By Darren Hefty
Before you start thinking you can’t afford to fertilize your soybeans, I want to walk you through some simple math. Let’s use 60 bushel soybeans as an example. Your yields may be higher or lower, so use the free Ag PhD Fertilizer Removal app to see the exact nutrient use for your crop.
Your soybean crop will remove from your field the pounds of each nutrient listed in the Grain column. Let’s look at the biggest two, the P & K.
Phosphate – 44 pounds per acre or 85 pounds of MAP – at $380/ton MAP, that’s $16/Acre.
Potassium (K2O) – 72 pounds per acre or 120 pounds of potash – at $290/ton potash, that’s $17/Acre.
Just to replace crop removal rates of a 60 bushel soybean crop would cost $33/Acre, and that’s just the P & K. Is it worth $33 or three to four bushels of soybeans to replace that in the soil? Absolutely it should be if you plan to continue farming that land. However, the question you should be asking is regarding the Total column. Can my crop actually pull 58 pounds of phosphate and 132 pounds of K2O potassium out of the soil in the 2 months of reproduction? A good soybean crop, at its peak, needs far more K on a per day basis than a good corn crop. For that reason, we work with many farmers who are seeing decent corn yields, with soybean yields struggling. We often think about a 2.5:1 or 3:1 ratio on corn yield to soybean yield. In other words, 240 bushel corn ground should be able to produce 80 to 96 bushel average soybeans. So what’s the solution if bean yields are low?
We suggest keeping parts per million at least at a good level AND targeting at least a 4% to 8% base saturation K level on your soil test. Base saturation tells you the balance of the positively charged nutrients that are dispersed throughout your negatively charged soil. If potassium is only occupying 1% of all the soil’s binding sites, your root system won’t be able to find enough K when it really needs it during flowering and pod fill. On our farm, we’ve built soils up to 4% K and our yields are much better than ever, but we’re now on the way to building them up to the 6% to 8% range to help us reach our goal of 100 bushel farm average dryland South Dakota soybeans. By the way, potassium in medium to heavy soil isn’t going anywhere if you don’t use it year one. It will stay there until your crop uses it, so think of it as a long-term investment in your soil if you are working on a build program.
Also, when it comes to phosphate, regulating soil pH and other soil factors can greatly improve the availability of the phosphorus in your soil. Target a soil pH in the 6.3 to 6.8 range and work from there.
Take another look at your soybean fertility plan today. If you’re not confident that you’re feeding your soybeans properly, you can always send us a soil test or a question at radio@agphd.com or just call Ag PhD Radio (844-44-AGPHD) weekdays at 2 pm Central. Fertility has helped us nearly double our yields in recent years, and our cost per bushel and water usage per bushel has greatly decreased at the same time. I encourage you to identify your nutrient shortages this season and try boosting the fertility in those areas to learn what works best on your farm.