If I want to make 12 tons of alfalfa, how many lbs of K and P do I need to apply?
Thanks, Alfalfa Grower
Dear Alfalfa Grower:
That is an easy and difficult question to answer.
Here is the easy answer: Alfalfa removes 9 lbs of P2O5 and 60 lbs K20 per ton of 90% dry matter produced. So, doing the simple math, in a perfect world, one will need to apply 108 lbs P2O5/ac and 780 lbs of K2O/ac.
The difficult part is to estimate the amount of P and K already present.
1. Our soils are relatively young geologically speaking. Soil minerals contain P and K occur naturally in our soils. Basically soil nutrients come in and out of plant availability. Plant availability of nutrients in the soil depends upon the soil chemistry. This is influenced by a number of factors: micro-organism activity, aeration, soil moisture, pH, soil texture, water quality, lime content of soil, root growth.
2. Higher yielding crops remove more nutrients from the soil.
3. Organic Matter from previous crops contribute to more nutrients in the soil.
4. Fertilizer applications contribute nutrients in the soil.
I have a library of books on this subject, but the answer is to monitor the soil P and K on a routine basis. Follow the applications of fertilizers’ to see the responses. Make up the differences to maintain high amounts of P and K in the soil so that will not limit yields.
Aron A. Quist, CPAg/SS