To Lime or Not To Lime

Nearly 20% of the samples received every year by the Soil Testing and Research Lab get a lime recommendation. Of all the crops we grow, soybean is probably the crop most affected by low soil pH as the activity of the nitrogen fixing bacteria slows down considerably and with it, the ability to form sufficient nodules to produce good yields.



The graphs show the cumulative yield response of soybean to Ag and pelletized lime. The objectives of this test were 1) to assess the yield response of soybean growing in soil with pH around 5.0, and 2) to compare the relative performance of Ag lime and pelletized lime.


Ag lime was applied at incremental rates equivalent to 700 lb/acre, while pelletized lime was applied at incremental rates equivalent to 500 lb/acre. Current lime recommendation for soybean in a silt loam is between 4,000 and 5,000 lb/acre Ag lime. At that application rate, a cumulative yield increase of more than 20 bu/acre was obtained. Pelletized lime at 500 lb/acre resulted in basically no yield gain after 2 years.


A significant yield increase was observed when pelletized lime was used, but at rates significantly higher than the 300 lb/acre commonly recommended by some fertilizer dealers. Results from this study clearly indicate that the best option to correct soil acidity is to use Ag lime.


A minimum of 1000 lb pelletized lime per acre were needed to obtain a significant yield increase, but such increase was only half of the yield gain observed when the full rate of Ag lime was applied. Lime tends to work best when applied preplant, at least 30 days before planting, and if it is incorporated.


Provided by:
Leo Espinoza, Associate Professor & Extension Soil Scientist
University of Arkansas
Division of Agriculture
lespinoza@uaex.edu - (501) 671-2168

Use of Poultry Litter on Soybean

Nathan Slaton, professor and director of soil testing with the University of Arkansas Division of Agriculture, talks about using poultry litter as a fertilizer option on soybean - taking advantage of the litter's phosphorus and potassium components.

What is the availability of P and K in poultry litter?
The availability of P and K in litter is generally believed to be similar to that in inorganic fertilizers like muriate of potash (0-0-60) and triple superphosphate (0-46-0). Some state recommendations suggest that only 60 to 90% of the P is available during the first year after application. Arkansas research with rice and soybean suggests that plants fertilized with equal amounts of P and/or K from poultry litter and commercial fertilizers take up similar amounts of P and K. Thus, we would recommend that the P and K in poultry litter be considered as 100% available during the first year. Because the nutrient content of poultry litter is not guaranteed, a representative sample should be collected and analyzed to determine how much P and K it contains per moist ton.

Will the nitrogen in poultry litter inhibit nitrogen fixation by soybean?
University of Arkansas scientists (and scientists at other universities) have researched soybean response to manure application and found that by the end of the season plants receiving manure had similar nitrogen uptake as plants that received no manure. Nodule formation may be inhibited or N fixation by the soybean nodules may be delayed when the soil contains moderate to high amounts of inorganic nitrogen from residual fertilizer or manure. However, so long as the proper bacteria or inoculum is present, nodules can still form and begin actively fixing atmospheric nitrogen when plant uptake or N loss pathways removes the extra inorganic nitrogen from the soil. In all the trials we have conducted in Arkansas, poultry litter has never reduced soybean yield, but, in several instances, has increased yields more than that of equivalent rates of commercial fertilizer.

I applied poultry litter in the fall and soil sampled several weeks afterward and my soil test results recommended that more P and K should be applied. What happened to the P and K from the poultry litter and do I need to apply more fertilizer?
This has been a common question for the last four or five years. If the nutrient content of the litter was known and the rate of litter application was uniform and sufficient for the soil, you must trust that the P and K are still in the soil and will be available to the crop. One must remember that the numbers on your soil test report represent neither the total amount of P and K nor the total amount of plant available P and K in the soil. The nutrient values on your soil test report are simply an index of availability with availability increasing as the soil test value (index) increases. Arkansas research shows that less than 50% of the elemental K fertilizer applied in the fall or winter, 3 to 5 months before soil sampling, shows up on your soil test report. Only about 20% of the applied elemental P was reflected on the soil test report 3 to 5 months after fertilization. There are multiple reasons for this. First, P and K fertilizer units are expressed as the oxide form (K2O and P2O5) rather than the elemental form (as expressed on your soil test report). For example, 100 pounds of triple superphosphate (0-46-0) contains 46 lb of P2O5, but only 20 pounds of elemental P. The depth of the soil sample would also influence how much of the applied P and K fertilizer could possibly increase soil test values. Finally, chemical and biological reactions in the soil will begin immediately after fertilizer application, transforming the solubility and availability of P and K in the soil.

Provided by:
Nathan Slaton, Professor & Director of Soil Testing
University of Arkansas
Division of Agriculture
nslaton@uark.edu - (479) 575-3910