Algae, sand, and phosphorus
On February 28, I planted ‘Wana’ manilagrass as two node rhizomes in sand-filled pots. All pots have had irrigation and rainfall. One pot has not been fertilized. One has received only N from urea. Another pot has gotten N and P, from urea and from diammonium phosphate (DAP). The fourth pot has gotten all the macronutrients—N, P, and K—from urea and DAP and potassium chloride (KCl).
![Algae covers the sand surface in pots supplied with P, 72 days after planting.](/media/algae_4pots_hu226cb1036ce840ae21cacbeaff78a318_454613_6300664f297f461797911b4f5874283a.webp)
It seems there is plenty of K in the sand and in the irrigation water, because the grasses haven’t had much growth response to added K. They definitely respond to P.
And so does the algae.
![The manilagrass grows rapidly when P is supplied, but P addition has caused algae to grow too.](/media/algae_p_hu33d8f2710ea4928d295bd08cdc05f6eb_560309_6367825b7cb1aaada83bd70853734969.webp)
There was a hint that this would happen less than two weeks after planting.
![Pots supplied with P started to show hints of algae growth 11 days after planting.](/media/no_algae_11_days_hu2bd00f883be8212864ba89c70cd22a1a_340777_fc6001576fea8953ef9c5ec7a12f2e9f.webp)
It’s obvious, by looking at how much the grass has grown in 72 days, that P supplied to this grass in this sand speeds establishment. Once grass is established, however, there are all kinds of reasons not to apply P, such as:
reduce algae risk
reduce pollution risk
reduce Poa annua invasion risk
save money
I’d look at the soil test P, and its change over time, comparing the soil P level to the MLSN guideline and evaluating turfgrass performance, before including any P in a fertilizer to established turf.
For more about this, see:
P deficient bentgrass from Doug Soldat
John Kaminski’s SlideShare about algae, especially slide #28