May 4, 2013

1 - Pure compost or organic matter without some mineral soil is often not a good potting soil.

Yard and Garden May 4, 2013

Have I made a mistake by planting iris in a mix of topsoil, manure (year-old bags) and compost, with a greater proportion of compost than topsoil or manure (4:2)? Some of the plants are in their original clumps of soil, but others I have separated are in this prepared soil.

I have also used this mix by placing already-sprouted plants in compostable pots, with their seedling starter soil to plant strawberries.

If this is a mistake, please indicate whether or not I can remedy it.

Also, these plants are in the higher Manzano Mountains (7600 ft.). The strawberries are in large pots; the iris plants are in a prepared bed.

Russell W.


When you say "topsoil", do you mean composted material sold as "topsoil" or mineral soil from your garden? That makes a big difference. Some things sold as topsoil are soils removed from a construction site and may just be a mineral soil of unknown composition and quality. In farming country, especially in other parts of the U.S. where soils are well developed, such topsoil is very useful. In New Mexico, this may, or may not, be useful material. When you say "compost", I assume you mean home-made compost from kitchen and garden wastes. This is a good material for gardening, but must be correctly made to minimize disease organisms and to assure that the composting is completed. Even when properly made, compost alone is rarely a good potting soil because it may hold too much water and compact to provide too little aeration for plant roots. Depending on the materials used to make the compost, it may also have high levels of mineral salts that can damage tender new roots. Some commercially produced compost is high in salt and must be diluted by mixing with mineral soil to allow plants to grow well. Finally, old bags of manure may have some of the problems mentioned for compost. Bagged manure may be very high in salts. If the manure was heat treated to sterilize it, that treatment may actually increase the salt problems. If the manure was not sterilized, or if it was sterilized and the bags were opened or torn, they may have become contaminated with various microorganisms. Some of these may be plant disease organisms. Pasteurization of the manure and compost by heating to 140 degrees for at least 1/2 hour will help manage the problem. Pasteurization is better than sterilization because it allows some beneficial organisms to remain and as they increase, they prevent colonization by plant disease organisms. If the materials you mentioned in your question were all the organic (compost and manure) based materials, they should be mixed with mineral soil. If they easily become soggy and hold water too long, use a very coarse mineral soil containing coarse sand or even small gravel. Sand blasting silica sand can also be used. The soil will provides some nutrients from the natural mineral salts in the soil and provide aeration necessary for good drainage and root development. In the iris bed, if this material was mixed with the native, mineral soil (no more than 50% organic material), the iris plants should do well unless there were disease organisms present. In the strawberry planters, if there was no mineral component added, you may need to dig the plants up and mix mineral soil with the material you made. Then replace the mixed soil and plants in the pots.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email:, office: 505-865-7340, ext. 113.


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