tapped in | ask the experts
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 P 2
0 5
), 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
YK ?
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 macroelements 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 be experimented with in an organic lettuce system .
Kind Regards , Lynette Morgan
Lynette Morgan is the owner of Suntec International Hydroponic Consultants and is based in New Zealand . She has been contributing articles to Maximum Yield for more than 10 years .
ORGANIC COMPOUNDS
Several organic compounds ( compounds containing carbon ) compose the organic fraction of nitrogen in soil . Soil organic matter exists as decomposing plant and animal residues , relatively stable products of decomposition-resistant compounds and humus . Nitrogen has accumulated in these various organic fractions during soil development .
– passel . unl . edu
22 Maximum Yield USA | February 2017