Spring soil testing: which tests are right for your vegetable farm?

Author: Natalie Hoidal, UMN Extension Educator, Local Foods and Vegetable Production

With so many types of available soil tests available, how do you know which analyses to select, and how often should you do each one?

As of the publishing of this article, the University of Minnesota Soils Lab is still processing samples. Due to COVID-19 precautions, they cannot accept in-person drop-offs, and all samples must be submitted through the mail. Due to reduced staffing, your sample may be delayed. 

Soil test forms have many options, it can be difficult to know which tests you need. 

There are a few nutrients and soil properties that can be measured reliably with a soil test, and should be done every 2-3 years.

Organic Matter

Organic matter is a good basic measure of soil health and is a reservoir of nutrients essential for plant growth. This test should be done every few years when organic matter levels are optimal. If your organic matter is low, consider a soil test every year to track progress in building organic matter.

pH

This test should be done every few years since pH significantly affects plant health, microbial activity, and nutrient availability. If your pH is not in the ideal range, consider a soil test every year to track your progress as you amend your soil.

Phosphorus (P) and Potassium (K)

Phosphorus and potassium are considered macronutrients. Plants use substantial quantities of these two nutrients for growth, and they are often included in a “basic series” analysis along with organic matter.

Neither phosphorus or potassium are highly mobile in the soil, though potassium can leach out of soils if they are very sandy. Assume that in the years between tests, your levels will only change slightly, except for what your crops directly remove. Too much phosphorus in the environment can contribute to degradation of surface water quality; if your soil test shows high levels of phosphorus, consider limiting the addition of compost inputs or using nutrient sources that are low in phosphorus.

Secondary macronutrients and micronutrients are rarely limiting to crop growth in Minnesota, and can be measured more sporadically.

Secondary macronutrients: Calcium (Ca), Magnesium (Mg), and Sulfur (S)

These nutrients are rarely limiting to crop production in Minnesota. In areas with very sandy or acidic soils and low organic matter, test more often. In areas with high organic matter and an optimal pH, test once to determine your baseline, and then only sporadically or if you notice deficiency symptoms. Physiological issues related to calcium deficiency (blossom end rot, black heart, tip burn) may be caused by a failure of uptake and translocation to young tissue and fruit rather than a soil deficiency. For these nutrients, use of lime when needed and building organic matter will help to improve nutrient concentrations.

Essential micronutrients: Boron (B), Zinc (Zn)

Most soils provide adequate amounts of micronutrients, and most plants only use a few ounces per acre of micronutrients each year. Sandy soils with low organic matter may be deficient in micronutrients, but excessive fertilization, especially with boron, can be toxic to plants. Test occasionally, or when you suspect deficiency symptoms.

Lead and soluble salts can be reliably measured with a soil test, but are only necessary for specific systems. 

Lead (Pb)

Image: Natalie Hoidal

Soils in urban areas, near busy streets, at former industrial sites, or within 10 feet of pre-1950s buildings may have a higher risk of elevated lead content. Soils with high lead levels are mostly a risk for children or for people who experience regular, direct soil contact; there may also be food safety implications for some crops. Get a lead test the first year that you plan to grow food in a new area if you suspect that it has been exposed to lead.

Soluble salts

Nutrient salts can easily build up in high tunnels since there’s no natural rainfall to flush out excess salts in the soil, and evaporation from the soil surface can bring dissolved nutrient salts to the soil surface, which can cause damage to plants. To avoid reaching damaging levels of soluble salts, test for them every year in high tunnels.

Image: Carl Rosen's nutrient management guide (see link below)

The reliability of nitrate nitrogen testing depends on your location. 

Nitrogen is highly mobile in the soil, especially in areas with high precipitation. Soil tests are not reliable for predicting nitrogen fertilizer needs in the eastern half of Minnesota due to higher rainfall rates than the western half of the state. Nitrogen fertilizer rates should be determined by the yield goal, organic matter content, and previous crop. See the Nutrient Management Guide for Commercial Fruit and Vegetables for crop-specific recommendations.

There is no rainfall in high tunnels, so this test is applicable for high tunnel producers.

There are a few available soil tests with limited reliability or direct applications to management. 

Cation Exchange Capacity (CEC)

CEC relates to a soil’s ability to to “hold on” to positively charged elements (cations); soil with a high CEC retains more cations than soil with a low CEC. The amount of organic matter and clay in soil is closely related to CEC. For growers who are very interested in soil health, doing this test every 8-10 years and measuring small changes over time may provide some soil health insight. However, by monitoring and improving organic matter, CEC should improve simultaneously.

Iron and Molybdenum

The tests available for iron and molybdenum are not reliable for making fertilizer recommendations Focus instead on monitoring for iron chlorosis (especially common in soils with a pH greater than 7.2) and molybdenum deficiency symptoms. Both nutrients can be applied through foliar sprays.

Copper and Manganese

Soil tests for copper are only reliable for organic soils. Manganese needs are determined by measuring soil pH.

More in-depth information on soil testing:

• Calculating fertilizer rates for fruit and vegetable crops article from Annie Klodd
• Video overview of how to read a soil test and make nutrient management choices accordingly, see Extension Educator Paulo Pagliari's recent Local Foods College Webinar. 
• For crop specific recommendations, see the Nutrient Management Guide for Commercial Fruit and Vegetable Growers in Minnesota by Carl Rosen. 
• Video series: soil and foliar testing in orchards and vineyards

Thank you to Anne Sawyer, Carl Rosen, Nic Jelinski, Chip Small, and Jennifer Nicklay for your input on these recommendations!