it is necessary to know the yield which can be achieved from the field. The crop yield will depend on the soil type, the crop management, and the weather during the growing season. An estimate of a soil=s potential yield is provided in the county soil survey manual. Average yields under two management levels are published. Use the yield level for good management. Table 1 shows the yield estimate for selected soils in West Virginia. Yield levels such as these are based on experience in the area and are a reliable first estimate. However, with improved plant varieties and management it is possible to exceed these published levels by 25% or more. Where improved management is used, crop yield records for the farm give more accurate information than general area-wide yield estimates.
In today's economy relatively high yields are required to keep machinery and overhead cost at reasonable levels. Increased hay yields reduce the total cost per ton of hay produced when machinery, fertilizer, and labor charges are considered (Fig 1). The way to minimize the cost of hay production is to manage for reasonably high yields, relative to the yield potential of the soil, and reduce machinery costs by harvesting as few cuttings as needed to achieve the potential yield, without reducing forage quality below an acceptable level. The forage quality required depends on how the forage is to be used. If forage is sold on a quality basis or used for dairy cattle, having high quality is more important than if the forage is to be fed to dry beef cattle.
Figure 1. Harvest and fertilizer cost per ton of hay relative to the number of cuts per year and yeild per acre.
To determine a crop=s fertilizer requirement, we need to know the crop=s response to soil fertility and to added fertilizer. Forage crop response to soil test phosphorus (P) or potassium (K) is shown in Figure 2 and Figure 3. These response curves are based on research in Ohio and New York with the soil test values calibrated to the WVU soil test system (Sperow and Rayburn respectively, unpublished data). These response curves show forage crop yields over a range of soil tests when no fertilizer is applied. Yields are expressed as a fraction of the maximum crop yield obtained with fertilization. This is called relative yield. As can be seen, if the soil test P is greater than 50 (Fig. 2) or the soil test K is greater than 100 (Fig. 3), the crop yield is not greatly influenced by fertilization. However, these are short term experiments and do not account for removal of plant nutrients by the crop.
Table 1. Estimated yield, under good management, for selected soils in West Virginia for clover-grass and alfalfa-grass hay and grass-legume pasture.
|Soil||slope||legume grass hay||pasture|
|Albrights silt loam||8-15||3.0||3.5||200|
|Berks shaly silt loam||3-8||3.0||3.5||200|
|Berks shaly silt loam||15-25||2.0||3.0||170|
|Clarksburg channery silt loam||3-8||3.0||3.5||200|
|Clarksburg channery silt loam||8-15||3.0||3.5||200|
|Dekalb channery loam||5-12||2.8||3.2||120|
|Dekalb channery loam||12-25||2.6||3.0||105|
|Ernest silt loam||8-15||3.0||3.5||140|
|Frankstown silt loam||3-10||3.5||4.5||165|
|Frankstown silt loam||10-20||3.2||4.3||160|
|Frederick cherty silt loam||3-8||3.5||4.5||165|
|Frederick cherty silt loam||8-15||3.2||4.3||155|
|Frederick cherty silt loam||15-25||3.0||4.0||150|
|Gilpin silt loam||8-15||3.0||3.5||200|
|Gilpin silt loam||15-25||2.5||3.0||170|
|Laidig channery loam||8-15||3.0||4.0||130|
|Laidig channery loam||15-25||2.5||3.5||200|
|Monongahela silt loam||3-8||3.0||3.5||200|
|Monongahela silt loam||8-15||2.5||3.0||170|
|Teas and Litz silt loams||3-8||3.0||3.5||130|
|Teas and Litz silt loams||8-15||2.8||3.2||120|
|Teas and Litz silt loams||15-25||2.4||3.0||100|
|Westmoreland silt loam||3-10||3.0||4.0||145|
|Westmoreland silt loam||10-20||3.0||3.8||135|
|Westmoreland silt loam||20-30||2.8||3.5||125|
|Wharton silt loam||8-15||3.5||2.9||175|
Figure 2. Effect of soil test P on forage crop yeild with out supplemental fertilizer as a fraction of the yield when adequate fertilizer applied.
If a crop is grown and no fertilizer is applied the crop will remove nutrients from the soil and the soil test will go down. Over a period of time crop yields will go down. The decrease in crop yields will depend on the initial soil test, nutrients removed in the harvested crop, the soil=s reserve nutrients, the release rate of reserve nutrients, and the length of time that fertilizer is not applied. It is easy to measure the first two of these variables and control the last. First, take a good soil sample to obtain an index of the soil fertility. Second, fertilizer nutrients removed in each ton of crop dry matter can be measured by forage testing, or use average forage test values for the type of crop grown (worksheet Table 1).
Figure 3. Effect of soil test K on forage crop yeild without supplemental fertilizer as a fraction of the yield when adequate fertilizer is applied.
When looking at crop fertilizer response, it is helpful to express fertilizer rates as a multiple of the plant nutrients removed by the crop. If the soil test is medium or high and P is applied at 1 times the crop removal rate, crop yields are 90-100% of maximum (Fig. 4). If no fertilizer is applied, crop yields will drop to between 50 and 80% of the potential yield. If cutting hay three times a year, a savings in fertilizer which reduces yields by half will increase the cost per ton of hay by 40% (Fig. 1, 3 cut system, 4 ton/a cost $42, 2 ton/a cost $59). On the other hand, applying more than the removal rate results in little yield increase. Soil test levels would increase but would return no value unless fertilizer inputs are reduced in the future to draw on this reserve fertility.
For potassium the effect is almost the same (Fig. 5). When the soil test is medium or high an application rate of 1 times removal, maintains crop productivity at maximum level. Even at a K fertilizer rate of 75% the removal rate, yield is near maximum. However, soil fertility will decrease. Again with no added K, the crop yield drops to 30 to 90% of potential over a 3 to 7 year period. The drop in yield without K fertilizer is determined by soil type, crop yield, and forage species.
Figure 4. Forage crop response to fertilizer phosphorus (P) on soils having a medium or high P soil test.
Figure 5. Forage crop response to fertilizer potassium (K) on soils having a medium or high K soil test.
Figure 6. Forage crop reponse to fertilizer phosphorus (P) on soils having a low P soil test.
When soil test levels are low, crop yields will be greatly reduced if fertilizers are not used. For soils testing low in P and K, when no fertilizer is applied crop yields drop to 10 to 40% of potential for P (Fig. 6) and to 30 to70% of potential for K (Fig. 7). For low testing soils, the cost to get near potential yield increases since the fertilizer rate needed is 1.5 to 4.0 times the removal rate rather than the 1 times removal rate needed on medium and high testing soils. On low testing soils, fertilizing at the removal rate gives a yield between 40 to 100% for P and between 80 to 100% for K. The actual yield response depends on the soil=s ability to fix these minerals and how low the test is at the start.
Figure 7. Forage crop response to fertilizer potassium (K) on soils having low K soil test.
Use the attached worksheet to determine the fertilizer rates needed on your farm based on your soil=s expected yield, soil test, and management goals. Identify the farm, field, crop, and soil type at the top of the worksheet.
Legumes should be used with grasses on pastures and hayfields to provide economical nitrogen (N) fixation and improved forage quality. Where N is used in place of legumes to grow grass hay or pasture, apply 150-180 pound/acre of N from chemical fertilizer or available N from poultry litter or livestock manure. If using chemical fertilizer, N should be split applied at 50-60 pounds per acre, 3 times during the growing season.
The lime requirement from the WVU soil test report is for raising the soil pH to 6.6. For grass and clover a pH of 6.0 to 6.2 is adequate. Figure 8 provides the adjustment factor for the required liming rate when a pH less than 6.6 is desired. Look up on the lower axis the soil test pH reported on the WVU soil test report (example 5.6). Move up to the line representing the soil pH you desire (example 6.2). Go over to the left axis and read the liming rate adjustment factor (example 0.6). Multiply this factor by the WVU soil test report lime requirement to obtain the lime requirement which should raise the soil pH to 6.2. If the soil test called for a lime requirement of 4 tons per acre, multiply 4 x 0.6 and the rate would be 2.4 tons per acre.
Figure 8. Adjustment to a lime requirement to raise te soil pH to 6.6, based on current soil pH and desired adjusted soil pH.
Yield and Management Goal Fertilizer Requirements
Perennial Forage Crops
Farm __________________________________ Field_________________________
Crop _________________________________ Soil Series_____________________
Calculated fertilizer requirement.
|1. Soil test level from soil test report.
(L, M, H, VH)
|2. Expected yield (tons/acre)||_____________||_____________|
|3. Nutrient removal rate/ton
(from Table 1)
|4. Nutrient removal rate/acre
(multiply line 2 x line 3)
|5. Application rate factor
(from Table 2)
|6. Fertilizer rate pounds/acre
(multiply line 4 x line 5 )
|Table 1. Nutrient removal rate in pounds per ton of material harvested at normal moisture or dry matter content. The nitrogen should be provided by the legume in perennial grass legume forage stands.|
|Crop||Moisture %||Dry Matter %||P2O5||K2O|
|Table 2. Application Rate Factor. Multiply this number by the nutrient removal rate to calculate the fertilizer rate per acre. Two rate factors are given. To maintain a medium soil test or minimize cost use the smaller value. To maintain a high soil test use the higher value.|
|Legume-grass Hay||Avg. Pasture||Well Managed Pasture|
Programs and activities offered by the West Virginia University Extension Service are available to all persons without regard to race, color, sex, disability, religion, age or national origin. Issued in furtherance of Cooperative Extension Work, Acts of Congress of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Robert Maxwell, Interim Director, Cooperative Extension Service, West Virginia University.
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