June 7, 2020

Pigeon Pea Fertility

Pigeon pea seedling
I had a neighbor, Kathryn, who specialized in growing plants that various butterflies fancied. During a tour of her small yard, she pointed out a pigeon pea plant (Cajanus cajan), which she appreciated both for its butterfly attracting capability as well as its good looks. I went home with a couple of pea pods, which I wanted to plant next to a problematic peach tree. Although I do favor the playful fluttering of butterflies and their pollinating proclivities, this is not why I wanted those pigeon peas. I was interested in their flair for fixing nitrogen. I was also hoping to get a little protein out of my backyard, in the form of beans.

Here's a simplistic rundown on nitrogen-fixing and why I was interested. Nitrogen is essential for all living things, because it is a major constituent of amino acids, which are what proteins and DNA are made of. When I think of breathing air, I first think of oxygen. But in fact, air is almost 80% nitrogen. Unfortunately neither plants, humans, nor any other animals can use this atmospheric nitrogen directly. Plants pull nitrogen in through their roots. From the plant's perspective, this nitrogen is critical. It is found in all plant cells, in plant proteins, and in chlorophyll.

There are several ways for you to make sure there's sufficient nitrogen in the soil to support the plants you grow. You could head out to the garden store and buy a nitrogen-rich fertilizer straight off the shelf. Or you could make your way to the nearest friendly horse farm or cattle ranch and bring some manure home. But, wouldn't it be awesome to add nitrogen to your soil without even leaving home? You could compost your food scraps. Another way is to grow nitrogen-fixing plants in the vicinity of your nitrogen-needy plants. In a nutshell, nitrogen sharing sparked my interest in pigeon peas.

After some consideration, I decided to start with new plantings. I had a promising pomegranate seedling. I planted a couple of inoculated pigeon peas near the pomegranate seedling and watched them grow up together. You generally need to inoculate legume seeds with the bacteria that they need, because it is actually these bacteria that fix the nitrogen. If the specific bacteria were already in the soil, there was no need, but I had no way of knowing. I purchased the suggested cowpea inoculant and hoped for the best. The peas germinated easily and were quite drought tolerant; I was off to a great start. Over time, the pigeon peas reseeded themselves adding to their population as the pomegranate grew larger.

If I let the pigeon pea plants grow after their own fashion, the plant itself would consume all the nitrogen, as it produced pods and seeds. So, it was important to prune them. I attempted to grow them until they were about to flower, cut them back, and dropped them on the soil. I could cover the ground around the young pomegranate with the stems and leaves from the pigeon pea, while the plants themselves continued to grow. As the chopped plant matter decomposed, it acted as protective mulch and added nitrogen to the soil. Pruning the pigeon pea is also a catalyst for its roots to release nitrogen.

Storm season has now arrived in Sarasota. With it comes a fertilizer ban from June 1 through September 30, which is an effort to reduce the excess nutrients that flow into our waterways, feeding algal blooms, such as red tide. With my pigeon pea plants, I can skip the fertilizer all year long. Compared to commercial fertilizer or transporting manure over many miles from the country to the city, nitrogen-fixing plants are an environmental standout.
Pods at various stages
Beans peeking out of the pod
Beans sprouted inside the pod
But I had another use in mind - growing protein for me to eat. The young green seeds are edible. I had read that the entire green pod was edible. And finally, as the pod matures the seeds inside dry out, providing dried beans. For me, this did not pan out so well. First off, there's the tradeoff between how much nitrogen you get versus how many beans you get. More of one meant less of the other. I had originally planned to let some plants go to seed and keep the rest pruned. But, as it turned out, it didn't matter. I didn't really care for the taste. Pigeon pea soup just didn't cut it for this picky eater. Perhaps there are tastier varieties. I haven't tried sprouting the seeds or grinding them into flour, so there's still hope.

If you live near me in the warmer, sandier part of Florida, pigeon peas are a well-adapted nitrogen-fixer to consider. If you live elsewhere, you will definitely want to do your due diligence to determine some appropriate alternatives. For starters, most pigeon pea varieties are not frost tolerant. One approach would be to look for a native species. Native nitrogen fixing plants usually don't need looking after, since they have already spent thousands of years adapting themselves to your particular region. I found this nifty plant-finding tool on the USDA website. You can put in search criteria, including nitrogen fixing capability, drought tolerance, your location, annual vs perennial, etc. I don't say this very often - awesome job, USDA!


DD said...

Great article - really helpful info

Unknown said...

Great information. Very good plant to have. Grow what you eat. Eat what you grow.