The best way of estimating N, P and K, in terms of reliability and precision, in organic fertilizers is through combustion or digestion (nitric acid or hydrogen peroxide) of the sample followed by analysis using suitable equipments (N-analyzer, AAS, ICP etc). Gravimetric or volumetric determination methods (e.g. ammonium molybdate or quinoline for phosphorous and sodium tetraphenylborate for potassium) are available. But, the applicability of such methods for liquid organic fertilizer may be reviewed. Sodium tetraphenylborate method is applicable to products of potassium content (K) equal to or greater than 100 mg per kilogram of salt. In case of organic fertilizers, the efficiency of the method is reported to be good for K content equal or more than 6%.

The simplest procedure for K is determination with ion-sensitive electrodes, like this one, which is in a form of a single device: http://www.horiba.com/laquatwin/en/lineup/index.html#k

The simplest way to determination of N, P, K in your plant liquid fertilizer is using some of N-P-K test kits but that can not provide reliable information about content of  nitrogen phosphorus and potassium in your fertilizer. For these purposes (for potassium K+ and nitrate NO3-) is better to use ion-sensitive electrodes, but if you want accurate results particularly for the content of phosphorus and total nitrogen it is best to use a methods which protocols I send to you in attachment (these methods are not simple)

Thanks [email protected] send me the document in attachment

Some of water or liquid fertilizers are manufactured in the country for creating niche of NPK fertilizers. But largely country is dependent on import and about 90% of requirement is imported either as finished product or raw material. Main suppliers of water soluble fertilizers to India are Russia, UAE, Malaysia, Belgium, Israel and China. Manufacturing and use of liquid fertilizers can save considerable energy spent on solidifying the fertilizer in the factory. National Fertilisers Ltd, Nangal had developed a liquid fertilizer ‘Ankur’ which was found to be as good as urea and calcium ammonium nitrate for wheat (Singh and Prasad 1985). Urea plants are recycling a part of their  effluents at a fairly high cost and even then have to strip off some urea before allowing it to flow in the plant water stream. A 1 000 tonnes/day (TPD) plant discharges about 20-25 m3/hr effluent containing 0.8 to 1.5% urea (by weight) and 4-5% ammonia (by weight). With some more effort, this effluent can be developed into a liquid fertilizer. Urea plant effluent can also be used for developing a slow release nitrogen fertilizer by co-precipitating it with neem oil. This technology was developed at the Indian Agricultural Research Institute, New Delhi and the product was named as Pusa Neem Gold Urea. It is a coral shaped golden yellow adduct (35% N as urea and 12% neem oil) and in a field trial at IARI gave 36% higher grain yield of rice at 120 kg N/ha (Prasad et al. 1998).

Please see my following publication:

Water soluble fertilizers in horticultural crops— An appraisal

https://www.researchgate.net/publication/306092504_Water_Soluble_Fertilizers_in_Horticultural_Crops_-_An_appraisal

Article Water Soluble Fertilizers in Horticultural Crops – An appraisal.

Dear Otieno,

You may go through the attached document for some important information.