Nitrogen (N) is directly related to yield. Have you ever heard this statement from your fertilizer dealer? Without any other limiting factors, as you increase the nitrogen rate you increase yields of non-legume crops.
Yes, eventually a point is reached where yields flatten out, but usually this nitrogen rate is so high that it is not economical to apply. Phosphorus (P) and potassium (K) are vitally important, but their role is primarily maintenance. A good analogy would be your truck. Let's say that nitrogen is gasoline, phosphorus is the battery and potassium is oil. Every time you pull into the station to fill your tank up with gas you don't change the battery and the oil. The battery and oil are important in the operation of the truck, but it is the gasoline that makes it run. In the plant, the nitrogen makes it grow. The ratio of N, P and K removed from the field in a ton of bermudagrass is approximately 4:1:2. You can see that more nitrogen is needed to produce a ton of forage than phosphorus and potassium.
If the above statement is true, then why are straight blends, such as 17-17-17, so popular? There are no crops in the region that use nitrogen, phosphorus and potassium in equal proportions. Have you ever taken the time to calculate the cost per pound of nutrient in a ton of fertilizer? If soil test results indicate that you do not need phosphorus or potassium, why spend money on them with a straight blend when you could reallocate those dollars towards nitrogen and produce a greater return for your fertilizer dollar?
Let's use the prices per pound of nutrient in the table and look at a cost comparison in producing three tons of bermudagrass using a straight blend and a blend prepared from a soil test. First, we know that that the soil's natural fertility can yield one ton of forage, therefore, we only need to produce two tons of forage with fertilization.
Second, we know that one pound of nitrogen will produce approximately 30 pounds of forage (67 lbs. N/ton bermudagrass). This means that we need to apply 133 pounds of actual nitrogen per acre to produce the extra 4,000 pounds forage to obtain the desired yield goal. Third, we will assume the soil test results called for the same rate of nitrogen, 30 pounds of phosphorus and 40 pounds of potassium.
Fourth, we will assume 100 acres of bermudagrass pasture. Last, we need to add the cost of pulling the sample and the lab charges to the blend developed from the soil test. We will assume $0.40 per acre to pull the sample and $24.00 for the cost of analysis (two samples). The total cost for soil testing is $64.00. All nitrogen will be priced as 34-0-0 to keep nitrogen cost at a constant in the comparison.
Since we need 133 pounds of nitrogen per acre to produce two tons of forage, it will take 782 pounds of 17-17-17 per acre. At $190.00 per ton, it will cost $74.10 per acre or $7,410 for 100 acres.
Using the blend prepared from the soil test, we will need a 133-30-40 per acre to meet our desired yield goal. It will take 391 pounds of 34-0-0, 65 pounds of 18-46-0 and 67 pounds of 0-0-60 per acre. The cost per acre will be $44.36 or $4,436 (soil test charges included) for the entire pasture.
You can easily see the savings of $29.74 per acre or $2,974 for the pasture. It costs $37.05 to produce a ton of bermudagrass using 17-17-17. The cost using 133-30-40 is only $22.12 per ton. Why spend money on phosphorus and potassium (maintenance) that may not be needed? The soil may be able to store some excess phosphorus and potassium, but the investment and returns should also be considered.
It is important to note that the normal application rate of products such as 17-17-17 is 150 to 200 pounds per acre. You can use our calculations to estimate yields. Applying 200 pounds of 17-17-17 provides 34 pounds of nitrogen per acre and 34 x 30 (lbs. of forage/lb. N) equals 1,020 pounds of bermudagrass production. The total production would only be 3,020 lbs. per acre (1,020 + 2,000lbs. natural production). This may explain why some forage producers are disappointed with forage yields.
This comparison clearly illustrates the importance of soil testing and applying only the nutrients that are needed to reach your desired yield goal. Being smart with your fertilizer dollar can greatly reduce your overall cost of production.