A Tale of Two Soils

Meet Lupinus microcarpus variety densiflorus! This California native is part of a species group that can be found all the way from Canada to Argentina. Even better, it is easy and fast to grow, since in the wild it is an annual - a plant that must grow from seed to flower and to mature seeds again in a season or two, and which dies and exists only as seeds in the ground for part of the year.

This year I thought I would help them grow even faster by planting them in a vegetable garden type of potting soil. That produced unexpected and bad results - growing them in a mineral poor, low nitrogen, fast draining soil make awesome plants, while growing them in rich garden soil results in simply not growing them. Why?

This is a tale of the right soil, the wrong soil, and how to tell which is which for a given plant.

We grow a lot of desert and California native plants. A lot of these are specialists in odd, highly inorganic soils, such as serpentine soils. Lupinus microcarpus var. densiflorus is not terribly fussy about soils, being found all the way from Canada to Argentina. What it demands is a well aerated soil.

Below are some of our seedlings in two very different soils. The square pot at the top shows a healthy seedling. The yellowing seedlings in the lower trough are not at all happy. These are growing side-by-side. Why the droopy yellow leaves on the lower plants?

The poor health of the lower plants is a direct result of the soil. It is a commercial potting soil (for containers). Tomatoes grow well in it, as do our parsley, nasturtiums, peppers, and beans. This species does not.

The sickly plants have brown roots. Healthy roots are almost always, at least when small, white. This is true for this annual Lupine - when we unpotted the plant shown above to examine it's roots, here is what we found:

This is the species planted in our "desert bulb mix", which is 3 parts Pearlite, 1 part silver 80 grit blasting sand, and 3 parts rehydrated coir (like peat moss, but sustainable)(parts are measured by volume here). Water drains quickly through it, there are low levels of nutrients, and it is very difficult to turn it sour or make it go anaerobic. Notice the long, white roots visible at the bottom center. These pots are 3" square and 7" deep for scale. This is a textbook healthy and happy young plant.

When we unpotted the others in the trough, we found completely different roots.

These roots are shorter and very brown. They are fighting for their life. Notice how dark the soil is and how it sticks to the starter pot. Compare the texture of this soil with the soil above.

This soil is almost pure compost as purchased - we added most of the sand and whatever pearlite it contains. Vegetables grow fantastically well in this stuff, desert plants are not as fond. Key things to notice - there are fewer air spaces, and a lot of fine grained organic material that forms a sticky "mud" for lack of a more concise term.

The effect of the soil on the plant is dramatic. Soils are not a one-size-fits-all affair. Watch the plant - we have read to some of our plants from various garden books, and it appears many plants are illiterate. They will, however, communicate clearly when they are happy and when they are not. Listen to the plant, take guidance from a book, but if the plant and book differ, then listen to the plant!

These are some photos of the species in the wild this year in California. Notice that this is a sandy soil, yet one that has a very low organic material content and a coarse, aerated structure.