BRIAN PAIGE
SOILS RECEIVES BEACON AWARD
BRIAN PAIGE
SOILS RECEIVES BEACON AWARD
Managing Over-Fertilized Soil: Have You Ever Had a Bad Haircut?
By Sam Corcoran, UMass Extension Soil and Plant Nutrient Testing Lab Manager Artie Siller, UMass Extension Soil Health Educator
“ My soil test says my nutrients are well-above optimum. How can I reduce the nutrients in my soil?” This is a question we receive at the UMass Soil & Plant Tissue Testing Lab at least once a week, all year long. Accumulating lots of nutrients in your soil is like getting a bad haircut. Happens fast, takes a while to grow out.
The first thing to do is consider the sample that was submitted:
Routine soil tests are for mineral soils( like you find in the backyard), made up of sand, silt, clay, and some organic matter. By contrast, the potting mix you buy in the store contains large amounts of organic material and typically should be analyzed using the saturated media test; this is also called the soilless media test or greenhouse test.
High organic matter in mineral soils can lead to lighter soil and an overestimation of nutrients. To determine if this is the culprit, check the“ scoop density” on your soil test report. We see scoop density values of 0.8 – 1.1 for many New England soils. If your scoop density is closer to 0.6 or less, multiply your results( ppm) by the scoop density. This adjustment will result in lower values( ppm) and give you a better estimate of the nutrient density in your soil. This may give values that are no longer considered to be above optimum. It is rough math but can give a clearer picture.
If you have ruled out all of the above scenarios, you can start thinking about managing your excess nutrients. There are four key steps:
Consider testing your soil for soluble salts. Excess fertilizer can result in fertilizer salts that cause plants to wilt. Large amounts of water can be used to leach these excess salts. Be aware that leaching is often unrealistic for landscape management or home gardeners due to the large volume of water required. Leaching often makes sense only for commercial growers experiencing crop loss.
Remove excess fertilizer with cover crops, like oats and tillage radish. These plants will suck up nutrients from the soil and hold onto the nutrients in their stems and leaves. Harvest the stems and leaves and throw them into the compost pile to slowly remove nutrients over time. Apply that compost to another spot in the garden that does not have excess nutrients.
If soil was recently fertilized or contains fertilizer beads, this can result in " spiked " samples. Scoop density is the weight of soil divided by the volume of the scoop, pictured here. Soils high in organic matter tend to be light soils and have a lower scoop density. When soils are very light, they are going to be less dense in your garden. Thus, the nutrients will be less dense( less concentrated). Right to left: high mulch perennial bed, OM = 21.3 % SD = 0.56; field loam, SD = 1.2, OM = 3.2 %; coarse sand, SD = 1.6, OM = 1.4 %.( SD = scoop density. OM = organic matter)
Be patient. Snowmelt, rain, and low-nutrient irrigation water will lead to natural leaching over a few years. This applies to calcium, magnesium, and nitrogen. Phosphorus can stick around for many years( even decades) when the soil has been extremely over fertilized.
Stop adding fertility and watch out for“ hidden” nutrients. For example, if you only need nitrogen, do not buy a 10-10-10 fertilizer that will also add potassium and phosphorus. If you use compost, use a nutrient-poor compost.( continued on page 22)
MALP Newsline | Spring 2025 21