by Nigel Melican
Marginal conditions occur when one or more of these ideal growing conditions is not met and where major corrective action needs to be taken to obtain a commercial yield. The symptoms of marginal conditions, however, vary from the obvious to the hidden, and the effects from debilitating to death. In the last 100 years of science based tea growing we have learned many techniques for minimizing the effects of less than ideal conditions – irrigation, shading, fertilizer, land drainage, soil acidification, all spring to mind – but be aware that though effective such techniques increase the cost of production, so have to be factored into your business plan.
Some examples and typical remedies for common marginal conditions are illustrated below under the heading Soil Conditions. Genetic options will be covered in a future post
a) Waterlogging and Drainage: tea is quite flexible in the type of soil it can tolerate – from sand to clay, peat to grit, but the soil must drain well. Any soil subject to even occasional waterlogging has to be considered as marginal. Tea is particularly susceptible to waterlogging, or to the anaerobic conditions that follow from having standing water in the soil. Anaerobis very quickly occurs during waterlogging. Just 30 hours of saturation is sufficient to reduce oxygen in the soil to 1%. In marginal areas where flood or channel irrigation is employed the surface water should never stand for more than 8 hours. The soil type has a distinct effect on oxygen status during wet periods. Russell demonstrated that the relatively much poorer performance of clay soils compared with sandy soils under natural rainfall conditions was due to reduction in available oxygen. Under similar annual precipitation silty clay soil could become waterlogged marginal while sandy soils provided both water and oxygen – however, in the same soils under very dry conditions the sandy loam would dry out and become drought marginal first.
Drainage: If there is any possibility of marginality due to waterlogging the soil must be artificially drained to drop the depth of the natural water table. Assam is particularly subject to waterlogging and much work has been done there to improve conditions for tea. Yields have increased by 45% since drainage was introduced in the mid 60’s. A high water table has an adverse effect on tea root development. A water table at a 36” depth compared with 18” doubled the bulk of feeder roots and plants showed twice the depth of rooting. When growing tea on ex-paddy field land in Pakistan the minimum water table depth aimed for was 5 ft. Certainly a water table higher than 3 ft induced waterlogging symptoms (stunted growth in shallow circles, and nitrogen deficient foliage). But a balance has to be struck between drainage and retention. I told a Pakistani small farmer that “tea hates wet feet”, and later found he was not irrigating his newly planted (and badly wilting) tea as he understood that tea does not like water!
Much of the Assam tea land is artificially drained to avoid waterlogging. Indian TRA (Tocklai) results show that the depth of drainage is important, with a 6” pipe drain at 5 ft depth being significantly better than a 2 ft open drain, both for increasing tea yield and in making more efficient use of nitrogen fertilizer. Yield was 800 kg Made Tea per hectare at the highest nitrogen dose for the deep drain, but only 300 kg MT/ha for the shallow drain.
Spacing of drains is always a compromise between cost and effectiveness. For pipe drains at 5 ft depth under Assam monsoon conditions a spacing of 70 to 90 ft is effective, while spacing at 115 ft and above is insufficient.
b) Marginal Soil pH: with such a narrow ideal range (pH 4.5 to 5.5) the soil pH is often found to be marginal for tea by being either too low (acid) or too high (alkaline) for optimum growth. Given adequate control of other factors bush growth and tea yield is proportional to pH, peaking at pH 5 and approaching zero at pH 3.0 and 7.5. High soil pH conditions are particularly marginal in Pakistan. For a given area of tea, if the costs of production are known and the break even yield can be calculated, then a farmer’s commercial viability is directly proportional to his soil pH.
In more traditional tea countries when tea soils are marginal by being too acid this is generally due to the continual use of sulfate of ammonia fertilizer as a source of nitrogen. In Assam and Sri Lanka, this has in many places caused the pH to drop as low as 3, giving a requirement for liming to maintain productivity. Either finely crushed limestone or Dolomite is usually used. To raise the pH by one point on a sandy soil requires 0.5 tons per acre, a loam soil 0.8 tons, and a clay soil 1.2 tons. Use of mildly acidic fertilizers (Urea) in place of the highly acidic sulfate of ammonia helps to maintain the higher pH though there is now evidence that, due to this change, some NE Indian soils are becoming sulphur deficient.
In Pakistan the natural soils in the few areas suited by climate for tea production are very much too high in pH. In a 1993 survey of 38,000 hectares of potential tea land in North West Frontier Province there were no Well Suited Soils (well drained at pH 4.5 to 5.6). Moderately Suited Soils (well drained at pH 5.7 to 6.5) were 18%, Poorly Suited Soils (poorly drained, pH 6.6 –6.8) were 4%, Not Suited (pH >7.0 and/or poorly drained) were 78%. The Tea Research Station that I helped set up in 1989 for Lever Brothers Pakistan Ltd was sited on Poorly Suited Soil (silt) combining a pH of 6.8 with poor drainage – it was ex-paddy land used for rice growing for generations, and though tea hates it, rice requires poor drainage. Such land must be adequately drained and acidified for tea to grow. Acidification techniques have long been applied to small land areas in East Africa (hut sites) and to slightly high pH areas in India but the Lever Brothers’ project in Pakistan was planned to plant at least 1,500 acres of tea on artificially acidified high pH soils (up to pH 8.0) on a commercial basis. To make this work we had to unravel the interactions between soil type and acidification, and to devise new techniques.
Remedial actions for pH reduction (acidification) most commonly utilise the sulfate ion through application of elemental sulphur and/or aluminium sulfate. The dose rate is highly dependent on the soil chemistry. Hut site acidification is traditionally applied in the planting holes, but with a project involving planting 8 million plants requiring acid soil a quicker acidification method was required. We incorporate the required dose into the soil at two levels (ploughing and harrowing). To achieve the target pH of 4.5 to 5.5 the sulphur requirement in Pakistan is around 6 times the dose suggested in India. On the typical silt soils the requirement was around 2.5 tons per acre for a 1.2 pH point drop. Sulphur is always applied well before planting and is finely divided to encourage quick break down.
Aluminium sulphate has an immediate effect on Pakistan soils at around the same dose rate as elemental sulphur. In Africa 3.9 times the sulphur dose is recommended, in India the TRA recommends 6 times the sulphur dose, and Russell calculates it should be 6.9 times. The relative inefficiency of sulphur in acidifying Pakistan soils is not yet fully understood – though it confirms that every soil is different.
Soil may also be acidified by the use of SOA (sulfate of ammonia) fertilizer as a nitrogen source. This is an expensive method for primary acidification, however, and excessive nitrogen is bad for young tea. However, the effect is slow but sure at 535 lb/ac, a fairly normal level for mature tea, giving a drop from pH 6.8 to 5.5 over a 24 month period.
As an example of acidification in practice – aluminium sulfate was applied to 9 acres of new tea land that had an initial soil pH range was 5.5 to 6.5. Before acidification, in May, this brought the pH down to an average of 4.4 and by August and it drifted up to the target pH 5.0 after one year. Reaction of individual plots varied, with gritty sand averaging 4.5, and silty loams about 1 pH point higher. Soil pH was then maintained using ammonium sulfate fertilizer. One of the downsides of aluminium sulfate acidification is the locking up of potassium, particularly under dry soil conditions (when K is particularly required). This effect is especially undesirable in the North Pakistan soils that are naturally low in K. With practice (and some trial and error) we learned to understand and control this.
Acidification of high pH soil has been shown to be essential for maximizing yield. Its effect on plant growth is, however, obvious right away from transplanting one year old nursery plants into the field. Using mean stem diameter as a growth indicator, plants at the time of planting had stems of ¼” diameter. Planting was done into control soil at pH 6.8, and into soils with two levels of sulphur acidification. At the end of the first growing season the acidified plants were 32 and 36% ahead of control plants in stem diameter.
Despite the distinctly marginal growing conditions in Pakistan and the extreme procedures required to modify them, the cup quality and market value achieved there was equal to imported China black teas.