Nutrient Formulations and PPM Levels for Different Crops
High-quality and balanced nutrient formulations are the basis of successful hydroponics and matching nutrient levels to the crop being grown is often one of the more technically challenging aspects of production. There are a huge number of variables when it comes to either choosing a nutrient product or making up your own secret recipe from the individual fertilizer salts. While there are many published nutrient formulations, selection of the most suitable is often fraught with mistakes and usually requires some fine tuning to optimize them for a specific growing system and crop.
Why Do Nutrient Formulations and PPM Levels Differ?
Nutrient formulations for all hydroponic crops contain the same basic essential elements — nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulphur (S), iron (Fe), manganese (Mn), zinc (Zn), boron (B), copper (Cu), and molybdenum (Mo).
Some formulations may contain additional beneficial elements and bottled nutrient products may also have other various non-essential additives. While all nutrient formulations have the essential nutrients, the levels of these can vary considerably depending on the crop being grown, the environmental conditions, minerals already present in the water supply, the type of substrate, and the stage of growth. The objective of any good nutrient formulation is to supply the plant with all the essential elements required in ratios close to what the plant will remove from the nutrient solution. This means a nutrient formula not only supplies all the plants’ requirements so no toxicity or deficiency occurs in any one element, but also that ions are removed from the solution in the ratios they are continually supplied. This is particularly relevant in closed systems that recirculate the nutrient solution such as NFT or DFT, while open systems are a little more forgiving of formulation imbalances since fresh solution is supplied at each irrigation.
Left: Young seedlings require a lower EC than more mature plants, thus the ppm of macro elements is lower. Middle: Vegetative tomato and other fruiting plants run different N to K levels compared to when the crop is fruiting. Right: Lesser grown hydroponic plants can be difficult to determine nutrient ppms and ratios.
PPM
Parts per million (ppm) is the most widely recognized and easily understood way of reporting the level of each element in the working strength nutrient solution. Comparing ppm levels (at the same EC) of different formulations is the most accurate way of checking and evaluating differences. Just looking at the differences in fertilizer weights between two separate formulations is not a good indication as most fertilizers supply more than one element in the solution (for example, calcium nitrate supplies both calcium and nitrate in a nutrient formulation while potassium nitrate supplies nitrate and potassium). PPM levels in the working strength nutrient solution differ between formulations due to the differences in each of the fertilizers they may contain, but are also dependent on EC. A solution with a higher EC has a higher amount of nutrients, and thus higher ppm levels of some or all of the elements than one with a lower EC. PPM levels are not only useful for comparing two or more different formulations, but also for interpretation of nutrient solution lab analysis reports. By comparing the nutrient sample lab results back to the original ppm of the feed nutrient solution, it can be determined which elements are increasing or decreasing over time and adjust these as required. This is a standard and ongoing process in commercial hydroponic production, not only to provide optimal nutrition for the crop being grown, but also to minimize fertilizer wastage. Most nutrient formulations need adjustment over time as crops move through different stages of growth or as seasons and growing conditions change.
Left: Strawberries are another crop which requires different formulations for each stage of growth to meet nutrient uptake targets. Middle: A heavy fruit load increases the requirement for potassium in crops such as tomatoes and ppm levels need to change with each growth stage. Right: Flowering crops also require different nutrient ratios when starting to bloom.
What Do Nutrient Formulations Contain?
Nutrient formulations are made up from a number of separate fertilizer salts with each providing different elements. A typical recipe for a basic hydroponic nutrient formation will contain fertilizers such as calcium nitrate, potassium nitrate, monopotassium phosphate, magnesium sulphate, iron chelate, manganese sulphate or chelate, boric acid or solubor, zinc sulphate or chelate, copper sulphate or chelate, and sodium or ammonium molybdate. Others may contain small amounts of the ammonium form of nitrogen or slightly different fertilizer slats. These are weighed out and dissolved into two or more stock solution (concentrates) which are further diluted, often at a ratio of 1:100 with water to make up the working strength nutrient solution. Many growers use nutrient formulation software that allows them to dial up the ppm they require of each element and then converts this into a formulation of fertilizer weights.
Such nutrient formulation software takes much of the hassle out of long and complex calculations and allows for rapid adjustments to nutrient recipes when required. While there can be quite pronounced differences in ppm levels between formulations, much of this is with the macro elements (N, P, K, Ca, Mg, S), as these are taken up in the highest quantities in plants. The trace elements, or micronutrients (Fe, Mn, Zn, B, Cu, Mo), may vary somewhat but to a much lesser degree than with the macros.
The table below shows the broad range of ppm levels that are commonly found in hydroponic nutrient formulations: Note that nitrogen is usually supplied as mostly nitrate (NO3-), however, a smaller percentage of ammonium (NH4+) may be used for specific purposes in some formulations.
Levels of Macro- and Micro-Elements in Hydroponic Nutrient Formulae
| Element | Level (ppm) |
|---|---|
| Nitrogen | 70-450 |
| Phosphorus | 20-100 |
| Potassium | 70-650 |
| Calcium | 70-240 |
| Magnesium | 20-95 |
| Sulphur | 20-100 |
| Iron | 1-6 |
| Manganese | 0.5-3 |
| Boron | 0.1-0.9 |
| Zinc | 0.1-0.5 |
| Copper | 0.05-0.1 |
| Molybdenum | 0.02-0.07 |
How Do We Know What PPM to Target for Different Crops?
Knowing exactly the ppm of each element a particular crop would be taking up at any one time is difficult to determine since so may factors influence uptake, however, nutrient formulations are generally quite generous with ppm levels and there is a good buffer or reserve of each element in a well-balanced formulation. There have been many decades of data on nutrient uptake for the main hydroponic crops such as tomatoes and capsicum and this has allowed many good formulations to be developed. For lesser-grown crops or crops new to hydroponics, obtaining a leaf analysis of a well-grown and healthy plant can be used as a starting point to develop a suitable nutrient formulation. There is also general data available on factors that influence nutrient uptake in most plant species. For example, higher light levels promote iron uptake, and formulations for crops in high-light or summer growing conditions typically require higher iron levels than those in lower light conditions. Temperature and humidity can also influence the ratio and overall uptake of certain elements, as can EC and pH levels.
Table 2 below shows some examples of general ppm values in the working strength solution for each element in some commonly grown hydroponic crops. Note: these are all grown at different EC levels.
Crop PPM Level
| Element | Lettuce | Tomato (fruiting) | Rose | Strawberry (fruiting) |
|---|---|---|---|---|
| Nitrogen | 140 | 370 | 202 | 158 |
| Phosphorus | 32 | 98 | 94 | 77 |
| Potassium | 100 | 513 | 114 | 262 |
| Calcium | 150 | 295 | 260 | 125 |
| Magnesium | 24 | 74 | 68 | 48 |
| Sulphur | 35 | 103 | 95 | 67 |
Vegetative and fruiting phases also determine how ppm levels should change over the life of crop — heavy fruiting hybrid crops of tomatoes, for example, take up significantly more potassium when carrying a good fruit load than when the plants are vegetative.
This potassium largely accumulates in the fruit tissue and is a factor which determines fruit compositional quality. Flowering crops also require bloom formulations with allowance for the additional elements incorporated into the flower tissue. While a bloom formulation provides a different ratio of elements compared to a vegetative-only formulation, the use of bloom or fruiting formulations is not what triggers the plant to start the flowering process (this is determined by multiple other factors such as plant age and maturity; for some species it is day length, for others it may be temperature, etc).
Table 3 shows some examples of how nutrient ppm levels and ratios change in a tomato crop from vegetative to the fruiting phase. Just how much the ratios change is determined by fruit loading.
Crop Stage of Development
| Element PPM | Tomato Vegetative | Tomato Fruiting |
|---|---|---|
| Nitrogen | 409 | 355 |
| Phosphorus | 75 | 106 |
| Potassium | 305 | 493 |
| Calcium | 428 | 289 |
| Magnesium | 72 | 90 |
| At EC | 3.5 | 3.5 |
A basic understanding of ppm levels and how these may differ over the life of a hydroponic crop is a good basis for working through the process of nutrient formulation and adjustment. Each growing situation is different so being flexible with nutrient ppm levels, knowing how to adjust these, and responding to changes in growth is worth investing a little time for maximum yields and quality from a hydroponic system.
