Tuesday, August 25, 2015
Soil texture and soil structure are properties of a soil that have major effects on a soils behavior, influencing important properties like the soil’s water holding capacity, its ability to retain and supply nutrients, the rate of water movement through the soil, and how much nutrient leaching will occur under different weather conditions.
Soil texture has is the relative proportions of sand, silt and clay sized particles in a soil. Knowing the amount of these particles in the soil lets us group the soil into a texture class. This has turned out to be extremely useful concept as just by knowing the textural class of a soil we understand many of its properties. For example sand instantly understand that the soil will have low water holding capacity, that water can move through it very quickly, and nitrogen will be very susceptible to leaching.
Of all the particles, clay tends to be the most important in determining the soil properties. This can be seen b examining the soil triangle as only about 25% of the soil needs to be clay to have “clay” included in the soils name, where as it takes 40-50% of the soil to have silt or sand included in the soil texture name. The reason this occurs is because the surface area of the particle has a major influence on this properties, and because clay sized particles are so much smaller than sand particles, the same mass of clay particles as sand will have more than a thousand times the total surface area of the particle.
Soil structure refers to the arrangement of soil particles into groupings, or aggregates. Soil aggregation is an important indicator of workability to the soil and is often used synonymously with the term tilth. The type of structure that develops is dependent on lots of factors – the soil texture, the amount of organic matter, roots, and even the type of clay particles present.
So where am I going with this and how does it relate to manure? Well, this question came up because of sand bedding for dairy cows. The question was how does continually application of sand laden manure impact the soil and could it even change the texture?
So a few facts to get us started, the first thing question we need to know is how much soil is there in an acre? To make this calculation we need to know how deep we are considering, I’m going to pick 4 inches as most tillage equipment would work to this depth. Assuming a soil density of around 75 lb/ft3 (1.2 g/cm3) this means we’d have about 540 tons of soil in an acre.
The next question becomes how much manure are we going to put on and how often. Although its hard to pick a typical manure, I’d estimate sand laden dairy manure to be around 5 lb P2O5/ton and have about 300 lb of sand per ton. If I was applying to provide a year’s worth of P2O5 I’d want to put on around 60 lbs, which means I’d be applying about 12 tons of manure, or about 2 tons of sand per year. Will this impact my soil?
Say that our soil was a loam (20% clay, 40% silt, 40% sand). This means that right now my soil has 108 tons of clay, 216 tons of silt, and 216 tons of sand. If I added 2 tons of sand, my soil texture would change to 19.9% clay, 39.9% silt, and 40.2% sand; an imperceptible change (certainly not one we’d pick up by soil testing as the variability in collecting a sample is much larger than that). However, keep doing this for 84 years, and your soil texture would be 15% clay, 29% silt, and 56% sand – a sand loam soil.
Is this a long time, most certainly – is the soil better or worse than it was before? It’s hard, to say as it depends on longs of factors, like the soil texture we started with, our climate, our crop rotation, and what soil conditions the plant we are growing most prefers, but it is different. If you asked me if I was more concerned about adding two more tons of sand of soil to my acre of field or about losing two tons of soil to erosion, I’d most certainly be concerned about the erosion; however, it’s important to think about our soils long term and how we can best manage those resources to meet our production and sustainability goals.