Soil Rehabilitation

After a prolonged period of monoculture under tea, the physico-chemical as well as biological properties of the soil deteriorate considerably. Old tea fields due for uprooting usually carry a lot of vacancies. The death and debility of the bushes are caused by diseases and pests and faulty agronomic practices, deterioration of soil fertility and physical conditions due to inadequate fertilization, erosion, poor drainage
compaction, etc., and old age is the cause of low yield of fields due for uprooting. Proper rehabilitation of the uprooted soil is a pre-condition for the success of any replanted operation.

The old tea fields, after uprooting, are put under rehabilitation crops for a period of 18-24 months. The rehabilitation crops used in Northeast India are Guatemala grass (Tripsacum laxum), Pusa Giant Hybrid Napier (Pennisetum purpureum), Citronella grass (Cymbopogon wintarianus).

Guatemala grassCharacteristics of a good rehabilitation crops are: (1) it should yield a large quantity of green material in a short period of time, (2) should be quick growing to be able to suppress the weeds, (3) should have more leafy growth so that its decomposition will be rapid and (4) should have a deep and fibrous roots system to penetrate deeper into the soil and improve soil structure. Guatemala grass has been mostly used in the tea estates of North East India. Benefits of rehabilitation can be summarized as:

  1. it adds organic matter and nutrients to the soil,
  2. helps in improving soil structure and thereby better aeration and moisture storage,
  3. draws nutrients from deeper soil layers and added to the top soil through the lopping,
  4. increases the activity of micro organisms
  5. conserves the top soil, and
  6. breaks the food chain of the primary root diseases.

Rehabilitation crops should be planted in spring in moist soil and manured with 15-20 tonnes of cattle manure per hectare or N:P:K at the rate of 60:30:60 kg/ha (10:5:10) mixtures @ 600 kg/ha initially, and @ 300 kg/ha after each lopping. It is strongly recommended that the lopping be left in situ. The rehabilitation period will depend on the build-up of organic carbon in the soil preferably above 1%. For achieving this normally a period of 1.5 to 2 years is required. If the growth of the rehabilitation crop is not satisfactory, the period may be extended by one more year.

Effect of rehabilitation crops on soil properties was studied at Tocklai. The results showed that roots of Guatemala grass proliferated down to a depth of 120cm and in the process reduced compaction (2.8-7.7%), increased water stable aggregates (24.4-162.5%), hydraulic conductivity (16.0-81.0%), porosity (0.0-15.4%) and organic matter status (5.4-119.6%). It is clear from this study that rehabilitation with Guatemala grass improved the soil health.