Plant Sap Testing

Plant Sap analysis is a chemical analysis of plant leaves and other organs. Plant analysis reflects both nutrient availability in the soil and the nutritional status of the plant.

Generally, the higher availability of a nutrient in the soil translates into a greater concentration of that nutrient in the plant.

However, the availability of a certain nutrient in the soil is not the only factor affecting its uptake by the plant. Other factors, such as temperature, humidity, soil moisture, plant diseases, etc., considerably affect the uptake of nutrients.

The most common uses of plant Sap analysis are verification of visual diagnostics and identification of nutrient deficiencies or excess.

Generally, the higher availability of a nutrient in the soil translates into a greater concentration of that nutrient in the plant. However, the availability of a certain nutrient in the soil is not the only factor affecting its uptake by the plant. Other factors, such as temperature, humidity, soil moisture, plant diseases etc., considerably affect the uptake of nutrients.

SOIL ANALYSIS VS. PLANT ANALYSIS

In most cases, there is a correlation between plant analysis and soil analysis results. A soil test can be used as a predictive tool to plan fertilizer needs (before planting, for example) and to evaluate the availability of nutrients in the soil.

It cannot indicate whether plants are able to uptake the nutrients and does not give any information about the nutritional status of the crop growing in the soil.

Plant Sap analysis provides information about the present nutritional status of the plant and helps in verifying the visual diagnosis of deficiency/toxicity symptoms. However, it does not provide enough information to explain the reason for a nutritional disorder.

Sometimes plant analysis might not be sensitive enough, since above a critical nutrient level, only a small change in the nutrient concentration in the plant occurs, despite a significant increase in the nutrient availability.

We offer several options which may include some or most of the following parameters:

• Levels of % Nitrogen (N), % Phosphorus (P)

• Levels of Nitrate (NO3) and Phosphate (PO4) 

• Potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S)

• Molybdenum, cobalt, silicon, selenium, ammonium (NH4-N)

• Brix, Pungency

The concentration of nutrients in the plant is not fixed, but constantly changes. It may vary from month to month and even from day to day. The concentration of nutrients even differs between various parts of the same plant.

In order to learn the rate in which a nutrient is absorbed, it is necessary to take samples from several plant parts at different growth stages.

Since many factors affect the interpretation of plant analysis results, it is essential to correctly take the samples. Samples should represent the plants' general population; otherwise, the results will be invalid, and money and work invested will be wasted.

The most important things to be considered in taking the samples are:

• The part of the plant sampled.

• The number of samples taken.

• Timing of taking the sample, i.e., the phenological stage of the plant.

Samples should be taken from plants at the same physiological stage and from the same parts of the plants.

Younger tissues will contain more N, P, and K, while in older tissues, one can find higher concentrations of Calcium, magnesium, manganese, and boron.

Plant Sap analysis results of adjacent plants may vary considerably, even if they were fertilized at the same fertilizer rates. Therefore, it is up to the grower to find, using field trials, the appropriate number and location of samples needed in order to get a valid result. Under conditions of nutrient deficiencies, the variance between the plants is considerably greater.

The nutrient content of the plant is expressed on a dry weight basis. Therefore, any condition that affects the dry weight of the collected sample will affect its nutrient composition. The macronutrients N, P, K, Ca, Mg and S are usually expressed as percentage or as mg/kg and the micronutrients as either mg/kg (ppm) or microgram/ gram.

There is a correlation between its concentration in the plant and the yield of the plant for each nutrient. 

Click here to see the NUTRIENT UPTAKE CURVE, which illustrates the correlation.                   

Field trials determine the relationship between a nutrient concentration and the corresponding yield.

From this curve, we can see that the yield is decreased below a certain critical nutrient concentration. An increase in the nutrient concentration above the critical concentration does not affect the yield until the nutrient concentration reaches another threshold, above which any increase of the nutrient concentration will result in yield reduction.

To identify a deficiency or an excess of a nutrient, the tissue analysis results are compared to known ranges of values and classified as Low, Adequate, High, Excessive, etc.

The concentration of a nutrient in the plant tissues does not reflect only its availability in soil. Still, it is affected by many other factors, such as the concentration of other nutrients in the plant tissue, competition between nutrients, and the mobility of the nutrient in the plant.

If you have additional questions, a specialist or educator at our office can quickly review your results with you over the phone.

Ask us your question HERE.