As you are probably well aware the price for agricultural inputs has increased dramatically. Fertilizer is no exception. The price of fertilizer is so high that several cattle producers have asked me if it is economical to apply fertilizer to pasture and hay ground this fall. The answer to that question is difficult because it depends on your specific situation. When considering the answer for your operation the first thing you should do is examine your soil test levels. If you have not taken soil samples within the past 3 years then you would be wise to collect new ones before making a decision. From the soil test results, determine what, if anything, is most limiting. In terms of soil pH, the minimum value depends on the type of forage you are producing. If it is alfalfa then I would recommend lime if the pH is below 6.0, a grass legume mixture can probably tolerate soil pH down to about 5.8, and a pure grass system can probably go down to pH 5.5 before yields are affected by pH. Similar statements could also be said for phosphorus (P) and potassium (K) nutrition - with alfalfa requiring the most and pure grass (fescue) requiring the least.
The University of Kentucky recommends P applications starting when the soil test P level drops below 60 lbs/acre and K when soil test K drops below 300 lbs/acre. If soil test levels are above 60 lbs P/acre and/or 300 lbs K/are, then the likelihood of a yield response to additional P and/or K fertilizer is extremely low, but if you want to be sure that P and K are not limiting, apply fertilizers as recommended. If you are conservative (assume some risk that P and K might reduce yield), then you might allow soil test levels to decline further. From small plot research, we know that once soil test P drops below 30 lbs/acre and/or soil test K drops below 200 lbs/acre, a yield response to added fertilizer is likely, therefore, these would be the minimum tolerable levels. In the range between 30 - 60 lbs/acre for P and 200 - 300 for K we make fertilizer recommendations mainly because of soil
variability. A soil sample represents the average value, but there are areas of the field below average and areas above average. The fertilizer recommendation in this range is designed to supply nutrient to those less fertile. As fertilizer prices continue to increase, grid soil sampling may also be beneficial to further refine whole-field sampling techniques.
For nitrogen the answer is easier. Research has shown that each lb of nitrogen applied in early to mid-August increase cool-season grass production by 20-25 lbs dry matter per acre. Therefore, if one pound of nitrogen cost less than the cost of 25 lbs hay on a dry matter basis then N should be applied. Of course this assumes adequate growing conditions this fall.
The source of nitrogen used in the fall is also important. Ammonium nitrate is most efficient fertilizer to use when stockpiling because it is not subject to volatilization. However ammonium nitrate is becoming harder to get and can be cost prohibitive. Research has shown that urea is approximately 79 to 89% as effective as ammonium nitrate. However, urea can be just as effective as ammonium nitrate, if a urease inhibitor like 'Agrotain' is used.