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Cotton Petiole Testing

15 November 2011 | Categories: Cotton Petiole Testing

It is now generally recognized that an appropriate way of monitoring and managing the effectiveness of a plant nutrition programme for high yield potential cotton is to track the concentration of nutrients in plant tissue over time. At any sampling time the concentration of nutrient in the tissue reflects the interaction of growing conditions such as water, light and heat availability and nutrient concentration available from soil.  Therefore, if growing conditions are not standardised as much as possible, measurement of the adequacy of a crop nutrition programme may be masked by variability introduced from differences in growing conditions between sampling times.

Much of the inconsistency experienced in the past appears to have resulted from a lack of precision and clarity in protocols for sample collection rather than incorrect critical nutrient concentration ranges. Lack of precision and clarity in sampling protocols leads to large variations in plant tissue nutrient concentration. These reflected the variability within a field than the more valuable change in concentration as a result of supply and demand across time.

The absolute value of plant tissue nutrient concentrations and the rate and direction of its change are all valuable information making good nutrient management decisions for cotton. Single measurements of plant tissue nutrient concentration increase the need to ensure growing conditions are optimal and therefore increase the risk of a poor decision. Multiple measurements over time provide more data, therefore multiplying the information with which management decisions can be made.

LOCATING SAMPLING REFERENCE POINTS

The Field

Select a location or locations in the field that are easy to relocate and preferably related to some features of the performance of past crops e.g. high or low vigour area, high yield area and\ or where other observations such as soil moisture and soil chemistry are being made. The combination of soil nutrient and water information, and plant tissue analysis provide a useful platform on which to make decisions.

Avoid areas of the field such as areas affected by head ditches or tail drains or leaking channels that may be atypical. This is unless the aim of the sampling programme is to monitor these areas.

Once the sampling reference point is located then a samples should be collected in a repeatable pattern within a specified radius of the reference point. A radius of 10 metres should provide low variability due to soil factors and an adequate number of plants (2000 – 5000) from which to obtain samples

The Plant

To reduce variability, standardisation of location on plants for collecting tissue is advisable particularly for N, P, K as these nutrients are highly mobile in the plant and generally have a large gradient in nutrient concentration between leaves from upper to lower leaves. Ideally leaves should be at the same developmental age and undamaged; standardisation in cotton is commonly for the “youngest mature blade” (YMB) on the main stem. Under different growing conditions this may occur at the third to the fifth leaf below the terminal.

CONDITIONS

To get the earliest indication of a crop unable to meet peak nutrient demand it is important to ensure that samples are taken under conditions where the growth rate is unlikely to be limited by any other factor other than nutrients. Hence, it is important that moisture availability and light intensity are similar at each sampling i.e. photosynthesis and transpiration are at or near maximum for the irrigation cycle. To achieve this standardisation of factors such as soil moisture (deficit), time of day and light conditions (cloud cover) should be specified and strictly adhered to for each sampling time. Allowable variance from these conditions should be set so that sampling can be rescheduled if prevailing conditions are outside those where reliable results may be gained.

PLANT PARTS

Leaf Blade (minus petiole)

Leaf blades are most commonly used where a full range of nutrients are to be analyses early and late in the season. There is a large body of interpretation data for leaf tissue analysis with critical concentrations or concentration ranges derived from crop productivity response but because leaves are storage organs for nutrients they may be slower to indicate a deficit in supply to the leaf than petioles.

Petiole

Petiole analysis is most useful for monitoring N and K across time. Water soluble nitrate-N and K are measured hence petioles are more responsive to changes in nutrient availability. This is because they are conduits for the movements of nutrients from the soil and lower leaves to meet current growth needs and therefore usually provide earlier indication of nutrient trends than leaves. Conversely petioles are also more likely to change rapidly in response to adverse sampling conditions. Petiole analysis is most useful in managing major nutrients just prior and during flowering and boll filling. There are few published interpretation studies that have developed critical concentrations or concentration ranges from field responses for nutrients other than N, P and K.

NOTE - It is vital that the petiole is removed from the leaf blade immediately after picking.

TIMING

When monitoring the progress of the cotton crop it is important to develop a sampling plan just prior to or immediately after sowing. This will allow planning of sampling activities around a known sowing data and estimated crop development 

It is important for interpretation that crop growth stage at sampling is correctly identified by either calculating Accumulated Growing Day Degrees or closely defining phenological development stages.

The following suggested timings are a guide to sampling that is most useful in managing crop nutrition.  Early leaf analysis timed to detect underlying nutrient issues that may limit ability to closely manage N and K during peak demand (80 – 90 days post emergence) whereas the later sample is suggested to assess the crops’ ability to maintain its nutrient status under maximum boll load. Petiole analysis is timed to span the start of and peak N and K demand to ensure tactical N and K applications decisions can be made in an informed and timely manner.

SAMPLE TIMING GUIDE

At a minimum it is suggested that sampling be conducted as follows

Sample Type	BPC Product Code	Growth Stage	Day Degrees
Leaf (YMB only)	PAA-01	Late Vegetative -Early Squaring	400-650
Petiole (from YMB)	PAA-10	Early Flowering	750 -800
Petiole (from YMB)	PAA-10		900-950
Petiole (from YMB)	PAA-10	First Max Size Boll	1000 - 1050
Leaf (YMB only)	PAA-01	First Open Boll	1450 – 1700

OPTIONAL

2 samples between 1100 and 1700 DD

The “optional” sampling times are less likely to provide timely information for tactical in crop management of nutrients in the current crop but may help in fine tuning the entire crop nutrition program in the following season.