FERTILIZING POT PLANTS

One of the things that never cease to amaze me is how many commercial nursery growers in India never fertilize their pot plants. Most of those that do either use an agricultural grade mixed fertilizer (such as Sufala, 15:15:15) or veer straight to the other end and use imported fertilizers costing Rs.50- Rs.100 per kg.

Why is it essential to fertilize pot plants? There are several conditions unique to growing plants in pots:

1. The root zone in pots is almost always limited and the amount of media that the plant roots can exploit is always smaller than it would be in the ground.

2. Pots are watered very frequently, often every day. This leaches away the soluble elements like Nitrogen and Potassium. Worse, the warm times of the year when the plant can utilize the maximum of nutrients are also when maximum watering is done with consequent leaching of the very nutrients the plant needs.

3. Ideally pots are filled with a low nutrient soiless media- this low nutrient level has its advantages in the initial growth stages but it also means nutrients must be added soon. Also, the components used usually do not hold and store as much nutrients as soil so there is the need to fertilize on a regular basis rather than give large amounts occasionally.

4. Growing plants in pots is a high value and high capital cost business where it pays to produce a good, saleable plant as quickly as possible. Fertilizer costs are negligible as compared to the quality improvement and cutting down of growing time.

Fertilizing plants is a relatively complex process but it isn't rocket science. Provided one understands the fundamental facts and issues in the process it is possible to substantially meet the nutritional needs of your plants and get decent if not the best possible results at very reasonable cost indeed.

The basic concepts that need to be understood are:

1. Organic & Inorganic fertilizers
2. Macronutrients and Micronutrients
3. Practical Issues- concentration & EC, mixing, problems, delivery and costs


ORGANIC & INORGANIC FERTILIZERS:

This is a complex issue for all but the most obvious fertilizers- those of animal or plant origin are organic, those of artificial or chemical origin are inorganic. Fortunately ornamental plant growers need not get deep into these issues, unlike growers of food crops.

For practical purposes:
Organic fertilizers are items like animal manures, blood & bone meals, leaf mould etc. Specialized growers, especially in the Far East make various organic teas from molasses, plant wastes etc.

The main points to remember are:

They are usually dilute fertilizers with low concentrations of plants nutrients

Nutrients are complex, undefined and variable

The Nitrogen is in the form if Ammonia ions which has several implications

Many nutrients are released slowly though this is not always the case. This slow release ability is one of the most important attributes for using organics in ornamental pot plant production.

May seem cheap by the ton but are costly in terms of nutrients contained

Supply humus, micronutrients and complex nutrients.

Inorganic fertilizers are either single chemicals- Potassium nitrate, Calcium nitrate, Monopotassium Phosphate, Urea etc or complex fertilizers like Sufala (15:15:15). The main points to remember are:

These are chemicals that can be defined: thus Potassium nitrate, subject to purity levels, is the same anywhere in the world and it makes absolutely no sense to pay 2-3 times more for imported material when local material of equal analysis is available. Despite all the hype, color brochures and fancy packing, it will give the same results as the local stuff.

The analysis for many chemicals is simple and obvious, the complex fertilizers being the exception. These complex fertilizers are manufactured for agricultural use and have the only advantage of being cheap due to government subsidy and must be used after trials and with caution.

The nutrient concentration is high- this means they must be used with care and adequate dilution. They may be costly by weight but are cheap in terms of nutrient content.

The nitrogen can be in Ammonia form but can be selected to be in purely Nitrate form too.

Nutrient availability is usually immediate but may be modified for slower release. We do not discuss coated slow release fertilizers like Osmocote here as they are too expensive in India.

In the vast majority of situations inorganic fertilizers will best serve the needs of the pot plant grower. The two situations where organic fertilizers are used are in the form of animal manures in the potting media and as pelleted chicken manure as a slow release fertilizer-these not only provide nutrition during growth in the nursery but are critical in providing post production nutritional support to the pot plant at the buyer's location.

MACRONUTRIENTS AND MICRONUTRIENTS:

For practical purposes, the fertilizing program needs to provide a limited number of elements to the plant. These are listed below along with examples of common chemicals that will provide that element.

The elements needed are divided into macronutrients and micronutrients. The former are required in large quantities compared to the latter. For example, we use 1-2 grams per one liter water of Calcium nitrate+ Potassium nitrate in the liquid feed to provide the macronutrients Calcium, Potassium and Nitrogen; compare this to 1-2 grams per 100 liters water of Copper Sulphate to provide the micronutrient Copper.

MACRONUTRIENTS:

Nitrogen (N): required in large amounts for plant growth. Plants lacking N show poor growth and are weak and stunted. Acute shortage leads to yellowing and fall of lower leaves. The most limiting element when plants are grown without inorganic feed but usually more than adequate in a well fertilized nursery.

There are two forms: Nitrate and Ammonia. Both provide N but nitrate is generally preferred and should form at least 50% of the N. Ammonia leads to excessive tall growth, soft tissue and can be toxic to many ornamental species. Reaction varies from species to species though- Scindapsus prefers Ammonia N.

Ammonia N also has a strongly acidifying action on the media pH and this is an issue especially with long term crops. Commonly used chemicals include Potassium nitrate, Calcium nitrate, Ammonium nitrate and Urea. The last two are pure Nitrogen- Ammonium nitrate providing 50:50 Ammonia and Nitrate N while Urea is 100% Ammonia N. All are totally soluble in water.

Phosphorous (P): One of the elements most misrepresented by fertilizer manufacturers over the years. It is an essential element and deficiency will give stunted plants with small, deep green leaves and poor flowering. However, relatively little P is required by most plants and there is no need for the high levels of P often seen in "Flower Booster" and similar fertilizers. Such high levels lead to excessively long and stretched growth of many plants. Even higher levels will lead to micronutrient deficiencies as the P reacts with them and makes them insoluble.

Another point to remember is that clay in soil will bind a lot of phosphorous: some soils are so poor in P that initial applications will all be totally absorbed to fill this "hunger". In soiless media P is not bound and is easily available as well as easily leached and will require regular applications.

Current thinking on P is to use it at very low basal levels (5-10% of the N level) and use it to control plant height and growth. If the plants are too stunted, increase P; if too tall, reduce P levels.

Commonly used chemicals include single Superphosphate (which is 50% Calcium sulphate and so it provides Calcium & Sulphur too), Monoammonium phosphate and Monopotassium phosphate. The last two are totally soluble forms. Monopotassium phosphate is also used for foliar sprays where it also has antifungal action.

Potassium (K): Also required in large quantities, it is important for structural strength and disease resistance in the plant and for flowering and fruiting. Deficiencies are seen as lower leaf edge burn followed by leaf drop and naked lower stems. As with N, deficiency of K will be unlikely in well fertilized nurseries. It is easily leached from the media (as well as leaves by heavy rain).

Commonly used chemicals are Potassium nitrate and Potassium sulphate. A cheap alternative is Potassium chloride (Muriate of Potash or MOP). Never use this for pot plants as chloride toxicity can ruin your plants.

Magnesium (Mg): this element is critical for chlorophyll molecule and deficiency is seen as interveinal chlorosis of lower leaves, often with bright red coloration followed by burnt interveinal areas and leaf drop. We have little experience with Mg deficiency since our soil and water are naturally high in Mg. A common chemical source is Magnesium sulphate (Epsom salts).

Calcium (Ca): Like the above elements, Calcium too is required in large amounts but is generally neglected and is commonly deficient. The symptoms of this are quite characteristic- starting with poor development and paleness of leaf tips and followed by dying (necrosis) of the tips. More severe deficiency will lead to extreme reduction in size of new leaves followed by tip death. Flower buds abort and any fruits show typical "blossom end necrosis" - the drying and death of areas furthest from the stalk and nearest the stigma. Calcium deficiency is a complex problem and can occur due to a number of reasons other than actual lack of Calcium in the media. It is more common than is generally recognized. Common chemical sources are Calcium nitrate, Calcium chloride and Calcium sulphate (Gypsum). Calcium nitrate is the best source, clean and fully soluble. Gypsum can be used as a media additive but avoid marine gypsum which has Sodium chloride. Often the only way to clear a Calcium problem is to spray- use Calcium nitrate or Calcium chloride at 5 grams per liter of water for this purpose.

K:Ca:Mg ratio is an important concept: this should be in the region of 5:3:1 and any significant deviation from this will lead to problems. For example, in our growing system, we use coir which is naturally high in K and low in Ca. Also our water is high in Mg. This leads to an imbalance in this ratio and Ca deficiency is a common problem. We use Calcium nitrate in the liquid feed and as foliar sprays to control this problem.

Sulphur(S): Another nutrient required in relatively large quantities but rarely short when plants are well fertilized. It is present in water from around industrial areas and cities and present in single superphosphate, gypsum etc. Deficiency is seen as a general, overall chlorosis.

MICRONUTRIENTS:

Iron (Fe): The micronutrient required in largest quantities. Deficiency is seen as a characteristic chlorosis of new leaves, which slowly go completely white. Very often the problem is more than a simple lack of Fe in the media. It can be root damage due to any reason, often water logging of the roots. It can also be due to high media pH at which the Fe becomes unavailable.

Common source is Ferrous sulphate- this is often useful but won't work if media pH is the problem. In this case the short term solution is to use chelated Iron, especially Fe-EDDHA if available. The long term solution is to lower media pH- use acidifying fertilizers, use nitric or phosphoric acids to lower irrigation water pH etc.

Manganese (Mn): deficiency symptoms are very similar to Fe: chlorosis of new growth. This is often accompanied by necrotic spots on the leaves. As I read somewhere- "only the very experienced or the very foolish will try to distinguish between Manganese and Iron deficiencies on symptoms alone". Manganese sulphate can be used as drench or a chelated form is used as a spray.

Copper (Cu): Quite a common problem when using high organic media containing coir, peanut shells etc because these materials bind copper. Deficiency affects new growth- leaves are small, narrow and with some paleness. Often new growth seems "sticky" and leaves get caught in each other. Flowers become small and quite characteristically, flower color fades markedly. Chelated Copper sprays can be used but a drench with Copper sulphate at 10-25 grams per 1000 liters of water works very well. We use this every 6 months and whenever a problem is seen (this is often in spring when temperatures are beginning to rise and plant growth increases). Excess Cu due to too much of the above drench will show Cu toxicity in the form of iron deficiency symptoms.

Zinc (Zn): Another micronutrient which often gives problems- new growth is affected with leaves becoming smaller, closely spaced (rosseting) and often one side of the leaf will be malformed, leading to a leaf twisted in one direction. Bright variegation like chlorosis may be occur. Flower and fruit drop is seen.

We drench with Zinc sulphate along with the Copper sulphate at the same rate as above to prevent any problems.

Boron (B): A non metallic element, often deficient in open well drained media and after heavy rains as it leaches easily. New growth is severely affected with basal areas of newly formed leaves showing abnormalities. The midrib is distorted and often the leaves look like the mid rib is too short for the leaf and the leaf blade is corrugated and pleated.

In severe cases buds drop and fruits abort and new growth ceases. Stems & fruits develop corky areas and may crack and exude gum.

It is difficult to differentiate between Calcium and Boron deficiency symptoms, especially when mild. Typically, however, boron affects the basal areas of the leaf and Calcium the distal or tip region.

Common chemicals used to supply B are boric acid (needs hot water to dissolve) and Solubor, an easily soluble, more concentrated form. Take care with B- toxicity is also common and many plants are particularly sensitive, e.g. Euphorbia milii

Molybdenum (Mb): rarely seen deficiency except in some vegetables (Cruciferae) and is a problem with Poinsettia. Sodium molybdate is used at extremely low concentrations as a preventive measure.

PRACTICAL ISSUES:

The above chemicals can be used to make a complete nutrient solution supplying all the plants needs. How much of each to use is also relatively easy to decide if you know the percentage of each element in the chemical you use. Details of this are beyond the scope of this article and can be found in any good text book.

Some practical guidelines are:


Keep it simple: use the minimum number of chemicals to provide all the nutrients. Complex formulations will not give better results.

Have a plan: for example, add Super phosphate to the growing media to provide P, Ca and S. Use a simple mix of Potassium and Calcium nitrate to provide K, Ca and N. Add Magnesium sulphate if Mg is needed. Micronutrients can be added as a drench as well as occasional foliar sprays.

Add nutrients as needed- e.g. N in the form of Ammonium nitrate to boost growth and green up foliage plants or P in the form of Monopotassium phosphate to get taller plants.

Remember some chemicals will react in solution, e.g. Calcium and phosphorous will react to give an insoluble powder that will settle out of the solution. Do not mix such solutions together, apply them separately.

Mix solutions thoroughly: we dissolve the powder in large tanks by adding the chemical to a little water and then starting the pump to fill the tank- the large amount of water falling in mixes everything.

Check the EC of the fertilizer solution: this is more important than blindly following a formula. The acceptable EC depends on the plant, the season etc but experience will tell you the level that is acceptable. About 3.0 micro siemens should be generally the maximum. In fact it is possible to feed by EC only- decide on what you need to use and add enough to reach your target EC. We generally use two parts of Calcium nitrate and one of Potassium nitrate and add enough water to get to an EC of 2 -3 depending on the crop and the season.

Getting the fertilizer solution to the plants is not easy. The best way is to use a measured mug or bottle and drench each pot individually- this becomes impossible with large number of pots. We use our watering wands to apply the fertilizer solution. This works best with square pots in our MP trays as there is little wastage. With round pots there is some waste. For larger pots and tubs we use the Junior watering wands to apply the fertilizer solution to each pot individually.

Drip irrigation can be used to deliver the liquid fertilizer. In this case use good grades of fully soluble chemicals and quality filtration.

Costs are much lower by using the specific chemicals. Most chemicals cost less than Rs.20/- per kg. Commercial fertilizers like Urea, Diammonium phosphate and Ammonium nitrate are much cheaper. Using imported fertilizers will not give better results except for some specialty chelates like EDDHA. Whenever trying out costly fertilizers, try local materials as controls. Very often plants are so starved that almost anything will give a response.

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