Fall Fertilizer

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By Bryce Roberts

  As you’re probably aware, the price for agricultural inputs has dramatically increased and fertilizer is no exception. In fact, the price of fertilizer is so high cattle producers are asking if it’s 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 three years, 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’re producing. If it’s alfalfa and the pH is below 6.0, you could apply lime. A grass-legume mixture probably can tolerate soil pH down to about 5.8 and a pure grass system probably can go down to pH 5.5 before yields are affected. 

Similar statements could also be made for phosphorus (P) and potassium (K) nutrition, with alfalfa requiring the most and pure grass, fescue, requiring the least.

The University of Kentucky College of Agriculture recommends P applications starting when the soil test P level drops below 60 pounds per acre and K when soil test K drops below 300 pounds per acre. 

If soil test levels are above 60 pounds per acre and or 300 pounds of K per acre, 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 and assume some risk that P and K might reduce yield, you might allow soil test levels to decline further. From small plot research, we know that once soil test P drops below 30 pounds per acre and/or soil test K drops below 200 pounds per acre, a yield response to added fertilizer is likely, therefore; these would be the minimum tolerable levels.

Now is the perfect time to sample your soils.  Sampling in the fall offers the advantages of good weather, allows time to plan for coming crops and gives lime, if needed, time to react with the soil prior to spring planting. 

However, it is not without some disadvantages. Seasonal fluctuations mean the pH and potassium levels are at their lowest during this time. Rainfall and crop nutrient uptake are factors in these fluctuations.

Understanding these seasonal fluctuations can aid in understanding and interpreting soil test results that vary from year to year or within the same year.

The pH levels in soils generally are lower during the summer and early fall. The reduction generally is attributed to soil drying, root and bacteria activity and nitrification of nitrogen fertilizers. The process is reversed as soil moisture increases.

Soil tests for potassium often have the lowest levels in the fall for grain crops. 

The difference is most profound if samples are taken immediately after harvest in a dry fall. As potassium from crop residue is washed back into the soil, levels will increase. 

Soil samples can be submitted to your Spencer County Cooperative Extension office for testing. For residents of Spencer County, there is absolutely no cost to you to have your soil tested.  

Thanks to the Spencer County Soil Conservation District, you are not charged the normal $5.25 per sample fee.  It takes approximately 2-3 weeks to get the results back to you.

Feel free to contact me at your Spencer County Cooperative Extension Service at 477-2217 or you can email me at broberts@uky.edu.  

You can visit the Spencer County Extension Services’ website at www.spencerextension.com.

Educational programs of the Kentucky Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability, or national origin.