Article by Dale Cowan
Agronomy Strategy Manager and Senior Agronomist
AGRIS Co-operative - CENTRAL OFFICE
Have you noticed unexpected results in your fall soil test reports? If so, you're not alone. About half of the soil tests this fall have shown nutrient levels below expectations. But how does this happen?
The simplest explanation is that crop nutrient removal has exceeded replacement rates over time. This imbalance occurs when harvested crops take up more nutrients than are being replenished through fertilizer or other nutrient applications. Soil tests measure your soil's ability to supply nutrients—not a specific amount of them—but its capacity to replenish what crops remove. For phosphorus (P) and potassium (K), soil test results are presented in parts per million (ppm), and shifting these values requires careful planning.
For example, raising the phosphorus level by 1 ppm requires up to 35 pounds of P₂O₅, and potassium levels need up to 20 pounds of K₂O per acre. At AGRIS, we use regionally tailored averages of 16 pounds of P₂O₅ and 6 pounds of K₂O per ppm change.
The example below demonstrates a common crop rotation with average fertilizer applications. Under these conditions, soil test levels drop by 6 ppm for phosphorus and 9 ppm for potassium over time. High yields can significantly influence this balance, accelerating nutrient depletion.
Application of Nutrients in Pounds per acre of Plant Food
| Crop |
Yield bus/acre |
N |
P2O5 |
K2O |
| Corn |
200 |
190 |
45 |
75 |
| Soybean |
70 |
0 |
0 |
0 |
| Winter Wheat |
110 |
115 |
30 |
35 |
| Total application |
|
305 |
75 |
110 |
| Crop REMOVAL |
ACTUAL Yield |
N |
P2O5 |
K2O |
| Corn |
220 |
198 |
92 |
59 |
| Soybeans |
75 |
300 |
63 |
100 |
| Wheat straw removed 110 |
120 |
144 |
72 |
66 |
| Total removal |
|
642 |
227 |
225 |
| |
N |
P2O5 |
K2O |
| Crop Removal Balance |
-337 |
-152 |
-115 |
| Change in soil test values using average of factors |
- |
(6ppm) |
(9ppm) |
Crop Removal Balance Example
Crop nutrient removal exceeded nutrient applications, resulting in a negative balance:
- Phosphorus (P₂O₅): -6 ppm
- Potassium (K₂O): -9 ppm
What Can You Do?
Start by reviewing your crop yields, nutrient removal rates, and fertilizer applications. Collaborating with our AGRIS Crop Specialists can help you refine your fertility management program, ensuring nutrient levels support your yield goals for the long term.
So, the key question remains: which direction are your soil test levels heading? Let’s work together to keep them balanced and productive.
Nutrient Management Plans
Your AGRIS Crop Specialist is here to help you with your nutrient management plans.
Find out more