Is your soil health improving? Metrics for assessing soil health over time

Author: Natalie Hoidal, UMN Extension educator, local foods and vegetable crops

I’ve been participating in a climate resilience planning cohort with 12 farmers, led by Land Stewardship Project. One of the topics that has consistently come up is how we can measure our soil health over time. Evaluation is important, because it helps us narrow down what’s working, what’s not, and where we should be spending your time, energy, and money. Often we rely very heavily on organic matter as a proxy for health, when there may be equally valuable tools and metrics.

The following article highlights different ways that you can measure your soil health over time, and evaluate the effectiveness of strategies like cover crops and reduced tillage.

Soil test metrics

One of the most common ways that vegetable growers assess their soil health over time is the % soil organic matter. This a useful metric because increasing soil organic matter improves your soil’s capacity to retain water and nutrients. However, it has limitations. We often assume that more soil organic matter is always better, and this may lead us to use expensive and not always beneficial practices, such as applying tons of compost every year in order to drive up that number as quickly as possible.

Soil being sampled for a soil test. Photo: CT Ryan Photography

In addition to measuring organic matter, make sure you’re also monitoring your soil nutrient levels. Excessive compost use can lead to high soil phosphorus, potassium, and salt levels in the soil (e.g. magnesium, calcium). This can in turn drive the pH up, making your soil more basic. Too many nutrients can also have negative environmental effects.

I attended a talk at the MOSES conference last week with Jamie Patton, an outreach specialist for Northeast Wisconsin with the nutrient and pest management program at UW-Madison. She shared that when we’re using soil building practices like cover crops and reduced tillage, a realistic goal is to increase our soil organic matter by about 0.1% per year. Many vegetable farmers are increasing their soil organic matter much more quickly with compost additions; this is fine, as long as other soil chemical properties like P, K, and salt levels are remaining at a sustainable level.

Physical measures

There are some simple tools that can help you to measure how your soil is functioning over time.

The Slake Test

In order to maintain organic matter in your soil, it’s important to build aggregate stability. Aggregates are chunks of soil held together by natural glues secreted from roots, earth worms, and microbes. Aggregates hold organic matter in more stable forms so that it doesn’t break down as quickly, and they improve the structure of your soil over time. The slake test is a simple and user-friendly way to measure aggregate stability by placing an aggregate or clump of soil over a mesh sieve in a jar of water and seeing how well it holds up. Ideally, conduct a slake test every year around the same time, and take notes on how long it takes the soil clump to break down.

Extension educator Anna Cates doing a slake test. Photo: Paul McDivitt

Infiltration test

An infiltration test measures how well your soil holds on to water. This is an increasingly important characteristic, as our Minnesota climate is becoming wetter, with more intense rain storms. Similarly to the slake test, you should do this test annually around the same time each year to measure progress over time.

The Sustainable Farming Association has a video showcasing how to do both of these tests.

Saving time and money, and achieving secondary goals

There are many less concrete, but equally important ways to measure the health and function of your soil. Many soil health practices are accompanied by other goals. For example, maybe one of your goals in planting a cover crop is to reduce weed pressure, and by using a cover crop, you can reduce the number of tillage passes in a season. This saves you time, and has an added soil health benefit.

Or, maybe you’re planting legume cover crops, and your use of cover crops for nitrogen has reduced your reliance on manure or a synthetic fertilizer, which in turn reduces your input costs and helps to balance the P in your soil.

Think about the functions of your soil, such as holding water. While you can measure this with an infiltration test, you can also measure this less analytically, using metrics like “Is that field that’s prone to flooding experiencing floods as often during heavy rainfall events?”

Consider your secondary goals, and try to come up with metrics for measuring success that account for those goals in addition to measuring your soil structure and chemistry. Try to choose a couple of goals that you can keep track of over time. Some examples include:

• How much time did you save?
• How much money did you save?
• Did a certain practice allow you to reduce your tillage?
• Did a certain practice allow you to reduce your fossil fuel inputs?
• Are you seeing less disease pressure in a given field?
• Are you seeing more pollinators on your farm?