lemons in the greenhouse

February 2022 -Dealing with Compacted Soil and Monitoring the Progress

Can soil biology grow our way out of soil compaction?

I’ve mentioned my issues with soil compaction a few times. The conventional practice for breaking up hardpan is tillage. In regenerative agriculture, avoiding the disturbance of soil is a key principle because of the detrimental effect on soil structure and biology, and tillage is disturbance to be avoided if possible.

An alternative approach to breaking up hardpan is to increase biology with the use of covers and compost extracts.

Figure 1 shows a typical moist winter soil profile in the greenhouse outside of the trees’ 2.5 cubic foot amended sweet zone. Note the two inch organic layer which is followed by an inch of gravel and an abrupt transition to silty clay with no root penetration. When it’s dry, a pickaxe is required to break it up.

Figure 1. A one-foot soil profile in the greenhouse. The topsoil was stripped 25 years ago.

To see how long it takes to break up the compaction layer with monthly applications of compost extract, we set up an experiment at four locations in the greenhouse. We started in the winter of 2021/22 hoping that adding compost extract before the soil heated would provide a jump start on increasing soil biomass in early spring. Though, we’re unlikely to see much of an effect until the soil temperature rises above 10℃ (Davidson & Janssens 2006).

Here’s our procedure:

1. Record the date, soil temperature and soil moisture level

2. Identify and record the compaction layer with a tensiometer

3. Take a soil sample at 3”- 6” level from three spots and mix

4. Penetrate the compaction layer at the drip line in six locations with a digging fork

5. Apply two gallons compost extract (0.25 lb. compost/10 gallons of rainwater)

6. Test the soil samples for F:B ratio and biomass, slake, earthworm count, root penetration

7. Repeat the test monthly

I’m currently enrolled in the Soil Food Web microscopy lab tech course and as I become more proficient at microscopy, I will add the soil food web microscopy assessment to step 6 of the study.

At present we are using the MicroBIOMETER soil testing kit for Fungal to Bacteria (F:B) ratios and biomass results. Here’s a video of Oliver, my charming high school assistant, carrying out a test (see video) with the kit.

In February 2022 we filmed our compaction study procedures (see video) at the drip line of a Cocktail Grapefruit tree which was planted three years ago in newly amended soil.

The soil results for the test are as follows:

Soil temperature: 9°C (48°F)

Soil Biomass: 220 units of µg microbial carbon / gram soil

Fungal: Bacteria: 0.4:1

According to the Soil Food Web succession stages defined in the following chart, the F:B ratio of this soil puts it into an early succession stage best suited for grasses, a long way from the 5:1 ratio suitable for fruiting trees.

Figure 2. Dr. Elaine Ingham's Soil Food Web succession levels.

I look forward to monitoring our progress through the growing seasons and sharing results in future blogs.