Nature has a way of doing this, through the symbiotic relationship between beneficial soil bacteria called Rhizobium and the root nodules of certain ‘nitrogen fixing plants’ like pulses and legumes and trees like acacia ( karuvelam ).
The root nodules of these plants and trees play host to Rhizobium bacteria, which absorb atmospheric nitrogen and convert them into ammonia, and subsequently to nitrogen-containing organic compounds, the nutrition needed by plants. Of course, nitrogen fixing plants are not the complete answer to the nutritional needs of your garden, you will still have to add manure to the soil initially. But over a period of time the nitrogen fixing plants will enrich and vitalise your soil.
Choices for home gardensSo, what are these nitrogen fixing plants? “Some such plants that could be grown in home gardens are the beautiful sangu pushpam , the nutritious fenugreek or vendayam , agathi keerai , clover, soybeans, alfalfa, hyacinth bean or mochai , avarakkai , groundnut, sweet peas, cowpea, French beans, broad bean, green gram, pigeon pea or red gram, chickpea or kondai kadalai , sun hemp, and senna. You can also plant local trees like karuvelam , babool, vanni , aanai gundumani , vaagai , false rudraksh, pungam , kodukkapuli , and kalyana murungai, ” says horticulturist Hariesh Krishnamurthy of Offshoot, a centre that educates people on gardening.
Apart from the sangupushpam , most of the plants mentioned above have a four/five-month life cycle, while the trees are, of course, perennial. In traditional agriculture, farmers plant a crop of nitrogen fixing plants between other crop cycles thus fertilizing their soil effortlessly and without expensive and energy-intensive factory fertilizers. But in a home garden, we can simply plant them alongside our other plants, and harvest them. “Post-harvest, chop them into small bits and put them back in the soil”, recommends Hariesh. Most of these plants are best planted in December or January and harvested by April. The second season involves planting in June-July and harvesting in October. These plants need around seven hours of good sunlight to nodulate well.
Nitrogen fixing trees are great for avenues too. Some like the alfalfa sport very beautiful flowers. Hariesh adds, “Some nitrogen fixing trees like pungam , aanai gundumani , kodukkapuli and kalayana murungai are ideal for homes as they will not attack the building’s foundation. You have to plant them the proper way, by digging three feet into the soil while planting or sowing.”
Rhizobium cultureSince by now, our city soil might have become bereft of Rhizobium bacteria, it is advisable to apply Rhizobium culture to the soil when planting leguminous plants. Since the bacteria have a natural affinity for the root nodules, we can simply mix the culture in water and scatter this into the soil. Rhizobium culture can be obtained from Government Horticulture Centres or from horticulturists.
