Cover crops and ground cover management

The undulating topography coupled with the high rainfall of the traditional rubber-growing belt predispose the soil towards erosion. In order to conserve soil and improve soil fertility, cover crops are grown and maintained. Leguminous ground cover enhances the growth and yield of rubber.

Advantages of cover crops

1. Prevent the beating action of rain and effectively reduces run off
2. Increase soil moisture and keep down the temperature during summer
3. Smother weeds
4. Add large quantities of organic matter and improve soil structure
5. Fix atmospheric nitrogen resulting in improved soil fertility

Common leguminous cover crops grown in rubber plantations

Pueraria phaseoloides

This is a vigorous twiner and creeper and forms a dense thick ground cover when established. It can withstand strong sun and smother weeds. Seed rate is about 3-4 kg per ha. A disadvantage is that it is grazed by cattle unless afforded protection.

Mucuna bracteata

It is a deep-rooted fast growing legume with moderate drought resistance and shade tolerance. A native of Tripura, it forms a thick luxuriant cover and suppresses all weeds. The dried leaves form thick mulch and it is less palatable to cattle. The seeds are comparatively big and the seed rate is 200 g/ha.

Calopogonium mucunoides

It is a twiner and creeper with tolerance for poor soils. It has a rapid initial growth and dies off during the dry months and is a prolific seeder. Seed rate is 3.0 to 4.5 kg per ha.

Centrosema pubescens

It is a perennial climber and creeper that can grow in shade. It grows slowly and is not tolerant to wet conditions. It smothers weeds. Seed rate is 5.5 kg per ha.

Since the seeds of cover crops often have very hard seed coats, it is advisable to have certain pre-sowing seed treatment such as acid treatment, hot water treatment, and abrasion treatment to obtain a high percentage of germination

Establishment of cover crop

Cover crops are generally established from seeds. However, Pueraria phaseoloides and Mucuna bracteata can also be propagated by stem cuttings. About 420 beds of 1.2 m x 1.0 m per ha are prepared immediately after the pre-monsoon rains. If cuttings are used, fresh cuttings two or three feet long should be planted when frequent rains are available during the month of June or July. On the other hand, if seeds are used, they should be sown in prepared patches during May after the pre-monsoon rains.

Seeds of cover crops have very hard seed coat, which delays or inhibits germination. Therefore pre-sowing treatment is done to ensure uniformity and higher percentage of germination. This also helps in reducing the toxic compounds present in the seed coat, which inhibit the nodule forming bacteria.

Acid treatment

Seeds are treated with concentrated sulphuric acid for a period of 10 min for P. phaseoloides, 30 min for M. bracteata and 20-30 min for C. mucunoides.

Hot water treatment

This method is suitable for the seeds of P. phaseoloides and C. mucunoides. Hot water treatment is carried out by pouring water at 60-80oC over the seeds in a container till all the seeds are submerged, for a period of 4-6 hours before sowing.

Abrasion treatment

Abrasion treatment is done by mixing the seeds with sand (about 1-2 times the quantity of seeds) and then grinding them gently in a mortar. Scarification of seeds by rotating in drums lined with sand paper can also be adopted. After treatment, the seeds may be soaked in water overnight before sowing.

The pretreated seeds are mixed with equal quantity of rock phosphate and sown in rows or in equidistant patches between the plant rows.

Powdered rock phosphate (20% P₂O₅) @ 150 kg. per ha is applied in two equal splits, the first one month after sowing and the second two months after the first application. In areas where the soils are known to be deficient in available potassium, application of a mixture of 150 kg of rock phosphate and 50 kg of muriate of potash is recommended.

Cover crop should not be allowed to grow in a circle of about 2 m diameter around the plants or along the contour terraces for 3 to 4 years to prevent its competition with the young rubber.