At the end of December, Indiana’s Department of Local Government Finance calculated the base rate assessment for farmland to be used for taxes in 2015. Predictably, the base rate increased to $2,050 per acre.

That’s a 16 percent increase over the pay-2014 value of $1,760, which was an 8 percent increase over 2013’s $1,630.

The base rate is the starting point for farmland assessments. Assessed value for each acre is the base rate adjusted by a soil productivity index and sometimes by an influence factor.

I mean “predictably” literally. The numbers used to calculate the base rate enter the formula with a four-year lag. To figure the base rate for 2015 taxes, the department used data on rents, commodity prices, yields, interest rates and costs for 2006 through 2011.

All those numbers were available a year ago. Run them through the formula, and your prediction should be right on.

That means it’s possible to predict the base rate for taxes in 2016 using data through 2012. It comes out to $2,420, an 18 percent increase from the 2015 value.

The 2017 base rate will use data through 2013. Almost all of those numbers are available now. The base rate for taxes in 2017 will be around $2,770, a 14 percent increase.

As a result of all these assessment increases, agricultural property taxes have risen by a third since 2007. Property taxes as a whole have fallen 15 percent.

Why the increases? Corn and bean prices were high until recently. Rents have been increasing. These numbers are used to estimate the income earned from farming an average acre.

Meanwhile, the national economy suffered a deep recession, so the Federal Reserve cut interest rates to rock-bottom lows.

The base-rate capitalization formula divides income by an interest rate. Income is up and the interest rate is down, so up goes the base rate.

Commodity prices have dropped, and interest rates may edge upward this year. This may reduce the income capitalization calculation for 2014. The four-year lag means that these new numbers won’t affect taxes until 2018.

Couldn’t you change the base rate formula to slow these increases? There’s a problem with that. The property tax is a tax on the value of property, and the value of farmland is going up.

Its average price has just about doubled since 2007, according to the Purdue Department of Agricultural Economics farmland value survey — on the Web at

The farmland assessment formula is constrained by the Indiana Supreme Court. In a December 1998 decision, the court said assessments must be based on objective measures of property wealth.

Every number in the base rate calculation is objective. That’s why you can get the data to predict the base rate in advance. Income capitalization is one of the recognized methods for determining the value of property.

Attempts to slow the base rate increase by changing the formula might not be defensible in court, if the changes were ever challenged.

The formula could be challenged. Some property tax experts have argued that farmland assessments actually are too low. The base rate per acre was $1,760 in 2013, for taxes in 2014, while the market value of an average farmland acre was $7,446.

So farmland is assessed at about a quarter of its market value. Reduce the four-year data lag to two years, so farmland assessments catch up with rising values, and the base rate still is only 33 percent of market value. Farmland gets a huge tax break from the assessment process, despite the base rate increases.

Homesteads are assessed at their full market values. But they aren’t taxed at those values. Homesteads get deductions.

Statewide, homestead taxable assessed value is only 43 percent of market value. In rural counties, where farmland is more than 10 percent of assessed value, only 38 percent of homestead market value is taxed.

Farmland gets its tax break above the line from the assessment process. Homesteads get their tax break below the line from big deductions. The farmland tax break is more generous than the homestead tax break, but not by as much as it may seem.

And tax break or not, farmland tax bills are likely to rise faster than homestead tax bills, at least through 2017.