Lower prospects for crop prices and higher costs of crop inputs are creating concerns among many Cornbelt farmers that 2012 income potential may be less than comfortable.  The choices are creatively increasing revenue or creatively reducing expenses.  Both of those could be achieved with an effort of minding your P’s & K’s.

Phosphorus (P) and Potassium (potash or K) are necessary for good corn yields, but can also be expensive. December 8, 2011 prices quoted by USDA’s Agricultural Marketing Service averaged $710 per ton for DAP and $645 per ton for potash in Illinois.  Cornstalks are full of P and K and if cornstalks are disked or chiseled back into the soil, their nutrients are returned to benefit the next year’s crop. 

However, if corn stalks are baled for livestock bedding or harvested as a biomass feedstock for an ethanol refinery, then most of the P and K will be lost, and will have to be replaced. But Iowa State University agronomists Antonio Mallarino and Ryan Oltmans report that P & K content in cornstalks changes over time and the timing of soil tests can be quite important in either saving on fertilizer application or increasing fertility to improve corn yields.

As the corn plant grows it absorbs P & K from the soil, most of which was applied as commercial fertilizer.  Most farmers will either use soil tests to determine P & K needs for the following year, or make an educated guess about needs, based on yield of the crop and how much it may have removed.  The Iowa State agronomists suggest that timing of any soil test may only be good that day, since weather conditions will change the rate at which P & K are returned to the soil.

Phosphorus content was initially measured in the corn plant at the time the black layer was formed in the kernel, and then at periodic times later as the corn stalk degraded.  At the initial test they measured from 7 to 45 lbs of phosphate per acre, with an average of 20 lbs.  By October that had faded to an average of 15 lbs, and then slowly declined to about 11 lbs per acre in the vegetative material on the ground.  The balance had leached into the soil and would be available for nutrient uptake by the next crop. Mallarino and Oltmans reported, “The amount of P in residue at grain harvest was 69 percent of the amount at the black layer stage, and by April the amount remaining in residue had decreased to about one-half (53 percent).”

Potash content in the corn stalk, leaves, and roots was washed out quicker by rainfall than was the phosphorous.  At the time of black layer formation in the kernel the potash amount in the vegetative material ranged from 75 to 225 lbs per acre, with an average of 140 lbs.  That declined to about 100 lbs at harvest, and had faded to less than 50 lbs per acre by the following April.  The agronomists reported, “The amount of K remaining in residue at grain harvest time was 69 percent of the amount at the black layer stage (proportionally similar to the value observed for P), by late fall the amount remaining in residue was 59 percent, and by April had decreased to38 percent.”

Their bottom line—which can make a difference in the amount of P and K that is applied for the next crop—is:  “There was large variation in the P and K concentrations and amounts in cornstalks across the sites, but mainly from the black layer stage to late fall. This high variation resulted from differences in soil-test levels, fertilizer rates, hybrids, rainfall and other growth conditions.”  The agronomists add, “Nutrient loss between the black layer and harvest was large for both P and K, loss from cornstalks between harvest and late fall were large only for K, and the overall proportional loss by spring was greater for K.”  However, they are quick to add that variations can be great from year to year and the best way to get a good idea of how much P & K are being removed from the field is to sample the cornstalks that are being removed.

Summary:
Since cornstalks are considered a valuable organic fertilizer that is returned to the soil, there is little doubt that removal of the stover will reduce the available nutrients available for the next crop.  But timing of that process can have a great impact on the added expense of replacing the nutrient or saving on the amount of fertilizer that has to be applied.

Source: FarmGate blog