Fall-Applied Phosphorus: Cost-Effective and Convenient
What to know about applying phosphorus in the off season
Phosphorus is necessary for plant growth. That’s not new information – after all, it is the “P” in the N-P-K macronutrient trifecta, along with nitrogen and potassium, of course. With every essential plant nutrient comes the risk of a yield and profit limiting nutrient deficiency. In the years when additional phosphorus applications are necessary, questions start to form around how much, what form and when to apply. The “when” piece of the equation can be easier to determine, with many benefits to applying phosphorus fertilizer in the fall.
Apply Ahead of Tillage
One of the most important things to note about fall phosphorus application is the ability to apply before fall tillage, which allows for maximum distribution of the nutrient within the root zone. P has very little mobility in the soil, so it’s important to apply it close to the heart of the root zone. Incorporating phosphorus into the soil with tillage, as opposed to applying on top of the soil, reduces the risk of runoff. Wet soils in the spring may preclude tillage, which makes fall application a safer choice as fall tillage is more likely. Even in no-till fields, fall application provides more time for P to slowly move beneath the soil surface before spring planting.
Know Your Soil pH
Soil pH levels have a direct impact on P availability, so maintaining a pH of anywhere between 6.0 and 7.0 is key when managing phosphorus levels. Actually, this is the optimum range for nutrient availability in most crops. When soil pH is lower than 6.0, more iron and aluminum are available to bind chemically with P, making it unavailable to plants.
In this case, an application of lime neutralizes soil acidity and adds calcium to help unlock some of the immobilized phosphorus. Lime takes three to six months to dissolve in the soil, making fall applications ideal to provide time for incorporation. The amount of lime is determined by soil test results, and varies with the source of lime being used.
Even in optimum pH conditions, only a very small fraction of P is in a soluble form that is available for plant uptake, even though there is often plenty of P in the soil. Soil testing can provide accurate pH results, further insight into P availability in the soil, and recommendations for application based on crop needs.
Soil Testing is Key to Cost-Efficient Application
The goal is to keep P at critical levels — the amount of a nutrient that allows 95-97 percent of a crop’s yield potential to be reached without any additional inputs of that nutrient. Because nutrients are removed via grain and stalks when the crop is harvested, soil testing in the fall after harvest provides an accurate measure of the current nutritional status in a particular field.
The cost of soil testing is nominal compared to the cost of fertilizer. Applying P only in the amount needed, if at all, can save money and reduce the chance of excess runoff into waterways. When soil samples are taken before harvest, the amount the crop removes can be calculated mathematically. For instance, soybeans remove 0.8 pounds of P per bushel; that amount needs to be replaced unless P levels are already high enough to allow this removal and still maintain the critical level.
If test results show P levels lower than the critical level necessary for the crop, additional P should be applied. It is also worth noting that potassium is also often applied in the fall, and it can be done at the same time. Under certain conditions, farmers using a corn-soybean rotation can apply P and K every two years.
Ideally, maintaining P (or any plant nutrient) at critical levels, rather than waiting for a deficiency and subsequent reduction in yield, maximizes profits.