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By Esper K. Chandler, CPAg, SSc and Noel Garcia, CCA

The importance of the Nitrogen Carbon (NC) ratio concept has been known since 1918 or earlier, as documented by the work of Kraus and Kraybill. Carbon sprayed on crops and soils, such as cotton, sugar cane, tomatoes, melons and other multi-fruiting crops, has been an effective way of slowing vegetative growth and speeding fruiting. Carbohydrate levels are directly related to Nitrogen levels. NC ratios can be adjusted in crops to enhance yields and quality. Regular petiole (sap) tests are the best way to know when to apply N and how much to apply.

The NC ratio of crop plants can be altered through cultural practices and by environmental factors. However, only three types of situations or ratios are generally present in the field, and are described as follows:

Type 1 – Low N – Moderate to High C: If the available N is low, but there is adequate opportunity for carbohydrate synthesis and adequate soil moisture, vegetative growth will be slow and fruiting will be poor or nonexistent due to lack of N. Such plants will have a yellow-green appearance, will be rich in stored carbohydrates, reduced sugars and sucrose (Brix), and will be very low in total N as well as nitrates (NO3).  They may also be relatively low in moisture despite adequate irrigation. Such plants will have very woody stems.

Type 2 – Moderate to High N – Low C: If, on the other hand, the supply of soil N is great and the availability of water is also great, and there is little opportunity for sufficient carbohydrate synthesis, the plants will be very vegetative or lush and may even produce an abundance of flowers which may drop without setting fruit.

Type 3 – Moderate N – Moderate to High C: The desirable situation is the moderate in-between one, when N is maintained at moderate levels determined by such external factors as adequate radiant energy, temperature (heat units) and soil moisture. Thus, Type 2 could be brought into proper growth as Type 3 by increasing the carbohydrate production potential – that is, by increasing the sun light intensity or the available Carbon concentration, drying soils, or by root pruning or removal of the soil N through application of a soluble carbohydrate to the foliage or soil. Also, Nitrogen or moisture applications can be withheld until plants indicate need.

Note, however, that plant nutritional requirements are constantly changing throughout the growing season.

An important method that brings the NC Ratio into balance favorable for fruiting (Type 3) is to apply highly available carbohydrates such as molasses or sugar (sucrose). Humic Acid products also contain very complex organic compound that contains alophilic acids and natural plant hormones that stimulate rooting and fruiting. Current investigators favor a molecular structure as in some organic products consisting of micelles of a polymeric nature, the basic structure of which is an aromatic ring of the “Di” or “Trihydroxyphenol” type bridged by O– , NH2++, N++, S–, and other groups, and containing both free “OH” groups and the double linkages of Quinones. The typical dark color of humic acids and their ability to form stable complexes with metals is consistent with this concept. In the natural state the molecule may contain attached protenaceous and carbohydrate residues, which also form stable complexes with metals.

Thus, the application of such organic products or other carbohydrates can shift a High N – Low C ratio to a Moderate N – Low to Moderate C ratio. This shift is accomplished by increasing C skeletons which then utilize available N. Also, as is the case with humic acids, Carbon use efficiency can be increased by reducing Carbon oxidation or by stimulating overall CO2 fixation.

One of the most important considerations in the use of Nitrogen fertilizers is their effect on utilization of carbohydrates in plants and how they influence the NC ratio. In general, these effects can be summarized as follows:

When Nitrogen fertilizers are supplied in large amounts, the level of carbohydrates will decrease, aiding excessive vegetative growth to occur.

But when the supply of Nitrogen is curtailed, carbohydrate levels will increase and result in more fruiting. This interdependence of Nitrogen supply and carbohydrate (Carbon) utilization can be readily understood in terms of metabolic requirements of Nitrogen assimilation.

When Nitrogen is incorporated into amino acids (in the NH2 form, as occurs in urea) intermediates of the TCA cycle (Krebs Cycle) are consumed, but the continual operation of the TCA cycle requires that these intermediates be regularly replenished. Replenishment can occur only at the expense of utilizations of sugars and their derivatives that are used to convert Nitrogen to the NH2 form.

These latter sugar compounds represent the “Carbon skeletons” for the biosynthesis of amino acids. Thus, the assimilation of inorganic (not NH2 / urea) Nitrogen represents a drain on carbohydrate reserves that needs, at this time, to be replenished for better fruiting. This need can be determined and accomplished with the proper use of TPSL®’s ASK THE PLANT® Petiole (Sap) Analysis.

By monitoring with ASK THE PLANT ® Petiole (sap) analysis, maximum fruiting or vegetative growth can be carefully regulated by the addition of Nitrogen or Carbon as needed in the proper amounts. Along with these, phosphates, potassium, as well as other minerals and micronutrients, can be monitored and supplied to obtain maximum growth and fruiting.

Use of plant growth regulators (stimulators) have shown that they can be programmed according to the needs of the plants as determined by systematic petiole tests for outstanding results. This manifests itself in earlier fruiting, heavier fruiting and an enhanced, larger, earlier, mature top crop in plants.

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