I would advise not entering the kelp rabbit hole or you'll end up buying 50-pound bags and mixing your own. Worst case would be harvesting your own from the North Sea. Just get whatever is convenient in your location.

Have you done any tests on your soil? Do you know if it is in need of particular supplements?

Raw Soil Data

Lets take a closer look at your soil, all results are provided in parts per million (ppm).

pH5.315.8-7LowTotal Nitrogen (N)2.097-18LowNitrate (NO3-N)0.63--Ammonium (NH4-N)1.46--Phosphorus (P)8.485-11OptimalPotassium (K)17.7638-72LowSulfur (S)2.227-16LowCalcium (Ca)134.2793-314OptimalMagnesium (Mg)35.8928-67OptimalSodium (Na)5.530.5-30OptimalIron (Fe)1.634-11LowManganese (Mn)5.564-12OptimalZinc (Zn)0.570.1-0.25HighCopper (Cu)0.020.07-0.28LowBoron (B)0.090.22-0.66LowNutrientYour ResultsOptimal RangeRating

Gibberish.

I'm not sure this recoding will be any better, bigge, but can you confirm these values?

Nutrient Your Results Optimal Range Rating

pH5.315. 8-7 Low

Total Nitrogen (N)2.09 7-18 Low

Nitrate (NO3-N)0.63--

Ammonium (NH4-N)1.46--

Phosphorus (P)8.48 5-11 Optimal

Potassium (K)17.76 38-72 Low

Sulfur (S)2.22 7-16 Low

Calcium (Ca)134.27 93-314 Optimal

Magnesium (Mg)35.89 28-67 Optimal

Sodium (Na)5.530. 5-30 Optimal

Iron (Fe)1.63 4-11 Low

Manganese (Mn)5.56 4-12 Optimal

Zinc (Zn)0.570 .1-0.25 High

Copper (Cu)0.02 0.07-0.28 Low

Boron (B)0.09 0.22-0.66 Low

Knowing the chemistry is nice, but in this case you're dealing with biological components. Seaweed is food for the beneficial microbes in the soil. That's a lot harder to quantify. I've looked into seaweed and never felt the need to use it. Ordinary organic fertilizer is a lot less expensive in my opinion. I have a friend in Phoenix who uses it in his organic program, and I don't see where his lawn excels above neighboring lawns. I'd say if you think you need to spend more money buying stuff, be absolutely sure you are watering deeply and infrequently, mowing properly, and fertilizing late May, early September, and late November before spending any money on frills.

Thanks for the advice, I don't think I'll be using it if it's not a big improvement

As I recall there was a golf course in SoCal, Palm Desert area, that kept getting washed out by flash floods. They had some success using kelp in restoring their turf, but their soil conditions were much different from normal. They were basically growing grass in washed sand and gravel with no nutrients at all.

I use a lot of seaweed, mostly in my compost, and I'm a big fan, but I wouldn't pay for it. A huge benefit fresh seaweed supplies is moisture, which you do not get with the dried product. It takes up rainwater in its little "pillows" every time it rains, then releases it a few days later. In the areas I don't walk, I use it as a smelly, slippery mulch.