Dear Thomas,

this is a complex issue but perhaps less so for perennials. See for example:

http://www.cropnutrition.com/efu-plant-analysis#overview

It says somewhere:

"Monitoring using plant analysis for perennial crops may involve sampling as infrequently as once per year, with sampling timing and procedures consistent from year to year. Monitoring for annual crops may require multiple samples during a single season. Where this is done, concentration standards need to be established for a series of growth stages".

You may also have a look at

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-90162003000200021

Best wishes, Kees

have a look at what we did with Hevea!

This issue is perhaps more complex in perennial crops than annuals , because of long juvenile period of the crops before the tree comes to flowering , nutrients stored in perennial framework of the trees and above all extended  annual growth cycle lasting for about 270-300 days( I am not too sure about rubber)  . The issue becomes more complex , since  nutrient pool  in soil does not commensurate with the changes in plant parts , designated index plant parts. Another issue is , perhaps of more over-riidng importance , is the inability ot diagnose the nutrient constraints within the same current season crop. this necessitated to propagate proximal sensing of nutrients , inflorescence analysis etc..This issue , we debated at length not long time back...   

Most important is to have a crop -based  nutrient standards to diagnose the nutrient constraints , simply nutrient status will not do any world of good to either soil or crop, as far as nutrient management is concerned. 

foliar diagnosis and analysis are the most used methods. But, the suitable date of foliar samples selection must be sited in the litterature. In the cas of the rubber tree , if their is no references you must proceed for many dates in the vegetatif cycle and for many seasons to determine the best date of foliar diagnosis. in general, the best date of foliar diagnosis correspond at the pic of vegetation. However, for some species, we can consider the floral diagnosis which allow us to estimate early  the plant nutrients need and to early prevent some nutrient deficiency.

The foliar diagnsis must be effected according an experimental protocol which is sited in the many references.

 Dr.Hasna,good answer.

Rubber cultivation is being expanded extensively into non traditional rubber growing 

areas such as Moneragala district in the intermediate zone of Sri Lanka. This study 

determines the nutritional status of these soils for the growth of rubber [Hevea

brasiliensis (A. Juss.) Muell. Arg.] and the degree of nutrient variability. Soil samples 

were collected from the surface layers (0 – 0.3 m) and the availability of N, P, K, and 

Mg status and the urease activity were determined. Leaf N, P, K, and Mg status in

samples collected from Kalutara, Kegalle and Moneragala regions, were also 

determined to compare the nutritional status of the plants among regions. Soils in the 

Moneragala region exhibited a very high variability with regard to exchangeable K 

(23 – 273 ppm) and exchangeable Mg (15 – 346 ppm) contents. The urease activity of 

the soils was very low (7 µg of NH4

+

-N released g-1 hr-1) compared to traditional

rubber growing areas. This highlights the need to evaluate the agronomic

effectiveness of urea as the N fertilizer in rubber growing soils in this region. Leaf N 

levels in the Moneragala region was significantly low but leaf Mg levels in all

regions were in the high range. The fertilizer requirement of rubber plants could be

highly site-specific in this region and therefore the currently used low K fertilizer 

mixture with a Mg source may not be equally effective in all parts of the region. 

Rubber cultivation is being 

promoted in non traditional areas in the 

intermediate zone of Sri Lanka (e.g. 

Moneragala district) to increase the

rubber production in Sri Lanka, to 

increase the forest cover in the region 

and also to improve the social status of 

the poor in these areas. Although it is

known that rubber plant can be grown in 

the intermediate zone of the Moneragala 

district, various aspects on efficient 

resource management to obtain the 

maximum economic benefits is yet to be 

studied in detail. One important input 

often been used in rubber plantations to 

increase the growth and yields of rubber 

is application of inorganic fertilizers.