Why Aren't My Nutrient Levels Going Up?

If your soil test results show that nutrient levels are not increasing year after year or are even dropping despite the fact that you are regularly fertilizing, there are several possible explanations. Here’s a list of why this can happen:

1. Matched Nutrient Needs of Plant – Soil System Perfectly

I think of this as simple put and take. This is when you put fertilizer out throughout the season and plant-soil system takes it up. You provided exactly what the plant and soil needed and no more. If you are trying to increase certain nutrients, continue to build soil health with additions of organic material and then increase inputs of the nutrient or nutrients that you’d like to raise.

2. Nutrient Removal by Plants

Lawn: If your lawn is growing vigorously and you regularly remove clippings (instead of mulching them back), you are also removing nutrients from the system. Over time, the nutrients taken up by the grass and removed from the site can exceed what you’re adding as fertilizer, especially if fertilizer rates are low or not matched to actual plant needs.

Garden & Fruit Production: Hopefully you had a bountiful and nutrient dense harvest and moved that nutrition from the soil and added it into your diet. Be sure that you’re following the soil test recommendations to replenish these nutrients next season.

3. Nutrient Leaching

Nutrients, especially nitrogen (esp. Nitrate - NO3-), potassium (K+), and sulfur (esp. Sulfate SO4-2) can be washed out of the root zone by heavy rainfall or excessive irrigation. This process, called leaching, moves nutrients deeper into the soil profile, beyond where plant roots can access them. As a result, even though you applied fertilizer, the nutrients may not remain available to your plants. Check out the video below for a discussion on nitrate leaching and cation exchange processes:

https://www.youtube.com/watch?v=GH5YShUrm5U

4. Nutrient Immobilization and Fixation

Some nutrients, like phosphorus, can become fixed or bound to soil particles, especially in soils with high clay content or imbalanced pH. This means the nutrient is present in the soil but not available for plant uptake. In such cases, you might add fertilizer, but the soil test doesn’t show an increase in available nutrients because they are chemically locked up (Ex. Calcium phosphate, aluminum phosphate, inter-layer clay fixation of potassium)

5. Soil pH Issues

If your soil pH is too high or too low, it can limit the availability of certain nutrients, even if they are present in adequate amounts. For example, a pH above 7 can restrict the availability of micronutrients and phosphorus, while a low pH can limit the availability of calcium and magnesium.

6. Losses Due to Volatilization or Denitrification

Some nutrients, particularly nitrogen, can be lost to the atmosphere through volatilization (conversion to ammonia gas) or denitrification (specifically conversion to nitrogen gas in waterlogged soils). These processes are accelerated by improper fertilizer timing, waterlogged conditions, or surface application of certain fertilizer types such as urea that is not watered in appropriately. To minimize volatile losses, water in nitrogen containing fertilizers, including organic fertilizers, with approximately one-half inch of water after application. To minimize denitrification, ensure that you have good drainage and avoid causing saturated or waterlogged soils.

7. Low Organic Matter and Poor Soil Biology

Soils with low organic matter or poor microbial activity may not cycle nutrients efficiently. Organic matter and active soil biology help retain nutrients and make them available to plants over time. Without this organic matter, nutrients can be lost more easily or remain unavailable to grass roots. Regardless of what you are growing, applications of some form of organic material at least annually are recommended to aid in nutrient retention, nutrient cycling, enhancing soil-water relationships, and enhancing life in the soil.

8. Fertilizer Application Issues

Applying fertilizer at the wrong rate, unevenly, at the wrong time, or using the wrong type for your soil and plants can also result in poor nutrient uptake or loss. Always follow soil test recommendations for timing, rates, and product selection and be sure to properly calibrate your spreader.

Summary

Even with regular fertilization, nutrient levels may not rise on your soil test due to leaching, nutrient fixation, pH imbalances, removal by mowing or harvest, atmospheric losses, low organic matter, or poor application practices. Addressing these underlying issues by improving soil structure, adjusting pH, increasing organic matter, using slow-release fertilizers, and matching application rates to plant needs can help ensure that your fertilizer investments result in healthier soil and better nutrient retention.