Variable-Rate P: Economic Analysis Tool

Variable-rate P and K increase costs to producers, both in soil sampling and in application. Where is this cost recouped? It has to be either by increasing crop yield, or by reducing P and K use enough to more than cover the cost of using this type of management.

A statewide network of on-farm small-plot experiments showed that under prevailing P and K management conditions, response to the current year’s P and K applications is near zero. This suggests that producers are managing P and K in such a way that the soil can supply all that the crop needs, and that it is not possible to increase yield by managing P and K differently.

Although variable-rate P and K management may sometimes reduce the total amount of P and K applied, I think there is potential to enhance these savings using the variable-rate approach.

In the network of experiments mentioned above, there was evidence of a small yield response to P in corn and soybean when soil test levels were below 30 lb P/acre. So why is the target level for soil test P set at 45 lb P/acre by the University of Missouri? I don’t know the history, but it makes sense to me that there should be a ‘cushion’ between the known ‘critical value’ and the recommended ‘target value’. This is because of the spatial variability of P in the field. Click here for an example.

Having a ‘cushion’ built into the target value for soil test P ensures that even the lowest-testing parts of the field are at full yield potential. Variable-rate management may offer a cheaper way to ensure that the whole field is at full yield potential.

By taking spatially intensive soil samples, the low-testing areas of the field can be located and fertilized to achieve their full yield potential. The ‘cushion’ is no longer needed. This means that a lower target level for soil test P can be used, and savings on fertilizer will be increased to where they should easily exceed the cost of variable-rate management.

Ray Massey and Peter Scharf have developed an economic analysis tool for variable-rate P and K (pdf capability and Flash Player required). The user can change the cost of P, the cost of K, and P and K target levels for both uniform and variable-rate management. The program will then calculate the 10-year Net Present Value (amount of money made/lost over 10 years including the interest cost of money spent) for any of five fields that were grid sampled with no previous variable-rate management. We suggest that soil test thresholds of 45 lb P/acre and 280 lb K/acre will produce full yields with uniform management, and allow the flexibility to miss a year’s application without losing yield. With variable-rate application, thresholds of 30 lb P/acre and 200 lb K/acre will also produce full yields and allow the flexibility to miss a year’s application without losing yield. If the producer is committed to applying P and K every year, these numbers can be reduced farther (but we don’t know how far).

P and K prices are lower now than when we developed this tool in January 2009. This means that the potential to save money with variable-rate P and K is now less than it was then.

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