Shortages of P and K can limit production of your crop

What is your P and K fertilizer program? If you’re basing your rate on what you have always done, have you ever questioned why you apply what you do? It’s pretty easy to get stuck in a rut and use the same fertility program year after year without looking at it more closely, as long as yields keep increasing.

If this sounds familiar, we should talk.

The basics of soil fertility haven’t changed much over the years, but that doesn’t mean we should take it for granted. Our plants are factories that need a certain amount of nutri­ents to operate at peak capacity, just like an automotive manufacturing plant. When something is in short supply—whether that is phospho­rus or a computer chip—both types of plants are going to throttle back production until they can get all the materials they need. How do we avoid this shutdown? In simple terms, it’s just a matter of under­standing supply and demand.

Let’s start with supply. The vast majority of nutrients are supplied to the plant from the soil solution. While the soil can store significantly more nutrients than our plants will demand in a single season, those nutrients are not all readily avail­able for uptake. Many nutrients are bound tightly with other com­pounds in the soil. They are essen­tially in long-term storage, which means that it takes a significant amount of time for them to arrive in soil solution for the plant to use. As the plant takes nutrients from the soil solution, replacement nu­trients are transferred from “storage” to restock the system. The processes that facilitate that movement are not instant.

Soil testing can help us manage and understand this process. Many diverse and complicated factors control how much of the total quantity of nutrients contained in the soil are available for plant uptake, making it nearly impossible to estimate what is readily available for a plant to use. Soil testing ap­proaches this problem in a different way. Chemicals are used to flush nutrients out of the soil sample, and then the nutrients that are re­leased are measured. The chemicals and methods are designed to closely mimic how a root would extract nutrients from the soil. By compar­ing how yield responds with the addition of readily plant available fertilizers versus the known soil test value, we can determine where the soil is providing enough nutrients for complete plant growth.

Again, a soil test is not necessarily an inventory of all the nutrients contained in the soil, it’s more like the amount of nutrients that are in soil solution ready for plant uptake. To relate it to the factory model, it tells us how many computer chips are actually sitting in the factory that we can use to build pickup trucks today.

If the soil test tells us about nutrient supply, why does demand matter? Demand tells us how much fertility is hauled away in a grain truck or a bale of hay. We use that number to understand what we need to put it back into inventory for the next season. Demand is fair­ly easy to calculate. You just need the nutrient concentration in the crop and the yield. It may surprise you to know that nutrient concen­tration is actually easy information to find. Many of the crops we grow are used in livestock feed, and much work has gone into analyzing that feed for its nutrient concen­trations to create a balanced ration. For the most part, the nutrient con­centrations we use for crop removal estimates are based on long-term averages from feed information.

Yield can come from several different sources, whether it is your long-term yield average, yield goal or actual yield data. We can use any of those numbers along with the nutrient concentration to arrive at the total nutrient removal we can expect.

Admittedly, it’s easy to ignore your fertility program and just do what you’ve always done without think­ing about it. But like most decisions on the farm, it pays to understand how fertility is going to affect your plants in the field. If they face a shortage, it doesn’t matter what else you do. Those shortages are going to limit production.

As you build your fertility plan this year, I encourage you to figure out your supply and demand to ensure plant nutrition doesn’t limit your production. Reach out to your local MFA if we can help you in this process.

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