Last month’s column stressed the importance of soil testing. Understanding what’s present and what’s missing in a field’s soil can help determine why some crops are thriving and others failing, and is the first step in determining what needs to be done to create the optimal growing conditions for desired crops.

Just about all soil tests provide NPK levels – Nitrogen, Phosphorous and Potassium. These three elements are critical building blocks to the growth of plants and test results showing insufficient amounts of any of these warrants attention.

Nitrogen (N) deficiency in the soil can result in crops that fail to thrive. Nitrogen is critical in protein development in plants, as well as in photosynthesis. Decomposing minerals in soil, as well as ambient nitrogen in the atmosphere, are the two primary natural sources of nitrogen, but are often insufficient for strong plant growth.

Understanding what’s present and what’s missing in a field’s soil can help determine why some crops are thriving and others failing, and is the first step in determining what needs to be done.


Nitrogen can be added to soil through adding composted organic matter, planting nitrogen-fixing cover crops or applying commercially available fertilizers. While compost offers a relatively low concentration of nitrogen and can’t offer a precise quantity with each application, it has the added benefit of increasing the soil’s organic matter and water retention. Planting legumes or other nitrogen-fixing cover crops and then plowing them into the soil can be very effective, but requires taking the field out of crop production for a period of time to do so. Chemical fertilizers, such as anhydrous ammonia, urea and ammonium nitrate, can provide a precise concentration of nitrogen in a form that is immediately available for uptake by plants, but over-application can damage crops. Careful attention must be paid to timing of applications and soil conditions to ensure proper absorption of the amendment.

Phosphorous (P) is equally critical to crop growth, particularly in early stages of plant development. Composted organic matter can provide some additional phosphorous, but applications of bone meal or rock phosphate, often treated with acidic chemicals to aid in activation, are more effective. Just as phosphorous promotes growth in soil, it will promote algae and other plant growth in water, so careful attention must be paid to avoid over-application which results in runoff into water sources.

A lack of Potassium (K) will negatively impact crop yield as well. A great deal of potassium found naturally in minerals in the soil is unavailable to plants, because it is released too slowly. Potassium chloride is a commonly used fertilizer to adjust available potassium levels, as is potassium-magnesium sulfate, particularly when a soil test shows a need for additional magnesium as well. Potassium-specific fertilizers are also often called potash.

A test will also reveal the acidity (pH) of the soil. Most crops thrive in soils with a pH level as close to neutral (7 on a scale of 0-14) as possible, but some prefer slightly more or less acidity. Soils too high in acidity greatly limit the availability of many nutrients to plants and tend to be high in elements that can be toxic and destroy plants’ root systems. Applying agricultural limestone is the most common and least expensive way to reduce soil’s acidity. On the other end of the equation, many micronutrients depend upon a certain amount of acidity in the soil to nourish the plants. Without them, the plants will show signs of stress. Farms with soil that tests reveal to be alkaline – very low in acidity – can use sulfur to lower the pH to optimal growing conditions.

The results of soil tests usually come with a suggested application amount for each type of fertilizers, usually in the form of pounds per acre. These prescriptions should take into account the type of crop being grown, as different plants have different needs. And with any fertilizer, water solubility of the amendment itself, weather conditions, soil type and level of aeration, soil temperature and timing during the growing season will all play a significant role in how well the plants are able to access and use the nutrients.

In addition, there is growing research into the role that naturally-occurring mycorrhizal fungi in the soil play in facilitating the transfer of nutrients between the soil and the plant roots, and other soil microbes produce critical CO2 along with nitrogen and phosphate for plant development. While there are some commercial sources for biological amendments, they are not as simple to integrate into soil as other fertilizers. There is a balance between how well these organisms can do their job and the amount and quality of organic matter available for them to colonize.

The final step in adding soil amendments goes right back to the beginning – another soil test. Compare the results of the tests over time to get a sense of whether or not the regimen of treatments is progressing as desired, keeping in mind that plants will use and deplete some of what is applied and runoff will also be a factor. Soil fertility management is an ongoing process, requiring constant tweaking to maintain optimal growing conditions.