Grassland Ecological Restoration – A soil nutrient ‘card game’

The idea of soil testing Western or Southwestern native grasslands and finding their nutrient thresholds for restoration is little understood and rarely done. Furthermore, it is considered "heretical" by scientific community to suggest adding nutrients to serpentine soils in particular.

But after restoring more than 800 acres of native grassland to 95% native cover, adding nutrients and organic matter have been critical for success, I have learned.

In many restoration projects, research has looked at what's going on above ground when trying to preserve and restore native grasslands, but rarely do projects do test to learn what is going on below ground. There is little study linking soil nutrients and organic matter tied to specific native species of grasslands plants.

But I have found threshold-nutrient levels -- and organic matter -- to be a requirement for the restoration and sustainability of Western and Southwestern grasslands.

After three decades of soil testing and successful application of nutrients and organic matter, I have found that as in a card game, each native plant needs to be dealt a very specific hand in order to “win” and be able to survive and thrive. This so-called "grassland ecological restoration soil-nutrient card-game" could be helpful for any Western or Southwestern grasslands-restoration project. 

Searching for the thresholds

In 1993, I began to consider the addition of soil nutrients as a critical component of grassland restoration while trying to reestablish native grasses on the 100-mile Tuscarora gas pipeline project (www.ecoseeds.com/greatbasin.html) for the Bureau of Land Management. The project was located north of Reno, Nevada. During the first year, locally harvested native seeds planted in test plots without benefit of any nutrients resulted in a 97% seedling failure.

We then ran soil samples taken from the top two inches of soil around existing, reproducing grass stands of each species we were attempting to plant. The critical issue was to find the nutrient thresholds for seedling survival of each species. The two-inch depth for testing was determined because that was where seedlings would normally germinate.

Using soil taken from the future pipeline site, we planted some of the locally collected seeds in two flats. In one flat, nothing was added with the seeds; the second had seeds plus organic matter and fertilizers.

Many arid-land planting failures are blamed on the lack of rainfall. To take that factor out of the experiment, we watered the two flats daily for three months. In the first flat with no amendments, seedlings grew two inches tall, but then all died of starvation. The seedlings in the amended flat thrived. (See photos of the two flats at www.ecoseeds.com/good.example.html.)

Subsequently, we planted small-scale, 3-foot-by-6-foot test plots at the pipeline site for the six different seeds we planned to use for the pipeline. We did not add any water. The next year, we did dozens of different test plots to determine the correct sowing rates, fertilizer and mulch applications, tailoring them for each species.

We found that some grasses, such as Poa and squirreltail, only needed a small amount of fertilizer and organic matter to get established, whereas the bluebunch wheatgrass and Great Basin wild rye needed large amounts of both.

Seeds sown in mosaics rather than mixes, with the added amounts of organic matter and nutrients required for each species per our test results, had a 100% native cover rate with no cheatgrass in six months.

In 2016, I began a native grassland and wildflower meadow restoration project at Kite Hill Wildflower Preserve in the Town of Woodside, located in San Mateo County, California, 36 miles south of San Francisco and 26 miles north of San Jose.

Taking samples from the top two inches of serpentine soil, I conducted 70 soil tests to determine the native species’ nutrient-threshold needs. I tested two-inch-deep samples from around reproducing plants, and samples from weedy areas of the preserve. The goal was to find what levels of nutrients and organic matter I needed to bring the weedy areas back into a native-threshold range.

The results were from the Waypoint Lab (Anaheim) A-01 nutrient tests and organic-matter soil tests. The results were striking. When nutrients and mulch were applied in the proper amounts, the natives were able to return. No seeds were added to these sites; plants re-established from the existing soil seedbank.

In perhaps the most dramatic example, in one location, a barren serpentine area degraded by human and animal encroachment was restored in one year to a 99% weed-free wildflower meadow. The restoration used only dormant seeds still in the soil and added only nutrients and organic matter.

Testing at Kite Hill, I determined there are four "suits" needed for restoring grassland nutrient and mulch levels on the serpentine soil: nitrogen, phosphorus, organic matter and pH, plus a “wild card.”

Below are some examples of how these four "hands" played out at the preserve:

The “Spades” of Nitrogen: The number of the nitrogen suit is in parts per million and ranges from 1 to 1,000 ppm in the top two inches of the serpentine soil. For example, California poppies needed a minimum of an 80 ppm to stay in the game, and ideally needed 120 ppm. Tidy tips have the widest range found so far on the serpentine soils at Kite Hill, needing a minimum of 10 ppm to stay in the game, and a minimum of 250 ppm to fight star thistle. Ideally, tidy tips showed it could use up to 1,000 ppm!

The “Clubs” of Phosphorus: At Kite Hill, an ideal phosphorus range is between 2 ppm and 90 ppm, with a few weed grasses -- the Brachypodium grass and Rat-tail fescue -- living at the bottom of that threshold and miner's lettuce thriving at 90 ppm. Tidy tips have the widest range, needing only 4 ppm to get established, but ideally would like 48 ppm to thrive in the serpentine soil.

The “Hearts” of organic matter: Very critical in California and absolutely critical in the Southwest where native seedlings needed at least a one-inch-deep layer on the surface for their survival, based on the test plots at the Tuscarora pipeline project. Observations from the test plots included that surface organic matter kept the surface moist while the seedlings were germinating.

Soil tests of samples from the Tuscarora site found that organic matter secured nitrogen like a sponge for the seeds to utilize when they are germinating in the top two inches of soil.

Subsequently, on the barren serpentine slope at Kite Hill, the 3 mm-deep layer of organic matter we applied the first year proved adequate for holding nutrient levels in place. The second year, however, the sun and rain washed about half of the nitrogen out of that small reservoir. In the future, we will be adding more mulch as a larger reservoir for the nitrogen on that site.

The “Diamonds” of pH levels: Testing showed that rather than a one-size-fits-all pH level for the grassland, the plants required a tailored pH based on individual species' needs. Kite Hill's serpentine pH ranges were from 5.8 to 7.4 and very rarely 8.0.

Some species are able to grow in a wide range of pH, while others are limited to very narrow ranges. There are three main groups: pH 7.0 to 7.3 (coyote mint, Lessingia, milkweed, Stipa, tarplants, tidy tips, native Vulpia); 6.5 to 6.9 (Clarkia, coyote mint, Lewisia, Lessingia, Marin dwarf flax, Mariposa lily, California poppy, squirreltail, tidy tips); and 6.1-6.3, which were the lowest ranges required for California poppies and Vulpia microstachys. No natives grow at Kite Hill in the top two inches of serpentine soil at a pH below 6.0, according to my tests.

Other takeaways: At pH 7.4 and above, some weed species grow, such as yellow starthistle, Brachypodium grass and the tallest wild oats, but none of the natives normally grow at this high alkali level. As a first step, something as simple as correcting the pH away from the weed levels could help get rid of some of these alkali-loving weeds and bring the pH back within the thresholds needed by the local natives.

Soil biocrusts posed interesting findings regarding pH at Kite Hill. I took soil samples of where the biocrusts were growing, and within a meter away where they didn't exist. Samples in the biocrust areas were a highly acidic pH of 5.8, compared to pH 6.2 in the adjacent soils where the biocrusts didn't grow.

Soil biocrusts are known to produce organic acids that help dissolve minerals so plants can absorb nutrients. The acid production helps the biocrust organisms and significantly contributes to the overall health and productivity of arid ecosystems, according to multiple published studies.

Biocrust acid production and nutrient-availability enhancement are important for ecosystem functioning. By improving nutrient cycling, especially phosphorus, biocrusts support plant growth in the nutrient-poor soils typical of dryland ecosystems. This function is crucial because many desert plants rely on these microbial communities for their survival. (Eldridge et al 1997, Eldridge 2011, Eldridge et al 2017, Eldridge et al 2020)

Biocrust nutrient and organic-matter thresholds

If we want biocrusts to return, we must find the soil nutrient and organic matter threshold levels for a particular site.

At Kite Hill, biocrusts regenerated on their own in 1-2 years in areas where the top two inches of soil contained adequate thresholds of nitrogen, phosphorus and organic matter. Seeding or inoculating the soil with biocrusts was not necessary for their regeneration or colonization.

I also found that biocrusts needed an inch of organic matter on the surface as their foundation and matrix. The biocrusts emerged from a barren serpentine slope after achieving the following thresholds:

-- Organic nitrogen fertilizer to raise nitrogen levels to >25 ppm threshold. (Non-biocrust soil one meter away was 20 ppm.)

-- Bone meal to raise phosphorus to >11 ppm threshold. (Non-biocrust soil one meter away was 6 ppm.)

--Organic material added to produce 12.6% organic matter (Non-biocrust soil one meter away was 9.2%.)

As one can see, often, it didn't take much additional nutrient or organic matter for biocrusts to recover.

Of note, usually when one is doing soil tests, one doesn't include the plant material. However, when doing a soil test where biocrust exists, I suggest one needs to include the biocrust as part of the sample, since the nutrient and organic-matter levels between a biocrust growing or not growing on a site are on such a thin razor's edge that one needs to know where that edge resides.

For example, in Winnemucca, Nevada, even when the biocrust was included in soil testing, thresholds were only 17 ppm nitrogen, 2 ppm phosphorus and 2.6% organic matter. One could miss the phosphorus the biocrust requires if only testing the soil.

This is an awesomely low threshold to meet, and it should be relatively easy to add these very small amounts of nutrients and mulch and have biocrusts grow back.

The “wild card”: Species thresholds

An intact, weed-free ecosystem is composed of different members, which occur in specific percentages of cover, or Species thresholds, according to my findings. These "wild cards" must be studied to understand how the plant community organizes itself. If one has a target species that one wants to restore, one needs to know its percentage cover as well as the cover percentages of its surrounding neighbors.

I determined the species thresholds by doing percentage-cover transects of the target species in 5%- 10%-cover increments. I studied the percentage cover of the other plants in the transect area in relation to the 5% cover of the target species, the 10% cover of the target-species, and so on. It's important to measure the interactions between different kinds of plants in an area as weed-free as possible -- 95% native cover or better is best. 

Here's how it sorted out in the Goldfields Meadow at Kite Hill, located next to the U.S. Interstate 280 fence on March 31, 2024, which was 100% native cover: 39% cover of goldfields; 25% Vulpia microstachys; 21% tidy tips; 6% Linanthus; 4% California poppy; 3% pepperweed; 1% native Plantago and 1% soap plant.

These species thresholds can be critical to a target plant's survival. Marin dwarf flax seedlings, for example, can easily be wiped out by allelochemicals produced by certain weeds (wild oats, annual rye, and willow leaved wild lettuce) when there is more than 3% cover of those weeds, according to my measurements.

Natives growing among Marin dwarf flax also appear to have a significant negative effect if their numbers are too great. Any amount of cover of soap plants, >20% cover of Stipa grass, or >10% cover of tarplants, all have negative effects. 

However, other natives growing among the Marin Dwarf flax plants can have a neutral to beneficial effect: blue-eyed grass, Brodiaea, Fremont’s star lily, Lessingia, Mariposa lilies, and silver puffs. Kite Hill's yampah can be good neighbor -- as long as this plant is less than 65% cover -- whereas >65% showed a reduced Marin Dwarf flax cover.

References:

--Eldridge, D. J., & Greene, R. S. B. (1994). “Microbial processes in biological soil crusts from two contrasting Australian ecosystems.” Soil Biology and Biochemistry, 26(3), 307-314.
--Eldridge D. J. et al. (2011) “Soil Biocrusts Enhance Nutrient Cycling and Plant Growth in Dryland Ecosystems.” Ecological Applications, 21(3), 671-681.
--Eldridge D. J. et al. (2017) “Biological Soil Crusts: A Key Component of Dryland Ecosystems.” Soil Biology & Biochemistry, 111, 85-95.
--Eldridge, D. J., & Greene, R. S. B. (2020). “Biocrusts: A key component of dryland ecosystems.” Frontiers in Ecology and Evolution, 8(1), 1-12

Copyright © 2025 by Craig Carlton Dremann,
The Reveg Edge, Box 361, Redwood City, CA 94064
Email craig@ecoseeds.com – Office 650-325-7333

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