Yesterday, I was talking with a company agronomist about their humic acid product and he was saying that the humic acid increased fertilizer efficiency enough that you could cut your fertilizer rates back by 20% and see no detrimental effect.
At the center of this claim is the fact that when fertilizer is broadcast, in the case of Phosphorous, approximately only 20% of the material is available to the plant in the year of application.The company agronomist claims that his product increases the percent available in the first year from 20% to about 50%, thereby allowing for reduced rates.
I asked him if using reduced rates would pull the soil levels down over time and he answered with a very confident “no”, but when we delved into the topic further, he could not explain how this would not happen.
I presented this agronomist with the scenario that if I applied 60 lbs. of P2O5 and the plant removed 80 lbs. of P2O5, where did the other 20 lbs. come from? He could not answer this simple question.
Of course it’s very obvious where the plant obtained the other 20 lbs. of nutrient; it came from the soils reserves. So if it came from the soils reserves, the soil test level will drop over time, or in other words, the soil is being mined of that nutrient.
I think it’s very important that people understand exactly what “efficiency” means. When “industry experts” talk about fertilizer efficiency, they are not talking about reducing the amount of nutrient the plant needs in order to produce a crop. Rather, they are talking about the portion of applied product that remains available to the crop in the year of application.
The take home message of all this is that if you apply less nutrient than the plant removes, soil test levels will fall and eventually yield will be adversely affected.