Last week I talked about organic matter and adding it to my garden. In case you're wondering why I'm so focused on that this year let me explain organic matter and it's roles in plant growth and soil health.
Let's first start with a common mistake as I outlined last week and I also mentioned above; what I am adding is organic materials and not organic matter. Organic materials are just what it sounds like; materials that will breakdown into organic matter, that will benefit the soil and feed the microorganisms and feed subsequent crops to feed people or animals in the future. Organic matter has decomposed from organic materials and will further decompose very little; perhaps five percent per year depending on soil conditions and treatments.
Organic matter (OM) has many benefits to look at. The OM holds moisture and releases it as the subsequent crops demand it. Clay soils hold water but it is usually unavailable for plant use; not like OM. Same with nutrients. OM helps eliminate compaction of soils and prevent crusting, which can negatively affect plant growth, and increase water filtration into the soil. During dry periods, soils with a high OM percentage have crops which show less drought evidence than soils low in OM. Often conventional corn fields will have some plants that are obviously affected by the lack of water; if the OM content were higher it would perhaps not be that much of an issue.
Here's some interesting facts I picked up while doing some research for this article: an acre of soil six inches deep weighs around 2 million pounds. 1% OM would weigh in at 20,000 pounds. Most of our soils on the farm average at least 5% OM so each acre contains ~ 100,000 pounds of organic matter. That's a lot of OM! It takes at least 10 pounds of organic materials to produce 1 pound of OM so in order to increase the OM total by 1% we would need to add 200,000 pounds of materials per acre. That's a lot of materials too! 200,000 pounds is 100 tons, which equals 200 yards of the average compost. 200 yards times $45.00/yard equals $9,000.00. That's a bit much for my yearly gardening budget but you can see where I'm going with this.
My garden is 7500 square feet and is at 3% OM. 7500 square feet 6 inches deep weighs in at 340,000 pounds. 3% of 340K equals 10,200 pounds. To increase it to 6% I will need to add 102,000 pounds of organic materials which will break down into 10,200 pounds of OM over time. Of course I won't do this all in the same year. I put on 18 yards of compost this year (average weight of 1000 pounds/acre) which added 18,000 pounds. Now my deficit is only 84,000 pounds. I put 25 yards of henhouse cleanings on (average weight 600 pounds/yard) which added another 15K pounds leaving me only 69K pounds short. The total poundage I added this year so far is 33,000 pounds. Seems like a lot and it is; I shoveled most of it at least twice. Two more years of adding this amount of materials and adding some for breakdown as outlined below and I'll have an ideal OM content in my garden.
Figuring I'll lose 5% of my total OM content per year I'll need to at least add enough organic materials to offset that number. 5% of 10,200 is 510 pounds and multiply that by 10 and you'll get 5100 pounds of organic materials I'll need to add every year just to maintain the OM content of the soil; and more to increase it. 5100 pounds is 5 yards of compost; that's manageable.
Like everything else there's lots of variables at work here. Like crops that add organic matter, the nutrient composition of crops tilled under, the frequency and method of tillage, the types of materials added and any nutrients that will be tied up breaking down the organic materials. The cleanings from my henhouses contains large amounts of soft wood shavings so the chicken manure nitrogen is used to help break down the sawdust and won't be available for the crop to use. Frequent tillage removes vast amounts of OM as does erosion.
Sources of organic materials are wide and varied. Besides the obvious ones of composts and manure other sources abound. Plant and weed debris, cover crops and leaves, organic mulches and straws are but a few amendments which will add to the OM content of the soils over time. Seaweed, kitchen wastes, old potting soils, rotted wood and basically anything that was once alive all add up to more organic materials to put in the garden.
This is not a quick fix but rather a long term process in the gardening world. Continually adding materials to increase the OM content of the garden is a way of life amongst gardeners and farmers. We understand, perhaps better than anyone else, the relationship of plants to soils to us. We too shall sometime add to the OM content of the soil.
Until next week, it's as good as it gets.
Brian
Thanks for the info. This is great!
ReplyDeleteBeing a technical person who enjoys gardening, I think this is a great article. One question though... If you lose 5% of the OM per year and you want an ideal percentage of 6% total OM, wouldn't you have to calculate the maintenance amount on that instead of the 3% deficiency amount that you want to achieve?
ReplyDeleteBill,
ReplyDeleteI've been thinking about this: you will typically lose 5% of your OM total each year. We need to add materials on a yearly basis to make up for this loss.
My soils are at 3% OM and I would prefer them to be around 6%. As my soils are deficient by 3% then I would want to increase my addition of organic materials to bring my OM content total to 6% and then add enough materials to make up for the 5% loss each year. I would add 5 yards of compost yearly once I reach my targeted goal of 6% total Organic Matter.
So in other words, I'll have to add a substancial amount of organic materials in the next year to improve my soil and get it to where I want it. Whether it be compost, manure, crop residue or cover crops, all materials are welcome. A yearly soil test will confirm whether or not my goals have been met.
Brian