Mastering Soil Amendments

With spring’s arrival, life gets a little busier for us horticulture Extension agents. Plants are greening, gardeners are itching to get outdoors, and the annual influx of soil test submissions has begun. Interpreted by agents with recommendations tailored to individual garden needs, these tests reveal nutrient levels, availability, and the necessary amendments. However, there are several considerations to keep in mind when amending soil. Whether new to gardening or brushing up on your skills, understanding soil amendments is not just a key but a powerful tool for ultimate success in your garden.
Begin by understanding the macronutrients: nitrogen, phosphorous, and potassium. These are advertised on store-bought fertilizers as three successive numbers, ultimately representing the proportions of each nutrient within the mix. Knowing how these nutrient proportions interact with plants and the soil is key.
As a gardener, know that nitrogen is the primary nutrient requiring regular replenishment, as growing plants actively absorb and deplete it. Alternatively, phosphorus and potassium are utilized much more incrementally. Because of this, soil tests often reveal extremely high amounts of phosphorous and potassium due to repeated applications where only nitrogen was necessary. Over time, this nutrient buildup leads to leaching, where excess nutrients run off into waterways, causing algae blooms that deplete water oxygen levels and harm wildlife.
Another essential factor revealed by soil tests is pH, which determines how absorbable available nutrients are. High soil pH (anything above 7.0 for vegetables) equates to physically present nutrients becoming “locked” in the ground, unable to be taken up by plants. In such cases, remediation with sulfur, an acidifier, becomes necessary.
Amending soil becomes even trickier when considering both the facets of nitrogen needing to be resupplied and that many gardeners may also prefer organic fertilization.
Known as “side dressing,” many vegetable crops require supplemental nitrogen applications throughout their growing time because the plants are actively depleting the supply to make a crop. However, it’s equally important to consider the crop type being fertilized and its growth stage instead of making blanket nitrogen applications.
Leafy greens depend heavily on nitrogen support to develop a crop, and as such, providing extra nitrogen when they’re half-grown is essential for optimal production. On the other hand, fruiting crops like tomatoes will likely have a negative response to nitrogen if not timed correctly because too much nitrogen leads to excess leaf development and diminished fruit development. For these crops, additional nitrogen should be applied sparingly—ideally when flowering begins or after the first ripe fruit is harvested.
Looking at organic methods of supplying nutrients, gardeners should remember that nitrogen is often low within organic products. Blood meal (12-0-0), cottonseed meal (6-0-1), and soybean meal (7-2-1) are all standard options, and the nitrogen supply within them is limited. As such, incorporation of composted manure then becomes the best option available, as these are reliable sources of higher organic N, P, and K. This should not be overdone, however, or a gardener will find themselves in the same scenario as someone overapplying storebought fertilizer, once again wreaking havoc on waterways and potentially harming their plants.
In the end, soil testing is the first step in having garden soil that supports your success rather than working against it. Properly implementing recommended amendments will then be a vital step two for a successful growing season.
Consult your local Extension Office for details on how you can complete a soil test.

by Anthony Reardon, Horticulture Small Farms Agent, 2025