Soil test related question

Asked September 13, 2018, 12:06 PM EDT

Hi: I had you do a soil test of an area we will be seeding. (If you have access, it's Report Number 70513.) It says that the Phosphorus is very high (100+ PPM), Potassium is High (219 PPM). Under the recommendations, it recommends 1 lb/1000 sq ft addition of phosphate before seeding. I'm wondering, if the P is already high, should I really be adding more to the soil before seeding. Thank you for your help. Amy S.

Hennepin County Minnesota

3 Responses

You are asking a very good question. I can understand your confusion since your soil seems to have a high phosphate level but yet you are told to add phosphate. To understand this apparent paradox, it is important to recognize that, in a general sense, phosphorus in the soil exists in two forms: insoluble and soluble. Only the soluble (in water) form can be taken up by plants. The insoluble form can eventually be taken up by plants but it takes a long time. In our yards and gardens, we often strive for rather rapid growth.

If you look at your soil test results, you can see that your soil had a pH of 6.1. This is on the acidic side. Under such conditions, phosphorus (P) forms very insoluble complexes with aluminum and iron. Consequently, movement of this phosphate into plants is very slow. Related to this, your test results indicated that phosphorus was determined by something called the Bray 1 test. This test is used when soil pH is below 7.4, as was your test. This test can be high under conditions of high insoluble phosphate.

The recommendation of adding phosphorus applies to fertilizers that you would purchase containing phosphorus in the soluble form.

This is rather like being on a raft in the middle of the ocean. You are surrounded by water you cannot drink but you could die from a lack of water.

Hopefully this helps. Follow the recommendation for phosphorus mentioned in the soil test results.

Good Luck!!

This is GREAT info. Thank you for explaining so thoroughly!

You are welcome. And by the way if you are seeding your yard, fall is the ideal time to do so provided that it is done as quickly as possible.

Good Luck!!