Preparing Land for Green Manuring in Organic Farming

Soil fertility doesn’t happen by accident – it’s built, carefully and deliberately, through choices made long before a single seed goes into the ground. In organic farming, green manuring is one of the most powerful ways to rebuild soil health without relying on synthetic fertilizers. Crops like sunhemp (Crotalaria juncea) and lucerne (Medicago sativa) are grown not to harvest, but to plow back into the soil while still green, feeding it from within. But here’s what most guides skip over: the success of green manuring hinges entirely on how well the land is prepared beforehand. A poorly prepared seedbed leads to patchy germination, shallow root growth, and ultimately, far less organic matter being returned to the soil. Getting land preparation right is not optional – it’s the foundation everything else rests on.

What green manuring actually does to your soil 🔗

Green manuring is the practice of growing specific crops and incorporating their biomass into the soil while still green. The un-decomposed plant material gradually releases nutrients – nitrogen, potassium, phosphorus, and more – into the soil, making them available to the next crop. Beyond nutrients, the decomposing organic matter feeds soil microorganisms, improves water infiltration, enhances aeration, and builds long-term soil structure. Whether grown as green manure or cover crops, legumes fix atmospheric nitrogen through biological nitrogen fixation, reducing dependence on commercial fertilizers and directly enhancing soil fertility. The benefits are both immediate and cumulative – each green manure crop leaves the soil a little better than it found it.

Green manure crops: sunhemp and lucerne 🔗

Not all green manure crops are alike. Two of the most widely used in organic farming – sunhemp and lucerne – have distinct profiles that directly influence how you should prepare your land for them.

Sunhemp: the fast-acting nitrogen fixer 🔗

Sunhemp is a tropical legume prized for its speed and nitrogen-fixing ability. Through atmospheric nitrogen fixation, sunhemp can accumulate more than 200 pounds of nitrogen per acre, while also adding 2.5 to 4 tons of organic matter when incorporated into the soil. It germinates rapidly – seeds typically germinate within 3 days under favorable conditions – and grows well across a wide pH range of 5.0 to 8.4, adapting to well-drained calcareous and acidic sandy soils alike. From a land preparation standpoint, sunhemp needs well-loosened, aerated soil with good drainage. It does not tolerate waterlogging. Seeds should be sown in a well-prepared, weeded seedbed to a depth of 2-3 cm, either broadcast or in rows. For green manure use, sunhemp should be plowed in approximately 2 months after sowing, around the time of flowering, when it decomposes most readily and delivers a positive nitrogen balance.

Lucerne: the deep-rooted soil conditioner 🔗

Lucerne, also known as alfalfa, is a perennial legume with an exceptionally deep taproot system that can penetrate several meters into the soil, breaking up compacted layers and drawing up nutrients from below the root zone of most crops. This makes it especially valuable on degraded or compacted soils. However, lucerne is more demanding in its seedbed requirements than sunhemp. Because lucerne seeds are small, the topsoil must be flat and fine – seedbed preparation should be approached similarly to that required for sugar beets. Compressed soils must be loosened before seedbed preparation begins, and fertilization and liming should be completed before any tillage operations start. Lucerne also favors neutral to slightly alkaline soils, so a soil pH check before land preparation is not just recommended – it’s essential.

The land preparation process: step by step 🔗

Land preparation for green manuring crops follows a clear sequence. Each step serves a specific purpose, and skipping any of them compromises the entire effort.

Step 1 – Soil testing before you start 🔗

Before any tillage, test your soil. Knowing the pH and nutrient status tells you what amendments are needed. Lime application should be completed before seedbed preparation – ideally 2-3 weeks before sowing – so it has time to react with the soil and create conditions that support Rhizobium bacteria, which are responsible for nitrogen fixation in legumes. If soil phosphorus is deficient, productivity of sunhemp and other legumes is enhanced on fertile soils, so correcting major deficiencies at this stage pays off significantly.

Step 2 – Primary tillage: plowing 🔗

The main purpose of plowing is to prepare the soil for planting by creating a loose, aerated seedbed, while also controlling weeds and improving water infiltration. For green manure crops, plowing to a depth of 20-25 cm is generally appropriate. This breaks up the topsoil, buries surface weed seeds and residues, and opens the soil structure to air and moisture. Plowing is also crucial for soil aeration – by loosening tough soil layers, it creates pores that ensure proper air pathways and help meet oxygen demands. For lucerne, if the field has been previously compacted (by heavy machinery or intensive use), a deeper initial loosening pass may be needed before standard plowing.

Step 3 – Secondary tillage: harrowing 🔗

Plowing alone is not enough. After the primary tillage, the soil is typically left with large clods that are unsuitable for small-seeded crops. Harrowing is defined as a soil preparation process that involves breaking up clumps, removing crop residues, and leveling the ground, performed after plowing and before planting to enhance soil structure and promote seedbed preparation.

Harrowing focuses on refining the surface layer of the soil to create an optimal seedbed – breaking up clods and leveling the soil surface, which helps create a fine tilth that promotes seed-to-soil contact and uniform germination. For green manure crops, two to three passes of harrowing at different angles are recommended to ensure thorough and uniform soil pulverization. A well-harrowed soil has a fine, crumbled, and level surface that is suitable for seedbed construction.

For sunhemp, a disc harrow or cultivator works well. For lucerne, given its tiny seed size, an extra pass with a rotavator may be needed to achieve the fine, powder-like surface structure these seeds require to establish successfully. Harrowing also helps incorporate any organic matter or amendments evenly into the soil, ensuring nutrients are available to emerging seedlings.

Step 4 – Leveling the field 🔗

After harrowing, the field should be leveled using a leveler or planker. An even surface is critical for uniform seed placement, consistent moisture distribution, and avoiding waterlogging in low-lying spots. Uneven fields result in patchy germination – some areas too wet, others too dry – which directly reduces the stand density and ultimately the organic matter returned to the soil.

Why a fine, friable seedbed matters so much 🔗

The whole point of plowing and harrowing in sequence is to create a fine, friable seedbed – soil that is crumbly, well-aerated, and free of large clods. This matters for green manure crops for several concrete reasons. First, seed-to-soil contact: small seeds like lucerne need firm, fine soil around them to absorb moisture and germinate evenly. Second, root development: sunhemp’s taproot and lucerne’s deep root system need loose soil to penetrate and grow vigorously. Third, weed competition: a well-prepared, evenly sown stand emerges quickly and shades out weeds, which is critical in organic systems where herbicides are not used. Green manuring, if well managed, suppresses the germination of weeds through shading and allelopathy, gradually reducing the weed seed bank in the soil.

Soil moisture and sowing depth 🔗

Even perfect land preparation fails if soil moisture is wrong at sowing time. For sunhemp, seeds should be sown at a soil depth of ¾-1 inch; at greater seeding depths, emergence and establishment will be affected. Sowing is most successful when soil temperatures are above 68°F (20°C). A light irrigation 2-3 days before sowing – if the soil is dry – helps create good germination conditions without causing waterlogging. For lucerne, sowing depth should be kept shallow (1-2 cm) given the small seed size, and the seedbed surface must be firm enough to ensure good contact without being compacted.

Timing land preparation within the crop rotation 🔗

Land preparation for green manuring crops should be planned as part of a larger rotation strategy, not as an afterthought. Warm-season green manure crops like sunhemp can be used to fill a niche in crop rotations, improve the conditions of poor soils, or prepare land for a perennial crop. In practice, this means preparing and sowing green manure crops during fallow periods or between main crop harvests and subsequent sowing dates. There should not be a long gap between digging in the green manure and planting the next crop – this prevents nutrients from leaching out of the soil before being taken up. Timing the land preparation correctly ensures that the green manure can complete its growth cycle and be incorporated at the right stage.

Incorporating the green manure crop 🔗

The final step – incorporating the green manure back into the soil – is itself a tillage operation and marks the return on all the land preparation effort invested. The biomass is incorporated with a plow or disk while still green; the plant matter releases large amounts of carbon dioxide and weak acids that react with insoluble soil minerals to release beneficial nutrients. Microbial activity from incorporation leads to the formation of materials that benefit soil health through improved aggregation, water infiltration, retention, and aeration. For maximum nitrogen benefit from leguminous crops like sunhemp and lucerne, incorporation should happen at the flowering stage – this is when nitrogen is most concentrated in the above-ground biomass and before it moves into seeds. If plants become too old and tough before incorporation, soil organisms find it harder to break down and decompose the material, reducing the efficiency of the whole process.

What do you think? Given that land preparation directly determines how much nitrogen and organic matter your green manure crop can ultimately return to the soil, how much attention does this step get on farms you’ve seen or worked on? And with crops like lucerne requiring more precise seedbed preparation than sunhemp, does that influence which green manure crops farmers tend to choose – and should it?

References

1. https://en.wikipedia.org/wiki/Green_manure
2. https://www.iaea.org/topics/improving-soil-fertility
3. https://edis.ifas.ufl.edu/publication/TR003
4. https://www.feedipedia.org/node/313
5. https://www.barenbrug.biz/forage/field-preparation
6. https://www.patelagroindustries.com/blog/ploughing-vs-harrowing-everything-you-need-to-know
7. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/harrowing
8. https://justagric.com/difference-between-plowing-and-harrowing/
9. https://www.naturland.de/en/producers/service-and-expertise/organic-news/4761-green-manure.html
10. https://attra.ncat.org/publication/overview-of-cover-crops-and-green-manures-2/
11. https://avrdc.org/download/v4pp/training-farmers/1-2-soil-manage/1-2-3-cover-crops-manure/green-manures-cover-crops_hdra1.pdf