Practicing crop rotation in your garden
Crop rotation is where crops such as zucchini pictured, are grown in different areas over a 12 month period or even longer. It has been used for hundreds of generations across the world, to ensure fertility, to manage pest control and to ensure the healthiest and most productive crops.
Using legumes such as peas and beans has long been recognized to increase the harvest of crops after the green manure. The role legumes play in crop rotation is vital to contributing organic matter to the soil in addition to enhancing soil life and contributing free nitrogen fertiliser to the soil.
This free fertiliser is fixed from the atmosphere by nitrogen-fixing bacteria called rhizobia, that live in a symbiotic (mutually agreeable) relationship, in nodules on the plant roots. These rhizobium convert atmospheric nitrogen to plant available nitrogen in return for sugars and other nutrients from the plant.
But the nitrogen is not all available as soon as the rhizobium makes it. As the roots are replaced or the plant dies, the nodules containing the nitrogen are shed into the soil, making it available for the following crop. So, we maximise this availability by digging in the roots and plant parts before the next nitrogen hungry crop (eg sweetcorn) is sown.
To enhance the ability of each legume plant to fix nitrogen, we can inoculate the seeds with the specific form of nitrogen fixing bacteria for that seed. The bacteria come in a small pack, mixed with fine peat powder. To inoculate the seeds, mix a little milk powder with the inoculant, then add just enough water to moisten the lot and make the inoculant stick to the seed, before sowing the seed. (Adding milk powder provides the calcium required for the rhizobia to work.)
There are 39 inoculant groups/strains. These are the common ones for garden vegies:
|F||Broad Beans, field peas, lentils|
|I||Mung beans, cow peas, snake beans|
|J||Lab lab, pigeon peas|
When purchasing green manure seeds from sources like Eden Seeds or Green Harvest, you’ll be supplied with a small pack of the inoculant. (Keep both seeds and inoculant in the fridge until sowing time to keep them fresh.)
Will your plants fix nitrogen without the rhizobium inoculant?
Several conditions are required for fixing nitrogen (N) by legumes. Many N fixing rhizobium are acid intolerant, so the regulation of pH with lime or dolomite is needed if your soil is under pH5.
If you’ve never grown a crop of legumes with an inoculant in the spot before, you may not have the rhizobium resident in the soil. You will need an inoculant.
If you have grown an inoculated crop of the same plant in the same spot in the past, you may have enough rhizobium in the soil already.
Legumes make great Green Manure crops
- Green manure crops are those plants that add nitrogen and other nutrients to the soil as they grow and when they are dug in
- Crop rotation typically includes a green manure crop every 4th It follows a root crop and once dug in, typically leafy greens follow.
- It is not true that growing a legume such as beans, next to a heavy feeder such as corn will feed the crop its nitrogen needs. Remember the bulk of the nitrogen is only available after the legume has died and been dug into the soil.
|Leguminous green manure crops for the Redlands|
|Sow in spring/summer|
|Lab Lab Dolichos purpureans||Legume
Trip factor with the vines.
|Mung Bean||Seeds edible.|
|Soy beans||Edible beans, Some Nematode control|
|Peanuts||Edible peanuts if left in the ground. For green manure dig in before nuts form. Trip factor.|
|Sow in autumn/winter|
|Broad beans||Edible beans, adds organic matter Frost tolerant|
|Fenugreek||Edible seeds Good in clay soils|
|Linseed||Seeds good for chooks.|
|Lupins||Sow with oats or barley|
|Subterranean clover||Perennial that grows in cool weather. Attracts bees|