I have organic sources of N-P-K on hand already. How would I go about mixing them to the correct N-P-K ratios?

Q:

I am growing several types of lettuce in my indoor vertical farm. I am interested in switching to organic fertilizers so that I can become Certified Organic. Along with several sources of organic micronutrients, I have the following organic sources of N-P-K on hand already: Sodium Nitrate Allganic (16-0-0), Tennessee Brown Phosphate (23 per cent P205), and Potassium Sulfate Soluble Fines GSL (0-0-50). All of these ingredients are OMRI approved. How would I go about mixing them to the correct N-P-K ratios? I have a 20-gallon tank and I am using LED lights and coco coir. I only want to use plant-based or mined minerals—no animal products—and I want my homemade hydroponic liquid fertilizer be 8-15-36. If my N-P-K percentages need to be changed, I am not opposed.

– Page

A:

Hello, Page. Thanks for the detailed information. Unfortunately, it’s not just a case of simply switching from normal hydroponic fertilizer salts to organic products in a recirculating system; there are many factors to consider.

First, although you have the N-P-K information for the organic products you want to use, check to see if they contain the other essential macro elements required for plant growth, mainly calcium and magnesium. Calcium in particular is difficult to obtain in a readily soluble form for use in organic hydroponics; most organic calcium fertilizers are slow release. So, the organic fertilizers you have might provide sufficient N-P-K but the plants may still not thrive due to a lack of calcium and magnesium. Also, without knowing the composition of the liquid micronutrient products, it’s not possible to say if these provide all the trace elements in the correct levels. For example, iron can be difficult to obtain in sufficient amounts and in the correct form for hydroponics.

Second, the sodium nitrate product you have, while it may be considered organic, would be fairly toxic in a recirculating system growing lettuce. It not only provides nitrate, but also a considerable amount of sodium that would build up rapidly in the recirculating system, eventually causing major growth issues. Sodium nitrate typically contains 16 per cent nitrogen and 27 per cent sodium, which is why there are restrictions on the use of sodium nitrate in organic production. Often, organic regulations for producers only allow 20 per cent of total nitrogen to come from sodium nitrate, so another source of nitrogen is required.

Next, the phosphate product you have is unlikely to be very water soluble and is likely to originate from rock phosphate. While rock phosphate is organically allowable, it isn’t intended to be a slow-release form of potassium and is usually not soluble enough for use in recirculating, NFT-type systems. Potassium sulphate, however, is soluble and is also used in non-organic hydroponics, so it can be used as a potassium and sulphur source.

The ratio of the three organic products you have listed would be required, theoretically, at the following percentages to give the approximate N-P-K you requested:

  • sodium nitrate – 28 per cent
  • phosphate – 36 per cent
  • potassium sulphate – 36 per cent

This would give an N-P-K of 4-8-18 (as elements) and 200 grams dissolved into 100 liters of water would give 90 ppm N, 166 ppm P, and 360 ppm K. However, as pointed out above, it would also give a toxic level of sodium, and the phosphate is unlikely to be readily available for plant uptake due to insolubility.

To make a usable organic nutrient formulation for hydroponics with a source of nitrogen that is not sodium nitrate, many growers start with an organic liquid base nutrient that is based on fish, blood, bone, or similar materials because they contain amino acids/proteins that are organic sources of nitrogen. The microbial actions in the substrate or soil break them down into nitrates for plant uptake. Alternatively, there are commercial organic liquid nutrient concentrates already on the market that could experimented with in an organic lettuce system.

Kind Regards,
Lynette Morgan

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Written by Lynette Morgan
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Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. Lynette is a partner with Suntec International Hydroponic Consultants and has authored several hydroponic technical books. Visit suntec.co.nz for more information.  Full Bio