Management of tree crop nutrition

12 March 2014 05:31 am - 0     - {{hitsCtrl.values.hits}}

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Unlike annual crops such as vegetables or field crops, the perennial nature of tree crops requires that their nutrient needs be maintained in a holistic and long-term manner.  Practices such as fertilizer application, water management and pruning can affect tree nutrition status for successive years. Tree crop nutrition management should seek to optimize nutrients at ideal levels in the tree that is neither deficient nor excessive.



Tea
The long-term exploitation of soil under the tea plantations (where many estates are >100 years old) has led to impoverishment of soil fertility and stabilization of yields, despite increasing application of external inputs such as fertilizers and pesticides.

Some of the soil degradative processes include: decreasing organic matter contents lower cation exchange reduced water-holding capacity loss in important soil biota (reduced up to 70 percent) acidification (pH down to 3.8) increases in toxic aluminium concentrations compaction of the soil surface soil erosion leaching of nutrients accumulation of toxins (polyphenols) from tea leaves.

Some experiments done in India have showed that:
A mixture of tea prunings, high-quality organic matter and earthworms was very effective at raising tea yields (more than application of fertilizers alone) due to its favourable effects on physical and biological soil properties; a bio-organic fertilization technique increased yields from 79.5-276 percent. The increase in yields by using a bio-organic fertilization technique ranged from 75.9-282 percent, representing a profit gain of up to US $ 5500 per hectare per year compared to conventional techniques.

Despite soil faunal depletion in intensive tea plantations, there is a potential for recovering their population and activities by applying various organic materials with optimal limitation. There is a significant relationship between the earthworm populations present in the field and total green leaf tea yields. The termite:earthworm ratio may be a good indicator for assessing soil degradation status.




Soil and plant analysis
Plant tissues, particularly leaves are analysed to determine the nutrient content in the sample and hence in the selected stand of the crop, with a view to utilizing the data to improve fertilizer use efficiency and to confirm visual symptoms.

The potential role of leaf analysis in fertilizer use includes evaluation of the rates of nutrient inputs needed; checking on nutrient deficiencies and any imbalance, interaction or antagonisms and determination of whether the fertilizers applied are being utilized by the plants.

Basically however, leaf analysis indicates the nutritional status of the crop at the time of sampling.

Soil analysis, on the other hand, has the advantage that it can measure the level of immediately available nutrients in the soil (nutrient intensity) and the soil’s ability to continue the supply of nutrients throughout crop growth (nutrient capacity).

Interpretation of soil analysis allows for assessing fertilizer needs, but it does not allow us to evaluate the efficiency or sufficiency of nutrient uptake to ensure optimal growth and productivity of the crop(s).

Working with rubber (Hevea brasiliensis) in Sri Lanka, I showed that the response to fertilizers and thus the nutrient needs, varied not only with leaf nutrient content, but also according to soil type and the nutrient content of the soil.

I also showed that where the soils of the trial sites had been well characterized and the trials well monitored, the results of a limited number of trials could be used elsewhere on similar soils.

Such adoption is based on relationships of yield to soil parameters and plant tissue analysis.

Fertilizer use has been developed on this basis, Sri Lanka in particular, not only for rubber but for other plantation crops.

Diagnostic criteria with varying levels of confidence for application to different perennial tree crops are now available.

As a consequence, many government agricultural institutions and a number of commercial ones in Southeast Asia have set up their own facilities for field sampling and observation, for soil and leaf analysis and eventually for calculating fertilizer recommendations for perennial tree crops.



Leaf nutrient content
It has been reported that at least for rubber and oil palm, there was sound evidence that the nutrient levels in leaf tissues were influenced by many factors other than inherent factors of soil.

These additional factors include: modifying factors through the soil; fertilizer inputs, type of vegetation cover; e.g. with legume cover there could be higher N, position of leaf tissue within the canopy; factors inherent in the tree; genetic make-up, age of crop, time of sampling; yearly (seasonal) variation, age of leaves and yield of crop (and hence nutrients drained from the system).

The effect of soil and modifying factors through soil are readily appreciated and needs no elaboration. However, other factors do need some discussion.




Time of sampling
Choosing a different time for sampling leaves of e.g. rubber can mean that the samples contain leaves of a different age. Yogaratnam working with rubber showed that the optimum leaf age for interpretation of nutrient status was around 100 days.

However, sampling could occur from around 90 to 160 days after leaf formation. H found a relationship between leaf calcium levels and leaf age and introduced correction factors for leaf age based on leaf and soil Ca values.

Computerized adjustments of leaf nutrient status to correspond to those at optimum leaf age, was subsequently made.

A more insidious variation is one due to seasonality. In rubber, it was showed that the year-to-year variation (even after correcting for leaf age) was quite large, even in plots which had not been given fertilizer and could cover the range from deficiency to sufficiency. In oil palm, too, seasonal and yearly variation in the nutrient contents in leaves has been found to be large.

Thus, when leaf analysis from commercial plantations in any one year is being interpreted, reference must be made to the analysis of leaves from control plots.




Soil analysis
Researchers reviewed the use of soil analysis for rubber and showed that researchers had established relationships between the trees’ performance and total N, NH4F/HCI extractable P (Bray II), sulphuric and perchloric acid extractable P, 6N HCI extractable K and Mg in the soil.

Thus, these soil indicators are generally monitored for rubber.

The same extraction procedures seem to be appropriate for other plantation crops such as oil palm, cocoa and coconuts.



Utilization of soil and leaf data
The relationship between leaf analysis and plant productivity is generally evident for most crops and an assessment of fertilizer needs can be based on such an analysis. However, for a cost-effective approach, leaf analysis has to be integrated with soil analysis.

This is because there may be instances where plant uptake of nutrients present in adequate amounts in the soil may be inhibited by the lack of another limiting element, e.g. uptake of K can be reduced by the lack of N. In a case of this kind, leaf analysis will reflect the need for N and K fertilizer.

Reference to soil analysis will indicate that the K reserves in the soil are adequate and thus K fertilizers need not be used. This allows a savings in the cost of inputs.
In addition to the above it is emphasized that to obtain best results, all analytical data available must be used together with other criteria, in particular the environment and growing conditions (soil type, slope, rainfall and soil cover), physiology of growth and productivity, yield, nutrient requirements of the crop and experimental data from fertilizer trials.

The uptake (immobilized in the growing trees, exported when the harvested crop or its by-products are removed, or returned to the soil when residues are recycled) features prominently in the assessment of actual needs.

Data on uptake by most perennial and other crops are readily available (IFA 1992).

A review of nutrient uptake by rubber, oil palm, cocoa and coconuts has also been published.

Up to the present, the integrated use of soil and leaf analysis on an extensive scale has only been used for rubber and coconut in Sri Lanka. For other crops, soil analysis is often used to monitor a build-up of nutrients, particularly P, to monitor long-term trends and to detect possible imbalances. The approaches used for rubber are dealt with in greater detail below.




Rubber
The first step was to category levels of nutrients into groups and to define levels of deficiency, sufficiency and excess.

Later, the level of nutrients was in soil categorized according to availability classes.

These nutrient levels give an indication of the likelihood of a response, although the actual levels can change over time.

At the same time, data interpretation to assess actual nutrient needs was standardized and the requirements for fertilizer rates according to soil and foliar analysis defined. This tabulation provides a sample of the development which has made possible a computerized approach to interpretations of soil and leaf data.
The rates are for areas yielding 1500 kg ha-1 y-1 of dry rubber. Methods to calculate the additional fertilizer needed for areas yielding above this level have also been developed.

Though these levels are generally used, researchers have also suggested the need for different critical levels for inland soils (mainly “low activity clay”, consisting of soils such as Oxisols and Ultisols) and for coastal soils (mainly soils with 2:1 lattice clays e.g. Inceptisols).

The critical range for the nutrients N, P and K given below reflect the differences for the two major soil groupings.

Some years later, researchers could demonstrate a relationship between soil properties including drainage, yield level, plant age and plant density and response to fertilizers.

On the basis of these relationships, they formulated equations which allow the estimation of N and K fertilizer requirements of oil palm.
However, these equations are not widely used in Sri Lanka.




Coconut
Soil analysis to assess the fertilizer needs of coconut is not fully developed. Generally, there is a dependence on leaf analysis, especially frond 14 of mature palms. The optimum range of the nutrients and fertilizer rates vary with the variety and thus there is a need to identify the genetic make-up of a given stand of palms.

(N. Yogaratnam can be contacted at treecrops@gmail.com)
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